ICONE ADDRESSING BORON DILUTION SCENARIO THROUGH RELAP5/3.3 ANALYSIS OF PWR SB LOCA

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1 Proceedings of the 17 th International Conference on Nuclear Engineering ICNE17 uly 12-16, 29, russels, elgium ICNE DDRESSING RN DILUTIN SCENRI TRUG RELP5/3.3 NLSIS F PWR S LC Patricia Pla S. Piero a Grado Nuclear Research Group (GRNSPG), University of Pisa / Technical University of Catalonia Pisa, Italy / arcelona, Spain Francesco D'uria, Carlo Parisi, Walter Giannotti, lessandro Del Nevo, Nikolaus Muellner, Marco Cherubini, Giorgio Galassi S. Piero a Grado Nuclear Research Group (GRNSPG), University of Pisa Pisa, Italy STRCT Reactivity accident scenarios can occur originated by internal boron dilution in the primary system of a nuclear pressurized water reactor type (PWR or ER). In essence the problem is caused by boron dilution following vaporization and condensation of the primary system coolant in case of decrease of primary system mass inventory, for example during a small-break loss of coolant accident (S-LC) that may include boiling in the core with condensation of steam in the steam generators. When the liquid level in the reactor vessel decreases below the hot leg elevation, steam begins to flow to the steam generators and condenses there. This steam carries no boron and thus boron in the cold leg loop seals begins to decrease. If for some reason this water plug with low boron begins to flow towards the core and enters it without any major mixing with the borated coolant, the result is a positive reactivity insertion. The paper presents an analysis by RELP5 Mod 3.3 code [1] of a small break LC of 2 cm 2 area in the lower plenum of a four-loop PWR nuclear reactor. The boundary conditions of the calculations consider the eight accumulator tanks available, two/four low pressure injection systems (LPIS) available, and two of the four high pressure injection systems (PIS) available. Sensitivity calculations were performed, regarding among other things, the boron in the Emergency Core Cooling Systems (ECCS) and reactor cooling system Regina Galetti CNEN (National Commission for Nuclear Energy) Rio de aneiro, razil Francesc Reventós Technical University of Catalonia arcelona, Spain (RCS) from Design asis ccident (D) to beyond D conditions. From the results obtained, in some calculations boron dilution is observed in more than one loop seal. The situation in which the plugs in the loop seals are transported to the core without mixing with other borated water led to a potentially hazardous situation for four calculations in which initial conditions were far from D. It is important to emphasize that the present study has not the objective of a safety analysis of the NPP involved, but it should be considered inside research activities regarding the boron dilution issue. 1. INTRDUCTIN Reactivity accident scenarios could originate through internal boron dilution in the Primary System (PS) of a nuclear pressurized water reactor type (PWR or ER) [2]. The potential for this scenario is more relevant for the reactor core at beginning of life (L), when boron added to the coolant has its maximum effect and therefore any dilution is significant. The problem is caused when the liquid level in the reactor vessel falls below the hot leg elevation; consequently steam begins to flow to the steam generators and condenses there. This steam carries no boron and thus its in the cold leg loop seals begins to decrease. If for some reason this water plug with low boron begins to flow towards the core and enters it without any major mixing with the borated coolant, the result is a positive reactivity insertion. 1 Copyright 29 by SME

