UPTEC F10 018

Examensarbete 30 hpMars 2010

Further development of moulding technology for underwater applications in nuclear reactors

Hanna Nygren

Teknisk- naturvetenskaplig fakultet UTH-enheten Besöksadress: Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0 Postadress: Box 536 751 21 Uppsala Telefon: 018 – 471 30 03 Telefax: 018 – 471 30 00 Hemsida: http://www.teknat.uu.se/student

Abstract

Further development of moulding technology forunderwater applications in nuclear reactors

Hanna Nygren

To be able to ensure quality, efficiency and safety in nuclear reactors, non-destructiveevaluations (NDE) are performed. The moulding technique, which has been studied inthis project, is an NDE method used to verify surface breaking cracks at variousobjects in reactor vessels.

The idea of moulding is to receive a copy of the replicated surface for microscopicanalysis. Within forensic science the moulding technique is used at crime scenes tocollect evidence and tie suspects to crimes. Underwater moulding, however, is anewly developed technique and WesDyne TRC is a pioneer in offering services withinmoulding for underwater purposes.

This project was initiated by WesDyne TRC to further their knowledge within themoulding technology. In the project, studies have been made at three importantparameters effect on cast quality using three different polymer compounds. Problemsduring moulding, such as crack detection failures and bubbles in the casts, raise thequestion whether the underwater moulding technique can be trusted to detectcracks.

Results from the experiments led to a greater insight into the problem with receivinghigh quality casts during underwater moulding. Only if a satisfactory cast is made, themoulding method can be trusted to detect defects down to the detection target inboth dry and wet environment.

To increase the surface quality of underwater casts a suggestion for mould design anda recommended moulding method was developed. In addition, one of the polymercompounds approved for use, turned out not to be suitable for underwater moulding.

ISSN: 1401-5757, UPTEC F10 018Examinator: Tomas NybergÄmnesgranskare: Staffan JacobsonHandledare: David Stenman

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CONTENTS 1 INTRODUCTION …………………………………………………………………….3

1.1 What is moulding technology for underwater applications? ……………………….3 1.2 Non-destructive evaluation …………………………………………………………4 1.3 History of the moulding technique ………………..………………………………..5 1.4 Problem statement ...…………………. ………………………...………………….6 1.5 This work ……….. …………………………………………………………………7 1.6 About the company …………………….. ……………………………………….....7 1.7 Objectives of the moulding technology ...…………………………………………..8 1.8 State of the art of moulding technology …..………………………………………...8

2 MATERIALS ………………………………………………………………………...10

2.1 The polymer compound …………………………………………………………...10 2.2 The dispenser gun …………………………………………………………………12 2.3 The mould …………………………………………………………………………12

3 METHODS …………………………………………………………………………...14

3.1 Pressure experiment ……………………………………………………………….14 3.2 Quality assessment ………………………………………………………………...15 3.3 Time experiment ………............…………………………………………………..15 3.4 Temperature experiment ……………............……………………………………..16 3.5 Experiments using a mould ……....……….……………………………...………..16 3.6 Crack detection ……………………………………………………………………17 3.7 Test samples ……………………………………………………………………….18

4 RESULTS …………………………………………………………………………….20

4.1 Influence of pressure ………………………………………………………………20 4.1.1 Notch quality ………………...........…………………………………………20 4.1.2 Surface quality ………..……………………………………………………...24 4.1.3 Limit pressure ………....…………………………………………………..…26 4.2 Influence of time …............………………………………………………………..28 4.2.1 CopyRite 7/30 Thixo …………………………………………………………28 4.2.2 CopyRite 7/30 ……………………….…...…………………………………...29 4.2.3 Mikrosil automix ……………………………………………………………..30 4.3 Influence of temperature …………………………………………………………..31 4.3.1 CopyRite 7/30 Thixo …………………………………………………………32 4.3.2 CopyRite 7/30 ………………...…….………………………………………...32 4.3.3 Mikrosil automix ……………………………………………………………..32

4.4 Comparison between polymer compounds ………..………………………………32 4.5 Analysis of moulds ………………………………………………………………..34

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4.6 Analysis of moulding method …………………………………....………………..37

5 DISCUSSION ………………………………………………………………………...40

6 CONCLUSIONS ……………………………………………………………………..42

7 SUGGESTIONS FOR FUTURE WORK ………………………………………..…45

8 ACKNOWLEDGEMENTS …………………………………………………………46

9 REFERENCES ……………………...……………………………………………….48

APPENDIX 1: MOULDING AT CONTROL RODS ……….………………………. 50

APPENDIX 2: MATLAB CODE …………………………….……………………….53

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1 INTRODUCTION 1.1 What is moulding technology for underwater applications?

The moulding technology is a non-destructive evaluation (NDE) method used to ensure quality, efficiency and safety in our nuclear reactors. Surface damage on nuclear reactor objects is revealed by making an imprint of the surface using a polymer compound. A copy of the damaged area is then obtained. When looking at the imprint using a microscope, the surface structure reveals cracks with high resolution6. Since the moulding technology for underwater applications is a newly developed technology, there are not many references. This is one of the reasons why the present work was initiated. Hopefully this master thesis will become a future reference. However, when the object and specific spot for underwater moulding has been determined, a mould that suits this area geometrically is designed and produced. Then a manipulator is used to position the mould, as can be seen in Figure 1. The polymer compound is pushed through a feeder pipe using compressed air. The mould is filled with the polymer compound that handles the displacement of water, which leaves the mould at the top. The cure time depends on the surrounding temperature and the mix of resin and hardener. When the compound has cured the mould is removed and brought up from the water with the manipulator. The cast is then studied using a microscope to verify defects.

Figure 1. Underwater moulding using a manipulator to place the mould. Underwater moulding distinguishes from moulding in dry conditions in the sense that water must be pushed aside. Therefore a suitable, fully covering mould is needed. In the

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nuclear reactor, the water depth at the area to be inspected can be 20 meters or more, which requires a suitable moulding technique.

1.2 Non-destructive evaluation

NDE is of great importance from a safety point of view1. Within the nuclear industry, safety is always a primary consideration. To be able to ensure quality, efficiency and safety in nuclear reactors, evaluations are performed. Non-destructive evaluation means that the object inspected is not damaged during the testing. Further, the analysis made within the moulding technology allows studies down to the microstructure level. Nuclear reactors and internals are traditionally inspected with NDE techniques such as ultrasonic, eddy current and visual inspection techniques10. The eddy current and the visual inspection techniques can be used to locate surface breaking defects. The ultrasonic method can also be used for this purpose but essentially it is used to tell whether objects have internal damages. The methods mentioned can indicate a damaged area but they cannot always tell whether there are scratches, tool marks or other geometrical indications. This may cause unnecessary repair work. To get a greater insight into the extent of the damage verify surface breaking cracks where other methods have indicated defects, the moulding technique is used.

Figure 2. Example of a surface breaking crack. Firstly, the moulding technique is used because it is well suited for use in hard to reach areas and secondly, no other method offers such clear information about the crack appearance. This is due to the ability of the moulding material to penetrate into the narrowest crevices and voids. Therefore it can be said that the moulding is more accurate and informative than alternative NDE techniques. Further, the moulding technique is applicable in the most inaccessible places and dangerous environments, which makes development of the technology important for proper inspections of our nuclear power plants.

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1.3 History of the moulding technique

The moulding technique is only a few years old within the nuclear industry but it has been used for a long time within criminal investigations, and in many other areas. At the State Forensic Laboratory (SFL), the moulding technique is called casting and it is used to tie suspects to crimes. Examples of casts used within forensic technology are footprints and traces of tires at crime scenes5.

