Simulation of thermal hydraulics accidental transients: evaluation of MAAP5.02 versus CATHAREv2.5

Transcription

1 1/12 Simulation of thermal hydraulics accidental transients: evaluation of MAAP5.02 versus CATHAREv2.5 J. Bittan¹ 1) EDF R&D, Clamart (F) Summary MAAP is a deterministic code developed by EPRI that can simulate the response of light water moderated nuclear power plants during accidental transients for Probabilistic Risk Analysis (PRA) applications. It can as well simulate severe accident sequences, including actions taken as part of the Severe Accident Management Guidelines (SAMGs). EPRI indicates that the latest version of the code MAAP5.02 benefits from major enhancements, in particular concerning the thermal hydraulics in the primary side. This code revision takes into account momentum equations allowing, to model thermal hydraulics transient with a good accuracy. EDF is interested in using MAAP5.02 as an incidental/accidental transient s simulation tool for the management of crises on its 58 PWRs. In particular, it could evaluate the time before core uncovers, core melting and fission product releases. In order to assess MAAP5.02 ability to simulate accidental transients prior to core uncovery, EDF has compared MAAP5.02 results to the code used as a reference to simulate thermal hydraulics transients in France: CATHARE. It is a system code for PWR safety analysis, accident management, definition of plant operating procedures and for research and development. It is also used to quantify conservative analysis margins and for licensing. It is based on a 2-fluid 6-equation model. The version of CATHARE used for the comparison is CATHAREv2.5_2 (one of the latest CATHARE versions). Several transient analyses on LOCA and non LOCA transients have been performed by EDF. Transients at full power conditions as well as in shutdown states have been considered. Here are some of EDF analyzed transients: LOCA (small break on cold leg), LOOP (Loss Of Offsite Power), SGTR (Steam Generator Tube Rupture). The transient analyses are done on a French type 3 Loops 900 MWe PWR reactor. The comparisons performed tend to prove that the discrepancies between MAAP5.02 and CATHAREv2.5_2 are rather small. Besides, MAAP5.02 is much faster than CATHARE. A. INTRODUCTION The latest version of the Modular Accident Analysis Program MAAP5.02 delivered at the end of 2013 ([1]), is presented by EPRI as benefiting from major enhancements compared to previous versions of MAAP4. In particular, the Thermal Hydraulics capabilities of MAAP5.02 are increased: the momentum equations are taken into account, which allows calculating the natural circulation flows that appear when the Reactor Coolant Pumps (RCPs) are tripped. MAAP5.02 enables to model each loop individually so that asymmetrical transient simulations are more realistic. The reactor meshing has also been refined, especially in the reactor vessel conducing to a better evaluation of thermal hydraulics phenomena (particularly when core uncovers). EDF is interested in using MAAP5.02 code as a management tool during crisis situations. It is thus necessary to evaluate MAAP5.02 ability to simulate accidental Thermal Hydraulics transients. It has hence been decided to compare MAAP5.02 to the reference code simulating Thermal Hydraulics accidental transients in France: CATHARE (that stands for Code for Analysis of Thermal hydraulics during an Accident of Reactor and safety

2 2/12 Evaluation, [2]). It is a system code for PWR safety analysis, accident management, definition of plant operating procedures and for research and development. It is also used to quantify conservative analysis margins and for licensing. CATHARE is based on a 2-fluid 6-equation model. The version of CATHARE used for the comparison is CATHAREv2.5_2 (one of the latest CATHARE versions). B. ACCIDENTAL TRANSIENTS SIMULATED WITH MAAP5.02 AND CATHAREV2.5 B.1 Description of analyzed transients Different types of accidental transients (heating and cooling accidents) have been analyzed in order to compare MAAP5.02 to CATHAREv2.5. Two different transients are discussed in this paper (the transients listed below are initiated at reactor full power): - Loss Of Offsite Power (LOOP), - Loss of Coolant Accident (LOCA). All analyses shown in this paper have been performed on a French type 3 Loops 900 MWe PWR reactor. All scenarios are simulated with MAAP and CATHARE codes until core fusion is started, i.e. when the boiled-up level in the core is lower than 50% (this is a simplified criterion). It is worth mentioning that for the considered scenarios, no safety injection is taken into account. B.2 Scenarios description B.2.1 Loss Of Offsite Power transient (LOOP) In this scenario, the reactor is initially at full power when a Loss of Offsite Power happens. It is conservatively assumed that the LOOP signal does not trip the reactor. Due to the LOOP, the Reactor Coolant Pumps (RCPs) trip instantaneously. On the secondary side, the main feedwater (MFW) is lost: the pressure in the primary side hence increases due to this loss in cooling efficiency. The reactor trip is obtained when the pressure in the pressurizer is higher than bar. The scram leads on the secondary side to a Turbine Trip (TT): the pressure in the Steam Generators (SGs) increases to the Main Steam Relief Train (MSRT) setpoint. No SG s Emergency Feedwater (EFW) and no operator actions are taken into account during this transient. B.2.2 Loss Of Coolant Accident transient (LOCA) The reactor is equally assumed at full power initially. A 2-inches break (small break) opens on one of the reactor s cold legs. The reactor trip is reached when the pressure in the pressurizer is lower than 135 bar. The threshold of 135 bar has been taken into account for the scram because the LOCA transient is much faster than the SGTR transient. On the secondary side, the scram drives to a turbine trip. It is supposed that 30 minutes after the reactor trip, the operator realizes a maximum cooling by fully opening SG s MSRT. The RCPs are tripped 1 minute after the reactor trip to avoid cavitation. It is supposed that MFW is lost when the break opens and that EFW feed the SGs until seconds after the reactor trips. Three accumulators are available and connected to the primary side. B.3 MAAP and CATHARE Reactor Coolant System (RCS) modeling B.3.1 CATHARE RCS modeling In CATHARE, each loop of the RCS is also modeled separately. The deck used to model LOCA and LOOP transients on a 3 Loops 900 MWe PWR reactor contains more than 400 nodes modeled. CATHARE has a 2-fluid 6-equation model, which means that in each node are performed calculations to determine flows, enthalpies and void fractions.