2 oron dilution is the process where a local decrease in boron in the PS occurs with (nearly) constant average boron ; De-boration and boration are the processes where a net loss or gain of boron from the PS occurs. During special S-LC scenarios boron dilution and deboration processes occur simultaneously. When boron dilution analyses are performed several aspects could be distinguished like: - formation of the diluted boron plug; - transport of the diluted boron plug; - mixing of the diluted boron plug; - de-boration (i.e. net loss of boron from the PS that results in a decrease in average boron ) and boration (i.e. net boron gain in the primary circuit associated with ECCS actuation); - reactivity feedback, necessarily associated with a threedimensional performance of the neutron flux and coolant distribution in the vessel and in the core region. Some of the aspects are mandatory associated with the use of specific type of tools, e.g. Computational Fluid Dynamics (CFD) codes to analyse the mixing of the diluted boron plug and 3D Neutron Kinetic codes for the reactivity feedback. It should be pointed that the study here presented refers only to the formation and transport of the diluted boron plug and boron dilution, boration and de-boration aspects, all associated with the use of system thermalhydraulic tools. ther studies performed at the University of Pisa have analysed the problem of boron dilution from the mixing (CFD) [3] and neutron kinetics point of view [2], [4]. The main objective of the study was to investigate with RELP5 Mod 3.3 patch3 code the formation of significant (of a certain amount of volume) non-borated plugs of water in the loop seals of a four loops PWR due to reflux-condensation mode after a S-LC of 2 cm 2 area in the lower plenum. This break size and location is considered a D in the specific type of reactor inside PWRs [5]. Some sensitivity studies were carried out to support the results. 2. INPUT DECK ND UNDR ND INITIL CNDITINS The original nodalization input-deck for Relap5/mod3.2.2 gamma code [6], [7], [8] was adapted to a nodalization for RELP5 Mod 3.3 patch3 (Figure 1). It represents a four loops PWR reactor type. The original input-deck for Relap5/mod3.2.2 gamma code underwent a qualification process documented at the University of Pisa [6], [7]. For S-LC calculations the number of nodes per each steam generator U-Tube bundle was increased and three parallel U-Tubes per each steam generator (not shown in Figure 1) were modelled because it was found to be of large interest for the understanding of natural circulation phenomena [8], which is also of importance in the analysis of boron dilution events. Eight downcomer stacks end up in a single stack of nodes into the lower plenum. This implies the prediction of full mixing in the lower plenum. The RELP5 Mod 3.3 code has been qualified for boron mass transport against boron transport experiments in the PKL Integral Test Facility [2]. The selected scenario is an S-LC in the lower plenum of the PWR reactor, with boron dilution in loop seals caused by reflux condensation. De-boration is more effective when the break is located in the lower plenum compared to break in the cold leg since boron is lost in the reactor vessel. The likelihood of this failure is due to the presence of bottom nozzles (e.g. instrumentation penetrations) in this zone of some PWR vessels. The boundary conditions consider the ECCS which comprises, depending on the calculation performed (Table 1): Eight (CC), two in each loop, one of them injecting into the cold leg and one of them injecting in the hot leg. Two (of four) PIS available in hot legs of loop 1 and loop2. Two (of four, in both cold and hot legs of loop 1 and loop2) or the four LPIS available. Figure 1 - PWR NPP nodalization overall view for RELP5 Mod 3.3 patch3 3. PERFRMED NLSES LC of break size of 2 cm 2 in the lower plenum was investigated in order to identify possible potential adverse conditions from unanticipated boron dilution. Table 1 summarizes all the calculations performed and boundary and initial conditions (IC). Table 2 shows the correspondence between Case ID Label shown in the Figures and all calculations. 2 Copyright 29 by SME

