Responses taken to fight the nuclear accident and to mitigate the consequences Hisashi NINOKATA Professor, Politecnico di Milano Department of Energy CeSNEF-Nuclear Engineering Division Nuclear Reactors Group Professor Emeritus, Tokyo Institute of Technology Focus on: The nuclear reactor core of units 1 3 of Fukushima Daiichi NPS (1F1, 1F2, 1F3) The events in a first few days
Looking back at that time, Very little information available in a first few weeks with only circumferential evidences --- ex. Radiation level in RB Heat balance calculations gave the idea ------------------------------------- With information disclosure under control, we (university professors, in particular) were given a role to explain what were going on at the 1F NPS, and at other NPS sites in Japan by all news media (TV, newspapers, radios, ) in particular on request by the public at symposiums, workshops, science cafes, lectures, conferences, inside and outside Japan Looking back at that time,
No damage from the earthquake The most likely assumption: the magnitude 9 earthquake did not damage reactor structure, pipes and cooling systems, and the important safety functions of all Fukushima Daiichi units of concern were all maintained In fact, IAEA walkdown examination of the Onagawa NPS, August 10, 2012, much closer to the epi-center than Fukushima, has revealed the nuclear power plant Remarkably intact Onagawa much closer to the epicenter 130 km offshore from the NPS
Onagawa Plant Onagawa vs Fukushima Ref. Isao Kato, Tohoku EPC, NUTHOS-9, Sept 12, 2012, Kaohsiung, Taiwan
Why at Fukushima Daiichi? Onagawa Unit-1 PCV is Mark-I, Unit-2 to 3 are of improved Mark-I and Fuk-2 four units are of Mark-II The newer design, the better prepared for tsunami with new knowledge on the tsunami history Fuk-1 units consist of BWR-3 and BWR-4 with Mark-I PCV Why at Fukushima Daiichi? In particular the Unit-1 of Fuk-I (1F1) was constructed based on the imported technology. After having digested the imported technology, at least that was the way we thought, Japan has spent more efforts in improvement and new development. Vulnerability of Fuk-I was pointed out against tsunami but has been put aside, given its First of a Kind nature in Tepco, given that it was constructed almost 40 years ago before many updates in regulations, given that constructing new defense was extremely expensive. The lessons would be useful for new nuclear countries who import foreign technology for a starter.
On reflection Before the Fukushima Daiichi accident, we trusted the improved NPP safety performance resulted in the health and safety of the public, given any of the anticipated accident scenarios In Japan, more so with more strict operation and maintenance requirements, with much more rigid and stronger anti-seismic structure and design/construction After the Fukushima Daiichi, the need to prepare for the unexpected and the unforeseen, including the beyond design basis, has become a most important issue, but too belatedly On reflection On reflection, it is evident that TEPCO and the broader nuclear industries were not prepared for unexpected and highconsequence situations to respond to maintain critical safety functions; or to implement effective emergency response procedures and accident management strategies under the extremely severe conditions encountered at Fukushima Daiichi. (INPO 11-005)
How severe was it? Beyond my description The following 7 slides are due to the courtesy of Dr. Sakae Muto of TEPCO Difficult to Access On-site Testimony As the tremendous aftershocks occurred, with our full face masks still on, we frantically headed off to the upper ground. While laying down cables at night, we were terrified that we might be electrocuted due to the outside water puddles. Wide crevices around there Scattered debris & Fire hoses Water injection by Fire Engine Tank adrift on the road 14
Response in Dark Control Room Shift Supervisor s Testimony When the power source failed, I felt completely helpless. Heated discussions broke out among the operators regarding whether it was important to remain in the control room or not. Connected portable Batteries 15 Difficulty of Venting Containment(1) Shift I Supervisor asked for volunteers Testimony: to As manually the work open required the high vent radiation valves. exposure, Young operators I decided to raised not involve their hands the young as well; workers. I was overwhelmed. Step1: SUCCESS MO 210 manually open valve Solenoid valve IA 電磁弁 AO AO 90 閉 IA AO AO ボMO 72 213 ンベ電磁弁 Cylinder Shift workers operation to Solenoid valve Ruptured ラプチャーディスク disc Broke at 0.549MPabs AO 83 AO 1 Closed Closed 0.549MPabs で破壊 閉 Closed 閉 Closed Cylinder ボンベ閉 Exhaust stack 排気筒 D/W maximum operating pressure: 0.528MPaabs D/W 最高使用圧力 0.528MPabs RPV D/W Venting ベント実施圧力 pressure: 0.954MPabs 0.954MPaabs Entry with self air set 16
