Measures against Earthquakes and Tsunamis in View of the Accident at Fukushima Daiichi Nuclear Power Station

Transcription

1 Nuclear and Industrial Safety Agency Ministry of Economy, Trade and Industry IEM 3: International Expert s Meeting (Earthquakes / Tsunamis) 4-7 September, 2012 Vienna Measures against Earthquakes and Tsunamis in View of the Accident at Fukushima Daiichi Nuclear Power Station Yoshinori Moriyama Deputy Director-General for Nuclear Accident Measures Nuclear and Industrial Safety Agency (NISA)

2 Contents I. Impact of the Earthquake / Tsunami on Fukushima Daiichi Nuclear Power Station II. Situation of Seismic Safety Assessment Activity in Japan before the Great East Japan Earthquake III. Re-investigation of Earthquakes / Tsunamis Assessment based on Findings and Lessons from the Earthquake / Tsunami IV. Approaches and Implementation of Safety Countermeasures related to Earthquakes/ Tsunamis V. Conclusion 1

3 I. Impact of the Earthquake / Tsunami on Fukushima Daiichi Nuclear Power Station 2

4 I.(1) Outline of the Accident at Fukushima Daiichi Nuclear Power Station Fukushima Dai-ichi NPS Table. Generation Facilities at Fukushima Dai-ichi NPS Occurrence: 14:46 March 11, 2011 Mw (moment magnitude): 9.0 Epicenter: approximately 130km off the coast of Sanriku (at degrees north latitude, degrees east longitude and 23.7km deep) Unit 1 Unit 2 Unit 3 Unit 4 Unit 5 Unit 6 Electric output (MWe) Commercial operation 1971/ 3 Reactor model BWR / / 3 BWR4 1978/ / 4 BWR5 1979/ 10 PCV model Mark-1 Mark-2 Number of fuel assemblies in the core

5 I.(1) Outline of the Accident at Fukushima Daiichi Nuclear Power Station Max. Acceleration Values Observed in Reactor Buildings of each Unit Loc. of seismometer (bottom floor of reactor bld.) Fukushima Daiichi Record Max. acceleration (Gal) Max. response acceleration to the design basis ground motion Ss (Gal) NS EW UD NS EW UD Unit *1 447 *1 258 * Unit *1 550 *1 302 * Unit *1 507 *1 231 * Unit *1 319 *1 200 * Unit *1 548 *1 256 * Unit *1 444 * *1:Each recording was interrupted at around (s) from recording start time *2:1Gal=0.01m/s 2, 981Gal=1G 3000 (h=0.05) Acceleration (gal) 加 速 度 (cm/s ) Period 周期 ( 秒 (s) ) Unit2 EW Response Spectrum Legend Observed DBGM Ss-1 DBGM Ss-2 DBGM Ss-3 4

6 I.(1) Outline of the Accident at Fukushima Daiichi Nuclear Power Station Assumed Height and Actual Height of Tsunami in Fukushima Dai-ichi NPS Fukushima Daiichi NPS The inundation height approximately 15m Destructive flood 10m D/G in the basement submerged. 修正中 4m 5.7m Onahama Port Fukushima Daiichi Sea water pumps were flooded. Damage to the buildings was slight. Flooding Counterflow Text added by NISA to published materials from Niigata Prefectural Technology Committee and Google 5

7 I.(1) Outline of the Accident at Fukushima Daiichi Nuclear Power Station Progress of the Accident (Outline Common to Units 1-3) Automatic reactor shutdown due to earthquake, loss of off-site power supply Soaking / depletion of battery, depletion of compressed air, etc. Emergency diesel generator started up and power supply was secured. Reactor was cooled by core cooling system. Shutdown of core cooling system Most of electric systems including emergency diesel generators and switchboards were unavailable due to tsunami. (Only one of emergency air cooling DGs in Unit 6 maintained its function) Station Blackout (On March 13, Unit 5 received power supply from Unit 6 on emergency basis. ) Fuels were exposed and melt down while cooling was not conducted. Hydrogen explosions occurred in reactor buildings at Units 1, 3 and 4. 6

8 I. (2) Impact of Earthquake on Facility Assessed by seismic response analysis using observation record. Conducted field validation for #5, which was not affected by hydrogen explosion or radiation. Estimated that facilities had remained in a state to maintain safety function at and immediately after earthquake. Major 7 facilities On-site check ( In PCV unit 5 ) Residual Heat Removal System Piping Reactor Containment Vessel Core support structures RHRS Pump Containing Reactor Pressure Vessel Main Steam System Piping Reactor Containment Vessel Reactor Pressure Vessel Control Rods (Insertionality) Main Steam System Piping Stopping Control rods (Insertionality) Core support structures Cooling Residual Heat Removal System Pump Residual Heat Removal System Piping 7