3 The Reference Calculation (case 3 in Table 1) has been selected taking into account a D situation regarding, among other conditions, ECCS actuation, boron and timing for secondary side (SS) cooldown. Main features of the Reference Calculation regarding the boron in D conditions are shown in Table 1. It was considered relevant to perform a sensitivity study on the initial RCS boron and on the boron in the ECCS in order to observe the influence in the possible boron dilution process in the loop seals. These calculations correspond to case 1 in Table 1, which is the case with further conditions from D ones in boron (in RCS and ECCS) and emergency actuation. Cases 4, 5 and 6 in Table 1 are sensitivity cases based in case 1 and imposing boron (in RCS and ECCS) to Reference Calculation values. Cases 2 and 8 have been studied based in case 1 and using the Relap option w1 in kinetic card 31 which indicates differential boron worth in $/ppm instead of boron reactivity given by a control variable as in all other cases. ll the cases have been executed with the RELP5 Mod 3.3 patch3 version. The Reference Calculation has been run also with a previous version of the code RELP gamma (8) (cases 9 and 1 in Table 1). ther sensitivity cases based on performed cases have been carried decreasing maximum time step from.1 or.5 to.1 or.5 (cases 7, 8 and 1 in Table 1). Regarding the LPIS actuation, the calculations could be joined in two different groups: Case 3 (Reference Calculation), case 9 and case 1 consider LPIS only available in loops 1 and 2, while the rest of calculations, cases 1, 2, 4, 5, 6, 7 and 8 consider LPIS available in all four loops. Table 1. Calculations performed in PWR Initial RCS Case N o IC boron conc. (ppm) 8 CC available, 4 LPIS 1 available, 2 PIS 2 available in loop 1 and loop2 2 " CC available, 2 LPIS and 2 PIS available in loop 1 and 148 ECCS boron (ppm) 1 in ECCS line WST tank 2 in the 1 in ECCS line WST tank 2 in the 22 in ECCS line WST tank 22 in the Notes eyond D conditions in boron (in RCS and ECCS) and emergency actuation Case 1 and w1 in card 31 to take into account reactivity due to boron Reference Calculation 4 loop2 8 CC available, 4 LPIS available, 2 PIS available in loop 1 and loop " 2 6 " CC available, 4 LPIS available, 2 PIS available in loop 1 and loop2 2 8 " CC available, 2 LPIS and 2 PIS available in loop 1 and loop " in ECCS line WST tank 2 in the 22 in ECCS line WST tank 22 in the 22 in ECCS line WST tank 22 in the 1 in ECCS line WST tank 2 in the 1 in ECCS line WST tank 2 in the 22 in ECCS line WST tank 22 in the 22 in ECCS line WST tank 22 in the Case 1 and initial RCS boron with D values Case 1 and ECCS boron with D values Case 1 and initial RCS boron and ECCS boron with D values Case 1 and reduced max time step Case 2 and reduced max time step Case 3 run with RELP gamma Case 9 and reduced max time step Table 2. Correspondence between calculations performed and Case ID Label CSE ID LEL 1 KWU_2CM2 2 KWU_2CM2_NCNTRLR 3 LP2RR56_R33 4 KWU_2CM2_R1 5 KWU_2CM2_R2 6 KWU_2CM2_R3 7 KWU_2CM2_TS 8 KWU_2CM2_NCNTRLR_TS 9 LP2RR56 1 LP2RR56_TS N o 4. MIN RESULTS F TE PWR S LC CLCULTINS The sequence of relevant events resulting from the PWR NPP S LC is given in Table 3 for the Reference 3 Copyright 29 by SME

