fractography of small punch tested samples ... - imr.saske.sk · m. kapus ňák, j. petzová ......

Powder Metallurgy Progress, Vol.15 (2015), special issue 167

FRACTOGRAPHY OF SMALL PUNCH TESTED SAMPLES FROM RPV MATERIALS

M. Kapusňák, J. Petzová, M. Březina, Ľ. Kupča

Abstract Small Punch Test (SPT) of irradiated samples has a great advantages in reducing the radioactivity of the irradiated materials, compared to standard testing procedures, mainly due to its small volume and also for the determination of material properties, which are highly dependent on the local microstructure e.g. heat affected zone. Based on the advantages mentioned above, the SPT procedure was included as a new method for evaluation of the real state of the RPV materials in the surveillance specimen programs (SSP) in Slovak nuclear power plants (NPP). These projects include, among other procedures and tests, more than eight thousand SPT samples from all RPV material types which were prepared for irradiation experiments during Slovak NPP’s operation in the reactor as part of SSP realization. Present paper summarizes the results obtained from fractographic analysis of selected SPT samples after their testing. Characterization and categorization of different types of fracture are used to justify the correct test procedure. Keywords: Small Punch Test, Reactor Pressure Vessel, Surveillance Specimen Programs

INTRODUCTION The most critical component of nuclear power plants (NPP) is the reactor pressure

vessel (RPV). RPV steel near the reactor core is a subject of irradiation degradation due to the fast neutron flux. Irradiation processes are rather complex but after all the damage of the steel crystal lattice lead to the changes of RPV mechanical properties as well as the shift of the transition temperature to higher values. Hence, monitoring of the RPV material properties changes due to irradiation must be provided during the whole NPP life-time.

Various standard tests are available for the determination of mechanical properties such as tensile strength, ductile-brittle transition temperature and fracture toughness, which are required for the structural integrity assessment of NPP components. However, the standard tests need relatively high amount of original RPV material and after irradiation the samples must be tested in the special hot laboratories due to their very high induced activity. In order to study irradiation damage, small samples are preferred because there are limitations of sample size in irradiation channels and great lack of original RPV materials.

EXPERIMENTAL METHODS

SPT Testing Procedure The principle of SPT testing procedure applied in a company VUJE a.s. is penetration

of the disk specimen by hemispheric rod. The small punch experimental configuration is

Michal Kapusňák, Jana Petzová, Martin Březina, Ľudovít Kupča, VUJE, a.s., Okružná 5, 918 64 Trnava, Slovakia

Powder Metallurgy Progress, Vol.15 (2015), special issue 168 presented in the Fig.1. Disk shaped specimen has 8 mm in diameter and 0.5 mm in thickness. The specimen holder consists of a lower and upper die and holder body. Using this specimen holder, the specimens are prevented from cupping upward during punching and, therefore, plastic deformation is concentrated in the region below the punch rod.

Fig.1. Schematic of the setup for the SPT.

Using relatively simple system with recorders of the load and deflection values, we can obtain the sufficient data about basic mechanical properties [1].

SPT Applications on Irradiated Samples Firstly, SPT method was used for evaluation of RPV material properties changes

causes by irradiation at the Slovak NPP in the frame of the project called the New surveillance specimen program (NSSP). This project was a part of an international project coordinated by IAEA for the irradiation embrittlement evaluation of older WWER-440/230 RPVs type. This reactor type was in operation at NPP V-1 in Jaslovské Bohunice.

Further, application of the small SPT samples is currently used in the international VUJE-Halden project. The main aim of this project is to compare the mechanical properties changes of basic RPV materials after irradiation in a power reactor and in a research reactor. All samples are irradiated in the Halden reactor to the neutron fluence, which is equivalent to one resp. three campaigns of WWER-440 RPV. The irradiated specimens are successively tested by SPT methodology in the IFE (Institute for Energy Technology) laboratories in Kjeller. The obtained results are compared with outcomes of surveillance sample programs at WWER-440 power reactors.

The most extensive application of the SPT method for testing of the irradiated samples is the new VUJE designed project called Advanced Surveillance Specimen Program (ASSP), which was prepared for all Slovak NPP units currently in operation. Brief description of this project and some first results are described in this paper.

At last, there are prepared new sets of samples, which will be used in the prepared surveillance programs for NPP units in Mochovce (units No.3 and No.4) which are, at this moment, under construction.

Advanced Surveillance Specimen Programs The new Advanced Surveillance Specimen Programs (ASSP) were prepared at all

Slovak NPPs reactors for assessment of reactor pressure vessel’s materials real state. Main goal of this program is monitoring state of RPV’s materials for the whole planned operational life-time.


Design of chains, measuring of irradiation temperature and neutron fluence is similar as in finished standard and extended surveillance specimen programs [2]. Innovations in these projects compared to previous are: • Besides the standard tests of mechanical properties there were included in the ASSP

material degradation monitoring also the new testing methods - Small Punch Test. • In ASSP, there were prepared samples not only from base RPV materials (base

material and weld metal), but also from austenitic materials of the internal parts of reactor (core shroud basket) and from austenitic cladding of RPV, too.

• Using of SPT and its methodology with a small size of samples allowed that the ASSP project includes the monitoring of potentially critical local areas RPV materials as the heat affected zones of the circumferential NPP´s welds and cladding welds.

• Implementation of IAEA recommended approach for mechanical testing evaluation - application of the "Master Curve" method from results of the fracture toughness on the COD samples.

