edited manuscript for barc report-jainendra

BARC/2018/E/008 BA

RC

/2018/E/008

2018

DECAY HEAT REMOVAL DURING REPLACEMENT OF EXPANSION JOINTS IN PROCESS WATER

LINE OF DHRUVA REACTOR

byJainendra Kumar, Y.S. Rana,

Tej Singh and P.V. Varde

Research Reactor Services Division

BARC/2018/E/006

BA

RC

/201

8/E

/006

GOVERNMENT OF INDIADEPARTMENT OF ATOMIC ENERGY

BHABHA ATOMIC RESEARCH CENTREMUMBAI, INDIA

2018

DECAY HEAT REMOVAL DURING REPLACEMENT OF EXPANSION JOINTS IN PROCESS WATER

LINE OF DHRUVA REACTOR

byJainendra Kumar, Y.S. Rana,

Tej Singh* and P.V. Varde

[email protected]

Research Reactor Services Division

BIBLIOGRAPHIC DESCRIPTION SHEET FOR TECHNICAL REPORT(as per IS : 9400 - 1980)

01 Security classification : Unclassified

02 Distribution : External

03 Report status : New

04 Series : BARC External

05 Report type : Technical Report

06 Report No. : BARC/2018/E/008

07 Part No. or Volume No. :

08 Contract No. :

10 Title and subtitle : Decay heat removal during replacement of expansion joints in process

water line of Dhruva reactor

11 Collation : 40 p., 14 figs., 15 tabs.

13 Project No. :

20 Personal author(s) : Jainendra Kumar; Y.S. Rana; Tej Singh; P.V. Varde

21 Affiliation of author(s) : Reactor Physics and Nuclear Engineering Section, Research Reactor Services Division, Bhabha Atomic Research Centre, Mumbai

22 Corporate author(s): Bhabha Atomic Research Centre,Mumbai - 400 085

23 Originating unit : Research Reactor Services Division,Bhabha Atomic Research Centre, Mumbai

24 Sponsor(s) Name : Department of Atomic Energy

Type : Government

Contd...

BARC/2018/E/008

BARC/2018/E/008

30 Date of submission : July 2018

31 Publication/Issue date : July 2018

40 Publisher/Distributor : Head, Scientific Information Resource Division, Bhabha Atomic Research Centre, Mumbai

42 Form of distribution : Hard copy

50 Language of text : English

51 Language of summary : English

52 No. of references : 8 refs.

53 Gives data on :

60

70 Keywords/Descriptors : DHRUVA REACTOR; EXPANSION JOINTS; PRIMARY COOLING CIRCUITS;

THERMAL EXPANSION; HEAT EXCHANGERS; AFTER-HEAT REMOVAL

71 INIS Subject Category.: S21

99 Supplementary elements :

Abstract : Dhruva is a 100 MWth research reactor with metallic natural uranium as fuel andheavy water as moderator, coolant and reflector. Demineralized light water is used as secondarycoolant which in turn is cooled by sea water. On the primary coolant side, there are three loopswith each loop consisting of a Main/Auxiliary Coolant Pump and heat exchanger. There arefour expansion joints (EJs) on process water lines to take care of thermal expansion of thelines. Water leakage was observed in one of the four expansion joints. In view of the observeddegradation and damage, it was decided to replace this rubber joint with a spare one. Thereplacement procedure required isolation of secondary side of the heavy water/process water(HW/PW) heat exchangers resulting in increase of the temperature of heavy water coolant andmoderator. Analysis was carried out to estimate the time available for the replacement jobwithout the heavy water temperature in the coolant circuit exceeding the predetermined limit.This report gives a detailed account of the analysis and the observations made during thereplacement job

DECAY HEAT REMOVAL DURING REPLACEMENT OF EXPANSION JOINTS IN PROCESS WATER LINE OF DHRUVA REACTOR

Jainendra Kumar, Y.S. Rana, Tej Singh* and P.V. Varde

Research Reactor Services Division Bhabha Atomic Research Centre, Mumbai 400085, India.

[email protected]

Abstract

Dhruva is a 100 MWth research reactor with metallic natural uranium as fuel and heavy water

as moderator, coolant and reflector. Demineralized light water is used as secondary coolant

which in turn is cooled by sea water. On the primary coolant side, there are three loops with

each loop consisting of a Main/Auxiliary Coolant Pump and heat exchanger. There are four

Expansion Joints (EJs) on process water lines to take care of thermal expansion of the lines.

Water leakage was observed in one of the four expansion joints. In view of the observed

degradation and damage, it was decided to replace this rubber joint with a spare one. The

replacement procedure required isolation of secondary side of the heavy water/process water

(HW/PW) heat exchangers resulting in increase of the temperature of heavy water coolant

and moderator. Analysis was carried out to estimate the time available for the replacement job

without the heavy water temperature in the coolant circuit exceeding the predetermined limit.

This report gives a detailed account of the analysis and the observations made during the

replacement job.

CONTENT

1 INTRODUCTION……………………………………………………………………. ........ 1

2 SYSTEM DESCRIPTION…………………………………………………………… ......... 2

3 REQUIREMENT OF SHUT DOWN COOLING………………………………….. ........... 2

4 EXPANSION JOINTS………………………………………………………………........... 3

5 MATHEMATICAL MODELING………………………………………………….. ........... 3

5.1 Thermal Hydraulics Model for Coolant………………………………………………….. ... 4

5.2 Estimation of Heat Transfer Coefficient………………………………………………….. .. 4

5.3 Overall Heat Transfer Coefficient for Pipes in Vault Water Region…………………… ..... 4

5.4 Evaporative Cooling from Moderator and Vault Water Surfaces…………………………. 5

5.5 Outlet Water Temperature………………………………………………………………… . 5

5.6 Heat Transfer from Coolant to Vault Water…………………………………………… ...... 5

5.7 Heat Transfer from Coolant to Moderator………………………………………….. ........... 6

5.8 Heat Transfer from Moderator Circuit to Vault Water…………………………………. ..... 6

5.9 Heat Transfer through Vault Water Heat Exchanger……………………………………… . 6

5.10 Heat Transfer from Coolant to Water Held-up in HW/PW Heat Exchanger……………… 7

5.11 Inlet Plenum Temperature………………………………………………………………….. 7

6 INPUT PARAMETERS………………………………………………………………… .... 7

7 RESULTS AND DISCUSSION…………………………………………………………. ... 8

8 SUMMERY AND CONCLUSION………………………………………………………. .. 11

NOMENCLATURE………………………………………………………………………………. . 12

REFERENCES…………………………………………………………………………………….. 14

List of Figures Sr. no. Title Page Fig.1(a) Simplified schematic of reactor core and surrounding regions 23 Fig.1(b) General Arrangement of Reactor Plan 24 Fig.1(c) Rubber Expansion Joint 25 Fig.2(a) Simplified cooling flow diagram 25 Fig.2(b) Simplified Flow Diagram of HW Loop #1 26 Fig.3 Turbine-1 Operation with only HX#1 online 26 Fig.4(a) Decay power variation with time after shut down 27 Fig.4(b) Decay power variation after 1 hour of shut down 27 Fig.5 Schematic diagram for coolant temperature estimations for decay heat

removal

28

Fig.6 Temperature variation with secondary side of HW/PW heat exchangers offline (after 1 hours of reactor shut down)