Difficulty of Venting Containment(2) Onsite Testimony When I climbed on top of the torus to reach for the high positioned valve, the soles of my boots quickly melted away. Couldn't approach, High radiation High temperature S/C vent valve (AO valve) R/B B1F Step2; Couldn t approach (Hi Rad, Hi Temp.) Step2 ; Use mobile compressor to remote open. 17 Voices from Operators In an attempt to check the status of Unit 4 D/G, I was trapped inside the security gate compartment. Soon the tsunami came and I was a few minutes before drowning, when my colleague smash opened the window and saved my life. In total darkness, I could hear the unearthly sound of SRV dumping steam into the torus. I stepped on the torus to open the S/C spray valve, and my rubber boot melted. The radiation level in the main control room was increasing 0.01 msv (1 mrem) in every 3 seconds but I couldn t leave I felt this was the end of my life. I asked for volunteers to manually open the vent valves. Young operators raised their hands as well; I was overwhelmed. Unit 3 could explode anytime soon, but it was my turn to go to the main control room. I called my dad and asked him to take good care of my wife and kids should I die. 18
Voices from Maintenance Staff We saw our car crashed by the explosion of the Unit 3. If we had gotten on the car a few minutes earlier, all of us would have been dead. We were replacing fire hoses when the explosion of Unit 3 occurred. We felt almost dying since many large rubbles were falling down to us.i urgently ran underneath a nearby fire engine. One of my colleagues got injuries in his leg and stomach. There were so many manholes opened by the tsunami. In order to lay cables, we had to proceed step by step carefully checking safety in the complete darkness. We were working in the Unit 3/4 control room when the explosion occurred. I was resigned to my fate. Dose rate was going up in the room after the explosion and we desperately tried to find places with lower dose rate. After replacing an air cylinder for the PCV ventilation of Unit 3, I heard sound of steam and saw white mist around us. I got into a panic for a while. 19 Sequence of Events after the Earthquake The Great East Japan Earthquake around 14:46, Mar. 11 th Reactor SCRAM due to the Earthquake (Automatic Emergency Shutdown) Loss of Off-site Power, PCV Isolation, D/G Started-up *1 Operation after SCRAM as Intended Operation after SCRAM as Intended Tsunami struck Fukushima-Daiichi & Fukushima-Daini NPPs around 15:20~, Mar. 11 th Fukushima Daiichi Units 1~3 Units 5,6 [Power] SBO w/o EDG LUHS [Seawater system] Not available [Power] D/G 6B start-up [Seawater system] Not available Fukushima Daini [Power] Off-site Power available [Seawater system] Not available *2 Unit 4 H 2 O injection & heat transport to S/C via HP system, e.g., RCIC/HPCI; DHR by IC Power supply from Unit 6 to Unit 5 Water injection via HP (Steamdriven) & LP systems Water injection via LP system (Alternative) (Freshwater & Seawater) Water injection via LP system Interrupted Injection and LUHS: no route secured for heat removal Heat removal secured by temporary power source & seawater pump Heat removal secured by temporary power source & motor replacement etc. Cold Shutdown Condition (Dec. 16) Cold Shutdown (Mar. 20) Cold Shutdown (Mar. 15)*3 Fukushima Daiich 1~4 Fukushima Daiichi 5,6 Fukushima Daini 1~4 *1 D/G:Emergency Diesel Generator *2 RHR Seawater System *3 Fukushima-Daini Emergency State was Lifted on Dec.26 tth 20
3/11 3/12 3/13 3/14 3/15 1F1 1F2 1F3 Earthquake/ tsunami IC on (A & B) 1503off man lly 1830 Open close 17~1800 TAF 2130 No access to IC Core melt (before mdngt) 1502 RCIC on w/o DC power RCIC valves not compl closed? Car batteries for instrumentation and to open SRVs 1506 RCIC on AM PM AM PM AM PM AM/PM Rad level high in RBs 3am RPV failure? 5am PCV failure? Prep vent RPV p falls down due to possible RPV failure high peak p pulse: by MFCI? Not likely but still under debate 21 1430 Vent successfuil? 1536 H2 expl. (20 hours) (14 hours) (>70 hours) 11am try PCV venting (not success) 1136 RCIC off 1230 HPCI on 0242 HPCI off 700TAF then Core metldown No coolant injection! started 841 PCV vent (insufficient) 908 SRV open 925 sea water injection (21 hours) S/C temp too high (no condensatn) 1312 Sea water inject (for ~12 hrs: insufficient) S/C temp high (sat) PCV pres high rupture level set too high; difficult to vent and open SRV PCV failure expcted due to excess temp Alternate water injection line (CRD pump, SLC pump lines) and PCV vent lines damag d by H2 detonat. No coolant injection! 