However, the principal ASSP project goal is to obtain reliable technical arguments for long-term NPP operation (i.e. the life extension of all our NPP units of at least 20 years), which is currently common in many countries around the world, such as Korea, USA, France, Russia but also in other countries using nuclear energy [3, 4].

EXPERIMENTAL MATERIALS For fractographic analysis there were chosen some types of SPT samples from

ASSP project in initial state and after two campaign irradiation in Unit 2 of the NPP Mochovce. Small SPT samples give the possibility for testing separately all parts of heat affected zone (HAZ) near the RPV weld metal.

Present paper illustrates fractographic analysis on SPT samples from following materials: • Weld metal of RPV, • first HAZ layer of the RPV base material (the nearest area to the weld No.4 of RPV) • and second HAZ layer of the RPV base material (the area next to the first layer).

RESULTS AND DISCUSSION

SPT results In the Table 1, there are summarized the tensile testing results obtained from SPT

tests for evaluated materials. The typical changes of the tensile properties due to irradiation are possible to observe from obtained results. The irradiation processes cause degradation of materials, consequently shift of the tensile properties towards the higher values.

Tab.1. Results of tensile properties of experimental materials in initial state and after irradiation during 2 campaigns in the RPV channels of the NPP Mochovce, unit 2.

Initial state After irradiation Material Rm

[MPa] Re

[MPa] Rm

[MPa] Re

[MPa] RPV WM 664 526 802 553 HAZ I. 711 621 964 671 HAZ II. 686 544 812 667


The Table 2 shows the values of the transition temperatures (FATT) evaluated by temperature dependence of the SPT energy obtained from SPT tests at various test temperatures. As the correlation coefficient, (TTSPT / TTFATT), value of 0.38 was used for weld metal and 0.45 for HAZ materials of RPV. There are visible changes of the transition temperature due to irradiation, specifically the shift of the FATT towards the higher values for RPV testing materials. The shifts of the SPT energy curves are illustrated in the figures 2, 3 and 4.

Tab.2. Shift of the transition temperature curves due to two-campaign irradiation in the RPV channels of the NPP Mochovce, unit 2

Transition temperature FATT [°C] Type of material Initial state After irradiation HAZ I. (weld No.4) -16.6 46.5 HAZ II. (weld No.4) -18.7 6.0 RPV WM 15.4 36.3

Fig.2. The shift of the SPT energy curves of RPV weld metal after irradiation in NPP

Mochovce, unit 2

Fig.3. The shift of the SPT energy curves of material from RPV HAZ I. after irradiation

in NPP Mochovce, unit 2

Fig.4. The shift of the SPT energy curves of material from RPV HAZ II. after irradiation in NPP Mochovce, unit 2


A - in original initial state

B - after two-campaign irradiation in NPP Mochovce, unit 2

Fig.5. RPV weld metal - comparison of fractographic surfaces on SPT samples tested on ambient temperature (level of upper shelf SPT fracture energy) and liquid nitrogen

temperature (level of lower shelf SPT fracture energy)




Fig.6. RPV HAZ I. - comparison of fractographic surfaces on SPT samples tested on ambient temperature (level of upper shelf SPT fracture energy) and liquid nitrogen

temperature (level of lower shelf SPT fracture energy).




Fig.7. RPV HAZ II. - comparison of fractographic surfaces on SPT samples tested on ambient temperature (level of upper shelf SPT fracture energy) and liquid nitrogen

temperature (level of lower shelf SPT fracture energy).


Fractographic analysis From all three evaluated types of RPV materials there were chosen typical samples

for fractographic analysis. To this analysis there were subjected SPT samples from the area of low-temperature tests, it means samples with brittle fracture surface, and from ambient temperature tests, hence samples with ductile fracture surfaces. These areas could be defined as the levels of lower or upper shelf energies on temperature dependence graphs of SPT fracture energy.

Figures 5, 6 and 7 illustrates possible comparison between fracture surfaces SPT samples from different RPV materials tested before and after irradiation. There is possible to evaluate that no significant changes caused by irradiation on the basic fracture surfaces were observed.

CONCLUSIONS Based on presented research we can summarize conclusions as follows:

• SPT methods are suitable for measurement of the basic mechanical parameters like tensile properties or evaluation of DBTT.

• With SPT tests before and after irradiation there were confirmed the expected changes in the mechanical properties of materials, i.e. shift of the FATT towards the higher values for RPV materials.

• Fractographic analysis of failed SPT samples offers additional information about failure mechanisms during the test.

Acknowledgements This contribution is the result of the project implementation „Long-term operation

of nuclear power plant type VVER 440 regarding the environment“, ITMS 26220220146 supported by the Research & Development Operational Programme funded by the ERDF.

REFERENCES [1] CEN: Small Punch Test Method for Metallic Materials. CEN workshop agreement

CWA 15627:2006, 2007 [2] Kupča, Ľ., et al.: Design of the ASSP project for the V-2 NPP in Jaslovské Bohunice.

Technical Report of VUJE No. 0360/90/96 (in Slovak) [3] IAEA: Ageing Management for Nuclear Power Plants and Research Reactors,

SAFETY GUIDE NS-G-DS382, 2005 [4] IAEA-EBP-LTO-20: Safety Aspects of Long Term Operation of Water Moderated

Reactors, Draft 15-02-2006

fractography of small punch tested samples ... - imr.saske.sk · m. kapus ňák, j. petzová ......

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