28


29


29


30


30


31


31


32

Fig.14 Temperature variation after 72 hours of shutdown cooling with Core cooling flow of 5000 lpm, a bypass flow of 400 lpm on the secondary side of the HW/PW heat exchangers and cooling of vault water not available

32

List of Tables Sr. no. Title Page Table.1(a) Input parameters (Two XCP pump operation) 15 Table.1(b) Input parameters (system dimension) 15 Table.2 Thermo-physical properties of materials 16 Table.3(a) Decay power variation with time (Reactor shut down from 100 MW) 16 Table.3(b) Decay power variation with time(Reactor shut down from 90 MW) 16 Table.4 Temperature variation when secondary side of HW/PW heat exchangers

is offline (after 1 hour of reactor shut down) 17

Table.5 Temperature variation when secondary side of HW/PW heat exchangers is offline (after 12 hrs of reactor shut down)

17


18


18


19


19

Table.10 Temperature variation with secondary side of HW/PW heat exchangers offline (after 96 hrs of reactor shut down)

20

Table.11 Temperature variation with secondary side of HW/PW heat exchangers offline (after 120 hrs of reactor shut down)

20

Table.12 Time to reach coolant temperature from 37 to 65 °C for different shut down cooling time with no secondary cooling to HW/PW heat exchangers

21

Table.13 Temperature rise with bypass flow on the secondary side of HW/PW heat exchangers

21

Table.14 Heavy water coolant temperature rise for different shut down cooling times with no regular secondary cooling to PW/HW heat exchangers (XCP-1 flow: 1380 lpm; Core flow: 5123 lpm; Vault cooling flow: 2170 lpm)

21

Table.15(a) Temperature variation with secondary side of HW/PWheat exchangers offline (after 12hr of reactor shut down)

22

Table.15(b) Temperature variation with secondary side of HW/PWheat exchangers offline (after 24 hr of reactor shut down)

22

1 INTRODUCTION

Dhruva is a 100 MWth research reactor using metallic natural uranium in the form of a seven pin cluster as fuel [1]. Heavy water is as moderator, coolant and reflector. To simplify the design and to ensure high integrity of the heavy water heat exchangers, an intermediate light water system is used before rejecting the reactor heat to the sea. The reactor core is contained in a cylindrical stainless steel vessel which is placed in a light water filled vault. Overall height of the calandria is 387 cm and is designed to accommodate 305 cm long fuel, lower and upper axial heavy water reflector thickness of 32 and 30 cm respectively and a gas space 20 cm long. A simplified schematic of reactor core and surrounding regions is given in (Fig.1(a)). The calandria design provides for 146 lattice positions arranged on a regular 18 cm square lattice pitch. Special provisions have been made in the calandria design to accommodate two engineering loops and three in pile creep and corrosion facilities. The remaining 141 sites are meant for loading fuels, shut-off rods and isotope tray/adjuster/pneumatic carrier rods and these sites are similar in design to allow for interchangeability. (Fig.1(b)) shows reactor core configuration in XY plane. Apart from the various in pile experimental facilities mentioned above, there are 15 beam holes for research/experimental purpose. The primary coolant system consists of three loops, with each loop having a main/auxiliary coolant pump and a heavy water/process water (HW/PW) heat exchanger. The secondary coolant system has six process water/sea water (PW/SW) heat exchangers and six PW pumps. There are four expansion joints (EJs) on process water lines to take care of thermal expansion of the lines (Fig.1(c)). On 16.12.2016, water leakage of around one lpm was observed from one of the expansion joints (EJ-4) at upstream side of PW suction line strainer. The reactor was shut down at 22:45 hrs and primary cooling water (heavy water) system, secondary cooling (process water) water system and tertiary cooling water (sea water) system was shut down as per normal procedure [2]. Physical checking of the leaky expansion joint, after removal of its secondary confinement in the form of Dresser Coupling (DC), showed some swelling of rubber material. Expansion bellow was observed to be damaged at two places (one at 3 O’clock position and other at 10 O’clock potion viewing from PW expansion tank side) due to piercing of expanded bellow material due to interference with flange nut and bolt stud. These expansion joints are 30 years old. In view of the observed degradation and damage, it was decided to replace this rubber joint with a spare one. Other three expansion joints were inspected after removal of dresser coupling and found normal on preliminary visual inspection. The rubber material of expansion joint was not touching any nut or bolt and the profile of material was similar to spare expansion joint. For replacement of the leaky expansion joint (expected time involved was about 10 hrs.), it was required to isolate the secondary side of the HW/PW heat exchangers. This will result in loss of secondary side flow in all three HW/PW heat exchangers, resulting in increase of the temperature of heavy water coolant and moderator. An analysis was carried out to estimate minimum shut down time required before starting the

1

job, and the expected temperature rise in heavy water and vault water. This report gives a detailed account of the analysis and the observations made during the replacement job. 2 SYSTEM DESCRIPTION

Dhruva reactor primary coolant system has three parallel loops with one Main Coolant Pump (MCP), one Auxiliary Coolant Pump (ACP), one Heat Exchanger(HW/PW) and associated pipelines.(Fig.2(a)) shows a simplified cooling flow diagram of the reactor whilesimplified flow diagram of one HW Loop (Loop#1) is shown in (Fig.2(b)). During normal reactor operation, heat generated in the core is removed by circulating the required flow by operating all the three MCPs. The heat is transferred to secondary coolant(PW)through three shell and tube type heat exchangers, one in each coolant loop; each heat exchanger is rated for 34.0 MW. In case of any interruption in the main coolant flow, the reactor trips on number of process and equipment parameters. The steady state shutdown cooling is provided by three numbers of ACPs located in parallel to MCPs. During auxiliary pump operation, the residual heat (decay heat) is rejected to the Emergency Cooling Water (ECW) system through main HW/PW heat exchangers. Each ACP is connected to two prime movers, motor and turbine mounted on the same shaft. Any of the prime movers can be operated. The motors are connected to class II buses (uninterrupted 415V AC). ECW from Over Head Storage Tank (OHST) flows through the turbines to run the ACPs. Subsequently the ECW passes through the secondary side of HW/PW heat exchangers and enters the dump tank from where it is pumped back to OHST by make-up pumps. If no turbine is operating, secondary side cooling is established through turbine by-pass line. (Fig.3) shows a simplified flow diagram of Turbine-1 Operation with only HX#1 online. Due to capacity limitation of OHST and class II buses, not more than two turbines or two motors are operated simultaneously.