3 B.3.2 MAAP5.02 RCS modeling 3/12 In MAAP5.02, each reactor loop is also modeled separately (which was not the case in MAAP4 versions). This allows simulating non symmetrical transient (LOCA, SLB ) more precisely. Figures 1 and 2 present the nodalization taken into account in MAAP5.02 for a 3 loops PWR: there are 39 nodes called water nodes (Figure 1) and 23 flow nodes (Figure 2). In water nodes energy and mass balances calculations are performed. Flows between nodes are calculated using flow nodes, which is a reduced nodalization of water nodes. Momentum equations are taken into account to calculate flows between flow nodes. MAAP5.02 modeling of the RCS appears as rather coarse compared to CATHARE. Many correlations are used in MAAP5.02; for example correlations in MAAP5.02 to estimate the void fraction in water nodes. Figure 1: Nodalization in MAAP5.02 for a 3 Loops PWR Plant Water Nodes Figure 2: Nodalization in MAAP5.02 for a 3 Loops PWR Plant Flow Nodes & Junctions C. RESULTS C.1 Loss Of Offsite Power transient (LOOP) As described in B.2.1, after the initial LOOP, the RCPs trip results in a degraded exchange between the primary and secondary side: the pressure in the primary side increases

4 4/12 and reaches the primary pressure trip threshold. Both MAAP and CATHARE model the increase in pressure in the first seconds of the transient (Figure 3). The maximum pressure reached in MAAP pressurizer is a little bit greater than in CATHARE (+1.5 bar). Figure 3: Pressure in the Pressurizer during the first minutes of transient (Pa) vs. time (s) After the reactor scram, the pressure in the primary side decreases and stabilizes around 145 bar in both codes. In fact, the reactor trip leads to a turbine trip in the secondary side and hence the pressure in the SGs reaches the MSRT setpoint (around 71 bar). As long as MAAP and CATHARE SGs are not emptied (around 3500 s) after the LOOP event, natural circulation flows (Figure 4) are calculated in each of the reactor s loops allowing to remove the decay heat. MAAP natural circulation flows are slightly greater than CATHARE (around 10% of overestimation). It can be observed that when soon after SGs are emptied, MAAP and CATHARE flows vanish. Figure 4: LOOP transient Natural Circulation Flows (kg/s) and vs. time (s)

5 5/12 It should be noted that even though MAAP flows are higher than CATHARE ones, the power exchanged at MAAP SGs is smaller than CATHARE ones (the decay heats considered in MAAP and CATHARE are identical), Figure 5. Figure 5: LOOP transient Power exchanged between the primary and secondary side (W) vs. time (s) This is caused by 2 distinct phenomena: in MAAP, it is possible to take into account the exchange that exists between the primary side and the reactor building whereas in CATHARE the primary side is considered as isolated from the reactor building. The MAAP calculation whose results have been presented until now takes into account a steady power exchanged between the primary side and the reactor building (2 MW). A MAAP sensitivity calculation without the aforementioned exchange (called MAAP_adiab) has been performed. For this calculation, the natural circulation flows are equivalent to the initial MAAP calculation. The exchange is also better in CATHARE because the masses of water in the SGs, while not emptied, are greater than in MAAP during the whole transient. When the masses of water of the SGs have significantly decreased compared to the beginning of the transient (around 1500 s), it can in fact be noticed that CATHARE primary side exchanges more than MAAP. A difference appears on the pressurizer pressure: MAAP_adiab pressure increases slightly more quickly than the initial MAAP calculation as the SGs empty (Figure 6). For all 3 calculations, the difference in opening Pressurizer Safety Relief Valves (PSRVs) time is smaller than 3 minutes, which is very low. It can also be noticed that both MAAP and MAAP_adiab calculations have an increase in pressure at the beginning of the transient (around 200 s). This is explained because it does exist a big increase of pressure at the secondary side (caused by the turbine trip). Thus the pressure in the primary side increases. When the SGs MSRT manage to maintain the pressure at the threshold (around 71 bar), the pressure in the primary side decreases.