9 I. (2) Impact of Earthquake on Facility Investigate whether temperature and / or pressure in pressure vessel or containment vessel varies in the case of slight leakage caused by damage to piping, etc. Earthquake SCRAM Changes of plant parameters in case of micro leakage ( cm 2 ) after earthquake Assumed Leak Point ( ) Sensitivity Analysis Comparison with actual plant behavior IC Start RPV PLR-A Recording Stop due to Tsunami SR/V PLR-B IC Reactor Pressure and Water Level (RELAP5) No significant difference in Reactor Pressure and Water Level in case of leak area less than 0.3cm 2 RPV Pressure [MPa,g] IC manually Stop Reactor Pressure IC manually Start/Stop (3 times) Plant data Zero LEAK (Analysis) 0.1cm2 (Analysis) 0.3cm2 (Analysis) Level above TAF [mm] Plant data (NR) Zero LEAK (Analysis) 0.1cm2 (Analysis) 0.3cm2 (Analysis) Reactor Water Level /11 14:45 3/11 15:00 3/11 15:15 3/11 15:30 3/11 15:45 3/11 16:00 3/11 14:45 3/11 15:00 3/11 15:15 3/11 15:30 3/11 15:45 3/11 16:00 TIME TIME Analysis of Leakage from PLR-B line(0.1~0.3cm 2 ) Fukushima Daiichi Unit-1 8

10 I. (2) Impact of Earthquake on Facility Conclusion of assessment at this moment - It is estimated that major facilities with functions important for safety remained in a state to maintain required safety functions at and immediately after the earthquake. - It remains, however, uncertain at this moment whether such damage that caused slight leakage to major facilities with functions important for safety occurred due to impact of this earthquake. It is necessary to continue investigation on the impact of the earthquake. 9

11 II. Situation of Seismic Safety Assessment Activity in Japan before the Great East Japan Earthquake 10

12 II. (1) Revision of Seismic Design Regulatory Guide Former guideline Consider active fault 50,000 years ago Expect Earthquake occurring directly underneath with a magnitude of 6.5. New Regulatory guide by 2006 amendment Require development of design basis ground motion taking latest knowledge into consideration Extended to 120, ,000 years ago Establish more severe Ground motion developed with non-identified seismic source. Conduct survey of active faults by means of literature searching, aerial photo Interpretation, field survey, etc. In addition to conventional investigation, conduct comprehensive active faults investigation using geomorphology. Ground motion assessment by empirical formula based on earthquake scale and distance from epicenter (Responsespectra) In addition to attenuation relation, adopt extensively a fault model as ground motion assessment method. Phenomena accompanying earthquake Tsunami, Surrounding slope stability Further, concept of Residual Risk is introduced. 11

13 II. (2) Outline of Seismic Back Check September 19, 2006 Nuclear Safety Commission amended seismic design review guide September 20,2006 NISA required the nuclear operators to re- evaluate seismic design of all existing NPPs according to revised guide. (Back Check) Geological survey Re-evaluation of Design basis ground motion (Ss) Seismic safety of NPP facility Ground stability Phenomena accompanied with earthquake (Tsunami, Slope stability) Nuclear operators started re-evaluation. July 16, 2007 Chuetsu - Oki Earthquake occurred in Niigata Prefecture. 12

14 II.(3) Post- Chuetsu - Oki Earthquake Response Outline of Chuetsu - Oki Earthquake Date and time of occurrence: July 16, 2007 Around 10:13 a.m. Epicenter:Offshore Chuetsu, Niigata Prefecture Scale of earthquake: moment magnitude 6.6 Depth of epicenter:17 km Epicentral distance : Kashiwazaki-Kariwa NPS Epicenter approximately 16 km Maximum acceleration observed on the reactor building base mat at Kashiwazaki-Kariwa NPS Observed Value ( ) indicates acceleration expected at design (Unit:Gal) SN Direction EW Direction Up-and-Down Direction #1 Unit 311(274) 680(273) 408(235) #2 Unit 304(167) 606(167) 282(235) #3 Unit 308(192) 384(193) 311(235) #4 Unit 310(193) 492(194) 337(235) #5 Unit 277(249) 442(254) 205(235) #6 Unit 271(263) 322(263) 488(235) #7 Unit 267(263) 356(263) 355(235) Observed response significantly beyond expectation at design (Observed example) 1 Unit Observed record :680 Gal Expected response at design:273 Gal Approx. 2.5 times Velocity(cm/s) Epicenter 30km 10km Epicenter Power Station 10km (cm) KK site 10 Unit 1 (EW) Unit 2 (EW) Unit 3 (EW) Unit 4 (EW) 30km 6 times Period (Sec) 周期 ( 秒 ) (h=0.05) (cm/s 2 ) Response spectrum derived from general attenuation equation Average of observed record Comparison between response spectrum derived from general attenuation equation from the scale of Chuetsu-oki Earthquake ( Black line) and that observed ( Blue line) 13