3 REQUIREMENT OF SHUT DOWN COOLING

After the reactor and MCP trip, it is stipulated that the coolant flow through the core should be more than 5000 lpm for the first 15 minutes and more than 2500 lpm thereafter (LCO clause# 5.6.1.11 of Dhruva Technical Specifications) [3].Thus, as long as irradiated fuel is present in the core, the minimum coolant flow through the core shall not be less than 2500 lpm (safety limit#3.2 of Dhruva Technical Specifications). Similarly, on the secondary side, with the reactor in shut down state, process water flow through the HW/PW heat exchangers shall not be less than 5000 lpm for first 15 minutes after shut down and not less than 2500 lpm subsequently (LCO clause# 5.10.5 of Dhruva Technical Specifications).Beside the technical specification requirements, it is important to note that for satisfactory operation of XCP on turbine mode, a flow of around 3200 lpm per turbine is needed. At an ECW flow of 2500 lpm through turbine, its speed will reduce from 1200 to 900 rpm which will result in a pump flow of 3166 lpm.

2

4 EXPANSION JOINTS

The Process Water/Emergency cooling water System of Dhruva comprises of two main 915 mm NB headers located in service building basement and sub-basement. Four numbers of rubber expansion joints with sliding supports have been provided on these headers to take care of thermal expansion, if any. During reactor operation, the secondary coolant is supplied through these headers to the HW/PW heat exchangers by operating Process Water Pumps (PWP). When the reactor is shutdown and process water pumps are off, the secondary cooling is provided by ECW system through OHST. The water from OHST runs through turbines into the inlet header of HW/PW heat exchangers. The outlet header of HW/PW heat exchangers is connected to dump tank (Fig.3).

In case of catastrophic failure of rubber expansion joints, core cooling may get affected as the leaky joints cannot be isolated without stopping secondary side cooling flow. To take care of such a situation, the expansion joints were encased with a DC type secondary containment such that the secondary containment will reduce the leak rate without significantly affecting core cooling. The DC was installed on EJ-4 in February-March, 2013. 5 MATHEMATICAL MODELING

After reactor shut down, the decay heat of the core is removed by auxiliary coolant system and the process water from OHST (after passing through turbine and secondary side of HW/PW heat exchanger) acts as ultimate heat sink. During the proposed EJ replacement job, secondary side flow to the HW/PW heat exchanger may not be available. In such a case, vault water will act as final heat sink and will receive the core decay heat from surfaces of the heavy water piping passing through the vault water region. Adopting a conservative approach, the following assumptions were made for the analysis. (i) Heat loss from the piping surfaces other than that in vault water (such as tail pipes in

upper service space, piping in loop room etc.)is assumed to be negligible (ii) Vault water is stagnant i.e. no recirculation flow is considered (iii) Radiative heat loss from primary coolant circuit is not considered

In absence of secondary side flow to HW/PW heat exchangers, there will not be sufficient heat removal from the hot coolant water coming out of the core. As a consequence, the coolant water temperature will rise each time it passes through the core. However, the outlet coolant water will lose some thermal energy to the vault water while passing through the pipes lying in vault water region. Therefore, the coolant temperature rise will be at a relatively slower rate.

For shutdown times beyond one hour, the change in decay heat see (Fig.4(a) & (b)) during coolant recirculation time (~5 minutes) is very small and hence the same is assumed to be constant during recirculation period. For simplicity, it was further assumed that, during coolant recirculation period, fuel, clad and coolant are in thermal equilibrium condition and decay heat is completely taken away by the coolant while passing through the core.

3

5.1 Thermal Hydraulics Model for Coolant

The analysis has been performed assuming peak temperature in the heavy water system do not crosses 70ºC, that essentially restrict the coolant in single phase region, therefore thermal hydraulics modelling has been carried for single phase of cooling water. The mathematical model includes general mass, momentum and energy conservation equations used in following form [4]. Mass conservation:

. 0v

t

Eq.(1)

Momentum conservation:

2. .v

v v P f vt

Eq.(2)

Energy conservation:

'' '''.( ) . .h

hv q q p vt

Eq.(3)

5.2 Estimation of Heat Transfer Coefficient

Suitable models for heat transfer across pipe surfaces have been considered. The well-known Dittus - Boelter [5] and Sieder-Tate [6] correlations are applied for estimation of heat transfer coefficients for pipes carrying hot water outside heat exchangers. The heat transfer coefficient for single phase flow of coolant in a pipe is expressed as:

.eff

e

Nu KH

D Eq.(4)

where, Nu is Nusselt number, K is thermal conductivity and De is the effective hydraulic

diameter of the flow channel. Nusselt number (Nu) is calculated using Dittus - Boelter correlation and Sieder-Tate correlation which includes a viscosity correction to account for heated wall effects.

0.8 0.40.023Nu Re Pr Eq.(5)

Sieder-Tate correlation is given as

0.14

0.8 0.330.023 b

w

Nu Re Pr

Eq.(6)

Effective heat transfer coefficient in heat exchangers are suitable adjusted for lower flow taking reference from the design values. 5.3 Overall Heat Transfer Coefficient for Pipes in Vault Water Region The effective heat transfer coefficient of a coolant pipe in vault water region due to conduction and convection processes can be represented as:

Eq.(7)

4

Where, is the effective heat transfer coefficient of coolant pipe and is the heat

transfer coefficient for inner side of the pipe, and are inner and outer radii of the pipe, is thermal conductivity of pipe material and is the natural convection heat transfer coefficient for stagnant water.

5.4 Evaporative Cooling from Moderator and Vault Water Surfaces

Evaporative heat loss from the pool surface exposed to the atmosphere is given by the following empirical expression Eq.(8) C = (25 + 19 va) Eq.(9)

Where, G is the evaporation rate of water (kg/h), C is a constant depending on air velocity above the pool surface, va is velocity of air above the water surface (m/s), A is the water surface area (m2) exposed to air, Xs is humidity ratio in saturated air at the same temperature as the water surface (kg/kg) and X is humidity ratio in the air (kg/kg).

A simple diagram illustrating the heat transfer mechanism from core to the coolant, vault water and moderator is shown in (Fig.5).

5.5 Outlet Water Temperature

Under steady state condition,

Eq.(10)

Where, is the inlet coolant water temperature, is the coolant outlet temperature, is

decay power of the core, is coolant mass flow rate (kg/sec) and is specific heat of the

heavy water coolant.

