Deconstructing the Nuclear Accident at the Fukushima-Daiichi Plant: What Went Wrong and What are the Prospects of Recovery?

Transcription

1 Deconstructing the Nuclear Accident at the Fukushima-Daiichi Plant: What Went Wrong and What are the Prospects of Recovery? Goldschmidt Conference 2011 Fukushima Review Session Prague, Czech Republic Edward Blandford, PhD Stanford University August 16 th, 2011

2 Aerial view of Fukushima-Daiichi NPP - before & after Before the Earthquake and Tsunami After the Earthquake and Tsunami Unit 5 Unit 6 Unit 1 Unit 4 Unit 3 Unit 2 Photo Source: TEPCO Photo Source: Air Photo Service Inc (Myoko, Niigata Japan)

3 Earthquake and aftershocks Ground motion of Fukushima NPP Earthquake time: 2:46 pm March 11, 2011 Location: Offshore Sanriku coast, 24 km in depth, M9.0 Earthquakes: M quake (March 11) M - 7 class 5 times M - 6 class 72 times M - 5 class 423 times Unit MWe Observed (max. gal) Design Basis (max. gal) N-S E-W Vert. N-S E-W Vert TOKYO tokyo.ac.jp/eqvolc/201103_to hoku/eng/#mesonet Earthquake Research Institute, University of Tokyo, Prof. Takashi Furumura and Project Researcher Takuto Maeda F-D NOTE: Scram set points by acceleration Horizontal= gal, Vertical=100 gal Seismometers were located in the basement of reactor building

4 14 NPPs along eastern Honshu coast affected by Tsunami Time sequence of Units 1-3 immediately following earthquake: Earthquake followed by Reactor SCRAM, LOOP, EDGs start, IC/RCIC in operation Tsunami followed by complete (AC/DC) blackout and (mostly) isolation from the Ultimate Heat Sink Earthquake Research Institute, University of Tokyo, Prof. Takashi Furumura and Project Researcher Takuto Maeda Source: A. Omoto, Fukushima Accident : An overview, ICAPP 2011, 3 May 2011;

5 BWR/3&4 with Mark 1 Containment BWR/3 (460 MWe, Unit 1) Mark I Containment (Drywell + Torus type Suppression Pool) Spent Fuel Pool on top floor of reactor building Isolation condenser for passive core cooling and HPCI (High Pressure Core Injection) (@Hi Pressure) Core Spray system (@Lo Pressure) after depressurization by SRV BWR/4 (784MWe, Units 2,3,4, and 5) Mark I Containment (Drywell + Torus type Suppression Pool) Spent Fuel Pool on top floor of reactor building RCIC (Reactor Core Isolation Cooling) & HPCI (High Pressure Core Injection) (@Hi Pressure) CS (Core Spray) & RHR/LPCI (@Lo Pressure) after depressurization by SRV Location of hydrogen explosions in Units 1 & 3 and location of fires in Unit 4 NOTE: Unit 6 is a BWR-5 with a Mark II containment

6 Initial impact on plant reactor safety systems Shutdown was secured by automatic shutdown of all control rods Transmission line was damaged by the earthquake Emergency diesel generators started but subsequently were lost due to Tsunami Station blackout Most of the cooling function of reactor and spent fuel pool were lost by the loss of power supply caused by Tsunami Source: TEPCO presentation, The Great East Japan Earthquake and Current Status of Nuclear Power Stations, April 25, 2011

7 Sequence of Events: Unit 1 Unit 1 Sequence of Events March 11 14:47 14:52 Earthquake, loss of offsite ac power, and plant trip Isolation condenser operated to cool reactor 15:03 Isolation condenser stopped operating 15:37 Tsunami and total loss of ac power SBO 15:37 Loss of ability to inject water to the reactor ~17:00 Water level below top of fuel --:-- Partial core damage (several hours after tsunami) March 12 14:30 Vent primary containment 15:36 Explosion results in severe damage to the reactor building (secondary containment) Source: Recommendations for Enhancing Reactor Safety in the 21 st Century: THE NEAR-TERM TASK FORCE REVIEW OF INSIGHTS FROM THE FUKUSHIMA DAI-ICHI ACCIDENT, US NRC