15 II. (2) Outline of Seismic Back Check September 19, 2006 Nuclear Safety Commission amended seismic design review guide September 20, 2006 NISA directed operators to conduct safety assessment of existing nuclear facilities July 16, 2007 Chuetsu - Oki Earthquake occurred in Niigata Prefecture. NISA compiled knowledge and findings from Chuetsu-oki Earthquake and directed the nuclear operators to follow ( December 2007). March 2008 Operators submitted interim reports of back check, etc. March 11, 2011 The Great East Japan Earthquake occurred. 14

16 III. Re-investigation of Earthquakes / Tsunamis based on Findings and Lessons from the Earthquake / Tsunami 15

17 III. (1) Characteristics of the Earthquake / Tsunami At this moment, the following appears to represent the characteristics of the earthquake / tsunami: Source area 15:08,Mar. 11 1km, M7.5 14:46,Mar km, M9.0 M9.0 15:25,Mar km, M7.4 15:15,Mar km, M7.3 ( JMA 5th report with modification by JNES) Distribution of aftershocks occurring in about 8 hours after the main shock It is estimated that the epicenter of the earthquake was in the area offshore of Miyagi Prefecture, the depth of the epicenter was 24 km, the earthquake scale was a moment magnitude of 9.0, the source area was 400 km or more and the width of the fault was 200 km. It is estimated that plate rupture commenced from the epicenter in the area offshore of Miyagi Prefecture and propagated to multiple epicenter areas, linking each other. 16

18 III. (1) Characteristics of the Earthquake / Tsunami (Cont.) At this moment, the following appears to represent the characteristics of the earthquake / tsunami: (Cont.) A wide-area earthquake occurred triggered by a rupture in an area offshore of Miyagi Prefecture because a boundary at a shallow plate along the Japan trench had been fixed and accumulated a long-year deformation, despite the traditional idea that no significant deformation should exist there due to gradual slippage. Although this was an earthquake of super-large scale with a moment magnitude of 9 from the viewpoint of long-period seismic motion, it had the same characteristics as an earthquake of a moment magnitude class of 8 from the viewpoint of short-period seismic motion. It is highly possible that the factor which affected the tsunami water level was a combined effect of large slippage (55 m less than 70 m) in a shallow area along the Japan trench and tsunami water level caused by propagation with time delay during destruction linkage of multiple source areas. Conduct re-assessment of earthquake / tsunami 17

19 III. (2) Revision of Regulatory Guide by Nuclear Safety Commission Contents to be added regarding safety assessment against Tsunamis The design standard tsunami shall be formed considering tsunami of the maximum scale based on the generation mechanism of tsunami and examination of the effects of propagation of tsunami, and then, it should be assumed to have significant impact to facilities. Basic concept should be to prevent tsunami from flooding into the site, etc. (socalled Dry Site ). In this case, flooding from intake openings, discharge, etc. where preventing from seawater inundation is difficult shall be limited to the extent that safety function of facilities shall not be affected. Contents to be added regarding seismic motion assessment Consideration of uncertainty (dispersion) such as an source area and/or scale of an earthquake between plates and an earthquake in an ocean plate. Safety of facilities against ground structure deformation and fault displacement caused by an earthquake shall be validated. 18

20 III. (3) Initiative of NISA (3)-1 Consideration of Guidelines for Safety against Tsunamis Investigate more specific guideline on assessment of tsunami height, etc. Estimate tsunami height, considering; Appropriate occurrence frequency Overlapping of waves with different frequency Tsunami sediments Consider Destructive wave force, water immersion height and water immersion range, etc. for design of tide embankment, watertight doors, etc. From the viewpoint of defense-in-depth, recognizing the existence of risk due to tsunami hitting beyond the design base of facilities, countermeasures should be taken to prevent serious damage to core and fuel in fuel pool. 19

21 III. (3) Initiative of NISA (3)-2 Investigation on Linkage of Active Faults Detailed investigation was conducted on situation of tectonics, stress, etc. regarding faults of which possibility of linkage had been denied. Tomari Power Station: Assume linkage with an active fault in sea area in front of site (approximately 160 km). Kashiwazaki-Kariwa NPS: Assume linkage with Nagaoka Plains West Edge Fault Belt (Approximately 130 km). Kakuda Yahiko Fault 91km Tomari PS Kashiwaza ki-kariwa Nuclear Power Station Kehinomiya Fault Katagai Fault 132km Tookamachi Fault Belt western part 41km In addition, uncertainty ( stress drop 1.5 times etc.) to be assumed. 20