5.6 Heat Transfer from Coolant to Vault Water

Heat transfer to vault water from the surfaces of three down comer pipes of the primary coolant circuit is given by:

Eq.(11)

Temperature rise in vault water ( during time

Eq.(12)

Where, is the heat transfer rate from coolant to vault water,hcv is the effective heat transfer coefficient of coolant pipe and is surface area of the pipes exposed in vault water, is total vault water inventory (kg) and is the specific heat of light water.

5

5.7 Heat Transfer from Coolant to Moderator

In Dhruva reactor, the primary coolant gets mixed with the moderator system through leakage of the coolant from the lattice cup, structural cooling and return of excess moderator to the coolant circuit. In the present analysis, heat transfer from coolant to moderator has been accounted by considering mixing of hot coolant water into moderator as follows:

Eq.(13)

Temperature rise in moderator water ( during time

Eq.(14)

Where, Q is the heat transfer rate from coolant to moderator, T is the coolant

temperature,T is the moderator temperature, is the structural cooling mass flow rate (kg/sec) from coolant to the moderator and is total heavy water inventory in moderator system. 5.8 Heat Transfer from Moderator Circuit to Vault Water

Heat transfer from moderator to vault water from the surfaces of moderator pipes exposed to the vault water has been considered as follows:

Eq.(15)

Temperature drop in moderator due to cooling in vault water during time

Eq.(16)

Temperature rise in vault water ( during time

Eq.(17)

Where, is the heat transfer rate from moderator to vault water, is the effective heat transfer coefficient of moderator circuit, is the moderator temperature, is the temperature of the vault water, is surface area of the moderator circuit exposed in vault water, is total heavy water inventory in moderator system, and is the specific heat of heavy

water, is the total water inventory in the vault and is the specific heat of light water.

5.9 Heat Transfer through Vault Water Heat Exchanger

The Vault water cooling system is designed to remove around 750 kW heat generated in the vault. The recirculation rate of 2300 lpm (138 M3 /hr) has been chosen to limit vault water outlet temperature to 50ºC to limit reactor vessel and shield concrete temperature. The vault water is circulated through a plate type heat exchanger to remove the heat generated in vault water. The cold water at temperature of 45ºC enters the vault at the bottom through a loop

6

and emerges out at a temperature of 50ºC from the top of the vault. Two heat exchangers are provided and normally one of them is in service. Under normal operating condition, ~2200 lpm flow is provided on the secondary side. However, to prevent any emergency cooling water flow through the heat exchanger, secondary side flow is stopped during reactor shutdown and hence heat is deposited in the vault water itself. Heat transferred ( from vault water to process water through heat exchanger is estimated

as follows: Eq.(18)

Temperature drop in vault water due to heat loss through heat exchanger during time

Eq.(19)

Where, is the effective heat transfer coefficient of the vault water heat exchanger, is

process water temperature, is the effective cooling surface area in the heat exchanger. 5.10 Heat Transfer from Coolant to Water Held-up in HW/PW Heat Exchanger

After isolation of the secondary side of HW/PW heat exchanger, a significant quantity of stagnant light water will be available on the shell side of the heat exchanger. Heat transfer from coolant to the held-up water will take place as per the following equation: Eq.(20)

Temperature rise ( in the water held up in the heat exchanger during time

Eq.(21)

Where,Q is the rate of heat transfer from coolant to stagnant process water in the heat exchanger, is the effective heat transfer coefficient of coolant pipe, T is the temperature of the process water and is surface area of the pipes exposed in heat exchanger, is total water inventory (kg) held-up in heat exchangers. 5.11 Inlet Plenum Temperature Considering the heat transfer of coolant to vault water, moderator and held-up light water in heat exchanger, the inlet plenum temperature of the coolant is given as:

Eq.(22)

6 INPUT PARAMETERS

Input parameters corresponding to operation of two XCPs, driven by motor as prime mover, have been considered for the analysis. The system parameters i.e. coolant flow, inventory, effective surface areas and dimensions are provided in (Table.1(a) &1(b)). Thermo-physical properties of different materials are given in (Table.2) [7&8].

7

7 RESULTS AND DISCUSSION

Thermal hydraulics analysis has been carried out for estimating the time margin (with respect to maximum coolant outlet temperature) available for the proposed valve replacement job. The analysis has been done for different cooling periods (defined as the time from reactor shut down to the point when process water from OHST is isolated). The estimated decay powers corresponding to different cooling times are given in (Table.3) and shown graphically in (Fig.4). In addition to the core decay heat, heating due to XCP pump operation (10 KW per pump) has also been considered. The results are described below.

Case-1: Normal operating condition

During normal operating condition at 100 MW reactor power, the total primary coolant flow rate remains ~66000 lpm while the secondary coolant (PW) flow rate is maintained at ~ 82000 lpm. The system consists of three shell and tube type heat exchangers, one in each coolant loop, thus each heat exchanger sharing the total load of 101.4 MW (100 MWt Fission Heat + 1.4 MWt pump heat). Hence, each heat exchanger is required to transfer 33.8 MW heat from primary coolant system to secondary coolant system. Accordingly, each heat exchanger is rated for 34.0 MW. The design of heat exchangers was carried out considering the reactor inlet temperature as 50ºC (which is same as heat exchanger D2O outlet temperature) and the reactor outlet temperature as 70ºC. Thus, the differential temperature across the heat exchangers of 20ºC on the primary side formed one of the design bases for heat exchangers. As regards to secondary side design bases, the minimum inlet temperature achievable is 39ºC and for a given viable flow rate of 33,333 lit/min (2000m3/hr), the outlet temperature is fixed at 53ºC, thus with a ∆T of 14ºC. The overall heat transfer coefficient for the normal operation is 1897.2 kcal/m2hr-ºC for the stainless steel tubes, existing geometry and the respective Reynolds number. The value of “Log Mean Temperature Difference (LMTD)” based on the inlet and outlet temperatures of D2O and H2O, discussed earlier, is 13.18 ºC.

Case-2: Normal shutdown cooling

As mentioned in section 3.0, under normal shutdown cooling, two XCPs are operational with a total flow of ~ 5100 and 3100 lpm in primary and secondary side of the HW/PW heat exchangers, respectively. Temperatures in the heavy water coolant and ECW remain ~ 38 and 37 °C, respectively after 1 hour of reactor shutdown. As the shutdown period increases, the temperatures approach the ambient values due to reduction in core decay power. As mentioned in case-1, the overall heat transfer coefficient for the normal operation (primary side flow of ~ 66000 lpm and secondary side flow of 82000 lpm) is 1897.2 kcal/m2hr-ºC. For reduced flow on the primary and secondary sides of HW/PW heat exchangers during shut down condition, this heat transfer coefficient has been accordingly adjusted for temperature estimations. The estimated values of temperatures are found to be near the observed values.

8

Case-3: Core cooling flow of 5000 lpm without secondary side flow in HW/PW heat exchangers and cooling of vault water not available.