Figure 3. A cast made from a footprint. Casting can also be made on safes after burglary. Instead of bringing the entire safe to the laboratory, a cast is made. A cast is easier to analyze than a metal surface that reflects a shining light. Then roughness on tools can be examined and matched against marks on the safe. Casting technology can also be used to examine whether a bullet is fired from a particular weapon5. The casting technique is applicable in many fields, not only within forensic science. In personal communication with Claes-Göran Bengtsson2 I was told the history of how the technology of casting spread from forensic inspection to the nuclear industry.

A beautiful day in 2002, a group of engineers at Ringhals came up with the idea that casting could be used to study defects on objects in nuclear reactors. The high resolution and all the possibilities associated with the moulding material gave them the idea that polymer compound could be applied on objects to detect surface breaking cracks. They went to the police department in Stockholm and asked about the technology used within forensic science to secure tracks near the crime scene. They found out that a moulding material named Mikrosil was used to do the replica. At Ringhals this polymer compound was applied to objects with known defects. The results of the initial trials were satisfactory and they decided to further try the technology in the nuclear power plant at Barsebäck. To use the Barsebäck facility was convenient since it has been taken out of public service for generating electricity and thus could be used as a test station. Here, they came up with the idea that

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the moulding technique could be used for underwater applications as well. In 2004 the moulding technique was accepted as an NDE method and today it is used to verify surface breaking cracks.

In order for the moulding technique to be accepted as an NDE method, qualification is needed. Qualification is a process in which a testing system and its usability for a particular type of test are assessed in a structured way19. Since 1994, qualification is required for testing systems that are to be used at reactor objects in Swedish nuclear power plants. The SSM (Swedish Radiation Safety Authority) rules state that qualification must be made by an independent and impartial authority approved by SSM20. Since 1996, the Swedish Qualification Centre (SQC) is the first and leading qualification authority in Sweden. A testing system includes equipment, testing procedure and personnel. The testing system will be qualified when it has demonstrated that it is able to detect, characterize, position and length size the damage that can occur in reactor vessels and associated mechanical devices. For this purpose, Ringhals AB has, together with FORCE technology, written a Technical Justification13 and a moulding Procedure7. A procedure is made as a guideline for qualified personnel to follow. Moulding performed in accordance with the Procedure shall give reliable results. The Technical Justification is made to prove the method can be trusted and to support the conclusions drawn in the Procedure. Here it is stated that surface-breaking defects can be detected, characterized and length sized according to the detection target in both dry and wet environment.

1.4 Problem statement

The Procedure and the underlying Technical Justification for the moulding technique have their shortcomings. For example, experimental results supporting the conclusions drawn in the Procedure are missing. Therefore, experiments need to be done to confirm the statements about the three important moulding parameters; pressure, time and temperature. The importance of pressure has been widely discussed. Whether the polymer compound should be forced into the defects or not raises questions. In the Procedure it says that ‘sufficient pressure shall be applied to ensure that no water is left between the moulding compound and the object’7 and that raises further questions about the amount of pressure that shall be applied during underwater moulding. It can also be read that ‘to avoid air bubbles in the moulding cast, a light pressure shall be applied from the backside after application’ which indicates that the surface quality is affected by the pressure parameter.

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Further, the temperature affects the curing process. In the Procedure it says that ‘for each 7 °C change in temperature the working and curing times will change with a factor of 2’7. This does not agree with the ‘normal’ rates of change according to Arrhenius’s law25 that says the times shall change with a factor of 2 for each 10 °C. In addition, the Technical justification states that ‘the curing time is not strongly temperature dependent in the specified range 15 °C – 50 °C’13, which can be discussed as the difference in cure time within this range is approximately 1 hour. The ‘working life’ of the polymer compound is the period of the cure process when it is in a liquid state and thus gives maximum resolution and penetration. The polymerization is initialized when the mixing process starts and since the distance from the cartridge to the subject surface can be several meters, the resolution and penetration will decrease with time. Therefore it is important to know how the quality of the polymer compound changes from the time of the mixing process until pressure can be applied against the surface to replicate. In the Technical justification it can be read that ‘during polymer compound application, it is essential to know the working and curing time of the relevant compound mixture’13 which makes time an essential parameter to study. Additionally, the characteristics of the three standard polymer compounds approved for use in accordance with the moulding Procedure have not been investigated. It can be concluded that the present incomplete moulding Procedure may cause two main problems; bubbles in the casts and failure to detect cracks. Thus, the main question is whether the method can be trusted to detect cracks down as small as specified by the detection target.

1.5 This work

• Three important parameters during moulding have been studied; pressure, time and temperature. • Three different polymer compounds have been compared. • The problem with bubbles in casts has been examined. • The problem with crack detection failures has been examined.

1.6 About the company

WesDyne TRC, which initiated this project, is a fully owned subsidiary of the nuclear technology company Westinghouse with 13 000 employees worldwide. The company is responsible for those parts of the operations involving non-destructive evaluation of nuclear reactors. WesDyne TRC has offices in the US, Sweden and Germany.

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WesDyne TRC is a leading supplier of services and products in qualified mechanized non-destructive testing for the international nuclear power industry. Operations include development, design, project management, manufacture, sales and marketing of inspection equipment for a number of non-destructive evaluation methods. The demanding environment in and around nuclear reactors put special demands on safety and quality of work performed in the delivery of service. This manifests itself in a high degree of automation and mechanization of remote controlled examination operations, involving advanced equipment carriers such as robots, manipulators and submarines. Now WesDyne TRC is a pioneer in offering services within moulding technology for underwater purposes.

1.7 Objectives of the moulding technology

The moulding technology is not only used to detect cracks, but also for moulding complex geometries where angles of objects are of great importance for evaluation using advanced technology 18. Instead of modelling important geometries the moulding technique can be used. Attempts have also been made using the moulding technology in drilling where the aim is to avoid spreading shavings or chips in the reactor vessel6. The polymer compound captures the particles from the drilling and after curing it can easily be removed. The objective of the moulding technology within this project is not to use it for drilling or to mould geometries but to detect cracks. According to the Procedure for crack detection, it has the following objectives7. • The detection target is cracks of length 1.5 mm and width 3 mµ . • Surface-breaking defects on a metal surface can be detected, characterized and length sized according to the detection target. The method says nothing about the depth of cracks. • The method is applicable in both dry and wet environment.

1.8 State of the art of moulding technology

The material used for moulding is a polymer compound. Mixing a polymer and a hardener initializes the polymerization. Practically mixing components from two separate tubes does this. The two parts are mixed with the right proportions to achieve the desirable properties of the compound.

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Within forensic science a spatula is used for manually mixing the two parts of the polymer compound. The surface to replicate is then processed with the mixed polymer compound like plastic padding5.

Figure 4. Manually mixing the two parts of the polymer compound using a spatula. For some applications, e.g. underwater moulding, it is not possible to replicate a hard to reach area using this specific moulding technique. For that reason a dispenser gun was used in laboratory environment to push the mixed compound through a feeder pipe down to the surface to replicate. Using a dispenser gun, the two parts get automatically mixed with the right proportions to achieve a certain working life and cure time7. Since the compound does not reach the surface to replicate immediately, the cure time is important. It is then advantageous to be able to automatically mix a polymer compound with the desired cure time.

Figure 5. A dispenser gun for automatically mixing the two-part compound. Within nuclear industry a dispenser gun is not practically applicable though. A larger force than the dispenser gun is able to apply is needed to push the polymer compound through a long feeder pipe. For those who want to learn more about the technology behind moulding at control rods, the approach is described in Appendix 1.