6 6/12 Figure 6: LOOP transient Pressure in the Pressurizer (Pa) vs. time (s) The PSRVs evacuate the decay heat but the mass inventory in the primary side continuously decreases in MAAP and CATHARE leading to a core uncovering. A difference in pressurizer geometry modeling (MAAP pressurizer height is 1 meter lower than CATHARE) between MAAP and CATHARE leads MAAP calculation to discharge water flow through PSRV around 10 minutes before CATHARE one. That difference explains the gap observed between MAAP and CATHARE core uncovering times (MAAP core uncovers around 10 minutes before CATHARE one), Figure 7 representing the boiled-up level in the reactor core (in %). Figure 7: LOOP transient Boiled-up water level in the reactor core (%) vs. time (s) It is noticed that for MAAP calculation the water level stops decreasing at around 6000 s. This phenomenon lasts about 300 s. This can be explained in MAAP because water from the pressurizer goes down to the surge line as the level in the pressurizer decreases and then goes to the core, Figure 8 (a negative flow indicates a flow getting down in the surge

7 7/12 line). An ascending steam flow coming from the core is still observed in MAAP, which explains that the core level continuously decreases even though a negative water flow is observed in the surge line. This phenomenon is not observed in CATHARE. In fact, in CATHARE, the level of water in the pressurizer is around 1 meter higher than in MAAP: the suction effect is lower in MAAP than CATHARE, causing water to go down to the hot leg in MAAP. Figure 8: LOOP transient Surge line water flow (kg/s) vs. time (s) The Table 1 summarizes the key times for the LOOP transient simulation for both MAAP and CATHARE codes. MAAP calculation results are very close to CATHARE ones. MAAP5 can hence model LOOP transient with a good reliability. Table 1: LOOP scenario key transient times C.2 Loss Of Coolant Accident transient (LOCA) As described in B2.2, a 2-inches break is assumed to initially open on one cold leg of the simulated reactor. This break is vertical and oriented to the bottom. Figure 9 shows the liquid and steam flows at the break during the transient first minutes. MAAP is consistent with CATHARE: MAAP maximal liquid flow is underestimated of roughly 3% versus CATHARE.

8 8/12 Figure 9: Liquid and steam flows during the first minutes of transient (kg/s) vs. time (s) After around 10 minutes of transient, CATHARE calculates a steam flow at the break whereas MAAP only calculates a liquid flow, Figure 10. This is explained because MAAP and CATHARE do not use the same correlations to calculate the liquid and steam break flows: MAAP uses a correlation taking into account a simplified fit to the Henry and Fauske model (Ref.[3]) whereas CATHARE takes into account Gros D Aillon correlation. Moreover, MAAP uses Schrock correlation (Ref.[4]) to calculate the steam quality at the break whereas CATHARE evaluates the quality thanks to balance equations in each node. Henry and Fauske model and Gros D Aillon correlation show discrepancies on flows at the break when the primary side is saturated or close to saturation conditions: CATHARE correlation underestimates liquid flows compared to MAAP correlation under those conditions. Figure 10: LOCA transient Liquid and steam flows at the break (kg/s) vs. time (s)

9 9/12 The break opening leads to the depressurization of the primary side and the injection of cold legs accumulators. Figure 11 shows the mass of water in each accumulator for both MAAP and CATHARE calculations. The MAAP_isoth calculation is the default MAAP discharge model: MAAP considers that accumulators discharge is isothermal. The MAAP_poly corresponds to a general polytropic discharge implemented by EDF in MAAP code (in fact an isentropic case is taken in this calculation). It can be noticed that MAAP default isothermal is too fast compared to CATHARE isentropic discharge. The EDF implementation in MAAP appears far better than MAAP s default one. Both MAAP_poly and CATHARE accumulators stop injecting in the primary side around s after the break has opened: at this time, SGs EFW injection is stopped leading the primary side to no longer be cooled. The pressurizer pressure hence stops decreasing explaining the accumulators stop injecting. As it can be seen in Figure 11, the amount of water injected by accumulators in MAAP_poly (named MAAP afterwards) and CATHARE primary sides is identical. Figure 11: LOCA transient Mass of water in each RCS accumulator (kg) vs. time (s) It is observed in Figure 12, that MAAP and CATHARE evaluate liquid flows at the break even though the cold leg loops are empty: these flows are the heat pipe flows. A part of the steam exiting the core recondenses on the SGs tube bundles leading to produce liquid water in the hot and cold legs while SGs tubes are colder than the steam in the primary side.