1100 H2 explosion 1325 RCIC off PCV p high No makeup until 1954 1730 TAF 1802 SRV op 1830 Whole core uncov. Makeup was delayed until 1954 Highest rad level at the main gate 600 H2 expl? Near S/C or #4 Large scale release of radactive materials 2000 a large portion of the core melt down to bottom head (est) PCV venting was higher priority at TEPCO In Short, Earthquake Tsunami SBO and LUHS RCIC/HPCI operation Loss Of Off-site Power (LOOP), MSIV closed = PCV isolation AC power from Emergency Diesel Generator (EDG) 1F1: Isolation Condenser (IC) automatic start 1F2, 1F3: Reactor Core Isolation Cooling (RCIC) - manual start for injection Primary Containment Vessel (PCV) Isolation RHR in service (RHR: Residual Heat Removal) Decay heat removal as planned and on the way toward the cold shutdown mode Station black out (SBO), RHR inoperable resulting in Loss of Ultimate Heat Sink (LUHS) 1F2, 1F3 under LUHS conditions; 1F1 LUHS after IC termination and HPCI not operable w/o aux cooling DC power lost except for 1F3 1F2 RCIC kept (its crippled) operation for 72 hours after tsunami w/o DC for valve control (RCIC line isolation valves possibly partially stack open) but unstable 1F3 RCIC continued operation for ~20 hrs, followed by HPCI for 14 hrs W/o UHS Core melt, RPV and PCV failures Core heat-up and melt due to long duration of uncovery after RCIC/HPCI termination for 1F2 and 1F3 Difficulties in RPV depressurization by Safety Relief Valve (SRV) opening w/o DC and air pressure; first priority was on the PCV venting Resulting in the delay in alternate low pressure injection; By the time of Suppression Chamber (S/C) venting success, PCV failure due to high temperature and pressure: LR (Large Release) By the time SRV relief valves opened, core damaged severely and H2 produced The worst scenario --- PCV failure and radiological release.
Responses to fight against the accident and to mitigate the consequences Unit 1 Isolation Condenser (IC) operation - 1 The unit 1 had two ultimate heat sinks: sea water through RHR circuit and air atmosphere through IC Two trains (A and B), four PCV isolation valves (see Fig. next slide) One train in use for the RPV pressure control before the tsunami by on-off strategy Train A Train B Heat sink =atmosphere
Unit 1 Isolation Condenser (IC) operation - 2 Train A Train B Heat sink =atmosphere On tsunami attack, IC was off with the one outboard valve (DC driven) closed; other valves were open but set to close with the loss of DC power (PCV isolation) When tsunami came and caused the SBO, and soaked all the DC batteries, all the valves were to close (fail to close); however, Actually the in-bound valves (AC-M driven) seemed to have remained partially open due to the loss of AC power Unit 1 Isolation Condenser (IC) operation - 3 At 6:27 pm, 3/11, with a dim revival of DC battery power, the operators opened the outboard isolation valve, successful Then why the operators closed the valve again? Closed worrying about the damage to the IC if all water was lost out of IC when the crew could not see the white steam coming out of the IC (according to TEPCO) --- --- If they trusted the water level in the shell side of the IC (water tank) and didnʼt stop the IC operation, the unit 1 core might have survived without serious damage; Note: even if the water tank is empty, no need to stop the IC operation After stopping the IC, the unit 1 was under the loss of ultimate heat sink condition
Core Meltdown --- Unit 1 Virtually nothing could be done for the Unit 1 with possible misreading of RPV water level; and the control room indications were unavailable; and no information fed and an optimism about the IC status by the site ERC (Too much trust on the passive safety) In this respect, IC is not perfect passive Pointed out: Communication between control room and site ERC, RPV water level reading, SAM drills/training etc. Core meltdown after a few hours of IC termination (core uncover started ~5pm, 3/11: fact much earlier than suspected) Radiation level was high already around 10PM in the RB Most of the fuels have melted and relocated to the bottom head, some leaked through the CRD/instrumentation guide tubes into the pedestal (CRD cavity room) Core Meltdown --- Unit 1 3/11 21:30 IC valve was open (according to TEPCO) Access to IC had been difficult due to the high temperature and high dose level PCV radiation level very high in the RB after midnight PCV venting took longer time due to lack of. 3/12 14:30 Venting finally done 3/12 15:36 H 2 explosion is after the core melt TEPCO continues sea water injection from 3/12 19:04 PM office reportedly suggested (ordered?) to stop sea water injection worrying recriticality event; TEPCO ignored the order (but pretended to obey for 19:25-8:20) However, as the day continues, Boron was added to address criticality concerns.
