Methodology of Treatment of Multiple Failure Initiating Events for Seismic PRA (2)Success Criteria Analysis for Multiple Pipe Break Accidents of a PWR *TATSUYA KUNISHI, HITOSHI MUTA, KEN MURAMATSU AND YUKI KAMEKO TOKYO CITY UNIVERSITY GRADUATE SCHOOL 1
Contents Background & Purpose Position of this study Analysis conditions Results & Consideration Conclusion & Future task 2
Background Multiple rupture events are not usually considered in PRAs of pressurized water reactors (PWRs) for internal events. Analyses assuming pipe break accidents in multiple location are not extensively performed. The possibility of Multiple Pipe Break Accidents occurrence by earthquake is very small, but the release of radioactive material etc. may be considered. Purpose Understanding the thermal hydraulic behavior in the reactor vessel and success criteria for core cooling systems considering beyond DBA 3
Position of this study In this study, we analyzed the following viewpoint and organized the findings. 1. Location (Location dependency) Effects of differences in location of the break points Confirm the influence by a combination of various break points. 2. Number of break points (Number of break points dependency) Relationship between fracture area and number of break points Confirm the difference in behavior due to the number of break points, assuming the Fracture area of the primary system piping is constant. 3. Fracture area (Fracture Area dependency) Relationship between the fracture area and the number of break points when changing the fracture area Confirm how the behavior differs depending on the size of the fracture area of the primary system piping from the viewpoint of 2. 4
Analysis Code Analysis Conditions RELAP (Reactor Excursion Leak Analysis Program) series Thermal hydraulic analysis code RELAP5-3D It is a well known and widely used code for thermal-hydraulic analysis. Model Plant The target model plant is 3-loop PWR (Output Power 2660MWt) 3-loops were separately modeled Detailed dimensions of the 3 loops PWR plant are not disclosed Lack of detailed dimension data were filled with simplified models. Validity of the plant model was checked by comparison with published LOCA analyses for similar PWRs. 5
Noding diagram Accident Scenarios Rupture of primary system piping Loss of ECCS function Pressurizer surge pipe(psu) Hot leg(hl) Crossover(CR) HPIS LPIS ACC HPIS LPIS ACC Pipe rupture is simulated by valve : Volume Cold Leg(CL) : Junction : ECCS`s HPIS LPIS ACC Noding diagram used for analysis 6
Analysis cases Number of points Location Fracture area ECCS operation 1 CR CL HL Psu Large~Small* area 2~ Combination of CR CL HL Psu Changing the fracture area (Combination of Large, Medium, Small*) HPIS 2 line LPIS 2 line ACC 3 line (Combination of ECCS actuation) Fracture Area * [m 2 ] Location Diameter*[m] Large Medium Small Crossover pipe 0.79 0.490 0.1225 0.0081 Hot Leg pipe 0.74 0.430 0.1075 0.0081 Cold Leg pipe 0.70 0.385 0.0962 0.0081 Pressurizer surge pipe 0.28 0.062 0.0154 0.0081 *:Large(rupture area of pipe inner diameter), Medium (rupture area of pipe inner diameter ½),Small is 2 inches rupture area 7
Analysis Flow 1Construction of Input Data and Benchmark Change scenario and reset piping combination 2Select the Scenario and Pipe Combination Understanding the thermal-hydraulic behavior & Grouping the analysis results that is Similar 3Analysis the selected conditions Definition of Success Criteria and Analysis Based on analysis result Definition of conditions for success of core cooling in progress scenario of accident 4 Clear the event progress and summarized the technical knowledge on Success Criteria of core cooling Applied to seismic PRA, contributing to safety improvement 8
Assumption of multiple piping rupture 1. Location (Location dependency) The fracture area are 100% rupture of the pressurizer surge pipe. Equal fracture is assumed for hot leg, crossover, cold leg. 2. Number of break points (Number of break points dependency) The fracture area are 100% rupture of the pressurizer surge pipe. In 2 break points, each points fracture area is 50% rupture of the pressurizer surge pipe. 3. Fracture area (Fracture area dependency) Fracture area twice as large as the pressurizer surge pipe in hot leg, crossover, cold leg. Evaluation Definition of core damage shall be when the surface temperature of cladding exceeds 1200 C. (about 1500 K) 9
Analysis Cases Number of points Location Fracture area ECCS operation 1 CR CL HL Psu Large~Small* area 2 Combination of CR CL HL Psu Changing the fracture area (Combination of Large, Medium, Small*) HPI 2 line LPI 2 line ACC 3 line (Combination of ECCS actuation) 10
Analysis Combination[Location] Number of break points 1 Piping Combination Crossover pipe (CR) Hot leg pipe (HL) Cold leg pipe (CL) Pressurizer Surge pipe (Psu) Case No ECCS operate HPI+LPI+ACCx3 HPIx2+LPIx2+ACCx3 No ECCS operate HPI+LPI+ACCx3 HPIx2+LPIx2+ACCx3 No ECCS operate HPI+LPI+ACCx3 HPIx2+LPIx2+ACCx3 No ECCS operate HPI+LPI+ACCx3 HPIx2+LPIx2+ACCx3 11
Analysis Results (No ECCS operate) 2500 Surface temperature of fuel cladding Temperature of fuel cladding [K] 2000 1500 1000 500 CL HL CR Psu Timings of temperature rise are different, however, temperature rise does not differ greatly each other CL break could represent because of it s the severest behavior, if simplification needed 0 0 100 200 300 400 500 Time [sec] 12
