Isolation Condenser; water evaporation in the tank and steam into the air. Atmosphere (in Severe Accident Management, both P/S and M/S)

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1 Loss of Ultimate Heat Sink ANS AESJ AESJ Fukushima Symposium, March h4, 2012 Hisashi Ninokata, Tokyo Institute of Technology Available ultimate heat sinks at 1F1~3 1F1 (Fukushima Dai ichi Unit 1) Sea water through hhx and RHRSW Atmosphere Isolation Condenser; water evaporation in the tank and steam into the air Atmosphere (in Severe Accident Management, both P/S and M/S) ADS or SRV opening to depressurize RPV Coolant make up PCV venting (release high temp and high pressure steam into the air) 1F2, 3 (Fukushima Dai ichi Unit 2 and 3) Sea water through HX and RHRSW Atmosphere (in Severe Accident Management, both P/S and M/S) the same as 1F1

2 Isolation Condenser for Unit 1 Train A Train B Heat sink =atmosphere Source: Boling Water Reactor (BWR) Systems (Modified) USNRC Technical Training Center No accident condition Normal Decay Heat Removal mode Source: Boling Water Reactor (BWR) Systems (Modified) USNRC Technical Training Center

3 Ultimate Heat Sink: No Diversity for Unit 2 & 3 Heat Sink Only sea water at present = No diversity common cause failure Atmosphere counted during SAM To atmosphere Rapture Disk HX RHRS Heat Sink Sea water Saturated S/C Source: Boling Water Reactor (BWR) Systems (Modified) USNRC Technical Training Center Diversity of Ultimate Heat Sinks Heat removals + Coolant make up if RCIC/HPCI stopped For containment cooling 1. Atmosphere 2. Sea or river To add 1. Atmosphere; or 2. Sea or river both with portable HXs 3. Underground water with pumps Each x2 (multiplicity) D/Gs for pumps Air or water cooling HX Source: Boling Water Reactor (BWR) Systems (Modified) USNRC Technical Training Center 3. Underground water Cooling + makeup

4 LOOP (Loss of Off site Power) PCV isolation EDG start up DHR mode [before tsunami] 1F1~3: EDG startup after LOOP 1F1 Isolation Condenser (IC); two trains A & B, DC power available; > 55 /hr then B train closed; pressure control by the A train; and HPCI standby 1F2 and 1F3 Reactor Core Isolation Cooling (RCIC) manually on; and HPCI standby; RHRSW to heat sink (sea), sea water pump with AC power available [After tsunami] SBO, no S/C RHR pumps w/o AC power: LossofUltimateHeat of Sink (Ocean) Isolation Condenser (IC) for 1F1 Reactor Core Isolation Cooling (RCIC) for 1F2 and 1F3 High Pressure Coolant Injection (HPCI) for 1F3 Make up and heat removal Make up but no heat removal capability (after S/C boiling) LUHS as a direct result of SBO because of no diversity Auxiliary cooling system down as well as mechanical components failure Loss of DC power prevented valve control (core make up and PCV venting, etc) Super SBO IC, RCIC, HPCi 8

5 LUHS and core damage LUHS results in S/C boiling ( containment cooling failure); RPV over pressure, SRV opening; Make up by IC or RCIC/HPCI; Termination of IC or RCIC/HPCI leading to the loss of coolant; Core exposure core melt meltdown RCIC Source: Boling Water Reactor (BWR) Systems (Modified) USNRC Technical Training Center After SBO, no S/C RHR pumps w/o AC power: Event Sequence after Loss of Ultimate Heat Sink (Ocean) Immediately after eventual loss of DC power (Super SBO), IC or RCIC and HPCI stop RPV pressure high SRV open depressurization and evaporation water level to the TAF rapid core uncovery fuel overheating clad oxidation and H2 generation cladding and fuel liquidus states and melting core meltdown No SAM all within an hour to a few hours Alternative make up should have been made before the core exposure [use fire engine pumps (in tandem), etc. and immediately when the RPV pressure lowered to 6~7 bars] Core melt due to delays in alternative make up to prevent core meltdown; and PCV failure/radiological release due to the delay in PCV venting; thus failed in mitigating the consequence of core meltdown 10

