Fukushima Daiichi NPP Accident

Transcription

1 Fukushima Daiichi NPP Accident Plant Design and Preliminary Observations K. Moriya and K. Sato Hitachi GE Nuclear Energy, Ltd. May 3, 2011

2 [Note] 1. The view expressed herein are not the official view by Hitachi GE Nuclear Energy, Ltd. 2. The information contained here is preliminary and needs to be confirmed by the Accident Investigation Commission to be set up later by the Government.

3 Contents 1. Design of Fukushima NPP 2. Earthquake and Tsunami 3. Event Progression 4. Possible Design Enhancements 5. Summary

4 1. Design of Fukushima NPP

5 Major Design Parameter of Fukushima1 1~4 Unit 1 Unit 2 Unit 3 Unit 4 Commercial Operation Reactor Design BWR 3 BWR 4 BWR 4 BWR 4 Rated Power (MWe) Thermal Power (MWt) 1,380 2,381 2,381 2,381 Isolation Cooling system IC RCIC RCIC RCIC ECCS Configuration HPCI (1) ADS CS (4) HPCI (1) HPCI (1) HPCI (1) ADS ADS ADS CS (2) CS (2) CS (2) LPCI (2) LPCI (2) LPCI (2) Primary Containment Vessel Mark I Mark I Mark I Mark I Operation Status at the earthquake occurred In Service Shutdown In Service Shutdown In Service Shutdown ECCS: Emergency Core Cooling System, HPCI: High Pressure Core Injection System, ADS: Automatic Depressurization System, CS: Core Spray System, LPCI: Low Pressure Core Injection System IC: Isolation Condenser, RCIC: Reactor Core Isolation Cooling System Outage 5

6 Evolution of BWR Design Standardized BWR (BWR 3/4/5) Improvement Safety Fukushima 1 1~6 16 Improvement Economy BWR 1 BWR 2 Jet Pump Dual cycle (with separator) External Recirculation pump With ihbuilt in i separator Reactor coolant Recirculation pump 6

7 Mark I Containment & Surrounding Structures Operating Floor Primary Containment Vessel ((PCV)) Drywell Spent Fuel Pool Reactor Pressure Vessel (RPV) Reactor Building Suppression Chamber (Wetwell) 7

8 Mark I Containment: design and improvement Technical Issues Concept/Countermeasures Basic Design Achieve compact design by having pressure suppression chamber Pool swell (short term), Condensation Oscillation i (mid term), Chugging Comprehensive experimental and analytical lstudies were conducted, d then individual plant evaluation Hydrodynamic y (long term) were was conducted based on these identified d as possible studies. Load during hydrodynamic loads to Actual implementation was LOCA containment after LOCA different from plant by plant SRV quencher issue (implementation of modifications, (condensation during or evaluation of design) transient) Integrity against Severe Accident Loss of integrity could be foreseen for possible beyond DBA Hardened Containment Venting capability was added per Generic Letter by USNRC in USA Same countermeasures were implemented in Japan 8

9 Important Systems coping with SBO Number of EDG DC Battery Capacity Non AC dependent Systems Unit 1 Unit 2, 3 Remarks hrs 8 hrs IC, HPCI RCIC, HPCI 1 DG was added to Unit 2, 4, and 6 in 1990 s as part of SAMG implementation Based on SBO coping evaluation (using different system, U1: IC, U2/3: RCIC/HPCI) Compliant with NSC s regulatory requirement for short term SBO (Guide 27: see below) Only DC battery power needed to operate Containment HVS HVS In1990s 1990s, Hardened Venting Systems were Venting installed installed installed in each units NSC Safety Design Guide 27: Design considerations against loss of power shall be designed that safe shutdown and proper cooling of the reactor after shut down can be ensured in case of a short term total AC power loss. Commentary to Guide 27: No particular considerations are necessary against a long term total AC power loss because of repair of transmission line or emergency ypower system can be expected in such a case. HPCI: High Pressure Core Injection System, IC: Isolation Condenser RCIC: Reactor Core Isolation Cooling System, HVS: Hardened Venting System 9

10 Design Feature of Isolation Condenser (Unit 1) Item Purpose Function System configuration Capacity Description Cooldown RPV water during isolation event (MSIV closure) without losing RPV water inventory After reactor isolation, sends reactor steam into condenser After heat exchange at condenser, returns condensed water into reactor Two division systems with Condensers, isolation valves, piping [see next slide] NOFO isolation valves in steam lines NCFC isolation valve in return lines All valves can be operated by DC power Two ICs can remove heat load of 6% of rated power IC Pool capacity is good enough for 10 hour operation without AC power for makeup water into IC tank 10