8 Sequence of Events: Unit 2 Unit 2 Sequence of Events March 11 14:47 ~14:50 Earthquake, loss of offsite ac power, and plant trip RCIC manually operated to inject water to reactor 15:37 Tsunami and total loss of ac power SBO March 13 --:-- RCIC continued to be used to cool reactor ~11:00 Vent primary containment March 14 13:25 RCIC stopped operating ~18:00 Water level below top of fuel --:-- --:-- March 15 Partial core damage (approximately 3 days after tsunami) Blowout panel open on side of reactor building ~06:00 Explosion; suppression chamber pressure decreased indicating the possibility that primary containment was damaged Source: Recommendations for Enhancing Reactor Safety in the 21 st Century: THE NEAR-TERM TASK FORCE REVIEW OF INSIGHTS FROM THE FUKUSHIMA DAI-ICHI ACCIDENT, US NRC

9 Sequence of Events: Unit 3 Unit 3 Sequence of Events March 11 14:47 15:05 15:41 March 12 11:36 Earthquake, loss of offsite ac power, and plant trip RCIC manually started to inject water into reactor Tsunami and total loss of ac power at site SBO RCIC stopped operating 12:35 March 13 HPCI automatically started injecting water into reactor 02:42 HPCI stopped operating ~08:00 Water level below top of fuel --:-- March 14 Partial core damage (approximately 2 days after tsunami) 05:20 Vent primary containment 11:01 Explosion results in severe damage to the reactor building (secondary containment) Source: Recommendations for Enhancing Reactor Safety in the 21 st Century: THE NEAR-TERM TASK FORCE REVIEW OF INSIGHTS FROM THE FUKUSHIMA DAI-ICHI ACCIDENT, US NRC

10 Sequence of Events: Unit 4, 5, and 6 Unit 4 Sequence of Events March 11 14:46 Earthquake, loss of offsite ac power, 15:38 Tsunami and total loss of ac power at site SBO March 15 ~06:00 Explosion in reactor building Unit 5 and 6 Sequence of Events (both units were shut down for periodic inspection) March 11 14:46 15:41 March 20 Earthquake and loss of offsite ac power Tsunami and total loss of ac power at site SBO 14:30 Unit 5 enters cold shutdown 19:27 Unit 6 enters cold shutdown Source: Recommendations for Enhancing Reactor Safety in the 21 st Century: THE NEAR-TERM TASK FORCE REVIEW OF INSIGHTS FROM THE FUKUSHIMA DAI-ICHI ACCIDENT, US NRC

11 Radiation release chronology Fukushima Daiichi Source: OECD Nuclear Energy Agency

12 Source of hydrogen Zirconium cladding reaction with steam to produce hydrogen becomes substantial at temperatures above 1000 C Volatile fission products released as noble gases (e.g. Kr) or aerosols (e.g. I, Cs) Fuel pellets melt at 2600 C Flammability limit of hydrogen gas Hydrogen Explosions are Chemical Explosions not Nuclear Explosions Source: 1) TEPCO presentation, The Great East Japan Earthquake and Current Status of Nuclear Power Stations, April 25, 2011, 2) MIT NSE webpage

13 Why hydrogen explosion right after venting? Possible Path 1 : Excessive leakage by over pressure at CV flange/airlocks Possible Path 2: Vent line Standby Gas Treatment System Reactor Building Source: A. Omoto, Fukushima Accident : An overview, ICAPP 2011, 3 May 2011;

14 What were the major factors contributing to fuel damage? 1) Elevation vs. Tsunami height Site ground level saved Onagawa units Elevation of air intake/exhaust of emergency diesel generator Physical location of emergency diesel generator /emergency equipment room/battery equipment 2) Availability of power Offsite power saved Fukushima-Daini site Air-cooled emergency diesel generators coupled with location/height saved Unit 6 Important to note that air-cooled emergency diesel generators were added for Units 2,4, and 6 in the 1990 s as a part of SAM modifications Units 3,5, and 6 at Fukushima-Daini were located at a higher elevation and escaped flooding 3) Implementation of accident management guidlelines by using thenavailable resources Saved Unit 5 spent fuel pools (makeup water) Source: A. Omoto, Fukushima Accident : An overview, ICAPP 2011, 3 May 2011;

