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SCENARIO 2

EPR 20 cm2 Cold Leg Break Transient Analysis in CATHARE and RELAP5 Calculation Codes

1.     Description of the problem

Scenario 2 represents modeling of a 4500MWth 4 loop Pressurized Water Reactor – the EPR. The 4 primary loops have been simulated separately. Pressurizer is connected to the hot leg in loop one and break is positioned in the broken cold leg 3, where the leak simulation is presented by a 20 cm2 break. Emergency core cooling system is provided by two low and medium head injection systems on 1 and 3 loop and four accumulators on each loop, which are connected to cold legs. Simulations is 4000s long.

In design basis accident (DBA) the most dangerous scenario is a loss of coolant accident (LOCA). One of those can be small break LOCA where high pressure injection systems should be sufficient to keep coolant inventory at the safe level. When rupture pipe occurs, turbine and reactor are tripped. Assuming that the plant is shut down, removal of the heat from the core can be provided by the steam generator to the secondary side. To run more conservative calculations only two SIS are working.

To perform thermal-hydraulic calculations the two system codes were used: RELAP5 and CATHRE. First code was developed for U.S. Nuclear Regulatory Commission (NRC) for use in rulemaking, licensing calculation and as a basis for NPP analysis. Code is dedicated to light water reactors to simulate steady states and transient in reactor applications. It is also appropriate, as calculation tool, during loss of coolant accident. CATHARE system code is also dedicated to thermal hydraulics calculations during accident of pressurized water reactor and safety evaluation. Code allows to perform best estimate calculations for LOCAs, steam generator failures and other transients as breaks in the secondary side or loss of residual heat removal system. CATHARE is a result of a joint effort of AREVA, CEA, EDF and IRSN.

2. Description of the model made in CATHARE

Model of the EPR in CATHARE has been developed by experts from AREVA company and has been distributed to the SARWUT project as a reference model. Model consists of four loops which are modeled separately. Safety systems, medium head injection system and low head injection system are modeled with use of gadget components, but during transient they work only on the loop with the break and the pressurizer. There are also four accumulators. All safety systems are attached to the cold legs at the length of around 5 m from the reactor pressure vessel (RPV). Break is modeled with use of boundary condition which is blind during steady state. It occurs around 5 m from the RPV. The pressurizer is one volume element which is attached to the hot leg by the surge line, which is one axial element, at the length of 5.5 m from the RPV. There are three boundary condition attached to the pressurizer with an appropriate model to simulate safety valve. The pressurizer can be approximated by one element, due to the fact there are empties at early stage of transient calculation, therefore it has no impact during later stage of transient. The RPV is modeled with use of 9 components, which 5 of them are volume components and 4 of them are axial components. Coolant flows into inlet plenum and is divided into 2 streams, but less than 1 % flows into upper head of RPV during steady state calculations. Water from the downcomer, which is modeled as axial component, enters lower volume. Lower volume models lower plenum of the RPV and free volume of the lower core supper structure. 95% of the total flow of the downcomer flow comes in the reactor core during steady state calculation and 5 % bypasses core through the two axial components. Core is divided into 59 segments but only 55 of them model active part of the core. Power of the reactor is set as a function of time. Characteristic of 55 segments of the fuel are the same only axial peaking factor is different at each elevation. Coolant is collected in the volume which models outlet plenum. Above there is a volume which models free volume of guide tubes. Steam generator is modeled with use of 6 components. Downcommer and riser part below U-tubes bending is divided into 2 parallel axial components. One simulates co-current part of the U-tube heat exchanger and the other simulates countercurrent part of the u tube heat exchanger. Mixture is collected in a very small volume and is distributed to the axial which simulates u tube banding and riser above bending. Model of the steam separator is set at the junction which connects riser with a volume, which simulates steam dome and free volume of the separator. Steam flows through an axial pipe and then is collected in the volume with boundary condition to simulate turbine. The nodalization scheme of the RPV and the loop are shown on the figures 1 and 2 respectively. Figure 3 shows the nodalization of the secondary side.

Figure 1. Reactor Pressure Vessel nodalization scheme.

Figure 2. Primary loop nodalization scheme.

Figure 3. Nodalization scheme of the secondary side.

3. Description of the model made in RELAP5

Model consists of four separately modeled, independent loops. The break is simulated using the valve in loop number 3 whereas the pressurizer is connected through the surge line to the loop number 1. There are 3 types of safety systems in the RELAP5 model of EPR: medium-head safety injection, low-head safety injection and the accumulator. In every loop all of them are connected to the one element (branch). The pressurizer spray lines are attached to the loops 2 and 4.

In RELAP5 model, the RPV is divided into several sections. There are two downcomers: one of them is attached to the cold leg with the break valve and the other one to the remaining loops. The core region with the bypass is modeled by three pipes, which simulate the heated section and two branches where the coolant enters the heated section or the bypass and escapes the core. The heat structures model the five regions: central zone, two average zones, the hot rod and the peripheral zone. The other modeled sections in RPV model are the lower and upper plenum, guide tubes and the upper head of the vessel.