During the EJ replacement job, due to non-availability of secondary side flow in HW/PW heat exchanger, coolant water temperature will increase with time. However, the outlet coolant water will lose some thermal energy to the vault water while passing through the pipes lying in vault water region. Therefore, the coolant temperature rise will be at a relatively slower rate. The rate of temperature rise will depend on the decay heat of the reactor core. It was expected that ~ 10 hours would be required to complete the job before normalizing the system. It was also required that heavy water temperature should not exceed 70 ºC. With these two conditions, analyses were carried out considering different shut down times to estimate the time for heavy water to reach 70ºC. The results of the analyses are given in (Tables.4-11) and shown graphically in (Fig.6-13). It can be seen from these results that, if the job is started after 1 hour of reactor shut down, the coolant water temperature reaches 70°C within 2.9 hours (Table.4). For longer shut down times, it will take longer time for the coolant to reach this temperature, as the core decay heat keeps on reducing with time. However, if the valve replacement job is taken up after 24 hours of reactor shutdown, the coolant water temperature in the hottest channel will reach 70°C after around 12.7 hours (Table.6). This time increases further to 20 and 30 hours for core cooling periods of 48 and 96 hours, respectively (Tables.8 & 10).

Calculations were also done for estimating the time taken by the coolant to reach a temperature of 65°C for different shut down times. The results are given in (Table.12). It can be seen from the table that, if the valve replacement job is taken up after 24 hours of reactor shut down, the coolant will reach 65°C temperature within 9 hrs. However, if heat transfer to stagnant water in HW/PW heat exchanger is considered, this time will increase to about 11 hours.

Case-4: Core cooling flow of 5000 lpm with a bypass flow of 300 lpm on the secondary side of the HW/PW heat exchangers and cooling of vault water not available.

Before starting the replacement job, it was decided to make an arrangement for a bypass flow on the secondary side of the HW/PW heat exchangers. Thus, analysis was carried out for bypass flows of 300 and 400 lpm. The analysis shows that (Table.13), the heavy water temperature rises to a peak value and then starts reducing, following the trend of core decay heat. The peak value of temperature depends on the reactor shutdown time. It is seen from the analysis that if the maintenance work is started after three days of shutdown core cooling, the temperatures in heavy water and vault water will remain within 50ºC for 400 lpm flow on the secondary side of HW/PW heat exchanger (Fig.14). If the maintenance work is started after four days of shutdown core cooling, the temperatures in heavy water and vault water will remain within 52ºC for 300 lpm flow in the secondary side of HW/PW heat exchanger.

It is worth mentioning that the replacement job was carried out after a shutdown period of 72 hrs and the peak temperature in the heavy water system was observed to be ~42ºC. Since,

9

the bypass flow was a temporary arrangement, no direct flow measuring device was available. However, to ensure secondary side flow of ~ 500 lpm, ultrasonic technique (UT) probes were used to measure the bypass flow. The observed flow using UT probes was ~1700 lpm. For benchmarking purpose, the above analysis was repeated for secondary side flow of 1700 lpm. The estimated value of peak temperature in heavy water system was 41.6ºC (Table.13) which is very close to the observed value of 42ºC.

Case-5: Core cooling flow of 5000 lpm without secondary side flow in HW/PW heat exchangers and cooling of vault water available with flows of 2300 and 2000 lpm on primary and secondary sides, respectively.

A preliminary assessment indicated that, for replacement of 4 EJs of PW system, around 10 days will be required to complete the job. Thermal hydraulics analyses for the decay heat removal during the job have been carried out. In the analysis, we have considered 10 days of reactor shut down cooling before taking up the replacement job. During this maintenance work, the secondary side flow in HW/PW heat exchanger will not be available. Analysis was carried out to estimate the rise in temperature of heavy water (D2O inlet plenum temperature, moderator temperature) and vault water. During the PW EJ replacement work, decay heat from the reactor core will be released in PW/ECW water (total inventory 2400 m3) via the root of vault water (i.e. cooling action of vault water on D2O lines passing through the vault region) and vault water heat exchanger, which will further be dissipated in to the open atmosphere through the PW pipe lines from dump tank to OHST and evaporative loss from the OHST pool surface. After 10 days of shut down, the total decay power including XCP pump power is estimated to be 136.5 kW. The analysis shows that if the maintenance work is started after ten days of shut down core cooling, the temperatures of heavy water and vault water will remain within 50ºC, provided the recirculation flow of vault water through the vault water heat exchanger is available with primary and secondary side flows maintained at 2300 and 2000 lpm, respectively. With this arrangement, the replacement work can be carried out over a period of 10 days. Analysis has also been carried out considering secondary side flow in the vault water heat exchanger as 1000 lpm. In this case, the temperatures of heavy water and vault water will remain within 52ºC. It is worth mentioning that the cooling effect of the pile block ventilation air flow in the upper service space, and Ion exchange cooler have been neglected in the analysis.

Case-6: Core cooling flow of 5123 lpm with a bypass flow of 1380 lpm on the secondary side of the HW/PW heat exchanger #1 and cooling of vault water available with 2300 and 2170 lpm flow on primary and secondary sides, respectively.

The replacement job for the EJs was taken up after a shutdown period of ~20 days whereas the analysis (Case-5) was done for a shutdown period of 10 days. In addition, an arrangement was made wherein a bypass flow of 1380 lpm was provided on secondary side of the HW/PW heat exchanger. After fifteen days of shutdown time, the estimated value of peak temperature was 39.6°C. The corresponding observed value was ~38°C (Table.14).

10

Case-7: Core cooling flow of 2500 lpm without secondary side flow in HW/PW heat exchangers and cooling of vault water not available.

During maintenance work on HW/PW heat exchanger #3, it was proposed to operate the reactor with only two loop sat 60 MW, if required. In such a case, during shut down, XCP#3 will be operating through loop#3 which has no heat exchanger. For preventive maintenance, one XCP (say XCP#1) is bypassed (not available) and primary and secondary cooling is maintained through other XCP#2 and XCP#3. In the event of failure of XCP#2, there will not be any flow on the primary sides of HW/PW heat exchangers #1 and #2. As far as core cooling is concerned, it will be maintained by XCP#3 which has no heat exchanger. Under these conditions, there will be no effective cooling of the primary coolant circuit. Accordingly, the operator has to normalize the XCP#1 (on preventive maintenance) in a time frame such that the coolant temperature does not exceed a predetermined value. Analysis was carried out to estimate the rise in temperatures of heavy water (D2O inlet plenum temperature, moderator temperature) and vault water when only one XCP is under operation. In the analysis, different shut down cooling periods of 12 and 24 hours were considered for decay heat estimation. It is assumed that reactor was operating at 60 MW and minimum coolant flow through the core during shutdown condition is 2500 lpm. It is further assumed that vault water heat exchanger is offline.