15 TEPCO released a roadmap towards restoration from the accident on April 17 th Established two steps as key targets Radiation dose is in steady decline (~3 months) Release of radioactive materials is under control and radiation dose is being significantly held down (~3-6 months) Roadmap established a set of immediate actions which are divided into three categories: 1. Cooling 2. Mitigation 3. Monitoring and Decontamination These three actions were required for five different issues: 1. Cooling the reactors 2. Cooling the spent fuel pools 3. Containment, storage, processing, and reuse of accumulated water 4. Mitigation of release of radioactive materials to atmosphere and from soil 5. Measurement, reduction, and announcement of radiation dose in evacuation order/planned evacuation/emergency evacuation preparation areas Source: TEPCO Roadmap Towards Restoration from the Accident at Fukushima Daiichi Nuclear Power Station, April 17 th, 2011

16 Current status according to TEPCO as of 7/17 Source: TEPCO Roadmap Towards Restoration from the Accident at Fukushima Daiichi Nuclear Power Station Revised Version, Append 1-3,July 19, 2011

17 Current status according to TEPCO as of 7/17 (cont.) Source: TEPCO Roadmap Towards Restoration from the Accident at Fukushima Daiichi Nuclear Power Station Revised Version, Append 1-3, July 19, 2011

18 There is a precedent for contaminated water treatment support from US Department of Energy Questions surrounding storage, treatment, and disposal questions were answered Sludge and contaminated water Treatment flowsheets Immobilization methods used Three Mile Island experience was provided Low activity treatment with EPICOR II High activity treatment with Submerged Demineralizer System Disposition Described new materials» Filters» Ion exchange resins & zeolites» Reverse Osmosis experience» Iodine removal (not used at TMI)

19 Some key questions about the water cleanup approach Water volumes and chemistry Complete isotopic distribution (gamma only) ph Salt Boric acid Oil composition or concentration Treatment Plan Required decontamination factor Waste Disposal Plan Disposal waste form or criteria Potential safeguards issue as Japan is a not a NWS Japan has provided a high-level plan to treat contaminated water at Fukushima Daiichi AREVA press release states they are clean up contractor

20 Approximately 20 million gallons of contaminated water in turbine buildings A state of stable cooling has finally been reached TEPCO has announced that it has treated approximately 6,190 tons of radioactive water Approximately 120,000 tons remain Source: TEPCO Roadmap Towards Restoration from the Accident at Fukushima Daiichi Nuclear Power Station Revised Version, July 19, 2011

21 Installation of Reactor Building Cover Source: TEPCO, Outline of the reactor building covering plan of Unit 1 at Fukushima Daiichi Nuclear Power Station,

22 The cleanup and investigation of Fukushima is an ongoing effort Hydrogen Explosions are Chemical Explosions not Nuclear Explosions Source: TEPCO website

23 Some key lessons learned from the accident Importance of a defense in depth philosophy where resources must be allocated to measures that improve system protection, mitigation, and emergency response Accident made global licensees and regulators re-evaluate whether their facilities have adequate protection from natural phenomena within the design basis Revisit the question of what is an acceptable coping time for station blackouts Importance of having an independent regulatory body and clear line of command Clear communication to the public about radiation levels is essential but challenging There were also some very positive lessons learned from the accident: - High performance of scrubbers in the wetwell preventing aerosol fission product release - Effective evacuation and food monitoring strategies greatly reduced impact to public health

24 Final Observations Severe natural events are possible, even those that are considered one-in-a-thousand years events The nuclear accident at Fukushima is an important contributor to the overall costs of the disaster» Likely greater than $100 billion from a total cost of ~$300 billion» But public health impact from accident is much smaller The most important lesson for managing beyond-design-basis events in existing infrastructure is to have planned ahead Modern reactors (Gen III or Gen III+) are more robust and can be operated more safely than the ones that have caused major accidents It is not clear at present how many of the safest designs will be built Mechanisms to facilitate and, where needed, enforce mutual learning among countries is not as effective today as it is within countries and may not be adequate to prevent avoidable disasters