For the each loop of the primary system, the hot legs are modeled by pipes and branches. Steam Generator inlet and outlet plenums are modeled by single volume. On the secondary side, the main feedwater enters the downcomer, which is modeled by annulus element, via branch, where is mixed with the recirculation flow. The combined flow then flows downwards downcomer, next upwards the riser which is modeled by pipe element and then is headed to the separator. Steam dome is modeled by the branch element and the turbine by the time dependent volume.

Figure 4. shows the nodalization scheme pf the RPV in RELAP5. Figures 5. and 6. present the primary and the secondary systems. The loop with the pressurizer is shown in the first one and with the break in the other one.

Figure 4. RPV nodalization scheme in RELAP5.

Figure 5. Nodalization of the primary and secondary sides in RELAP5: loop with the Pressurizer.

Figure 6. Nodalization of the primary and secondary sides on RELAP5: loop with the break.

 

4. Steady State conditions and the Transient results

This section includes the information about the parameters under the Steady State which are presented in Table 1. and the changes occurring during the Small-Break LOCA, presented on graphs.

The most significant changes are in mass flow per loop, liquid level in pressurizer and RPV downcomer and the recirculation flow in Steam Generator. The other parameters are in relatively good agreement. Differences showed in the last column of Table 1. are related to RELAP5 parameters with the assumption that the CATHARE values are closer to actual parameters.

Table 1. Comparison of the Steady State calculated parameters in the CATHARE and RELAP5 codes.

Parameter

Unit

CATHARE Value

RELAP5 Value

% difference

Primary coolant system

Mass flow/loop

kg/s

5198.90

4851.96

6.67

Hot leg pressure

MPa

15.85

15.79

0.38

Core dT

0C

34.91

37.74

8.11

Hot leg temperature near vessel

0C

332.14

337.85

1.72

Cold leg temperature near vessel

0C

297.33

300.11

0.97

Upper head pressure

MPa

15.97

15.88

0.56

Pressurizer

Liquid level

m

8.18

7.14

12.71

Liquid temperature

0C

347.17

346.32

0.24

Upper head pressure

MPa

15.75

15.84

0.57

Steam Generator Secondary Side

Downcomer level

m

15.70

13.86

11.71

Steam dome pressure

MPa

7.75

7.73

0.26

Feedwater temperature

0C

230.02

230

0.01

Feed water mass flow rate

kg/s

669.12

663.75

0.80

Downcommer liquid temp

0C

297.72

293.1

1.55

Recirculation flow

kg/s

1591

1266

20.42

Accumulator

Total volume

m3

47.00

43.74

6.93

Initial volume of water

m3

35.00

31.74

9.31

Pressure

MPa

4.50

4.20

6.66

The transient results are presented below. Figure 7. presents the pressure change in the primary and secondary systems. On the Figure 8. the water level in the reactor core calculated in two applied codes is showed. Next two charts present the core power as a fraction of the nominal power and the cladding temperature change during the transient.

Figure 7. Pressure change in the primary and the secondary system.

 

Figure 8. Calculated core water level change.

Figure 9. Thermal power after reactor SCRAM.

Figure 10. Calculated cladding temperature change.

5. Conclusions

Obtained results show that calculations done in RELAP5 are in good agreement with nominal parameters of EPR modeled in CATHARE code. During SBLOCA pressure in primary side, pressurizer and core level and the integrated break flow are very similar what is shown on the plots above. Some discrepancies are probably caused by differences in Nodalization and code calculation schemes.

Preparing calculations for both codes is very good activity, very helpful in the development of the knowledge and skills in the NPPs safety analysis field.

For the future, a sensitivity and uncertainty analysis should be performed to find weak points of the modeling.

Table 1. Comparison of the Steady State calculated parameters in the CATHARE and RELAP5 codes.

Parameter

Unit

CATHARE Value

RELAP5 Value

% difference

Primary coolant system

Mass flow/loop

kg/s

5198.90

4851.96

6.67

Hot leg pressure

MPa

15.85

15.79

0.38

Core dT

0C

34.91

37.74

8.11

Hot leg temperature near vessel

0C

332.14

337.85

1.72

Cold leg temperature near vessel

0C

297.33

300.11

0.97

Upper head pressure

MPa

15.97

15.88

0.56

Pressurizer

Liquid level

m

8.18

7.14

12.71

Liquid temperature

0C

347.17

346.32

0.24

Upper head pressure

MPa

15.75

15.84

0.57

Steam Generator Secondary Side

Downcomer level

m

15.70

13.86

11.71

Steam dome pressure

MPa

7.75

7.73

0.26

Feedwater temperature

0C

230.02

230

0.01

Feed water mass flow rate

kg/s

669.12

663.75

0.80

Downcommer liquid temp

0C

297.72

293.1

1.55

Recirculation flow

kg/s

1591

1266

20.42

Accumulator

Total volume

m3

47.00

43.74

6.93

Initial volume of water

m3

35.00

31.74

9.31

Pressure

MPa

4.50

4.20

6.66

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