4 Calculation, case 3. The break is imposed at time s of the transient initiation after 3s of steady-state. Table 3. Resulting sequence of events CLCULTED EENTS TIME FTER TRNSIENT INITITIN (s) reak opening. SCRM power curve enabled. Start of main coolant pumps coast down. Main steam line valve closure 2. Feedwater valve closure 1. PIS starts in loop 1 and loop Pressurizer emptied 17. Upper plenum in saturation condition 17. ccumulators injection starts In hot legs -Not injecting because pressure setpoint was not achieved In cold legs -Not injecting because isolated 5s after ECC criteria Minimum boron in loop seals 23. Loop seal clearing (first occurring) loop 1 loop 2 loop 3 loop Not occurring ccurrence of minimum PS coolant mass 171. LPIS starts Not occurring, LPIS is not active in loop 3 and loop 4 and active in loop 1 and loop 2 but LPIS are not injecting because pressure set-point was not achieved reak two phase flow 6. The Figures (time trends) per each calculation or code run in Table 1 are shown below. Figures 2 and 3 show PS and SS pressure for all calculations performed. Initial depressurization is rather fast leading to emptying of the pressurizer and to saturated conditions in the hottest parts of the PS. The PIS trains available initiate on PS pressure trip. PS pressure increase (Figure 2) in four cases, this is related to power increase and explained later. The different behaviour of SS pressure (and as consequence PS pressure till 25s) in Reference Calculation and cases 9 and 1 (which are based on the same initial conditions than Reference Calculation) from all the other calculations is due to the earlier imposed actuation of the SS cooldown of 1 K/h (Figure 3). Figure 4 shows normalized (to the steady-state value) PS coolant mass. It reflects the balance of mass lost from the break and increase from various ECCS injections, i.e. PIS, and LPIS in that order. The loss of mass is larger at the beginning of the transient for calculations in which the SS cooldown actuates later, since PS pressure is maintained higher and less mass is injected by PIS. etween 63s and 78s, depending on the calculation, the borated tanks that feed PIS were emptied in some cases, so PS mass decreases but earlier PS mass is maintained due to injection. In Reference Calculation and cases 9 and 1, PIS are available in loop 1 and loop 2 and LPIS are available in loop 1 and loop 2 but the LPIS are not injecting, even if pressure set-point was achieved in case 9 and 1, since the borated tank water was already emptied by PIS in these loops. In these cases the are injecting in the hot legs of all loops; but are not injecting in the cold legs of all loops due to isolation 5s after ECCS criteria. In case 1 and all the rest of cases PIS are active in loop 1 and loop 2, all four LPIS are active; borated tanks water for LPIS in the loop 3 and loop 4 are available and start injecting at about 9s. In these cases the eight in hot and cold legs are all available and injecting at about 8s. Pressure (MPa) Pressure (MPa) WinGraf KWU_2CM2 p231 KWU_2CM2_NCNTRLR p231 LP2RR56_R33 p231 KWU_2CM2_R1 p231 KWU_2CM2_R2 p231 KWU_2CM2_R3 p231 KWU_2CM2_TS p231 KWU_2CM2_NCNTRLR_TS p231 LP2RR56 p231 LP2RR56_TS p231 Figure 2. Primary System Pressure in all cases WinGraf KWU_2CM2 p KWU_2CM2_NCNTRLR p3541 LP2RR56_R33 p KWU_2CM2_R1 p3541 KWU_2CM2_R2 p3541 KWU_2CM2_R3 p KWU_2CM2_TS p3541 KWU_2CM2_NCNTRLR_TS p LP2RR56 p3541 LP2RR56_TS p Figure 3. Secondary System Pressure in all cases 4 Copyright 29 by SME