Results of the analysis are summarized in (Table.15(a)&15(b)), respectively. It is seen from the analysis that if the process water flow is stopped after 12 hours from reactor shut down core cooling, the temperatures in heavy water system will reach from 38 to 70 ºC in ~ 2.0 hours. Similar analysis shows that if the process water flow is stopped after 24 hours from reactor shut down core cooling, the temperatures in heavy water system will reach from 38 to 70ºC in ~ 3.5 hours. 8 SUMMERY AND CONCLUSION

Thermal hydraulics analyses have been carried out to estimate the temperature of primary coolant during replacement of expansion joints in the process water lines of Dhruva reactor. Since the replacement job requires isolation of secondary side flow in the primary heat exchangers, the coolant temperature will rise with time. The analysis was aimed at arriving at suitable shutdown period (and sufficient time margin for the job) after which the job could be started. The replacement job for EJ-4 was taken up in December, 2016 after a shutdown period of 72 hours. The observed value of peak heavy water temperature was ~ 42°C which was close to the estimated value of 41.6°C. Remaining EJs were replaced in February, 2018 after a shutdown ~ 15 days. Again, the peak temperature of heavy water was observed to be about 39.5°C which is in close agreement with the estimated value of 39.6°C.

11

NOMENCLATURE

EJ HW PW SW

ECW OHST LCO DC

v P

f

F

h ''q

'''q

effH

Nu

K De Re

Pr

b

w

G C Va A Xs X

Expansion Joint Heavy Water Process Water Sea Water Emergency Cooling Water Over Head Storage Tank Limiting Condition of Operation Dresser Coupling Density of coolant Velocity of coolant Coolant pressure Acceleration due to gravity Force per unit volume Enthalpy Surface heat flux

Volumetric heat generation rate Dissipation function

Heat transfer coefficient for single phase flow of coolant in the heated channel

Nusselt number

Thermal conductivity Effective hydraulic diameter of the flow channel Reynolds number Prandtl number

Viscosity of coolant at bulk coolant temperature

Viscosity of coolant at wall temperature

Heat transfer coefficient for inner side of the pipe

Inner radius of the pipe Outer radius of the pipe Thermal conductivity of pipe material Natural convection heat transfer coefficient for stagnant water Evaporation rate of water (kg/h) Constant depending on air velocity above the pool surface Velocity of air above the water surface (m/s) Water surface area (m2) exposed to air Humidity ratio in saturated air at water surface temperature (kg/kg) Humidity ratio in the air (kg/kg) Inlet coolant water temperature Coolant outlet temperature

Decay power of the core Coolant mass flow rate (kg/sec)

12

hcv

Q T T

Q

T

Specific heat of the heavy water coolant

Heat transfer rate from coolant to vault water Vault water temperature Effective heat transfer coefficient of coolant pipe in vault region Coolant recirculation time Surface area of the pipes exposed in vault water Total vault water inventory (kg)

Specific heat of light water

Heat transfer rate from coolant to moderator Coolant temperature

Moderator temperature Structural cooling mass flow rate (kg/sec) from coolant to moderator Total heavy water inventory in moderator system

Heat transfer rate from moderator to vault water Effective heat transfer coefficient of moderator circuit in vault region Moderator temperature Temperature of the vault water Surface area of the moderator circuit exposed in vault water Total heavy water inventory in moderator system Specific heat of heavy water

Total water inventory in the vault Specific heat of light water Heat transfer from vault water to process water through heat exchanger

Effective heat transfer coefficient of the vault water heat exchanger

Process water temperature Effective cooling surface area in the heat exchanger Temperature of the water held up in the heat exchanger Heat transfer from coolant to stagnant process water in heat exchanger

effective heat transfer coefficient of coolant pipe Temperature of the process water Surface area of the pipes exposed in heat exchanger Total water inventory (kg) held-up in heat exchangers

13

REFERENCES

[1] Agarwal, S.K. et al., “Dhruva: Main design features, operational experience and utilization”, Nuclear Engineering and Design, 236(7-8):747–757, 2006.

[2] “Report on replacement of rubber expansion joints in Process Water and Emergency

cooling water system”, DH/T-2 /1164/2018, dt. June 22, 2018 (Divisional Report), Reactor Operations Division, Bhabha Atomic Research Centre, 2018.

[3] “Dhruva Technical Specifications for Dhruva Operations”, dt. November, 2014.

(Divisional Report), Reactor Operations Division, Bhabha Atomic Research Centre, 2014.

[4] Todreas, N.E. and Kazimi, M.S. “Nuclear Systems Volume I: Thermal Hydraulic

Fundamentals”, 2nd Ed., Pennsylvania: Taylor and Francis, 1993. [5] Dittus, F.W. and Boelter, L.M.K. “Heat transfer in Automobile Radiators of the

Tabular type”, (Series: University of California Publications in Engineering, v.2 no.13), Berkeley: University of California Press, 1930.

[6] Seider, E.N. and Tate, C.E. “Heat Transfer and Pressure Drop of Liquids in Tubes”,

Industrial & Engineering Chemistry, 28(12):1429-1435, 1936. [7] “Research reactor core conversion guidebook, Volume 3: Analytical verification

(Appendices G and H)”, IAEA-TECDOC-643, Vienna: International Atomic Energy Agency, 1992.

[8] Crabtree, A. and Siman-Tov, M. “Thermophysical properties of saturated light and

heavy water for advanced neutron source applications”, ORNL/TM--12322, Oak Ridge National Laboratory, 1993.

14

Table.1(a). Input parameters (Two XCP pump operation) S/n Input parameters 1 XCP Pump heat (kW) 20 2 XCP flow (lpm) 5000 2 Flow through moderator (lpm) 900 3 D2O coolant recirculation time (sec) 356 4 Initial inlet plenum temperature (°C) 37 5 Initial vault water temperature (°C) 37 6 D2O hold up in coolant circuit (Te) 35 7 D2O hold up in moderator circuit (Te) 39 8 H2O held up in 3 HW/PW heat exchanger (Te) 24.6 9 Vault water inventory (m3) 175 10 Over all heat transfer coefficient (W/m2/K) 200 11 Surface area of moderator in calandria (m2) 8.95 12 Surface area of moderator in dump tank (m2) 7.21 13 Surface area of vault water (m2) 27.7 14 Recirculation flow of the vault water (lpm) 2200 15 Recirculation flow of the D2O water (lpm) 690

Table.1(b). Input parameters (system dimension)

S/n System Dimension Surface area in water (m2)

Heavy water coolant circuit 1 3 out comer of dia 300

mm 300 mm ID, 10 mm thick, Total length (11+6.2+6.2=23.4) m

22.05

2 3 inlet of OD: 300 mm 300 mm ID, 10 mm thick, Total length (5.3+2.5+2.5=10.3) m

