1. 0 Overview of Fukushima-Accident Analysis ERMSAR 2012, Cologne (Germany) March 21 – 23, 2012 JNES Masanori FUKASAWA

2. 1 Contents 1.1F ※ 1 Accident Analyses (Plant behavior) at JNES 2.Plant Behavior Analysis using MELCOR A)Results at IAEA Ministerial Conference (June 2012) and Problems B)Revised Analysis 3.Primary System Behavior during IC operation 4.Hydrogen Mixing and Explosion in Reactor Building (R/B) 5.Conclusions ※ 1F: Fukushima Daiichi

3. 2 Published Analyses and Evaluations ”Report of Japanese Government to the IAEA Ministerial Conference on Nuclear Safety,” June 2011. (June Report), JNES-RE-2011-0002. Documents of Hearings at Nuclear Safety Commission (NSC) and Nuclear and Industrial Safety Agency (NISA). Analyses and Evaluations submitted to NISA (published on JNES web). Accident AnalysisPlant BehaviorFP Release Getting chronology information together Event tree analysis of the accident Possibility of recriticality Reactivity constraint by sea water Time before fuel damage in SFP Salt precipitation MELCOR analysis Primary system behavior during IC operation Hydrogen mixing and explosion MCCI in case water injection stops Possibility of PCV failure by H 2 deflagration H 2, O 2 concentration FP release and dose evaluation Influence in case water injection stops FP release and EPZ Estimation of FP release and dose based on monitoring data FP release in case of venting 1. 1F Accident Analyses (Plant behavior) at JNES

4. 3 Analytical configuration: Code: MELCOR1.8.5 2. Plant Behavior Analysis using MELCOR 6 volumes of primary system 4 volumes of containment 5 volumes of reactor building to simulate FP transfer. Junctions of S/R valves, vacuum breaker, PCV leak, W/W vent Further (not depicted), activated cooling systems and assumed leak to simulate transports of steam, coolant and FP.  1F1 ： IC  1F2 ： RCIC  1F3 ： RCIC, HPCI Objectives: To figure out plant behaviors of 1F1 – 3 and enhance safety measures.

5. 4 ① IC stop 、② water injection 、③ W/W vent open 、 ④ W/W vent close 、⑤ Sea water injection 、 ⑥ Increase of PCV leakage 1F1: Lower coolant injection case Water injection (fire pump, F/P) by 3/15 is 88 m 3 2.A) Result of June Report and Problems (1F1)  IC actuation is limited and water level decreases at an early stage.  Core melts before alternate water injection.  RPV failure is calculated at 5 hrs.  Most core is calculated to melt and slump to PCV floor.  RPV failure timing (MELCOR default model calculates early failure.)  Actuation of W/W vent at 1st attempt (3/12 10:17). Analytical results Problems W/W vent Date Water level (mm) P/S pressure (MPa) Time (hr) ○, △： Measured data P/S Pressure Water level P/S pressure and water level

6. 5 1F2: In case PCV confinement maintains D/W pressure could not be reproduced in case PCV confinement maintained. →PCV leakage was assumed. ① RCIC manual actuation ② SBO ③ Change of RCIC water source from CST to S/P ④ RCIC stop ⑤ Sea water injection ⑥ S/R valve1 open ⑦ S/R valve open ⑧ explosion D/W pressure increases due to temperature rise of S/P. Water source of RCIC is switched from CST to S/P. Assumption of analysis in June Report 2.A) Result of June Report and Problems (1F2) D/W pressure Date D/W pressure (MPa) Time (hr) ○ ： Measured data