6 1F1 core meltdown March 11, 6 8pm 1F2 after March h14, 1F3 ~ March h14 11 Require DHR w/o AC/DC power With DC power lost, termination of IC or RCIC/HPCI resulted in a rapid core exposure and fuel melt With DC power continuously available (recharged or replaced or..), the core would survive with make up water RHRS or other alternative ti heat removal system employing forced convection cooling did not work as a result of tsunami flood, and Therefore, DHR systems that work w/o AC/DC power would render higher reliability of the decay heat removals (DHR) under the SBO condition Current GEN III+ NRx meets this requirement; so do sodium cooled fast reactors and SMRs One of the strong candidates to overcome the long term SBO is Natural Circulation Decay Heat Removal (NCDHR) system NCDHR system should survive the earthquake and tsunami as the added defense in depth 12 December 6, 2011 Hasanuddin University

7 Passive cooling Example Isolation Condenser (BWR) Passive Containment Cooling by Natural Circulation (BWR & PWR) IRIS (Integrated Primary System Reactor): ADS+PSS+LTCS; EHRS ESBWR: IC and PCCS with Gravity Driven Cooling System Good practice in 2F NPS Fukushima-Daiichi NPP Accident, GLOBAL2011, Makuahri, Japan Koji Okamoto

8 D/G air intake filter D/G operated Sea water falling onto D/G power panel Fukushima Daini Unit#1 TEPCO web page pg D/Gand related system had been flooded Power panel Photos on Nov. 26, 2011 Source: Global 2011 Koji Okamoto Loss of Ultimate Heat Sink Fukushima Daini #1 Just after tsunami (Mar. 11, 15:34) Vl Valve RCIC Turbine pump C/V R/V Fuel Control Rod S/C LUHS Declared (Mar. 11, 18:33) Heat exchanger Motor Power Panel Sea water Cooled by RCIC (Mar. 11, 15:36) S/C temp. increase Source: Global 2011 Koji Okamoto

9 Loss of Ultimate Heat Sink Fukushima Daini #1 Accident Management C/V R/V Vl Valve Fuel Rapture Disk Control Rod Ready for Vent (Mar. 12, 18:30) Depressurization (Mar. 12, 03:50) S/C (Mar. 12, 00:00) MUWC Make up water system water Heat exchanger Motor Power Panel MUWC, MUWP,.. (Mar. 12, 06:20) Sea water Auxiliary Water Injection Source: Global 2011 Koji Okamoto Loss of Ultimate Heat Sink Cold Shut down (Mar. 14, 17:00) / C/V R/V Vl Valve Fuel Fukushima Daini #1 (Mar. 14, 01:24) Heat exchanger Sea water Control Rod S/C Motor Power Panel replace Power Car P/C Source: Global 2011 Koji Okamoto

10 8 tsunami Reactor Pressure [MPa] RCIC Depressurization Fukushima Daini #1 Recovery from Loss of Ultimate Heat Sink Cold shutdown 0 3/11 3/13 3/15 3/17 3/ Vent ready 0 Aux. water S/C Press. (kpa) RHR restart 140 S/CTemp Temp. (C) S/C cooling lost 0 3/11 3/13 3/15 3/17 3/19 Japanese government Report for IAEA (Sept. 2011) Source: Global 2011 Koji Okamoto Summary for Loss of Ultimate Heat Sink LUHS has relatively large time margin if AC power is available Super SBO should be prevented under any conditions For quicker recovery/restoration, relevant backup components should be prepared onsite/offsite Seawater pump should be protected in waterproof building Air-cooling System might be considered for improving the diversity of the heat sink, especially, S/C and Spent Fuel Pool Natural circulation decay heat removals will be a key to avoid LUHS when SBO and postulated super SBO (SAM) W/o built-in or alternative heat sinks, Alternative coolant injection must be provided; and both Depressurization of RPV by opening RPV (in the absence of ADS) and PCV venting must be made possible manually ; Source (modified): Global 2011 Koji Okamoto

11 Some other comments from the disaster AC power restored 10 days later but SC/RHR not. No alternative Ultimate Heat Sink was provided but leakage from PCV to RB/TB basement compartments. SAM guidelines, emergency response procedures, safety design guidelines must be reinforced and containment vent, core cooling system (natural circulation capabilities), for long term SBO [due to tsunami in this case] to be implemented Risk of NPP, the mistakes committed, success,.. were learned and better understood from the disaster Lessons Nuclear power plants are made much safer by putting the lessons from Fukushima-Daiichi into practice 21 END H. Ninokata