9.7

3 3 tube sheet cooling: 100 mm

100 mm ID, 6 mm thick, Total length (7.8+7.8+4.9=20.5) m

6.44

Heavy water/Process water heat exchanger 1 8245 coolant tubes OD: 9.5mm length: 4967mm, thickness: 1.47mm 1158 Moderator system 1 2 Moderator return

lines OD: 150 mm, Thickness: 7.0 mm, Total length: (3+3=6 m)

2.83

2 Calandria main shell OD: 3758 mm, Thickness: 19.0 mm, Total length: (1150 mm)

13.54

3 Calandria bottom reflector

OD: 2876 mm, Thickness: 38.0 mm, Total length: (320 mm)

2.89

4 Moderator dump line OD: 200 mm, Thickness: 8.0 mm, Total length: (3.23+3.23 = 6.46m)

4.06

5 LCP Line OD: 100 mm, Thickness: 6.0 mm, Total length: (6+3.7=9.7m)

3

15

Table.2. Thermo-physical properties of materials

S/n Thermo-physical properties D2O H2O SS 304 1 Density (kg/m3) 1076 971.8 8030 2 Specific Heat (kJ/Kg/K) 4.17 4.22 0.5 3 Thermal conductivity (W/m/K) 0.56 0.58 16.2

Table.3(a). Decay power variation with time (Reactor shut down from 100 MW)

Time from shut down (hours)

Decay Power (kW)

Total Power (kW) (Decay+ pump power)

1 1208.5 1228.5 12 505.5 525.5 24 385.7 405.7 36 326.3 346.3 48 288.4 308.4 72 240.4 260.4 96 209.8 229.8 120 187.9 207.9

Table.3(b). Decay power variation with time (Reactor shut down from 90 MW)

Time from shut down (Days)

Decay Power (kW)

Total Power (kW) (Decay+ pump

power) 1 347.1 367.1 2 259.6 279.6 3 216.3 236.3 4 188.8 208.8 5 169.1 189.1 8 132.1 152.1 10 116.5 136.5 15 91.2 111.2

16

Table.4. Temperature variation when secondary side of HW/PW heat exchangers is offline (after 1 hour of reactor shut down)

Time from

shut down (hours)

Total Power (kW)

Hottest channel outlet

Temperature (°C)

Inlet Temperature

(°C)

Moderator Temperature

(°C)

Vault water Temperature(°C

)

1.0 1228.5 41.6 37.0 37.6 37.0

1.2 1158.3 46.0 41.7 39.7 37.1

1.5 1077.1 50.5 46.5 43.4 37.2

1.9 996.6 55.1 51.4 48.2 37.6

2.5 909.7 60.7 57.3 54.2 38.2

3.1 846.3 65.3 62.1 59.2 39.0

3.9 782.9 70.3 67.4 64.5 40.3

6.2 663.5 80.2 77.7 75.1 45.0

Table.5. Temperature variation when secondary side of HW/PW heat exchangers is offline (after 12 hrs of reactor shut down)

Time from

shut down (hours)

Total Power (kW)


Temperature (°C)

Inlet Temperature

(°C)


(°C)

Vault water Temperature

(°C)

12.0 525.5 39.0 37.0 37.3 37.0

12.8 513.4 45.1 43.2 41.7 37.2

13.7 500.9 50.0 48.1 46.5 37.7

14.9 485.7 55.3 53.5 51.8 38.7

16.3 470.0 60.2 58.5 56.7 40.2

18.0 452.1 65.2 63.5 61.7 42.5

20.3 432.3 70.0 68.3 66.6 45.7

27.1 387.0 80.0 78.5 76.9 55.8

17


Time from

shut down (hours)

Total Power (kW)


Temperature (°C)

Inlet Temperature

(°C)


(°C)


(°C)

24.0 405.7 38.5 37.0 37.2 37.0

25.2 398.2 45.1 43.6 42.4 37.3

26.5 390.7 50.1 48.6 47.3 38.0

28.1 382.0 55.0 53.5 52.1 39.3

30.2 371.0 60.0 58.6 57.2 41.6

33.1 358.0 65.1 63.7 62.3 44.9

36.7 343.8 70.0 68.7 67.3 49.3

45.8 314.5 80.0 78.9 77.5 60.5


Time from

shut down (hours)

Total Power (kW)


Temperature (°C)

Inlet Temperature

(°C)


(°C)


)

36.0 346.3 38.3 37.0 37.2 37.0

37.5 340.8 45.0 43.8 42.7 37.4

39.1 335.2 50.0 48.7 47.5 38.3

41.2 328.0 55.1 53.9 52.6 40.0

44.1 319.2 60.1 58.9 57.6 42.8

47.9 308.8 65.1 63.9 62.6 46.9

52.3 297.7 70.0 68.8 67.6 51.8

62.7 276.1 80.0 78.9 77.8 62.9

18


Time from

shut down (hours)

Total Power (kW)


Temperature (°C)

Inlet Temperature

(°C)


(°C)


(°C)

48.0 308.4 38.2 37.0 37.2 37.0

49.8 303.9 45.1 44.0 43.0 37.5

51.7 299.3 50.0 48.9 47.8 38.5

54.2 293.3 55.0 53.8 52.7 40.5

57.8 285.7 60.0 58.9 57.7 43.8

62.3 276.8 65.0 63.9 62.8 48.4

67.6 267.5 70.0 69.0 67.9 53.6

79.0 250.3 80.0 79.0 78.0 64.6


Time from

shut down (hours)

Total Power (kW)


Temperature (°C)

Inlet Temperature

(°C)


(°C)


(°C)

72.0 260.4 38.0 37.0 37.1 37.0

74.3 256.9 45.1 44.1 43.2 37.6

76.7 253.3 50.0 49.0 48.1 39.0

80.3 248.4 55.0 54.0 53.1 41.6

85.2 242.2 60.0 59.1 58.1 45.7

90.9 235.4 65.0 64.1 63.1 50.7

97.1 228.6 70.0 69.1 68.2 56.0

110.2 216.1 80.0 79.2 78.3 66.8

19

Table.10. Temperature variation with secondary side of HW/PW heat exchangers offline (after 96 hrs of reactor shut down)

Time from

shut down (hours)

Total Power (kW)


Temperature (°C)

Inlet Temperature

(°C)


(°C)


)

96.0 229.8 37.9 37.0 37.1 37.0

98.8 226.9 45.1 44.3 43.5 37.7

101.8 223.8 50.0 49.2 48.3 39.4

106.4 219.5 55.0 54.2 53.3 42.6

112.2 214.3 60.0 59.2 58.3 47.1

118.8 208.8 65.0 64.2 63.3 52.2

125.9 203.4 70.0 69.2 68.4 57.6

140.3 193.5 80.0 79.3 78.5 68.2

Table.11. Temperature variation with secondary side of HW/PW heat exchangers offline (after 120 hrs of reactor shut down)