7. 6 2.A) Result of June Report and Problems (1F2) 1F2: Lower coolant injection case with PCV leakage Water injection (F/P) by 3/15 is 213 m 3  D/W pressure is well simulated on assumption of PCV leakage (50 cm 2 ) at an early stage.  RPV failure is calculated at 80 hrs because water injection by F/P is not enough.  Higher FP release due to assumed early PCV leakage Analytical results D/W pressure ① RCIC manual actuation, ② SBO, ③ Change of RCIC water source from CST to S/P, ④ RCIC stop, ⑤ Sea water injection, ⑥ S/R valve1 open, ⑦ S/R valve open, ⑧ explosion Date D/W pressure (MPa) Time (hr) ○ ： Measured data D/W Pressure

8. 7 2.A) Result of June Report and Problems (1F2) D/W pressure ① RCIC manual actuation, ② SBO, ③ Change of RCIC water source from CST to S/P, ④ RCIC stop, ⑤ Sea water injection, ⑥ S/R valve1 open, ⑦ S/R valve open, ⑧ explosion Date D/W pressure (MPa) Time (hr) ○ ： Measured data  Measured D/W pressure does not increase even S/R valve opened.  On the other hand, pressure increases in calculation.  D/W pressure at this stage is not simulated due to assumed PCV leakage.  Some heat removal (instead of PCV leakage) possibly occurred by then. Problems 1F2: Lower coolant injection case with PCV leakage Water injection (F/P) by 3/15 is 213 m 3 D/W Pressure

9. 8 1F3: Lower coolant injection case Water injection (F/P) by 3/17 is 369 m 3  Much H 2 is produced due to water level decrease when S/R valve opens.  RPV failure is calculated at 79 hrs because sea water injection is not enough.  FP is released to environment through W/W vent. Analytical results ① RCIC actuation 、② RCIC stop, ③ HPCI actuation, ④ HPCI stop, ⑤ S/R valve open, W/W vent open, water injection, ⑥ W/W vent close, ⑦ W/W vent open, ⑧ Sea water injection, ⑨ W/W vent close, water injection 、⑩～⑭ W/W vent open ⇔ close 2.A) Result of June Report and Problems (1F3) S/R Valve open P/S pressure and water level Date Water level (mm) P/S pressure (MPa) Time (hr) ○, △： Measured data P/S Pressure Water level

10. 9  During RCIC operation, D/W pressure is underestimated. (inverse trend to 1F2)  Measured pressure drops when HPCI actuates.  During HPCI operation, water level is not clear.  Amount of produced H 2  Explosions of 1F3 and 1F4 are possibly attributed to H 2 produced in 1F3 2.A) Result of June Report and Problems (1F3) D/W pressure Problems Date D/W pressure (MPa) Time (hr) ○ ： Measured data D/W Pressure

11. 10 RCIC steam exhaust pipe Hot water Hot water flow Major problems; D/W pressure underestimation during RCIC operation and pressure drops after HCPI actuation. Latest information and examination  RCIC continuous operation using return line to CST. →Assume S/P thermal stratification by RCIC exhaust steam. (see Figs.) Simulated by upper/lower S/P nodes and RCIC exhaust to the upper. HPCI initiated, steam exhausted to lower node assuming lower temp. of the water near HPCI exhaust pipe.  PCV spray during HPCI  Similar pressure transition between P/S and S/C after 42.4 hrs. →Assume RPV failure at this time. 2.B) Revised Analysis (1F3: Analytical Conditions)

12. 11 Improved matching with measured data  D/W pressure increases during RCIC operation  Depressurization due to S/C spray （ No depressurization if thermal stratification not considered because of lower spray flow rate) Remained problems  Modeling of S/P thermal stratification; investigation using CFD  Further investigation is needed for PCV leakage and W/W vent, which have large influence on FP release. 2.B) Revised Analysis (1F3: Analytical Result) Date D/W Pressure (MPa) P/S pressure (MPa) Time (hr) P/S Pressure D/W pressure ○ ： Measured data P/S and D/W pressures S/C Spray