Analysis Results (HPI+LPI+ACCx3) Temperature of fuel cladding [K] 800 700 600 500 400 300 200 100 CL HL Surface temperature of fuel cladding CR Psu Cooling timing are different because of RV pressure behavior and cooling order is CL, CR, HL and Psu And the CL takes more time to cool down 0 0 100 200 300 400 500 Time [sec] 13
Analysis Results (HPIx2+LPIx2+ACCx3) 700 Surface temperature of fuel cladding 600 Temperature of fuel cladding [K 500 400 300 200 100 CL HL CR Psu Behavior is same as the previous case HPI+LPI+ACCx3. Cool down little bit faster because of the capacity of injection 0 0 100 200 300 400 500 Time [sec] 14
Conclusion for the Location From the results, Severer in the order of Cold leg > Crossover > Hot leg Pressurizer surge pipe Different behavior depending on the location and the severest one is CL break Location of break could affect plant behavior 15
Analysis Combination [break points] Number of break points Piping Combination Case 1 Cold leg pipe (CL) 2 CL+HL(Same loop) CL+HL (Different loop) CL+CL No ECCS operate HPIx2+LPIx2+ACCx3 No ECCS operate HPIx2+LPIx2+ACCx3 No ECCS operate HPIx2+LPIx2+ACCx3 16
Analysis Results (No ECCS operate) Location There is dependency Location exists in the dependency case of influence different loops Temperature of fuel cladding [K] 2500 2000 1500 1000 500 0 Surface temperature of fuel cladding CL Temperature of fuel cladding [K] 2500 2000 1500 1000 500 CL+HL(Different loop) 0 100 200 300 400 500 Time [sec] 0 Surface temperature of fuel cladding CL CL+CL 0 100 200 300 400 500 Time [sec] Temperature of fuel cladding [K] 2500 2000 1500 1000 500 0 Surface temperature of fuel cladding CL CL+HL(same loop) Same Behavior No dependency at breakpoints 0 100 200 300 400 500 Time [sec] 17
Analysis Results (HPIx2+LPIx2+ACCx3) Surface temperature of fuel cladding It take more time to cool for the 2 points break case Temperature of fuel cladding [K] 700 600 500 400 300 200 100 0 CL CL+CL 0 100 200 300 400 500 Time [sec] If all ECCS inject into RV, the core will cool faster Surface temperature of fuel cladding Surface temperature of fuel cladding Temperature of fuel cladding [K] 700 600 500 400 300 200 100 0 CL CL+HL(Different loop) 0 100 200 300 400 500 Time {sec] Temperature of fuel cladding [K] 700 600 500 400 300 200 100 0 CL CL+HL(same loop) 0 100 200 300 400 500 Time [sec] 18
Conclusion for the Break points From the result, 1 point break of CL and 2points break CL+CL are quite similar behavior, so NO dependency could exist in number of break points However, 2 points break of the combination of different loops, location dependency could exist There is no Dependency of Number of break points in the same position However, Location dependency could exist in the different position 19
Analysis Combination[Fracture area] Fracture area Piping Combination Case 100% Pressurizer surge pipe 200% Pressurizer surge pipe Cold leg pipe (CL) Cold leg pipe (CL) No ECCS operate HPI+LPI+ACCx3 HPIx2+LPIx2+ACCx3 No ECCS operate HPI+LPI+ACCx3 HPIx2+LPIx2+ACCx3 20
Analysis Results (No ECCS operate) Surface temperature of fuel cladding Temperature of fuel cladding [K] 2500 2000 1500 1000 500 CL CL200% Timing of temperature rise are different However, In different of fracture area, temperature rise rate does not differ greatly 0 0 50 100 150 200 250 300 350 400 Time [sec] 21
Analysis Results (HPI+LPI+ACCx3) 800 Surface temperature of fuel cladding 700 Temperature of fuel cladding [K] 600 500 400 300 200 100 CL CL200% The bigger fracture area size, the faster the clad temperature rise However, ECCS could operate earlier that core could be cooled down faster than smaller size of fracture area 0 0 50 100 150 200 250 300 350 400 Time [sec] 22
Analysis Results (HPIx2+LPIx2+ACCx3) Temperature of fuel cladding [K] 800 700 600 500 400 300 200 100 CL CL200% Surface temperature of fuel cladding Same as ECCS injected case with single failure The bigger fracture area size, the faster the clad temperature rise However, ECCS could operate earlier that core could be cooled down faster than smaller size of fracture area 0 0 50 100 150 200 250 300 350 400 Time [sec] 23
Conclusion for the Fracture area From the results The size of fracture area is different, the temperature rising point is different However, the temperature rise does not differ greatly regarding to the fracture area Even in all ECCS operate or the case considering single failure of ECCS, cooling speeds are not different greatly Rupture fracture area could affect plant behavior 24
Summary Conclusion Considering to define Success Criteria, the followings need to be discussed Location of break could affect plant behavior Cold leg break is the severest case. There could be no dependency of the number of break points in the same position Fracture area dependency need to be noted in considering success criteria. Future task In this presentation, primary piping ruptures are focused on, but consideration should also be given to the combinations with secondary piping such as steam generator tube. The technical knowledge on Success Criteria of core cooling should be summarized, and it should be applied to seismic PRA 25
Thank you for your attention. 26