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2 MATERIALS 2.1 The polymer compound

The polymer compound belongs to the group of silicone rubbers. The material is created when two components are mixed together by a mixing nozzle, ensuring that component A, containing the monomer chains, and component B, containing the hardener, will get a good mixture11. The following text is a brief description of polymers. For those who want to learn more about the nature of the blended polymers it is referred to “Introduction to Synthetic Polymers” by I.M. Campbell4. When the two parts get in contact, cross-links start to form between the monomer chains and a polymer is created. The amount of hardener affects the properties of the polymer. This polymer compound will form an elastic rubber material, which means that the material returns to its original form after it has been stretched and released.

Figure 6. Part A contains the monomer Figure 7. Part A and part B are mixed chains. together. Cross-links are formed between the monomer chains. The polymerization immediately starts when the two parts are mixed. The times for the cure process are determined by the amount of additives that either slows down or speeds up the process. Additives are also used to obtain a thixotropic polymer compound and different colour pigments. The working life of a polymer compound is the period of the cure process when the compound is in a liquid state. It is during this period that maximum resolution and penetration into cracks is obtained. The working life is an important parameter to consider during underwater moulding since the compound does not reach the surface to replicate immediately. A satisfactory result can only be promised if the mould is completely filled before the working life is expired. The cure time is the time for the cure process; i.e. the time from the compound is applied onto the object to it is ready to be removed from the replicated surface. This time is temperature dependent so that increasing temperature means shorter cure time and shorter working life.

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Polymer compounds from two different suppliers have been tested. Kjell Carlsson Innovation delivered a two-part compound named Mikrosil automix. The supplier developed Mikrosil for optimal microscopic characteristics. Therefore it is used within the forensic science all over the world. The Mikrosil automix compound is a 1:1 part A to part B, both containing colour pigments.

Figure 8. The two-part polymer compound Mikrosil automix. The other supplier, Engineering & Forensic Technology (E&FT), manufactures a two-part compound named CopyRite with different grades for many industrial applications. In this project CopyRite 7/30 and CopyRite 7/30 Thixotropic have been used. The numbers can be explained as follows: 7 is the maximum working life in minutes and 30 is the cure time in minutes for the compound at a temperature of 25 °C. The CopyRite compound is a 10:1 part A to part B, which means there is a flow restrictor at the part B outlet. The mixing nozzle will then receive the correct amount of each part when pulling the trigger at the dispenser gun. Here part A contains the colour pigment.

Figure 9. The two-part polymer compound CopyRite. The CopyRite Thixotropic was developed especially for working on vertical, overhead or curved surfaces and many material scientists now use thixo grades for most on-site replication. The thixo grades are designed not to flow as readily11. E&FT also manufactures CopyRite with faster and slower process times. For applications such as feeding through long pipes or temperatures higher than 25 °C, the 7/30 grade may not stay liquid long enough. For this reason, polymer compounds with slower cure grades such as 10/40 or 15/60 are also manufactured.

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2.2 The dispenser gun

For laboratory experiments dispenser guns (DS 50 for Mikrosil automix and DS 51 for CopyRite) were used to apply the polymer compound. A mixing nozzle ensures that the contents of the two tubes get thoroughly mixed before application. The mixing nozzles are specific to each compound type. They have a specific number of helixes to ensure complete airless mixing of the two components as they leave the cartridge, which means all the air inside the mixing nozzle is expelled a-head of the polymer compound. According to the manufacturers of the CopyRite compound, E&FT 11, the first millilitres of the polymer compound should be discarded to prevent inadequately mixed compound being dispensed onto a critical ‘one-shot’ subject to be replicated. This is due to the significant difference in viscosity (25 Pa s and 1 Pa s respectively) between the two components. In extensive quality control tests at E&FT it has been shown that the first few millilitres of the compound usually is set okay but still they are discarded before carrying out cure time and flow tests.

2.3 The mould

The mould is made of a clear substrate, e.g. Plexiglas (PMMA) to enable the monitoring of the filling phase using a camera. It is manufactured to suit the geometry of the replicated object. The mould size is about 50 mm x 50 mm to cover a surface large enough17. Inside the mould a calibration disk is mounted with reference defects of width 1mµ . When a cast has been made, the cast quality is controlled using these reference defects. If the reference defects are fully visible and the surface quality is satisfactory at both sides of the cast, then the polymer compound is able to detect cracks of width ≥ 3 mµ 7. Sealing strips are mounted on the mould to ensure that no polymer compound leaks in case the geometry of the mould does not exactly fit the object to replicate17. The idea with the sealing strips is also to allow remaining air and water to be removed from the mould when the compound gets pressurized against the surface to replicate.

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Figure 10. The current mould design used for moulding at control rods. At the bottom of the mould there are two entrances, one for the polymer compound and one for the compressed air used for aeration of the mould. At the top of the mould there is an outlet for air and water.

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3 METHODS 3.1 Pressure experiment

Polymer compound was applied to a test block with three notches using a dispenser gun. Then a pressure was applied at the compound. The application pressure was calculated as

A

mg

A

Fp == (1)

where m is the weight of the item placed on the compound, g is the acceleration due to gravity and A is the area of the test block. In order to make the corresponding pressure calculation in water you have to compensate for the displaced water according to Archimedes’s principle24. Experiments were performed at a water depth of 200 mm.

Figure 11. Pressure experiment in water where polymer compound was applied to a test block with three notches. Then a pressure was applied at the compound by

using a weight. Here the water is medium 1 and the item used as weight is medium 2, then according to Archimedes’s principle

2112

2

1

121 Vm

mmVV ϕ

ϕϕ=⇒=⇒= (2)

where V – volume m – mass ϕ – density, which gives

( ) ( )A

gVm

A

gmmp 21212 ϕ−

=−

= . (3)

Several casts were made using different pressures on the compound, and the resulting qualities were compared.

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3.2 Quality assessment

The casts from the pressure experiments were graded with respect to notch quality and surface quality. Both qualities were graded from 0 to 4. The notch quality was graded looking at the penetration into the notch. The notch had the grade 0 if it was visible and higher grades if the penetration was deeper. The highest quality grade was 4, which corresponded to maximum penetration into the notch.

Figure 12. A notch will get the grade 0 if it is visible and 4 if maximum penetration into the notch has been achieved. The surface quality was graded looking at the bubble diameter, d. The surface had the grade 0 if there were very large bubbles and 4 if bubbles could not be seen. The definition of the grades was as follows:

Figure 13. The surface quality will get the grade 0 if there are large bubbles on the cast and 4 if no bubbles can be seen. Since 1.5 mm is the minimum length of the detection target, the bubble diameter, d, should be less than this length for the surface of the cast to be satisfactory. Otherwise a bubble could cause a crack detection failure.

3.3 Time experiment

The influence of the working life on the quality of the cast was studied using Swedish crowns due to the complex but well defined surface pattern of these items. Since the maximum working life at 20 °C is approximately 10 min, a time interval of 0-11 min was identified as the time range of interest. Therefore, polymer compound was applied at each

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crown and then a sufficient pressure (2.5 kPa) was applied 0, 1, 2, …, 11 min after application. The working life has also been discussed with E&FT.

3.4 Temperature experiment

The effect of temperature on cast quality was studied using a temperature controlled heated bath with built in thermostat. Casts were performed at 15 °C, 20 °C, 25 °C, 35 °C, 40 °C and 50 °C, which spans the whole specified temperature range of the moulding technique. At each of these temperatures, different pressures were applied on the test block with notches. The test block had the same temperature as the water and the polymer compound was placed in an oven before application in order for it to receive the water temperature. Also the weights used had the same temperature as the water. Then casts made with the same amount of pressure were compared to see how the moulding temperature affects the quality. Experiments were also done using a mould. Further, the change in cure rates with temperature for the CopyRite compound has been discussed with E&FT.

3.5 Experiments using a mould

To improve the surface quality of casts, the mould design was considered. The moulding Procedure states nothing about mould design, which turned out to be one of the most important parameters during underwater moulding. The mould design is important not only to solve the bubble problem but also to get a well-prepared replication surface.

