Lessons Learned from Fukushima-Daiichi Accident (Safety Measures and PSA Utilization)

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1 PSAM Tokyo Lessons Learned from Fukushima-Daiichi Accident (Safety Measures and PSA Utilization) April 15, 2013 Akio Komori (Takafumi Anegewa) Tokyo Electric Power Company All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 0

2 1. PRA application before Fukushima-daiich accident 2. Key findings of Fukushima-daiich accident regarding earthquake and tsunami 3. The change of design philosophy in TEPCO after Fukushima-daiichi accident 4. Practical and effective PRA utilization for external events All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 1

3 1.1 Risk assessment before 1F accident - Internal events Chronology 1979 TMI accident 1986 Chernobyl accident 1992 Regulatory request of Accident Management (AM) development 1994 AM review report 1997 Implementation of PRA to PSR 2002 Effectiveness evaluation report of AM measures Activities in TEPCO 1Deterministic approach Safety measures based on deterministic safety assessment 2PRA introduction Preparations for internal events PRA approach Development of AM measures 3PRA application PRA for Periodic Safety Review (PSR) Effectiveness evaluation report of AM measures Risk monitor All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 2

4 1.2 Accident Management Measures applied in 90 s Basic concept: Utilize existing systems and components as much as possible. Function Shutdown Water injection Heat removal Electric power supply AM measures Alternate reactivity control (Recirculation Pump Trip and Alternate Rod Insertion) Alternate water injection to RPV and PCV (through Make Up Water Condensation System and Fire Protection System) Automatic Depressurization System Alternate Heat Removal PCV Hardened Vent Addition EDG Alternate power source(480v) cross-tied from adjacent units All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 3

5 1.3 PRA evaluation of AM effectiveness Reduction Core Damage Frequency is around 70 %. reference:effectiveness evaluation report of AM measures(2002) 1.00E % % 1.00E+01 10% % Enhancement of depressurization function Before the development of AM measures After the development of AM measures Enhancement of power supply Enhancement of shutdown function 1% 1.00E+00 % 1.00E % % 0.01% 1.00E-02 % TQUX LOCA TQUV TB TW TC Total Enhancement of water injection function Enhancement of heat removal function example of 1F Unit 2 All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 4

6 1.4 Risk assessment before the 1F accident - External events Chronology 1978 Establishment of Reg. Guide for Reviewing Seismic Design 2002 Establishment of JSCE method 2006 Revision of Reg. Guide 2007 Niigata-Chuetsu-Oki Earthquake 2007 Establishment of Standard for Seismic PRA Procedures 1 2 Activities in TEPCO 1Deterministic approach Design based on historical record 180gal seismic acceleration 3.1m Tsunami height based on 1960 Chili tsunami Seismic back-check based on the guide ( gal) Tsunami re-assessment based on JSCE (Japan Society of Civil Engineers) method Seismic back-check based on the revised Reg. Guide( gal) Reflection of lessons learned 2PRA introduction Start of residual risk assessment All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 5

7 1.5 Revision of seismic evaluation and safety measures Continuous enhancement of seismic durability of SSCs. the Regulatory Guide for Reviewing Seismic Design of Nuclear Power Reactor Facilities has revised in 2006: Upgrade of evaluation method of seismic acceleration (Introduction of Ss) Quantification of residual risk of FP release and exposure to the public Start of back-check of seismic resistance Submittal of the Interim report between 2008 and 2009 (Increase seismic resistant margin) Evaluation of seismic PRA was started as residual risk evaluation since seismic PRA standard was issued by AESJ (2007) Niigata- Chuetsu-Oki Earthquake happened, and seismic acceleration exceeded previous assumption (2007) Implemented additional countermeasures Seismic isolated building for on site emergency response center Fire engines Inlets for alternate water injection line All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 6

8 1.6 Tsunami height evaluation Tsunami height evaluation based on JSCE methods. Unit Tsunami Height m Design Basis (Chile tsunami) 2002 Tsunami source located off the coast of Miyagi Pref Tsunami source located off the coast of Fukushima Pref. Record of March 11th, F Maximum run up height: Maximum run up height: F Spent too much time to evaluate possibility of tsunami source existence. Limited effort had been done for tsunami measures. All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 7

9 1.7 Seismic PRA procedure Seismic Hazard Analysis Probability of exceedance Seismic acceleration Reasonable hazard according to record Sequence analysis 1Frequency of earthquake based on hazard analysis 2Plant level fragility Seimic Fragility Analysis Accumulate damage frequency Sequence analysis Seismic acceleration Frequency 1 2 CDF Seismic acceleration All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 8