GE BWR, Mark-I Nuclear Reactor After Tsunami SBO and LUHS [units 2 and 3] Major components that do not require AC: RCIC/HPCI, SRV Enthalpy build up in S/C Boiling; no condensation and no scrubbing PCV radiation level high PCV p and T high Need PCV venting Temperatures Pressure Radiation level.. HX SBO Sea LUHS by tsunami RHRS RCIC/HPCI NO DHR Source: Boling Water Reactor (BWR) Systems (Modified) USNRC Technical Training Center
Core Meltdown --- Unit 2 (day 3/13-14) High (and low!) pressure water injection by RCIC (w/o DC), enthalpy build up in the suppression chamber (S/C) w/o heat removals At the site Emergency Response Center (ERC), PCV venting was a first priority to dump the decay heat; then RPV depressurization by SRV opening for coolant injection by alternate pumps --- (reasonable) 3/13 0810 Manually opened MO-valve of the PCV DW ventilation line (25% open and stand-by) 3/13 1100 To open the AO-valve of the S/C (WW) ventilation line, the E-M valve was forced to open with the power from a mobile generator in the control room; however, PCV pressure not high enough to rupture the rupture disk on the S/C vent line 3/14 1100 The H2 explosion (unit 3) damaged much of the S/C vent line and fire engine injection line set ups Core Meltdown --- Unit 2 (day 3/13-14) 3/14 1325 RCIC off after 72 hours of staggering operation PCV pressure high; no coolant make up until 19:54 3/14 17:30 TAF An order of open SRV was issued by the PM office w/ an advise from NSC Alternate water injection line was not yet ready when SRV opened (3/14 18:02); and this SRV opening was suspected to accelerate the core meltdown and result in very likely worse accident consequence 3/15 Manually tried to open the DW vent line valve; a few minutes later confirmed the valve closed; S/C vent not yet due to the low S/C pressure
Core Meltdown and Release --- Unit 2 3/14 18:30 Whole core uncovery suspected Water injection delayed by ~2 hours (3/14 19:54) When injection was made, RPV pressure went high again due to evaporation, disabling further injection As the day continued, Boron was added to address criticality concerns. Misunderstanding again on criticality Then failures of RPV are well suspected Followed by PCV failure due to high temperature (> 450 deg C) and high pressure steam and gas mixture of high radioactivity, a large scale radioactive materials release was well-suspected (3/15 ~ 8am) Core Meltdown --- Unit 3 20 hours of RCIC operation, followed by the 14 hours of HPCI operation; 3/13 02:42 HPCI terminated; restarting efforts in vain Then, a long time duration of no coolant make up was suspected due to difficulties in opening ADS-SRVs PCV pressure high. During this period, safety valves opening resulting in lowering the water level rapidly Efforts on venting and SRV opening were continued SRV did not open due to its mechanism w/o both air pressure and DC power RPV back pressure high impedes the fire engine pump injection
Core Meltdown --- Unit 3 3/13 ~8am Core exposure and meltdown afterward 3/13 9:08am SRV opening was said questionable because RPV pressure rapid reduction before the reported SRV opening and after strong pressure spike ~9am: due to MFCI (?) not likely but still under discussion 3/13 ~9:25am Borated fresh water injection (~ 1220) 3/13 1300~ below TAF and not recovered 1312 Sea water injection --- could not recover TAF In spite of 12 hours of sea water injection efforts, ERC recognized water level was kept below TAF Circumferential evidence for RPV failures 3/14 11am H 2 detonation 1F1 1F2 1F3 3/11 3/12 3/13 3/14 3/15 Earthquake/ AM PM AM PM AM PM AM/PM tsunami S/C temp high (sat) IC on (A & B) Rad level PCV pres high 1503off man lly high in RBs 1430 Vent rupture level set too 1830 Open-close 3am RPV successfuil? high; difficult to vent failure? and open SRV Highest rad 17~1800 5am PCV 1536 H2 expl. PCV failure expcted level at the TAF failure? due to excess temp main gate 2130 No Prep vent access to IC Core melt (before mdngt) 1502 RCIC on w/o DC power RCIC valves not compl closed? Car batteries for instrumentation and to open SRVs 1506 RCIC on 1136 RCIC off RPV p falls down due to possible RPV failure high peak p pulse: by MFCI? Not likely but still under 36debate (20 hours) (14 hours) (>70 hours) 11am try PCV venting (not success) 1230 HPCI on 0242 HPCI off 700TAF then No coolant injection! Core metldown started 841 PCV vent (insufficient) 908 SRV open 925 sea water injection (21 hours) S/C temp too high (no condensatn) 1312 Sea water inject (for ~12 hrs: insufficient) Alternate water injection line (CRD pump, SLC pump lines) and PCV vent lines damag d by H2 detonat. No coolant injection! 1100 H2 explosion 1325 RCIC off PCV p high No makeup until 1954 1730 TAF 1802 SRV op 1830 Whole core uncov. Makeup was delayed until 1954 600 H2 expl? Near S/C or #4 Large scale release of radactive materials 2000 a large portion of the core melt down to bottom head (est) PCV venting was higher priority at TEPCO