Figure 14. Bubbles in a cast. Experimental moulds have been used for moulding on both neutral surfaces and at real cracks. Moulds of different shapes and of varying thickness have been used. The mould was brought underwater and placed over the surface to replicate. The experiments were done at a water depth of 200 mm.

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Figure 15. Underwater moulding in laboratory environment. Several casts were made using different mould designs and moulding methods. The casts were compared and conclusions were drawn regarding optimal mould design. Also a recommended moulding method was prepared.

3.6 Crack detection

A Leica stereomicroscope was used for crack detection. Examination of the cast was done in a raster pattern with steps corresponding to the width of the microscope image. The light source was adjusted so that the cast was illuminated with an angle as low as possible for optimal crack detection. The cast was also rotated 90° in order to make examination in both directions.

Figure 16. A crack on a cast as detected with the microscope.

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Cracks were length sized using a computer with a special Leica software program.

3.7 Test samples

The effect of three important moulding parameters on cast quality was studied using three different test samples (1-3). Mould analysis and analysis of moulding method were done using two test samples (2, 4).

1. The effect of pressure and temperature on cast quality was studied using a test block with three notches of different depths. The notch dimensions can be seen in Table 1.

Table 1. The notch dimensions.

Notch 1 Notch 2 Notch 3 Length [mm] 35 35 35 Width [mm] 0.15 0.15 0.15 Depth [mm] 1.0 0.5 0.2

Figure 17. Test block with three notches.

2. The effect of pressure and temperature on cast quality was also studied using a test block borrowed from Ringhals AB. It had a crack of width approximately 4 mµ and length 23 mm. Further, analysis of moulds and moulding method was made using this crack.

3. The effect of time on cast quality was studied using Swedish crowns.

4. Since notches do not have the same characteristics as cracks, analysis of

moulds and moulding method was performed using a thermal fatigue crack. A cylinder-shaped rod of stainless steel was used to make experiments as close to reality, when moulding at control rod shafts, as possible. The crack depth was 1.9 mm. The width and length can be seen in Figure 19.

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Figure 18. The thermal fatigue crack used for mould analysis.

Figure 19. The crack opening vs. the crack length.

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4 RESULTS 4.1 Influence of pressure

Each of the three notches and the surface of the casts were graded using the quality assessment described in chapter 3.2. The quality at each specific pressure was plotted. Then lines were fitted to the plotted points using cubic interpolation.

4.1.1 Notch quality

The ability of the three polymer compounds to penetrate into notches of different depths varies with pressure in dry environment. Comparing the graded casts in Figure 20, 21 and 22, there is a difference. At equal notch quality the application pressure varies slightly between the polymer compounds.

0 1 2 3 4 50

0.5

1

1.5

2

2.5

3

3.5

4

Pressure [kPa]

Qua

lity

Notch 1Notch 2Notch 3

Figure 20. Notch quality vs. pressure in dry environment for CopyRite 7/30 Thixo.

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0 1 2 3 4 50

0.5

1

1.5

2

2.5

3

3.5

4

Pressure [kPa]

Qua

lity

Notch 1Notch 2Notch 3

Figure 21. Notch quality vs. pressure in dry environment for CopyRite 7/30.

0 1 2 3 4 50

0.5

1

1.5

2

2.5

3

3.5

4

Pressure [kPa]

Qua

lity

Notch 1Notch 2Notch 3

Figure 22. Notch quality vs. pressure in dry environment for Mikrosil automix.

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It can be seen from Figure 20, 21 and 22 that the notch quality gets better with increasing pressures. This means that the polymer compound should be pressurized into the defects. Also a deeper notch needs a higher pressure for maximum penetration. The ability of the polymer compound to penetrate into notches in dry environment is more than satisfactory for very low pressures, especially using CopyRite 7/30 and Mikrosil automix. The low viscosity of the CopyRite 7/30 compound gives high quality of penetration into notches for low application pressures, as can be seen in Figure 21. Further, the relatively high density of the Mikrosil automix compound explains the high notch quality achieved for low application pressures in Figure 22. The underwater pressure experiments revealed that water filled notches is not a problem using CopyRite 7/30 Thixo and Mikrosil automix, as can be seen in Figure 23 and 25. The ability of these polymer compounds to penetrate into notches of different depths varies with pressure also in wet environment. Using CopyRite 7/30 on the other hand, the quality of penetration into notches appears randomly. Looking at the graded casts in Figure 24, it can be seen that this non-thixo grade of CopyRite does not work well for underwater purposes. The low viscosity of the compound explains its unreliable behaviour during underwater moulding. It tends to float out to the sides instead of penetrating into water filled notches.

0 1 2 3 4 50

0.5

1

1.5

2

2.5

3

3.5

4

Pressure [kPa]

Qua

lity

Notch 1Notch 2Notch 3

Figure 23. Notch quality vs. pressure in wet environment for CopyRite 7/30 Thixo.

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0 1 2 3 4 50

0.5

1

1.5

2

2.5

3

3.5

4

Pressure [kPa]

Qua

lity

Notch 1Notch 2Notch 3

Figure 24. Notch quality vs. pressure in wet environment for CopyRite 7/30.

0 1 2 3 4 50

0.5

1

1.5

2

2.5

3

3.5

4

Pressure [kPa]

Qua

lity

Notch 1Notch 2Notch 3

Figure 25. Notch quality vs. pressure in wet environment for Mikrosil automix.

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In wet environment a higher pressure is needed to achieve high notch quality. Comparing the notch quality of casts performed in air with the ones performed under water, top graded casts had a pressure difference of about 2 kPa.

4.1.2 Surface quality

The application pressure also affects the surface quality of the cast. With increasing pressure, the surface quality will increase and there will be no visible bubbles on casts made in dry environment. In wet environment though, the casts will never achieve the same quality as the ones made in air. It can be seen in Figure 26, 27 and 28 that the environment affects the surface quality. During underwater moulding, the quality will not reach a level higher than 2 because some water is always left between the polymer compound and the object to replicate. When applying a pressure at the polymer compound, the water is pushed aside which explains why the surface quality increases with higher pressure. During underwater moulding though, the water needs to be headed off in a better way to receive higher quality grades.

0 1 2 3 4 50

0.5

1

1.5

2

2.5

3

3.5

4

Pressure [kPa]

Qua

lity

AirWater

Figure 26. Surface quality vs. pressure in dry and wet environments for CopyRite 7/30 Thixo.

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0 1 2 3 4 50

0.5

1

1.5

2

2.5

3

3.5

4

Pressure [kPa]

Qua

lity

AirWater

Figure 27. Surface quality vs. pressure in dry and wet environments for CopyRite 7/30.

0 1 2 3 4 50

0.5

1

1.5

2

2.5

3

3.5

4

Pressure [kPa]

Qua

lity

AirWater

Figure 28. Surface quality vs. pressure in dry and wet environments for Mikrosil automix.

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In Figure 27 the surface of the casts made in dry environment achieves high quality for low pressures using CopyRite 7/30. Due to the low viscosity of the compound, air is easily pushed aside. For this non-thixo grade of CopyRite, the surface quality gets better with increasing pressure but during underwater moulding, the polymer compound gives results of varying character since a water film tends to form on the cast for high pressures as well as low. Additionally, the polymer compound tends to get very thin for high application pressures. During moulding in wet environment using Mikrosil automix, as can be seen in Figure 28, water is pushed aside effectively, probably due to the high density of the compound and the slightly higher viscosity than for the non-thixo grade of CopyRite.