5 PS Normalized mass (-) 1.6 WinGraf KWU_2CM2 NRMSS KWU_2CM2_NCNTRLR NRMSS 1.4 LP2RR56_R33 NRMSS KWU_2CM2_R1 NRMSS KWU_2CM2_R2 NRMSS 1.3 KWU_2CM2_R3 NRMSS KWU_2CM2_TS NRMSS 1.2 KWU_2CM2_NCNTRLR_TS NRMSS LP2RR56 NRMSS 1.1 LP2RR56_TS NRMSS Figure 4. Normalized Primary System Coolant Mass in all cases In order to observe local boron decrease (boron dilution) in the liquid water, boron and water mass were calculated along the loop seal pipe for the four loops seals. The correspondence of control variables that take into account the boron present in the loops and the loop number in the Figures is the following: loop 1(cntrlvar361) loop 2(cntrlvar362) loop 3(cntrlvar363) loop 4(cntrlvar364) These control variables take into account the physical volume of the individual RELP nodes inside each loop seal. The correspondence of control variables that take into account the water mass present in the loops and the loop number the Figures is the following: loop 1(cntrlvar351) loop 2(cntrlvar352) loop 3(cntrlvar353) loop 4(cntrlvar354) In some cases boron decreases in the long term, below the initial values, showing boron dilution in the loop seals. This occurs for cases in which initial conditions are far from D (see Table 1), this is case 1 (Figure 5) and the sensitivity cases based on case 1: case 7 (Figure 11), case 2 (Figure 6) and case 8 (Figure 12). It also occurs for two sensitivity cases based on case 1 in which D values were imposed to the initial RCS boron or ECCS boron, case 4 (Figure 8), case 5 (Figure 9). oron dilution is observed, in general, in more than one of the four loop seals of the NPP, in all cases. The loop seals clearly have a certain mass of water, as shown and explained afterwards. In all cases minimum values of boron never arrive to zero. The minimum values are later restored by the actuation of PIS. oron dilution in the loop seals is not observed in case 3, Reference Calculation (Figure 7) and the sensitivity cases based on Reference Calculation: case 9 (Figure 13) and case 1 (Figure 14). Water mass was calculated along the loop seal pipe for the four loops seals, it is not shown for all cases. In cases in which water mass is low in the loop seal, this indicates that there is presence of vapour in the loop seal (which carries no boron), thus there is no presence of liquid water free of boron (boron dilution). For example, in case 1 cntrlvar351(figure 15) shows decrease of water in loop 1 at about 2s, this indicates that no boron dilution occurs in loop 1, even that cntrlvar361 (Figure 5) shows boron decrease in this loop seal in the same period of time. In the long term water is present in all loop seals, so decrease of boron indicates boron dilution occurrence. In case 6 there is no boron dilution occurrence at about 2s. Cntrlvar351 (Figure 16) shows decrease of water in loop 1, so presence of vapour, no boron dilution occurs in this loop seal, even that boron decreases in this loop in the same period of time, cntrlvar361 (Figure 1). The situation in which one or more loops seals contain a significant amount of diluted water can lead to a potentially hazardous situation when these plugs are transported to the core without mixing with other borated water. This is confirmed by the reactor power increase observed in Figure 19 for four calculations. These are the cases in which initial conditions are far from D in Table 1, case 1 and the sensitivity cases based on case 1: case 7, case 2 and case 8. Total reactivity feedback and reactivity feedback from boron density changes in all cases are shown in Figure 18 and Figure 17, respectively. In total reactivity feedback positive values (return to criticality) can be observed in the long term for the four cases that show power excursion, cases 1, 7, 2 and 8. The reactivity contribution due to boron dilution is about 1$ in those cases. oron in the lower plenum top part is shown in Figure 2. In the long term, it can be observed for the four cases 1, 7, 2 and 8 that boron has decreased from 1.5 (initial value) to below 