13. 12 2.B) Revised Analysis (1F2: Analytical Conditions) Major problems; D/W pressure after S/R valve open. (Some heat removal instead of PCV leakage) Latest information and examination  Tsunami water flooded at a depth of boots length in RCIC room (same level as S/P torus room) at 1:00, 3/12 and increased at 2:00.* →Assume S/P heat removal by flooding water 60% heat of RCIC exhaust steam is removed.  Early PCV leakage is not assumed. Instead; Small leakage at 70 hrs because measured D/W pressure slightly decreases. Enlargement of leakage at 90 hrs when large pressure drop is measured.  RCIC injection rate is adjusted to simulate time when water level comes down to TAF.  Assume S/P thermal stratification (Same as 1F3). *TEPCO, “Report regarding factual information related to the investigation results of the accident situation at Fukushima Daiichi Nuclear Power Plant,” Dec. 22, 2011.

14. 13 P/S pressure and water level A:RCIC manual actuation, B:SBO, C:Change of RCIC water source from CST to S/P, D[1]:RCIC stop, F[3]Sea water injection, E[2]:S/R valve1 open, G[5]:S/R valve open, H[10]explosion 2.B) Revised Analysis (1F2: Analytical Result) P/S pressure is also simulated by adjusting RCIC injection rate Date Water level (mm) P/S pressure (MPa) Time (hr) P/S Pressure Water level ○ △： Measured data

15. 14 0.6 cm2 of leakage Enlargement of leakage (32 cm 2 ) 2.B) Revised Analysis (1F2: Analytical Result) P/S and D/W pressures High D/W pressure is reproduced. Calculated pressure increase becomes lower and consistent with measured data.  Steam through S/R valve flows to lower level of S/P, whose temperature is lower due to thermal stratification. Date Water level (mm) D/W pressure (MPa) Time (hr) P/S Pressure D/W pressure ○ △： Measured data

16. 15 3. Primary System Behavior during IC operation Isolation Condenser (IC) is a unique system for reactor cooling in unit-1, and worked at the initial stage of the accident. (ceased due to AC/DC valve power loss by the tsunami) RELAP5/mod3 analyses were performed to investigate the IC behavior.  IC functioned properly to the original design. (Not impaired by the earthquake)  Sensitivity analysis shows that the core uncovery could have been avoided by continued operation of IC after the tsunami. IC system (Unit-1) Initial stage of the accident (after earthquake) Assuming IC continued operation Reactor pressure Water level above TAF

17. 16 Hydrogen gas concentration 4. Hydrogen Mixing and Explosion in R/B JNES is conducting analyses of hydrogen gas mixing and detonation in Reactor Buildings (R/Bs) for investing explosion phenomena during Fukushima accident.  MELCOR for hydrogen source evaluation, FLUENT(CFD code) for hydrogen gas transport and mixing, and AUTDYN for structural analysis of detonation  The objectives are to better understand the phenomena that took place in Unit 1 and Unit 3, and assess and improve the methods and tools. CFD model of Reactor Building Mixture gas velocity

18. 17 Some results from the analysis of detonation at unit 3 4. Hydrogen Mixing and Explosion in R/B Debris Velocity R/B Pressure  The amount of hydrogen gas leaked into R/B is estimated to be approximately 1 ton.  If it is assumed that the leakage took place at S/C or D/W’s penetration, overall detonation behavior is well reproduced.  With initial velocity 70 m/s debris is supposed to reach at about 250 from the top of R/B at 7.1 seconds. This photo is quoted from Fukushima- chuo TV This photo is quoted from TEPCO web

19. 18 JNES has been conducting various analyses of the Fukushima accident. Plant behavior analysis using MELCOR improved by assuming S/P thermal stratification and latest information for 1F2 and 1F3. P/S behavior analysis of 1F1 using RELAP5/mod3 shows IC functioned properly to the original design. (Not impaired by the earthquake) Detonation analysis with the assumption that leakage took place at S/C or D/W’s penetration well estimates overall R/B behavior of 1F3. 5. Conclusions