4.1.3 Limit pressure

Finally, finding a lower limit pressure, where satisfactory casts can be promised, will summarize the results from the pressure experiments. A lower limit pressure was found by grading the casts with 0 for unsatisfactory and 1 for satisfactory. In order for the cast to be satisfactory, the surface quality has to reach a grade of 2, otherwise bubbles or water films will cover an area large enough to cause a crack detection failure. The limit pressures varied between dry and wet environment but also between the three polymer compounds, as can be seen in Figure 29, 30 and 31.

0 1 2 3 4 50

0.2

0.4

0.6

0.8

1

Pressure [kPa]

Qua

lity

AirWater

Figure 29. Cast quality vs. pressure in wet and dry environments for CopyRite 7/30 Thixo.

27

0 1 2 3 4 50

0.2

0.4

0.6

0.8

1

Pressure [kPa]

Qua

lity

AirWater

Figure 30. Cast quality vs. pressure in wet and dry environments for CopyRite 7/30.

0 1 2 3 4 50

0.2

0.4

0.6

0.8

1

Pressure [kPa]

Qua

lity

AirWater

Figure 31. Cast quality vs. pressure in wet and dry environments for Mikrosil automix.

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According to Figure 29, the lower limit pressure using CopyRite 7/30 Thixo is approximately 1.5 kPa in dry environment and 3 kPa in wet environment. The reason for the pressure difference is that a larger force is needed to displace water than what is needed to displace air. It can be seen in Figure 30 that the lower limit pressure using CopyRite 7/30 is about 0.5 kPa in dry environment. In wet environment a satisfactory result cannot be guaranteed at a limit pressure since a water film tends to form on the cast for high pressures as well as low. Further, according to Figure 31 using Mikrosil automix, the lower limit pressure is approximately 1 kPa in dry environment and 3.5 kPa in wet environment.

4.2 Influence of time

The CopyRite 7/30 compounds have a maximum working life of 7 min at 25 °C and for every 10 °C above/below this temperature; the working life is halved/doubled. This relationship can be described by formula (4). Max working life (T) ( ) 10/ C2527 T−°⋅≈ min (4) Further, the cure time is 30 min at 25 °C and for every 10 °C above/below this temperature, the cure time is halved/doubled. This relationship can be described by formula (5). Cure time (T) ( ) 10/ C25230 T−°⋅≈ min (5) The corresponding times for Mikrosil automix are unknown according to the supplier.

4.2.1 CopyRite 7/30 Thixo

When compound was applied at the crown, as described in chapter 3.4, it formed a lump. During pressure application before working life had expired, the compound floated out and covered the crown. The longer it took before pressure was applied, the higher compound viscosity. If the working life had expired before pressure application, the compound remained a lump. Thus it could be determined whether the working life had expired or not by looking whether the compound covered the crown.

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Figure 32. Sufficient pressure was applied at the polymer compound 0, 1, 2, …, 11 min after application. In a room temperature of 20 °C the maximum working life is 9.9 min according to formula (4). However, it can be seen in Figure 32 that 5 min after application, the compound was not completely in a liquid state when pressure was applied. That means the true working life was only 50% of the maximum value. Consequently, the true working life at 25 °C should be about 3.5 min instead of 7 min. This relationship can be described by formula (6). Working life (T) ( ) 10/ C25275.0 T−°⋅⋅≈ min (6) This means the quality of the thixotropic polymer compound decreases quite rapidly with time. Compound movement is important to reduce the viscosity of thixotropic liquids. By processing the compound the working life can be increased.

4.2.2 CopyRite 7/30

The non-thixo grades behave differently during application since they have lower viscosity from start than the thixo-grades. This compound flows until the crown is covered rather than forming a lump, which explains the result in Figure 33.

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Figure 33. Sufficient pressure was applied at the polymer compound 0, 1, 2, …, 11 min after application. Before pressure was applied, the compound had already covered the crown. If the compound was still in a liquid form during pressure application, it continued floating over the crown-edge. According to Figure 33 the working life was expired at 8 min, which agrees with theory since the average working life is approximately 75% of the maximum working life, according to E&FT. This relationship can be described by formula (7). Working life (T) ( ) 10/ C252775.0 T−°⋅⋅≈ min (7) Consequently, in a room temperature of 20 °C the working life should be 7.4 min, which is confirmed by this time experiment.

4.2.3 Mikrosil automix

During pressure application, this polymer compound behaves like the non-thixo grades of CopyRite in the sense that it flows until the crown is covered rather than forming a lump. It has a slightly higher viscosity though.

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Figure 34. Sufficient pressure was applied at the polymer compound 0, 1, 2, …, 11 min after application. The working life and cure time are unknown but according to the experiment in Figure 34, the times seem to be a little longer than for the CopyRite 7/30 compounds. The working life was expired at 9 min when the compound did not flow over the crown-edge as pressure was applied.

4.3 Influence of temperature

The cure rates for the CopyRite polymer compounds do follow Arrhenius´ law since the chemical reactions fall under the ‘normal’ rates of change. This means that for each 10 °C change in temperature above/below 25 °C, the cure rates are halved/doubled. Consequently, the cure time is strongly temperature dependent within the specified range 15-50 °C. During moulding in different temperatures, the working life of the polymer compound is an important parameter to consider. If moulding is to be performed at the higher temperatures within this range, the use of the polymer compounds approved in the Procedure is not a good choice.

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Depending on application time and moulding temperature, a polymer compound with suitable working life should be selected. Therefore, a polymer compound with slower process time is recommended when moulding is to be performed at temperatures in the upper part of this range.

4.3.1 CopyRite 7/30 Thixo

A cast cannot be performed at a temperature of 50 °C without affecting the result. The working life of the compound is expired before any pressure can be applied. It turned out that the polymer compound should not be used for moulding at temperatures above 35 °C. If moulding is to be performed at a temperature of 35 °C, the cartridge should be placed close to the mould since a maximum working life of 3.5 min gives a true working life of about 1.8 min according to formula (6) in chapter 4.2.1. This time is needed if the compound is to be transported through a feeder pipe before reaching the mould. Additionally, the time for filling the mould has to be considered.

4.3.2 CopyRite 7/30

A cast can be performed at a temperature of 50 °C without affecting the result but it is not recommended since the working life is expired in less than 1 min. To get a reasonable application time, mouldings should not be performed at temperatures above 40 °C. If moulding is to be performed at a temperature of 40 °C, the cartridge has to be close to the mould since a maximum working life of 2.5 min gives a true working life of about 1.8 min according to formula (7) in chapter 4.2.2. This time is needed if the compound is to be transported through a feeder pipe before reaching the mould.

4.3.3 Mikrosil automix

A cast can be performed at a temperature of 50 °C without affecting the result but it is not recommended since the working life is expired relatively fast. How the temperature affects working life and cure time is unknown for this polymer compound.

4.4 Comparison between polymer compounds

Three different polymer compounds have been used in the experiments. A comparison was made for the user to take into consideration before performing casts. In Table 2 limit pressures, working life and recommended upper temperature limit are stated for each polymer compound.

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Table 2. Values from experimental results regarding three important parameters; pressure, time and temperature for three polymer compounds. Polymer compound Limit

pressure in air [kPa]

Limit pressure in water

[kPa]

Working life (T) [min]

Upper temp. limit

[°C]

CopyRite 7/30 Thixo ~1.5 ~3 ( ) 10/25275.0~ TC−°⋅⋅ 35 CopyRite 7/30 ~0.5 *N/A ( ) 10/252775.0~ TC−°⋅⋅ 40 Mikrosil automix ~1 ~3.5 **N/A 40

* In wet environment a satisfactory result cannot be promised at a limit pressure. ** The working life and cure time are unknown but according to experiments they seem to be a little longer than for the CopyRite 7/30 compounds. From experimental results two positive (+) and two negative (-) properties about each polymer compound can be said. • CopyRite 7/30 Thixo

+ It is suitable for vertical moulding.