1., the diluted water coming from the loop seals was transported to the core inlet without mixing with other borated water. In case 4 and case 5 boron dilution was shown in the loops seals in the long term where boron is below the initial values (case 4 Figure 8 in all loops, case 5 Figure 9 in loop 2), however no power excursion was observed. In Figure 2 it can be observed that for these two cases the relative decrease of boron from the initial to the long term value is small. In these two cases the diluted water coming from the loop seals was transported and mixed with other borated water along the downcomer path, and thus no hazardous situation was observed. 5 Copyright 29 by SME

6 2.5 WinGraf WinGraf oron in LS (R5 units) KWU_2CM2 cntrlvar361 KWU_2CM2 cntrlvar362 KWU_2CM2 cntrlvar363 KWU_2CM2 cntrlvar364 oron in LS (R5 units) KWU_2CM2_R1 cntrlvar361 KWU_2CM2_R1 cntrlvar362 KWU_2CM2_R1 cntrlvar363 KWU_2CM2_R1 cntrlvar Figure 5. oron in loop seals in Case 1 Figure 8. oron in loop seals in Case WinGraf WinGraf oron in LS (R5 units) KWU_2CM2_NCNTRLR cntrlvar361 KWU_2CM2_NCNTRLR cntrlvar362 KWU_2CM2_NCNTRLR cntrlvar363 KWU_2CM2_NCNTRLR cntrlvar364 oron in LS (R5 units) KWU_2CM2_R2 cntrlvar361 KWU_2CM2_R2 cntrlvar362 KWU_2CM2_R2 cntrlvar363 KWU_2CM2_R2 cntrlvar364 Figure 6. oron in loop seals in Case 2 Figure 9. oron in loop seals in Case WinGraf WinGraf oron in LS (R5 units) LP2RR56_R33 cntrlvar361 LP2RR56_R33 cntrlvar362 LP2RR56_R33 cntrlvar363.5 LP2RR56_R33 cntrlvar364 oron in LS (R5 units) KWU_2CM2_R3 cntrlvar361 KWU_2CM2_R3 cntrlvar362 KWU_2CM2_R3 cntrlvar363 KWU_2CM2_R3 cntrlvar364 Figure 7. oron in loop seals in Case 3, Reference Calculation Figure 1. oron in loop seals in Case 6 6 Copyright 29 by SME

7 2.5 WinGraf WinGraf oron in LS (R5 units) KWU_2CM2_TScntrlvar361 KWU_2CM2_TScntrlvar362 KWU_2CM2_TScntrlvar363 KWU_2CM2_TScntrlvar364 oron in LS (R5 units) LP2RR56_TS cntrlvar361 LP2RR56_TS cntrlvar362 LP2RR56_TS cntrlvar363 LP2RR56_TS cntrlvar Figure 11. oron in loop seals in Case 7 Figure 14. oron in loop seals in Case WinGraf WinGraf oron in LS (R5 units) KWU_2CM2_NCNTRLR_TS cntrlvar361 KWU_2CM2_NCNTRLR_TS cntrlvar362 KWU_2CM2_NCNTRLR_TS cntrlvar363 KWU_2CM2_NCNTRLR_TS cntrlvar364 Water mass (kg) KWU_2CM2cntrlvar351 KWU_2CM2cntrlvar352 KWU_2CM2cntrlvar353 KWU_2CM2cntrlvar Figure 12. oron in loop seals in Case 8 Figure 15. Water in loop seals in Case WinGraf WinGraf oron in LS (R5 units) LP2RR56 cntrlvar361 LP2RR56 cntrlvar362 LP2RR56 cntrlvar363 LP2RR56 cntrlvar364 Water mass (kg) KWU_2CM2_R3 cntrlvar351 KWU_2CM2_R3 cntrlvar352 KWU_2CM2_R3 cntrlvar353 KWU_2CM2_R3 cntrlvar354 5 Figure 13. oron in loop seals in Case 9 Figure 16. Water in loop seals in Case 6 7 Copyright 29 by SME

8 5. WinGraf WinGraf Reactivity ($) KWU_2CM2 cntrlvar322 KWU_2CM2_NCNTRLR reacrb LP2RR56_R33 cntrlvar322 KWU_2CM2_R1 cntrlvar322 KWU_2CM2_R2 cntrlvar322 KWU_2CM2_R3 cntrlvar322 KWU_2CM2_TS cntrlvar322 KWU_2CM2_NCNTRLR_TS reacrb LP2RR56 cntrlvar322 LP2RR56_TS cntrlvar322 oron (kg/m3) KWU_2CM2 boron2811 KWU_2CM2_NCNTRLR boron2811 LP2RR56_R33 boron2811 KWU_2CM2_R1 boron2811 KWU_2CM2_R2 boron2811 KWU_2CM2_R3 boron2811 KWU_2CM2_TS boron2811 KWU_2CM2_NCNTRLR_TS boron2811 LP2RR56 boron2811 LP2RR56_TS boron Reactivity ($) Figure 17. Reactivity feedback from boron density changes in all cases Power (W) WinGraf KWU_2CM2 reac KWU_2CM2_NCNTRLR reac LP2RR56_R33 reac KWU_2CM2_R1 reac KWU_2CM2_R2 reac KWU_2CM2_R3 reac KWU_2CM2_TS reac KWU_2CM2_NCNTRLR_TS reac LP2RR56 reac LP2RR56_TS reac -3. Figure 18. Total Reactivity feedback in all cases x 1 9 WinGraf KWU_2CM2 rktpow 4.5 KWU_2CM2_NCNTRLR rktpow LP2RR56_R33 rktpow 4 KWU_2CM2_R1 rktpow KWU_2CM2_R2 rktpow KWU_2CM2_R3 rktpow 3.5 KWU_2CM2_TS rktpow KWU_2CM2_NCNTRLR_TS rktpow 