10 1.8 Tsunami PRA procedure Probability of exceedance Accumulate damage frequency Hazard curve Tsunami Height (m) Sequence analysis Fragility Analysis Tsunami height Point :Hazard curve is correct? Different tsunami height due to location and scale of tsunami source Different tsunami height due to landform Different tsunami height in case of same scale of earthquake and tsunami Frequency Sequence analysis 1Frequency of Tsunami based on hazard analysis 2Plant level fragility All Rights Reserved 2013The Tokyo Electric Power Company, Inc CDF Tsunami height 1

11 1. PRA application before Fukushima-daiich accident 2. Key findings of Fukushima-daiich accident regarding earthquake and tsunami 3. The change of design philosophy in TEPCO after Fukushima-daiichi accident 4. Practical and effective PRA utilization for external events All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 10

2.1 Deficiency in DID protection against external events Facts: Underestimate tsunami height for design base. Site level was not high enough to prevent inundation of tsunami as the 1 st layer of DID.

12 2.1 Deficiency in DID protection against external events Facts: Underestimate tsunami height for design base. Site level was not high enough to prevent inundation of tsunami as the 1 st layer of DID. Equipments as 3 rd,4 th barriers of DID layer were disabled by tsunami. (common cause failure mode) Lessons Learned: Necessary to enhance DID for external events Basic policy of safety enhancement: Define Design Extended Condition (DEC) for each DID functions physical barriers (1), shutdown (2), cooling (3), confinement (4) All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 11

13 2.1 Inadequate high-pressure injection in SBO Facts: Isolation condenser in 1F1 didn t work well after Tsunami hit. Core melted down in 5 hours after scram High-pressure injection systems worked 3 days for 1F2 and one day for 1F3. Couldn t depressurize RPV and lined up alternative injection systems while RCIC & HPCI were operating. Lessons Learned: Necessary to enhance high pressure injection function which is very important after SBO. Basic policy of safety enhancement: Extend DB-SBO duration to 12hours and enhance high pressure injection functions All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 12

2.3 Revision of design requirements for PCV Facts: 1F Unit 1,2,3 containment were breached by high temperature and pressure resulted from core damage, and radioactive materials were released.

14 2.3 Revision of design requirements for PCV Facts: 1F Unit 1,2,3 containment were breached by high temperature and pressure resulted from core damage, and radioactive materials were released. Design requirements of PCV as the 4 th barrier were not clearly defined. Original design requirements for PCV as a 3 rd layer were based on LOCA Lessons Learned: PCV designed based LOCA cannot withstand core meltdown condition. Basic policy of safety enhancement: Enhance the design requirements for PCV and related equipments as a 4th barrier of DID in melt through condition. All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 13

15 2.4 Impact of the earthquake on isolation condenser in 1F1 PCV pressure after the earthquake was stable. Record of charts indicated pressure boundary integrity. RPV pressure after the earthquake responded properly following IC operation. Isolation Condenser worked well. No damage due to the earthquake were found on safety features of R/B of 1F5 and 1F6 by walk down inspection. No damage due to the earthquake were found on any equipments in T/B of 1F1, 1F2 and 1F3. All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 14

2.5 Recorded charts of 1F1 * 4 2 5 3 1 0 2 4 6 8 (Mpa) RPV Pressure 1 Scram by the earthquake at 14:46 2

to IC shut down 5 Pressure change due to IC operation (assumed) * The recorder may have stopped due to

16 2.5 Recorded charts of 1F1 * (Mpa) RPV Pressure 1 Scram by the earthquake at 14:46 2 Pressure increase due to MSIV closed 3 Depressuring due to IC operation at 14:52 4 Pressure increase due to IC shut down 5 Pressure change due to IC operation (assumed) * The recorder may have stopped due to the tsunami PCV Pressure 8 10 (kpa) All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 15

1 Hydrogen recombiner of FCS device and tube 2 Ventilation duct and flooding protection chamber:

17 2.6 Field investigation around Isolation Condenser in 1F1 No damage on main tanks, piping, flange, valve of 1F1 IC. 1 Hydrogen recombiner of FCS device and tube 2 Ventilation duct and flooding protection chamber: connect with SFP wall 3 IC vent line:small diameter tube(3/4inch) included high temperature steam 4 Conduit All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 16

18 1. PRA application before Fukushima-daiich accident 2. Key findings of Fukushima-daiich accident regarding earthquake and tsunami 3. The change of design philosophy in TEPCO after Fukushima-daiichi accident 4. Practical and effective PRA utilization for external events All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 17