+ It is suitable for underwater moulding. - Not suitable for transporting long distances through a pipe due to its

slightly higher viscosity. - Since the viscosity is higher than for the other polymer compounds, the quality of this compound decreases faster with time. For the same reason quality decreases faster also at higher temperatures. • CopyRite 7/30 + It is suitable for moulding in dry environment. + It can relatively easy be transported through a long pipe because it is not as time and temperature sensitive as the thixo grade due to its low viscosity. - Not suitable for underwater moulding. - Not so good to use for making vertical casts because it flows readily and in the case of a mould-leakage it will run down to the bottom of the reactor vessel. • Mikrosil automix + It is suitable for moulding in both dry and wet environment. + It has more pigment that gives optimal microscopy characteristics in terms of enhanced three-dimensional feeling.

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- Residues of the compound must not be left in the reactor vessel due to the compound substances.

- During microscopy, cracks are more difficult to find because they are not reflected as clearly as with the other polymer compounds.

4.5 Analysis of moulds

The movement of the polymer compound is characterized by its complex fluid dynamics. A well-designed mould, however, provides compound movement along the surface to replicate. For optimal cast quality the compound should be smeared out and pressurized against the surface to be replicated, which makes the penetration into cracks better due to decreased viscosity of the compound. A mould that does not prepare the surface for replication in a good manner can be seen in Figure 35. The polymer compound will not get pressurized into the defects because of the depth of the mould. The mould depth also causes a relatively large amount of water, which must be removed to obtain a satisfactory cast. A deep mould means that you lose the desirable motion of the polymer compound, which ends up in air and water being mixed into the compound.

Figure 35. Poor mould design that Figure 36. A well-designed mould that contributes to air and ensures removal of air and water entrapments. water in a satisfactory manner.

35

For optimal crack detection possibilities, air and water entrapments should be prevented using the mould in Figure 36. Here air and water will be pushed in front of the polymer compound in direction towards the exit. Heading it in a certain direction without mixing it with the compound does the displacement of air and water. The depth of the mould makes it possible for the compound to displace water. From pressure experiments in chapter 4.1 it was concluded that a limit pressure is needed for the cast quality to be satisfactory. In order for the compound to be pressurized against the surface, the mould should not be deep. The Procedure states that a cast should be at least 0.5 mm thick, which means the mould does not need to have any depth because of the thickness of the seal. The mould should be provided with a seal, which closes tightly to prevent water from entering the mould. The seal should be relatively thick to be able to compensate for placement over a weld or other irregularities and still close tightly. The thickness of the cast, given by the seal, will meet the requirement of 0.5 mm. A seal proposal made of the rubber material EPDM (Ethylene Propylene Diene Monomer) can be seen in Figure 37. It should work as a part of the mould that does not need to be replaced. The seal is reinforced by polymer compound to withstand sealing of irregularities.

Figure 37. A seal proposal for replication of surfaces with irregularities.

Further, for the mould design to facilitate the export of mainly water but also air, a well-designed mould does not have sharp edges where bubbles arise in the cast. Since bubbles tend to form in the outer edges of the cast, a suggestion of an oval shaped mould was worked out. This mould proposal can be seen in Figure 38.

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Figure 38. The mould proposal is oval-shaped to facilitate evacuation of water. Since water entrapments are a problem during underwater moulding, the number of outlets has also been investigated. With an increasing number of outlets, as can be seen in Figure 39, there will be less air and water entrapments in the casts. Experiments were made providing the mould with several outlet channels in order to facilitate drying of the surface to replicate. This resulted in much extra work that ultimately resulted in uncertainty regarding whether the mould is filled with polymer compound or not. More than one outlet facilitates drying of the surface but hampers the moulding process by making the filling phase difficult to monitor.

Figure 39. A mould with several outlets. Figure 40. A mould with several inlets. Experiments were also made providing the mould with several inlet channels to obtain a more uniform filling technique. Then the polymer compound ended up mixed with water because it no longer travelled as a coherent part towards an outlet. Consequently, more than one inlet, as can be seen in Figure 40, had no advantages. It turned out that the fewer components used to make casts of top rating, the better. Therefore, the optimal 50 mm x 50 mm mould design had one inlet at the bottom where polymer compound entered the mould. The inlet was directed orthogonal to the surface to

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replicate in order for the compound to be pressurized against it. Then the compound travelled as a coherent part through the mould in direction towards an exit where air and water was evacuated. The inlet pipe diameter was chosen to 6 mm since this is the width of the mixing nozzle opening and therefore provides the best flow characteristics. The outlet pipe diameter was chosen to 4 mm, which is smaller than the inlet pipe to obtain a pressure in the mould during the filling phase. Using smaller pipe diameter at the outlet, an even larger pressure was obtained but then the likelihood of a mould leakage was significantly increased.

4.6 Analysis of moulding method

Problems arise during underwater moulding when polymer compound fails to replace water. The problems manifest themselves in bubbles and water films at the casts that can cause a crack detection failure. Therefore, underwater moulding should be replaced by dry moulding. Experiments have shown that satisfactory casts can be obtained by letting the polymer compound replace water but to improve the quality of underwater casts, the water should be removed from the inside of the mould. When succeeding in emptying the water filled mould, the quality of the casts made underwater will be similar to those made in dry condition. Removal of water is made using compressed air. After aerating the mould, some water drops will still be left against the surface to replicate. To facilitate drying of the last remaining water a surface tension-lowering agent is added. By adding alcohol, the surface tension of the water is reduced and casts can be made with absence of water bubbles. Therefore, a recommended moulding method has been worked out using the experimental layout in Figure 41.

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Figure 41. Experimental layout for the recommended moulding method. In order for all water to be removed, the mould has to be aerated as well as the inlet pipe for the polymer compound. That is why aeration and polymer compound have one single inlet. Otherwise, water will enter the mould through the feeder pipe after it has been emptied. Then isopropanol, which has surface tension-lowering properties, is added through the inlet pipe for aeration to facilitate the removal of water. When the mould is filled with polymer compound, a check valve at the aeration inlet prevents the compound from entering the aeration pipe. This leads to a recommended moulding method, which involves five steps:

1. To prevent the polymer compound tube from being affected by compressed air during the following steps, compound shall be pushed through the inlet pipe until it reaches the T-coupling.

2. Aerate for ~5 s to empty the mould. 3. Add 5-10 ml alcohol to facilitate drying of the surface to replicate. 4. Aerate for ~30 s to dry the surface thoroughly.

5. Fill the mould with polymer compound during constant movement. When polymer compound is pushed through the mould, whether it is after the mould has been pre-empted or not, constant movement is required for any remaining water to be removed. For this reason compressed air should be used to transport the compound instead of dispenser guns. This also contributes to increasing speed of the polymer

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compound through the mould, which is important for reducing the viscosity of the compound and thus to get satisfactory penetration into cracks. Another important finding is that the polymer compound should be placed as close to the mould as possible to avoid long distance transportation. Time experiments in chapter 4.2 showed that the viscosity of the compound increases with time, which means the quality of the compound, will decrease if it has travelled through a long feeder pipe before the mould is reached. Therefore, the use of a mould design that minimizes the transportation distance saves both time and polymer compound while the quality of the casts is improved. Additionally, when moulding is to be done at higher temperatures, the compound must be placed near the mould for a cast to be performed at all, according to the temperature experiment in chapter 4.3. Even if moulding on only a few meters depth, this solution should be preferred. The polymer compound tube should not be placed upside down though since the viscosity differs quite a lot between the both parts of the compound. Then the low viscosity part will run ahead of the high viscosity part, which will cause an unmixed compound.