11 Schematic of Isolation Condenser (Unit 1) Atmosphere Atmosphere Div. A Div. B 2B DC125V VALVE 2A DC125V VALVE IC Tanks 1B 1A 10A 10B AO 1A AO 2A MSIV MSIV To Condenser From FP From MUWC ~ PLR DC125V VALVE 3B 4B DC125V VALVE 3A 4A 11

12 Design Features of RCIC (Unit 2, 3) Item Purpose Function System configuration Capacity Description Injectwater into RPV during isolation event (MSIV closure) After reactor isolation, sends reactor steam to RCIC Turbine, which injects water into RPV Condensed C d d reactor steam (drained d water from RCIC Turbine) is discharged to Suppression Pool Turbine,, Pump, pp piping, valves, water sources (CST and S/P) [see next slide] Auto start at RPV low water level (L2), then stop at RPV high water level (L8) Primary water source is CST then switch to S/P when reaching high water level (+5 cm NWL) in S/P All valves and governor in turbine pump can be operated by DC power RCIC can operate without AC power using DC power only for 8 hours depending on load reduction in an SBO 12

13 Schematic of RCIC* Main Steam Line Turbine Stop Valve Turbine Trip Valve Feed Water Injection Valve AO RCIC Pump HO Steam Regulator Valve RPV Steam Turbine Drain Trap S/P CST S/P Suction Valve CST Suction Valve *HPCI (High Pressure Core Injection System) was also available as AC independent system 13

14 Overview of Severe Accident Countermeasures Severe Accidents Countermeasures Anticipated Transient Without Scram Alternative Reactivity Control System (ATWS) (Alternative Rod Insertion System and Recirculation Pump Trip System) SA events with loss of core cooling and/or loss of molten core cooling Alternative Water Injection System (Utilizing Fire Protection System etc.) Hd Hydrogen Generation Inerting Pi Primary Containment Vessel by nitrogen Loss ofdecay heat removal Station Blackout Hardened Containment Venting System Alternative Electric Supply System (Inter connections with ihadjacent unit) In Japan, all units installed SA Countermeasures in late 1990s 14

15 SA Countermeasures in Fukushima Alternative Water Injection System did provide water into either RPV (or PCV) by using existing systems (RHR/LPCI, MUWC, FP) from several water sources Hardened Containment Venting System did remove decay heat from containment either from Wetwell or Drywell Eiti Existing System Added System Makeup Water System SGTS Hardened Containment Venting System RHR Fire Protection System Connection from Other sources

16 2. Earthquakeandand Tsunami

17 Capability of each units for each events Event Category Unit 1 Unit 2 Unit 3 Unit 4 Design Basis 180 Gal 180 Gal 180 Gal 180 Gal Seismic Max. Beyond DB Capability 489 Gal 441 Gal 449 Gal 447 Gal Max. Actual Seismic 460 Gal 550 Gal 507 Gal 319 Gal Initial Design Basis 3.1 m 3.1 m 3.1m 3.1 m Tsunami Revised Design Basis 5.7 m 5.7 m 5.7 m 5.7 m At Actual ltsunami 14 m 14 m 14 m 14 m Design Basis N/A N/A N/A N/A SBO Beyond DB Capability 10 hrs 8 hrs 8 hrs 8 hrs Actual SBO Days Days Days Days 17

18 Damages by earthquake and Tsunami Incident Major Damages Remarks Earthquake Tsunami Collapsed pylons leading to Loss of Off site Power Washed outmanystructures along bay sea water pumps, intake structures and several tanks leading to loss of all ECCS Water penetrated Turbine Buildings leading to damaging DGsandSwitchgears (SBO) Moved many debris and rubbles onto site road leading to unavailability of access to units Damaged local roads made it difficult to approach Fukushima site from outside Automatic shut down and hot stand by achieved ed in all units, even having SBO after Tsunami hit the site Transition from hot stand by to cold shut down is not yet to be achieved due to unavailability of many important equipments after Tsunami hit the site, besides unavailability of transportation to the site Tsunami caused huge damages, while Earthquake caused LOPA 18