Time from

shut down (hours)

TotalPower (kW)


Temperature (°C)

Inlet Temperature

(°C)


(°C)


)

120.0 207.9 37.8 37.0 37.1 37.0

123.2 205.4 45.1 44.3 43.6 37.8

126.8 202.7 50.0 49.2 48.4 39.8

132.3 198.8 55.0 54.2 53.4 43.4

139.1 194.3 60.0 59.2 58.4 48.2

146.5 189.6 65.0 64.3 63.5 53.5

154.1 185.2 70.0 69.3 68.5 58.7

169.8 176.9 80.0 79.3 78.6 69.2

20

Table.12. Time to reach coolant temperature from 37 to 65°C for different shut down cooling time with no secondary cooling to HW/PW heat exchangers

Time from shut

down (hours)

Total Power (kW)

Time (hours)

Heat transfer to vault water and moderator circuit considered

Heat transfer to vault water, moderator circuit and HW/PW heat exchanger

considered

1 1228.5 2.0 3.1 12 525.5 5.9 8.1 24 405.7 9.0 11.9 36 346.3 11.8 15.0 48 308.4 14.2 17.8 72 260.4 18.8 22.7 96 229.8 22.7 27.1 120 207.9 26.3 31.0

Table.13. Temperature rise with bypass flow on the secondary side of HW/PW heat exchangers

By pass flow (lpm)

Peak inlet temperature(°C) for different S/D time 24 hrs 48 hrs 72 hrs

300 52 50 48.7 400 50 45.1 44.7 1720 - - 41.6

Table.14. Heavy water coolant temperature rise for different shut down cooling times with no regular secondary cooling to PW/HW heat exchangers

(XCP-1 flow: 1380 lpm; Core flow: 5123 lpm; Vault cooling flow: 2170 lpm)

Shut down Time(hours)

Total Power (kW)

Initial temperature in coolant heavy water

Peak temperature in coolant heavy water

Estimated Observed

240 136.5 36.2 40.3 - 360 111.2 36.2 39.6 39.5 480 95.6 36.2 39.2 -

21

Table.15(a). Temperature variation with secondary side of HW/PW heat exchangers offline(after 12 hr of reactor shut down)


Total Power (kW)


Temperature (°C)

Inlet Temperatur

e (°C)


(°C)


(°C)

12.0 333.4 38.0 38.0 38.0 38.0 12.5 328.6 57.8 55.3 42.6 38.1 13.0 324.0 61.8 59.4 46.8 38.3 13.5 319.6 65.6 63.1 50.8 38.5 14.0 315.5 70.0 66.6 54.4 38.9 14.5 311.5 72.2 69.9 57.8 39.2 15.0 307.8 75.2 72.9 61.0 39.6 15.5 304.1 78.0 75.7 63.9 40.1 16.0 300.7 80.5 78.3 66.6 40.5

Table.15(b). Temperature variation with secondary side of HW/PW heat exchangers offline (after 24 hr of reactor shut down)


Total Power (kW)


Temperature (°C)

Inlet Temperatur

e (°C)


(°C)


(°C)

24.0 259.1 38.0 38.0 38.0 38.0 24.5 257.1 53.5 51.5 41.6 38.1 25.0 255.2 56.7 54.8 44.9 38.2 25.5 253.3 59.7 57.8 48.0 38.4 26.0 251.5 62.6 60.7 50.9 38.7 26.5 249.7 65.2 63.3 53.7 39.0 27.0 248.0 67.7 65.8 56.2 39.3 27.5 246.2 70.0 68.1 58.6 39.6 28.0 244.6 72.2 70.3 60.9 40.0 28.5 243.0 74.2 72.4 63.0 40.4 29.0 241.4 76.1 74.3 64.9 40.9 29.5 239.8 77.9 76.1 66.8 41.3 30.0 238.3 79.6 77.8 68.5 41.8 30.5 236.8 81.1 79.4 70.2 42.3

22

Fig.1 (b). General Arrangement of Reactor Plan

24

Fig.1(c). Rubber Expansion Joint

Fig.2(a). Simplified cooling flow diagram

25

Fig.2(b). Simplified Flow Diagram of HW Loop #1

Fig.3. Turbine-1 Operation with only HX#1 online

26

20 40 60 80 100 120

200

400

600

800

1000

1200

De

cay

Pow

er(k

W)

Time after S/D (Hr)

Fig.4(a). Decay power variation with time after shut down

1.0 1.2 1.4 1.6 1.8 2.0

0

200

400

600

800

1000

1200

1400

Dec

ay P

ower

(kW

)

Time from S/D (Hrs)

Fig.4(b). Decay power variation after 1 hour of shut down

27

Fig.5. Schematic diagram for coolant temperature estimations for decay heat removal

5 10 15 20

40

50

60

70

80

Coolant temperature

Vault water temperatureTem

per

atur

e (C

)

Time from S/D (Hrs)

Moderator temperature

Fig.6. Temperature variation with secondary side of HW/PW heat exchangers offline (after 1 hours of reactor shut down)

28

15 20 25 30

40

50

60

70

80

Vault water temperature


Coolant temperatureT

em

pera

ture

(C

)

Time from S/D (Hrs)


25 30 35 40 45 50

40

50

60

70

80



Coolant temperature

Tem

pera

ture

(C

)

Time from S/D (Hrs)


29

40 45 50 55 60

40

50

60

70

80



Coolant temperatureT

em

per

atu

re (

C)

Time from S/D (Hrs)


50 55 60 65 70 75 80

40

50

60

70

80



Coolant temperature

Tem

pera

ture

(C

)

Time from S/D (Hrs) Fig.10. Temperature variation with secondary side of HW/PW heat exchangers offline

(after 48 hours of reactor shut down)

30

75 80 85 90 95 100

40

50

60

70

80



Coolant temperature

Te

mpe

ratu

re (

C)

Time from S/D (Hrs) Fig.11. Temperature variation with secondary side of HW/PW heat exchangers offline

(after 72 hours of reactor shut down)

100 110 120 130 140 150

40

50

60

70

80



Coolant temperature

Te

mpe

ratu

re (

C)

Time from S/D (Hrs)


31

120 130 140 150 160 170 180

40

50

60

70

80



Coolant temperature

Tem

pera

ture

(C

)

Time from S/D (Hrs)


250 300 350 400 450

38

40

42

44

46

48

50

Te

mp

era

ture

(C

)

Time(Hr)

Peak Coolant outlet

Inlet temperature

Vault water

Moderator

Fig.14. Temperature variation after 72 hours of shutdown cooling with Core cooling flow of 5000 lpm, a bypass flow of 400 lpm on the secondary side of the HW/PW heat

exchangers and cooling of vault water not available

32

edited manuscript for barc report-jainendra

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