3 LP2RR56 rktpow LP2RR56_TS rktpow Figure 2. oron in lower plenum top in all cases In the four cases with return to criticality and power excursion core dryout is observed, even reaching very high rod cladding temperatures (case 1, Figure 21). Cases 2, 7 and 8, as already mentioned, are sensitivity cases based on case 1. Rod cladding temperatures are much lower in Case 2 (Figure 22) compared with case 1 (Figure 21). oth calculations should be equivalent from the point of view of many quantities (PS pressure, PS mass ); the differences are inside the uncertainties of the calculations. The difference in rod cladding temperatures are because both calculations are very close to the threshold of Critical eat Flux (CF) and RELP calculates CF on or off. In other words, this is a typical cliff-edge phenomenon or bifurcation phenomenon. Due to these differences, and to confirm the results, other sensitivity cases based in case 1 and case 2 were performed decreasing maximum time step (from.5and.1 to.5 and.1), these are case 7 (Figure 23) and case 8 (Figure 24). In case 8 based in case 2 it is possible to see dryout that before did not appear. Figure 19. Reactor power in all cases 8 Copyright 29 by SME

9 14 WinGraf WinGraf Temperature (C) KWU_2CM2httemp KWU_2CM2httemp KWU_2CM2httemp KWU_2CM2httemp KWU_2CM2httemp KWU_2CM2httemp KWU_2CM2httemp KWU_2CM2httemp KWU_2CM2httemp KWU_2CM2httemp Temperature (C) KWU_2CM2_NCNTRLR_TS httemp KWU_2CM2_NCNTRLR_TS httemp KWU_2CM2_NCNTRLR_TS httemp KWU_2CM2_NCNTRLR_TS httemp KWU_2CM2_NCNTRLR_TS httemp KWU_2CM2_NCNTRLR_TS httemp KWU_2CM2_NCNTRLR_TS httemp KWU_2CM2_NCNTRLR_TS httemp KWU_2CM2_NCNTRLR_TS httemp KWU_2CM2_NCNTRLR_TS httemp Figure 21. Rod surface temperature at different core elevations in Case 1 Temperature (C) WinGraf KWU_2CM2_NCNTRLR httemp KWU_2CM2_NCNTRLR httemp KWU_2CM2_NCNTRLR httemp KWU_2CM2_NCNTRLR httemp KWU_2CM2_NCNTRLR httemp KWU_2CM2_NCNTRLR httemp KWU_2CM2_NCNTRLR httemp KWU_2CM2_NCNTRLR httemp KWU_2CM2_NCNTRLR httemp KWU_2CM2_NCNTRLR httemp Figure 22. Rod surface temperature at different core elevations in Case 2 Temperature (C) WinGraf KWU_2CM2_TS httemp KWU_2CM2_TS httemp KWU_2CM2_TS httemp KWU_2CM2_TS httemp KWU_2CM2_TS httemp KWU_2CM2_TS httemp KWU_2CM2_TS httemp KWU_2CM2_TS httemp KWU_2CM2_TS httemp KWU_2CM2_TS httemp Figure 24. Rod surface temperature at different core elevations in Case 8 Cases 9 and 1 are sensitivity cases based on the Reference Calculation (case 3) and run with previous version of the code, the RELP gamma. Results of case 9 and 1 for many quantities are, in general, similar to Reference Calculation. To put into perspective the issue of an unwanted plug of liquid water free of boron it is useful to note the following volumes of the different components of the PWR considered in this study: the volume of the U-part of a loop seal is about 5.4 m 3 (about 7 m 3 from the SG out till the pump), the volume of the downcomer is about 3 m 3, the volume of the lower plenum is about 23 m 3, the volume of the active core is about 21.5 m 3. Thus the situation in which one or more loops seals contain a significant amount of diluted water can lead to a potentially hazardous situation, particularly, if these plugs are transported to the core active part without mixing with other borated water. ther similar results found in literature can be comparable. The French IRSN performed calculations for 9 MWe French PWR Framatome 3 loops [9] considering the most penalizing case a S LC in the cold leg of 9 cm 2 and three simultaneous slugs of about 9 tones and 5 ppm boron in each loop entering in the vessel. The methodology included CFD calculations to consider the mixing in the cold leg end, downcomer and lower plenum. For this situation a potential return to criticality was not excluded. Figure 23. Figure 37 - Rod surface temperature at different core elevations in Case 7 5. CNCLUSINS The paper presents a study to investigate using RELP5 Mod 3.3 patch3 the formation of significant non-borated plugs of water in the loop seals of a PWR due to refluxcondensation after an S LC. From the results obtained, in some calculations boron dilution is observed in one or more loop seals. The situation in which the plugs in the loop seals 9 Copyright 29 by SME