19 3.1 DID enhancement policy Tsunami destroyed 1 st & 3 rd barriers of DID completely. necessary to enhance each layers of DID Necessary to use Design Extended Condition (DEC) for each DID layer in order to consider safety enhancement measures. To cope with tangled and complex Beyond DBE condition, DEC design requirements should promote diversity and flexible measures. In order to enhance the reliability of high pressure injection and RPV depressurization function, SBO should be treated as design base, then single failure criteria should be applied. All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 18

20 3.2 Measures in each DID layer for Tsunami Levels of DID Purpose (Important Function) 1 st Prevention of occurrence of anomaly (Physical barrier) 2 nd Prevention of escalation of anomaly (Shutdown) Prevention of core 3 rd damage (Cooling) Mitigation of accident 4 th after core damage (Confinement) Design Base Site elevation, Embankment, Tidal wall, Tidal board No additional system [Cooling] - additional high pressure water injection system besides RCIC - increase DC battery capacity for RCIC [Depressurization] No additional system Design Requirement DEC(promote diversity & flexibility) - Water tight doors to limit water inundation to significant areas - Water discharge pump at safety significant areas No additional system [Cooling] - portable DC battery for RCIC -CUW and MUWC enable by power supply vehicle -fire engine, Diesel Driven pump, Movable heat exchanger, hardened w/w vent, filter vent (before core damage) [Depressurization] - dedicated DC battery for SRVs - increase N2 capacity and pressure - compressor - additional diverse depressurization method - substitute spray, pedestal water injection, top head flange cooling - filter vent (after core damage) Originally defined as DEC - passive hydrogen recombine system in reactor building Newly added as DEC All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 19

21 3.3 Phased Approach for DID enhancement Design requirements for safety measures should be determined according to required time and available alternative equipments. Early phase: limited human resource and difficulty in the field access Later phase: difficulty in cope with only installed equipments due to complex situation Accident occurrence 12hrs 72hrs Support from off-site Response by portable equipments and AM Responses by installed equipments Complexity of accident progression [time] [Time margin: Small] - Installed equipments - Shift team [Time margin: Medium] - Portable equipments - On-site team [Time margin: Large] - Support from off-site - Off-site team All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 20

22 3.4 1 st layer of defense in depth The flood by tsunami is prevented and the measure which protects power sources and other important apparatus is implemented. Tidal board Water-tight door Tidal wall Embankment : Preventing inundation of site Tidal wall : Preventing inundation of building Water-tight door : Preventing flooding of critical areas (~60 places) Spent Fuel Pool Start up Transformer (Low Voltage) Tidal board (under consideration) Emergency D/G, Power Supply panel Waterproof treatment at Cable trays Waterproof treatment at Pipes Waterproof treatment : Preventing flooding of critical areas (~ 300 places) All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 21

23 3.5 3rd layer of defense in depth HP water injection Depressurization spare gas cylinder LP water infection and SFP cooling Fire engine Various power supply means GTG Turbine Water Lubricant pump Assure means of heat removal Power supply vehicle Charge DC power supply Emergency HV power supply panel Assure water sources Critical are a Alternative sea water heat ex. (deployed on high ground) Emergency HV power supply panel Water reservoir All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 22

24 3.6 4th layer of defense in depth Preventing primary containment vessel damage Controlling hydrogen ~ 原子炉ウェル Reactor well Preventing release of radioactive materials Fire 消防車 engine R/B top vent Passive autocatalytic recombiner Top head flange cooling Hydrogen detector Filter venting All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 23

25 1. PRA application before Fukushima-daiich accident 2. Key findings of Fukushima-daiich accident regarding earthquake and tsunami 3. The change of design philosophy in TEPCO after Fukushima-daiichi accident 4. Practical and effective PRA utilization for external events All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 24

26 4.1 Effectiveness evaluation of countermeasures in 2F4 Core Damage Frequency (/y) 1.00E E E E E E E E-11 Before Tsunami measures Less then 1.0E-11 After Tsunami measures Around 10-3 /y decrease LOCA TB TQUV TQUX TW Total Water tight measures in Hx/B and R/B Enhancement of functions using alternative facility All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 25

27 Conclusion Estimation of natural hazard has large uncertainty. Deficiency in DID protection caused Fukushima-daiichi accident. Elimination of cliff-edge effect is essential for external events. Each DID layer should have measures based on DEC. Diversity and flexibility are important for these measures. Effectiveness of safety measures can be evaluated using PRA. As 4 th layer DID measures, TEPCO adopts following measures even if PRA doesn t indicate reasonable cost vs. benefit results. Filtered venting system Passive Autocatalytic Recombiner All Rights Reserved 2013The Tokyo Electric Power Company, Inc. 26

Development of SAMG Verification and Validation for Kashiwazaki-Kariwa(KK) Nuclear Power Station

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