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5 DISCUSSION The examination of the influence from the three key parameters – pressure, time and temperature – on the quality of the cast does not give exact measurements. However, the scatter due to the subjectivity of the experimenter is small in comparison to the large differences between the tested polymer compounds. Pressure experiments showed that the surface quality of casts made in dry condition was different from those made in wet condition. The lower limit pressures found differ between casts made in dry and wet condition due to a higher pressure is needed to replace water than air. The limit pressures also varied between the polymer compounds due to their different characteristics in terms of density and viscosity. Time experiments showed that the quality of the polymer compound decreases faster with time than stated on the tubes. The minimum working life of the polymer compound has now been changed to become a maximum working life, a knowledge that may be relevant for future construction of moulding equipment. The actual working life of the CopyRite compounds is about 75% of the maximum working life. The working life of the thixo grades of CopyRite were only 50% of the maximum working life but processing the compound can increase it. The thixotropic compound becomes thick like a solid when left standing but it flows like a liquid when a sideways force is applied. Therefore, shear forces will decrease the viscosity of the compound26. However, the time for the newly mixed polymer compound to reach the surface to replicate should be as short as possible. Since the reaction between the two components starts immediately after contact, the quality of the compound will decrease with time and therefore, the transport distance should be minimized. A successful cast is guaranteed only if the mould is filled before the working life is expired. Temperature experiments showed that the polymer compounds, approved for use according to the Procedure, are not suitable for moulding at temperatures > 40 °C. If moulding is to be performed at temperatures of 40-50 °C, then a polymer compound with slower process time should be chosen. For example, CopyRite 10/40, CopyRite 15/60 and CopyRite 50/180 are manufactured for these purposes. Examination of the various characteristics of the polymer compounds, approved for use according to the Procedure, was first seen as a low priority task. But after moulding the three different compounds in both dry and wet environment, clear differences were found. The question is therefore whether CopyRite 7/30 (non-thixo) should be used for underwater moulding, looking at the mixed results it presents? It cannot be neglected that the non-thixo grade performs less well at casts performed in water. The non-thixo grade has lower viscosity than the other compounds and therefore it is believed to be less effective with water. What E&FT has to say about this discovery is that ‘it is probable that the non-thixo grades will disperse readily into the water and thus

41

could be expelled with the water’, which agrees with the experimental results regarding the behaviour of this compound during underwater moulding. A suggestion from E&FT is that this is an important parameter to consider during underwater moulding using a non-thixo grade. Further, it was found that water films and bubbles are the problems with the underwater moulding technology. Since it is the water that causes the trouble, a proposal is to transform the wet moulding into a dry. By cleaning the mould with isopropanol, which has surface tension lowering characteristics, water will easily be removed from the inside of the mould and the number of bubbles in the cast will be reduced. Experiments revealed that the moulding method is as important as the mould design. Then when the mould is to be placed at an angle where crack indication has been given using eddy current testing, it is not known with certainty where the zero-reference is placed, which means the mould could be placed at wrong position. Therefore, a way to ensure that the cast is made at the correct position is needed. Since a water film tends to form at the cast, it is also important that the object's surface texture can be seen on the cast. A blank area on the cast means cracks cannot be detected at this area of the cast.

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6 CONCLUSIONS

• The polymer compounds CopyRite 7/30 Thixo and Mikrosil automix are recommended for underwater moulding purposes.

• The polymer compound CopyRite 7/30 (non-thixo) gives results of varying character during underwater moulding. A thin water film tends to form over the cast when water repels the polymer compound, as can be seen in Figure 43. Therefore, this grade is not suitable for underwater moulding.

Figure 42. A crack detected at a cast Figure 43. A water film covers the area of using CopyRite 7/30 Thixo. the crack’s position, which causes a crack detection failure

using CopyRite 7/30 (non-thixo).

• Caution should be taken against blank areas on underwater casts where the object’s surface structure has vanished. Only if a satisfactory cast is made, the moulding method can be trusted to detect defects down to the detection target in both dry and wet environment.

• From what has been seen in the experiments, a crack detection failure can be

explained in two ways:

1. Wrong positioning of the mould.

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2. Water film on the cast.

• The surface quality of underwater casts can be improved from a satisfactory grade 2 up to a top-grade of 4 by using a well-designed mould and a good method for moulding. The recommended moulding method involves two important steps; aeration of mould and addition of surface tension-lowering agent (isopropanol).

• To ensure compound movement along the surface to replicate, the mould should

provide a thin cast. The polymer compound should also be headed as a coherent part through the mould, towards an exit. One inlet and one outlet proved to be sufficient to achieve a high quality cast using a mould of 50 mm x 50 mm.

• For each 10 °C of increase/decrease from 25 °C, the working life and cure time of

the CopyRite 7/30 compounds are halved/doubled, as described by the following formulas:

Max working life (T) ( ) 10/ C2527 T−°⋅≈ min Cure time (T) ( ) 10/ C25230 T−°⋅≈ min It is recommended to keep the working time within 75% of the given max

working life.

15 20 25 30 35 40 45 500

5

10

15

20

25

30

35

40

45

50

55

60

65

Temperature [degrees Celsius]

Tim

e [m

in]

Cure timeWorking life

Figure 44. Cure time and working life vs. temperature for the CopyRite 7/30.

44

• Casts should not be made at temperatures > 40 °C using these polymer

compounds because the outcome is adversely affected by the rapid cure time.

• Satisfactory casts require pressures of at least 0.5-1.5 kPa in dry conditions and 3-4 kPa in wet conditions (not for the non-thixo grade of CopyRite).

• The seal that surrounds the mould should keep air and water out. It should also be

flexible enough to seal against irregular geometries such as over a weld.

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7 SUGGESTIONS FOR FUTURE WORK Since limit pressures for application at the polymer compounds have been found, the pressure being applied during use of the moulding tools should be measured. Furthermore, the moulding tools should be adjusted for the polymer compound to be placed near the mould. Then less amount of compound will be required to make a cast and time is saved because the compound does not have to travel through a long feeder pipe. In addition, the compound will have better quality when it reaches the mould. The tube should not be placed upside down though since the viscosity differs quite a lot between the both parts of the compound. Polymer compounds with increased process times should also be approved for use in order to avoid time constraints during moulding at high temperatures. On site it would be beneficial to have formulas or graphs where working life and cure time easily can be read. Since the polymer compounds have different characteristics, recommendations are needed to tell what they are suited for. For example, the non-thixo grade of CopyRite proved not to be suitable for underwater moulding. To find the optimal polymer compound for underwater applications, a PhD project could be started. Then a recommended moulding method was also presented, which should be tested using the moulding tool. The recommended method had aeration of the mould and addition of surface tension-lowering agent (isopropanol) as two important steps. The method should be practically enforceable since small amounts of isopropanol are allowed during moulding in the reactor pool. To avoid crack detection failures in the future, some work needs to be done to prevent it. The two possible reasons for failures raise suggestions for future work:

1. A way to ensure that the cast is made at the correct position is needed. 2. Guidelines for satisfactory casts should be given. For example, a water film of the

size of the detection target is a warning sign. Therefore it is important that the replicated object’s surface structure is clearly visible on the cast.