10 10/12 Figure 12: LOCA transient Liquid and steam flows at the break after s (kg/s) vs. time (s) Figure 13 and Figure 14 show heat pipe flows respectively in hot and cold legs. By stripping the data of those 2 Figures, it appears that MAAP and CATHARE calculate roughly the same heat pipe flows. It is to be noticed that MAAP stops calculating heat pipe flows when SGs are emptied (around s in MAAP calculation) whereas in CATHARE heat pipe flows continue to be calculated with a smaller value even thought SGs are emptied (around s in CATHARE calculation). Figure 13: LOCA transient Heat pipe flows Hot legs (kg/s) vs. time (s) In MAAP, only three quarters of the heat pipe flows go to the break while all CATHARE calculated flows go to the break (Figures 12 and 14). This has an impact on the time the core uncovers and the core uncovering kinetics.

11 11/12 Figure 14: LOCA transient Heat pipe flows Cold legs (kg/s) vs. time (s) It can also be noticed on Figures 13 and 14 that CATHARE flows oscillate. Condensate flows are in fact small and difficult to evaluate: MAAP condensate flows calculations are simplified compared to the CATHARE ones, which take into account 6 equations and are much more accurate. Figure 15 presents the boiled-up water level in the reactor core for MAAP and CATHARE calculations. MAAP code predicts a final core uncovering around 45 minutes before CATHARE, which can be explained because MAAP overestimates liquid flow at the break from 500 s to 1500 s (Figure 10). MAAP code also sees a rise in core water level around s caused by a draining of water from the intermediate loop 2 of the reactor into the core, that is not seen by CATHARE code. MAAP core emptying kinetics is slower than CATHARE because in MAAP only a part (three quarters more precisely) of the heat pipe flows of the affected loop is lost at the break. It can be noticed on Figure 15 that MAAP calculation predicts a rather deep core uncovering at around 2000 s whereas CATHARE predicts a smaller core uncovering later at around 4500 s. In fact, MAAP overestimates the break liquid flow between 500 s and 1500 s, which explains this phenomenon. This happens later in CATHARE because the accumulator discharge is slower. In both MAAP and CATHARE codes, the cores refloodings are the aftermath of accumulators discharges. Figure 15: LOCA transient Boiled-up water level in the reactor core (%) vs. time (s)

12 12/12 The Table 2 below summarizes the key times for the LOCA transient simulation for both MAAP and CATHARE codes. Some discrepancies appear but MAAP calculation results are relatively close to CATHARE ones. MAAP gives more conservative results than CATHARE. Table 2: LOCA scenario key transient times D. CONCLUSIONS This paper shows that MAAP5.02 code can simulate with a good accuracy accidental thermal hydraulics transients compared to CATHAREv2.5. MAAP5.02 can model both heating and cooling accidental transients very quickly compared to CATHARE code. On a same computer, MAAP calculations run 7 to 10 times faster than CATHARE, which takes into more nodes and more equations than MAAP. Very small discrepancies appear on LOOP transients. For small LOCA transients, MAAP5 slightly overestimates the liquid flow at the break, which leads to an earlier core uncovering compared to CATHARE. The heat pipes flows lost at the break in MAAP are although slightly smaller than those calculated by CATHARE which causes MAAP core uncovering kinetics to be longer than CATHARE. However, MAAP calculations remain conservative. References [1] EPRI, MAAP5 Modular Accident Analysis Program for LWR Power Plants (2013) [2] Robert, Farvaque, Parent and Faydide, CATHARE 2 V2.5: a fully validated CATHARE version for various applications, NURETH-10, Seoul, South Korea (2003) [3] Henry, R. E., and Fauske, The Two-Phase Critical Flow of One Component Mixtures in Nozzles, Orifices, and Short Tubes, Journal of Heat Transfer, pp. 179 (1971) [4] Schrock, V. E. and al., Steam-Water Critical Flow Through Small Pipes From Stratified Upstream Regions, Proceedings of the Eighth International Heat Transfer Conference, San Francisco, CA, Vo. 5, pp (1986)