10 are transported to the core without mixing with other borated water led to a potentially hazardous situation for four calculations in which initial conditions were far from D. ll these results should be understood in a research context and not as a safety analysis of the concerned NPP reactor type. It should be pointed out that for RELP5, full boron mixing is assumed in the single nodes, and nodalizations are also coarser than in other approaches, such as using CFD codes. These assumptions could lead, in the case of system codes, to non realistic predictions of full mixed zones of borated and non-borated water, which could be nonconservative. Thus CFD analysis starting from the end of the cold legs at the inlet of the vessel, downcomer and lower plenum is another approach that could be adopted in analyzing this problem in certain type of analysis, unless with system codes a conservative situation is found. CKNWLEDGMENTS cknowledge is given to the EC (Sixth Framework Programme (Euratom), Intra-European Fellowship) for the financial support to the study here presented. REFERENCES [1] U. S. Nuclear Regulatory Commission, RELP5/MD3.3 code manual. olume II, Information Systems Laboratories, Inc. Rockville, Maryland, Idaho Falls, Idaho, US, March 26. [2] F. D uria, G.M. Galassi, W. Giannotti, D. raneo, M. Cherubini,. Del Nevo, TE RN ISSUE IN PWR ND ER-1, ECD/NE/CSNI PKL PRECT, PKL nalytical Workshop - University of Pisa, Italy, ctober 11-12, 25. [3] RE, FR, UNIPI, Gidropress, TCIS PRECT R2.2/2 - Development of safety analysis capabilities for ER-1 transients involving spatial variations of coolant properties (temperature or boron ) at core inlet - TSK 4 REPRT, November 26. Restricted. [4] ngelo Lo Nigro, ntonino Spadoni, Francesco D uria (Mechanical, Nuclear and Production Engineering Department, University of Pisa), na Maria Sanchez ernandez (Chemical and Nuclear Engineering Department, Polytechnic University of alencia) 3D Neutron Kinetics to ddress the oron Issue and nalysis of &W PWR Scenarios, PKL nalytical Workshop, Pisa, ctober 12, 25. [5] Final Safety nalysis Report, Central Nuclear lmirante Álvaro lberto, Unit 2, Eletronuclear, razil. Restricted. [6] F. D uria, G. M. Galassi, est Estimate nalysis and Uncertainty evaluation of ngra-2 plant LLC D, University of Pisa Report, DIMNP - NT 433(1), Pisa (I)-rev.1, uly 21, CNEN Contract TERM No.12-21, PRCESS No.1661/2. Restricted. [7] F. D uria, G. M. Galassi, est-estimate nalysis of ngra-2 plant TWS Event Category, University of Pisa Report, DIMNP - NT 526(4), Pisa (I)-rev.4, uly 24, CNEN Contract TERM No.-21, PRCESS No.548/22. Restricted. [8] F. D uria, G. M. Galassi, DESIGN F PKL RN-DILUTIN TRNSIENTS, ECD/NE - SET PRECT 24, DIMNP NT 529(4), University of Pisa, Italy, uly 24. [9] Francine Camous, Patrick Chatelard,. Guillard,. Marchand, nne-irginie Schwarz, Nathalie Messer, Roberto Freitas, Luben Sabotinov, Laurent Foucher, ean-pierre enoit, Philippe Dufeil, Frank Dubois, IRSN, Safety ssessment, code validation and R&D studies at IRSN using the bestestimate CTRE 2 code - Eleventh International Topical Meeting on Nuclear Reactor Thermal ydraulics (NURET- 11), Popes Palace Conference Center, vignon, France, ctober 2-6, Copyright 29 by SME