19 SBO comparison of assumptions and reality AC Power Recovery DC Power Operating Actions Control Room Presumed SBO AC Power can be restored by either offsite power (OSP) or DG restoration within about few hours No extended loss of DC was assumed SAMG actions was assumed Habitability should be maintained even in SBO Many plant parameters should have been available to monitor Fukushima Accidents OSP restoration was not available due to damages (believed to be mainly by Tsunami even local train station was washed out by Tsunami) DGs could not be restored because of heavy damages mainly by submergence of Tsunami water Tsunami washed away entire sea water cooling capability for good DC power management was necessary to control plant for days Damage (believed to be mainly by Tsunami) made it very difficult to access systems and components, which delayed actions Unable to control in extended SBO condition from Main Control Room Only a few plant parameters available to monitor Tsunami exceeded design assumption led to a severe SBO beyond control 19

20 3. Event Progression

21 Summary of Current Status of Unit 1 Estimation of RPV Water Level ΔT of RPV (structures: 125 ºC) and steam (FW nozzle: 200ºC) can be used to estimate uncovered portion of core 温度測定定値 Uncovered Portion: ~10 % feedwater nozzle N4B-end vessel bottom head 露出割合の評価 露出割合 % Inside Containment DW is in saturation condition DW Pressure: 190kPa Steam Press: 150kPa (Pre N2 inj) Sat. Temp: 110 ºC HVH: ºC 125 C PLR: ºC Water level in DW is about the bottom of RPV as of April 26 3 月 18 日 3 月 20 日 3 月 22 日 3 月 24 日 3 月 26 日 3 月 28 日 3 月 30 日 4 月 1 日 4 月 3 日 4 月 5 日 4 月 7 日 4 月 9 日 4 月 11 日 4 月 13 日 4 月 15 日 21

22 Summary of Current Status of Unit 2 Estimation of RPV Condition RPV Water Level Almost table since 3/26 although changing water injection rate (15 >13 > 7 t/hr) 2/3 of core is submerged but configuration is not unknown RPV Integrity There are signs of possible RPV leakage Inside Containment Water Level In between lower DW level switch and equator of DW bulb portion Possibly full in S/P PCV Integrity Overall Structural Integrity is maintained Leakage from S/P to R/B is possible 22

23 Summary of Current Status of Unit 3 Estimation of RPV Condition Inside Containment RPV Water Level Almost table since 3/23 although changing water injection rate (20 >7 t/h) 2/3 of core is submerged Water Level In between lower DW level switch and equator of DW bulb portion Possibly full in S/P but configuration is not unknown PCV Integrity RPV Integrity Overall Structural Leakage possible Integrity is maintained There is no clear sign of Leakage to R/B is RPV bottom failure possible 23

24 Unit 4 Situation Spent Fuel Pool has about 2 MW heat load but water level is maintained (filled up on 4/27) to keep the pool cooled Pool water samples were taken and it has been confirmed that there should be no severe fuel damage Rather, Fukushima Background was measured (short lived Fission Product, which should have never been existing in Unit 4 Spent Fuel after four months of shut down) Source: TBS (provided by TEPCO) 24

25 4. Possible Design Enhancements

26 Possible Design Improvement Category Observations Design Improvements SA Prevention (Core cooling function) Safety related SSCs (Systems, Structures, and Components) were submerged dby extreme Tsunami Redefine Tsunami water level depending on each site conditions If necessary, general arrangements and water tightness ih could be improved SA Mitigation Capability p yduring extended Accessibility y from outside of the site SBO was deemed to be Back up electrical systems improved SFP Cooling Difficult to access to SFP when beyond design base Tsunami hits the plant Simplified water injection systems independent of existing systems 26

27 5. Summary

28 Summary Design of Fukushima Daiichi NPPs GE BWR 3/4with Mark I Containment, which was designed in 1960s Incorporated Severe Accident Countermeasures in 1990s Less damage by earthquake and much more damages by Tsunami Principal consequence of Earthquake, which was about the same as/slightly larger than newly defined Ss in Japan, appears to have been Loss of Off site Power Relative magnitude of Tsunami versus current design basis was greater than earthquake. This caused extended SBO, and other unrecoverable, devastative damages for both preventive actions and recovery actions from severe accident 28

29 Summary (continued) Current Damage Status In U1 U3 U3, core was severely damaged, but RPV believed not to be breached Containment integrity seems to be maintained except for U2, and this can be assessed from long term leakage Possible Design Enhancement Preventive Measures: General arrangement, water proof doors Mitigatory Measures: Capability to cope with extended SBO SFP Cooling System: Simple water injection i system independent d to current systems 29