22 III. (3) Initiative of NISA (3) -3 Consideration of Uncertainty in Seismic Motion Assessment Sufficient geographic, geological, geophysical investigation to establish basic source model. Uncertainties still remain. 3 categories of uncertainties - Epistemological uncertainty due to lack of data or knowledge. - Aleatory uncertainty due to natural phenomenon. - Uncertainty associated with modeling. It is necessary to fully investigate what type of uncertainty exists regarding each parameter to establish a source model such as fault length, fault dip and stress drop. Reflecting experts opinions. 21

23 IV. Approaches and Implementation of Safety Countermeasures related to Earthquakes / Tsunamis Immediate safety measures Stress test Investigation of countermeasures against earthquakes / tsunami based on this accident. Countermeasures against severe accidents (external events) Back-fitting system 22

24 IV. (1) Outline of Immediate Safety Measures Goals (Desired Level/Extent) Preventing fuel damage and spent fuel damage even if AC power supplies, seawater cooling functions and spent-fuel storage pool cooling functions are all lost. Examples of Specific Measures (Short Term Measures) Securing Equipment Deploying power generator vehicles Deploying fire engines Deploying fire hoses Preparing procedural manuals for emergency responses utilizing the above-mentioned equipment Implementing training for emergency responses based on the procedural manuals 23

25 IV. (2) Stress Test in Japan Objectives In order to improve the safety of nuclear power plants and to secure the assurance and trust of the general public and local residents in particular, NISA implements safety assessments in reference to the stress tests implemented in European countries. Summary Primary assessment Evaluate safety margins of safety systems, structures and components to endure the events beyond design basis, for nuclear power plants under periodic inspection and ready for start-up. Secondary assessment Conduct comprehensive safety assessment to identify potential weak points for all nuclear power plants, as done in European Stress Test. 24

26 IV. (3) Consideration of Countermeasures against Earthquakes / Tsunamis based on This Accident Examples of countermeasures against Earthquakes / Tsunamis to be taken in the future in line with occurrence and sequence of the accident. Countermeasures concerning offsite power From the viewpoint to lower risk of severe accidents; Enhance seismic and against-tsunami reliability of offsite power systems and substation facilities Outer power grid : tower, foundation Offsite power station: switch gear, transformer, their base Switchyard: switch gear, transformer, their base, tie-line between units Power facilities and cooling facilities inside power station For common cause failure; Dispersed facility arrangement ; Different location (elevation) In different building For resistance against flooding; Water tightness of buildings and electric power panels Water resistant power line For emergency cooling; Portable alternative residual heat removal system which is not affected by earthquakes or tsunamis. 25

27 IV. (4) Countermeasures against Severe Accidents (External Hazards) Basic idea of response to external hazards Natural hazard Aircraft crash Terrorism All SSCs with safety function Tolerance (to external events) Durability (actual) Intensity Early detection Shutdown power down Maintenance of safety function (1) Margin (2) Frequency of accidents ( 10-7 )* (>10-7 )* (Notconsidered) Protection, Isolation Intentional Detection Security order Detection Delay Management Specific safety equipment Isolation, Dispersion (from external events) Special Safety Facility (Temporary name) (Commonalities?) Seismic isolation, (3) Dispersion Watertight structure, Diversification (Water-cooled + Air cooled) Robustness Dispersion Safety function Recovery, Alternatives (4) Recovery Alternative equipments (mobile) (* unit: reactor year) 26

28 IV. (5) Back-Fit System There is now no legal system in Japan that requires back-fit for items related to basic designs such as design ground motion. Back fit system is to be introduced in line with the legal system revisions based on the Fukushima Daiichi nuclear accident; Requiring as legal obligation Implementing by setting deadline Important to promptly apply new scientific knowledge to safety measures. 27

29 V. Conclusion A lot of factors that affected Fukushima Daiichi nuclear power station due to the earthquake and tsunami have been identified. However, it is not yet sufficient because of the difficulties in accessing the plant site, and continuous examination should be pursued. Effort will be made; - To accumulate scientific knowledge of earthquakes and tsunamis - To incorporate this knowledge into the safety measures for nuclear power stations. - To share results investigated in Japan to the international community through IAEA ISSC program, etc. - To contribute to incorporating this knowledge to IAEA safety guides, etc. 28

30 V. Conclusion (Cont.) Recognizing the existence of uncertainties, implementation of safety measures to compensate for uncertainties are required. It is important to take a comprehensive approach for the improvement of nuclear power safety as a total system. - Hazard evaluations of earthquakes and tsunamis - Safety measures such as accident management - Emergency preparedness and disaster mitigation - Risk communication with residents, etc. It is important to address efforts through cooperation among the specialists of different fields and by sharing knowledge and information. We hope discussion in this IEM 3 will improve the knowledge summarized at 1 st Kashiwazaki International symposium on Seismic Safety in

31 Thank you for your attention 30