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8 ACKNOWLEDGEMENTS First of all, I would like to thank my supervisor David Stenman for giving me the opportunity to do my master thesis at WesDyne TRC and also for helping me to structure my project. I would also like to thank Anders Werner at WesDyne TRC for having me involved in all meetings concerning AVT and in the investigation of the moulding technique in Oskarshamn. It provided me with new contacts that inspired me. During the project I have seen the moulding technology being used in the real world of nuclear power in Oskarshamn. Fredrik Svedberg is thanked for making this a pleasant experience for me, but also for showing his interest in this project and for helping me whenever help was needed. Then I would like to show my gratitude to Peter Harrå, Lehard Kastre, Magnus Rydström and Martin Sandberg for introducing me to the moulding technique and to your reflections regarding this technique, but also for helping me with various issues during the project. Thank you for sharing your experiences with me! I am also grateful for all the help I got from Torbjörn Eriksson with manufacturing of moulds and other practical issues. Further I would like to thank Petri Luukkonen for his assistance concerning issues related to materials. Milan Poznic is thanked for sharing his knowledge of water filled cracks and for providing me with crack blocks for my experiments. All the other colleagues at WesDyne TRC are thanked for creating a great atmosphere. I like to be at work thanks to you! I am grateful for having been introduced to the customer's issue regarding the moulding technology by Lars Johansson at OKG. Thank you also for helping me to find out what materials are allowed in the reactor hall! Further I would like to show my gratitude to Claes-Göran Bengtsson, Peter Berg and Ivar Jonsson at Ringhals AB for sharing the history behind the moulding technique with me, but also for providing me with test samples and for showing me how your microscope works. Thank you for giving me the opportunity to present my project at your meeting concerning the revision of the Procedure at FORCE in Denmark! Then I would like to thank Keith Murray at E&FT for having answered all my questions regarding the polymer compound CopyRite, which enabled me to understand the nature of the blended polymers.

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I would also like to thank Kjell Carlsson at the police department in Stockholm for visiting me at WesDyne to introduce his polymer compound Mikrosil and for discussing the casting technique with me. I want to say thank you to my examiner at Uppsala University, Tomas Nyberg, for accepting this project as my master thesis. Finally, when I was writing this report I had some good advice and feedback that helped me. Therefore, my substance reviewer and professor in materials science at Uppsala University, Staffan Jacobsson, is thanked for helping me with the report.

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9 REFERENCES 1. Barsebäck Power Station, Forsmark Power Station, Oskarshamn Power Station,

Ringhals Power Station, “PBM2”, http://www.vattenfall.se/www/vf_se/vf_se/Gemeinsame_Inhalte/DOCUMENT/196015vatt/815691omxv/819774vxrx/876156vxrx/876168fors/1880848pak/1880894pbm/P02109597.pdf, 2001

2. Bengtsson C.G., personal communication, Ringhals AB, 2009/2010 3. Borggreen K., personal communication, SydTek, 2009 4. Campbell I.M., “Introduction to Synthetic Polymers”, Oxford University, 1994 5. Carlsson K., personal communication, Kjell Carlsson Innovation, 2009/2010 6. Emerius M, personal communication, Rejlers, 2009/2010 7. Helsted C., “General Procedure for Moulding VT of Surface Breaking Defects in

Metallic Materials, Detection, Characterization and Length Sizing of Defects”, FORCE Technology, 2004

8. Hilborn J., Bowden T., “Tvåkomponents epoxi, möjligheter och begränsningar”, Uppsala University, 2008

9. Johansson L., personal communication, OKG, 2009/2010 10. McIntire P., ”Nondestructive Testing Handbook (2nd edition) volume 7”,

American Society for Nondestructive Testing, 1991 11. Murray K.E., personal communication, Engineering & Forensic Technology,

2009/2010 12. Ny Teknik, “Styrstavsfel stoppar reaktorer”,

http://www.nyteknik.se/nyheter/energi_miljo/karnkraft/article445998.ece, 2008 13. Olsen H.O., Helsted C., “Technical Justification Procedure for application of

moulding materials in nuclear power plant installations”, FORCE Technology, 2004

14. Olsen H.O., “Reactor Pressure Vessel and Internals Become Visible by Means of Remote Moulding Inspection, AVT”, ECNDT, 2006

15. Petrie C.S., Walker M.P., O’Mahony A.M., Spencer P., “Dimensional accuracy and surface detail reproduction of two hydrophilic vinyl polysiloxane impression materials tested under dry, moist, and wet conditions”, University of Missouri-Kansas City, 2003

16. Popescu-Pfeiffer M., Wiechers B., Lenz H., “Application of Molding VT Technique on Surface Breaking Defects in Wet Environment”, Westinghouse Electric Germany, 2007

17. Sandberg M., “Manual-Moulding tool Control rod extension”, WesDyne TRC, 2009

18. Skoglund L., personal communication, WesDyne TRC, 2009 19. SQC, http://www.sqc.se/kvalificering/, 2010 20. SSM, http://www.stralsakerhetsmyndigheten.se/Allmanhet/, 2010 21. Svedberg F., “Oskarshamn 3 – Kvalificerad AVT av styrstavsskaft – Jämförelse

mellan torr och våt avgjutning”, WesDyne TRC, 2009 22. Svensson A., personal communication, ABO-verken, 2009

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23. Tosello G., Gava A, Hansen H.N., Lucchetta G., Guarise M., ”Filling analysis in micro injection moulding using weld lines as flow markers”, Technical University of Denmark, 2007

24. Wikipedia, “Archimedes´ principle”, http://en.wikipedia.org/wiki/Archimedes%27_principle#Archimedes.27_principle, 2010

25. Wikipedia, “Arrhenius´ equation”, http://en.wikipedia.org/wiki/Arrhenius_equation, 2010

26. Worsley School, “Thixotropic Materials”, http://www.worsleyschool.net/science/files/thixotropic/materials.html, 2010

27. Zuljan D., Grum J., “Non-destructive metallographic analysis of surfaces and microstructures by means of replicas”, University of Ljubljana, 2005

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APPENDIX 1: MOULDING AT CONTROL RODS A manipulator is used to perform underwater moulding at objects in radioactive environment17. The manipulator used for moulding at control rods consists of two aluminium bars. A round plate at the top holds the bars together, provided with an angle indicator and a depth stop, and a mould at the bottom, which can be seen in Figure 1.

Figure 1. The tool used for underwater moulding at control rod shafts where a represents two aluminium bars held together by a round plate b with an angle indicator. The tool is mounted with its depth stop c resting on the bayonet mount of the shaft d. Then the mould f is pressurized against the surface to replicate by a pneumatic cylinder e.

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When a cast is to be made at a control rod shaft, height and angular position of the area to be inspected is already known since it has initially been detected using other inspection techniques. The mould is positioned at the detected area for visual verification. The moulding tool is mounted around the control rod shaft with its depth stop resting on the bayonet mount of the shaft17, as can be seen in Figure 2. The height is adjusted so that the mould is placed at the height given for the detected cracks. Then the angular position is adjusted using the angle indicator.

Figure 2. A round plate a with depth stop b resting on the bayonet mount of a control rod shaft. The plate is provided with an angle indicator c to set the position.

After the mould has been placed at the desired position, it gets pressurized against the surface, which is to be replicated, using compressed air. A compressed air panel, which can be seen in Figure 3, is used to regulate the pressure on the mould, the pressure on the polymer compound and the aeration of the mould. These pressures can be regulated up to 6 bars.

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Figure 3. The compressed air control panel. The mould gets aerated to remove the water while the polymer compound is transported through the feeder pipe. When the polymer compound reaches the mould, the aeration is turned off and the mould is filled with compound. An underwater camera monitors the sequence of events and when the polymer compound reaches the outlet pipe, the compressed air supply for the compound is closed. Then the pressure on the mould is increased to squeeze the compound into the cracks. When the polymer compound has cured, the pressure on the mould is turned off and the manipulator is brought up from the water. The mould is then removed from the manipulator and the cast can be analyzed.

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APPENDIX 2: MATLAB CODE %Matlab code: Figure 44 clear; clear all ; %Specified temperature range T = linspace(15,50); %Working life tmw = 0.75*7*2.^((25-T)/10); %Cure time tc = 30*2.^((25-T)/10); %Plot commands plot(T,tc, 'black' ); hold on; plot(T,tmw, 'red--' ); axis([15 50 0 65]); grid on; xlabel( 'Temperature [degrees Celsius]' ); ylabel( 'Time [min]' ); legend( 'Cure time' , 'Working life' );