Summary: LUHS Fukushima path to Core Melt - 2 After LUHS, PCV venting delay or failure with various reasons in dumping out the accumulated energy from PCV was fatal, resulting in PCV damage Subsequent delayed RPV depressurization with difficulties in opening relief valves (SRV), w/o ADS, and alternate low pressure coolant injection difficult no coolant injection for long hours Core exposure by continuous safety valves (SRV) opening and possible depressurization due to possible leakage paths formation at RPV and other primary boundaries (Unit 2 and 3) Core melt PCV failure -- Eventual lower PCV pressure -- due to most likely leakage path formation in unit 2 (due to excess temperature, ) Irony that the RPV depressurization was achieved by RPV failures and the PCV venting due more likely to possible PCV failures that made the low pressure injection possible and stabilized the degraded core with the atmosphere as ultimate heat sink 1F1 core meltdown 3/11, 6-8pm 1F2 3/14~15, 1F3 ~ 3/13 in the morning
Any success path to no core damage with LUHS? Easy to say if IC were not terminated ; if RCIC and HPCI were not stopped manually,, etc. These afterthoughts are not totally correct True that the reactor core seemed to have survived as long as IC or RCIC/HPCI were operating but this does not mean that the core could avoid core damage and melting w/o heat sinks F & B might be an answer as shown next but with many ifʼs. Extremely slim success path w/o IC, RCIC/HPCI under LUHS conditions -- F&B (SAM: Severe Accident Management) I. Depressurize RPV to 6~7 bars immediately after termination of IC, RICI or HPCI forcing SRV (ADS) open to alternate pump capability --- with DC + High Pres. Air or N 2 required II. W/o delay, inject make-up water by alternate high power diesel pumps or fire engine pumps III. Carry out PCV venting to release the enthalpy transported by the steam out of the RPV into the atmosphere (heat sink) --- Filtered vent system Repeat these procedures; all these actions must be done smooth and absolutely w/o delay when the level well above TAF Wait for RHRSW restoration If RHR is not restored for one week or so, it would be difficult to keep the nuclear reactor core intact
Possible success path to save the cores Success possibility extremely slim Nevertheless, it seems to me that TEPCO was following the path in principle Frequent strong aftershocks and tsunami warnings, tsunami debris, no lighting in the darkness, w/o much information,, and reportedly some frequent intervenes hindered the TEPCOʼs efforts at the site ERC and operation control room activities from the very beginning of the accident Natural and man-made hazards Extremely low frequency but large consequence Q event Why this tsunami risk was ignored? Very low frequency -- perception Underestimated the historical records Cost consciousness of TEPCO (non-nuclear sectors) Why resulted in the nuclear disaster? 1. (SBO=)LUHS; for BDBE, lack of diversity of ultimate heat sink 2. Lack of D-i-D 4 th layer (mitigation of Q by Accident Management / Risk Management after the breach of the 3 rd layer) 3. Neglect of the TMI lessons, IAEA recommendation on regulatory system, 4. Due to Overconfidence by regulatory body, utilities, nuclear professionals,.., in DBE safety, high reliability in power grid system in Japan,, Complacency 4 th Disaster
Information Control All these core meltdown facts were not disclosed until May 15, 2011 by TEPCO Press Release TEPCO was well aware of the meltdown from the beginning; so were NISA/JNES NISA/JNES did not or could not disclose the information against their intention (my guess) Post-Fukushima Immediately after the accident, installation of the counter-measures against higher tsunami attack and SBO, E/March 2011 Roadmap to stabilization Many lessons learned and safety improvement; hardware, and software, regulatory system Nuclear power plants are made much safer by putting the lessons from Fukushima-Daiichi into practice
Post-Fukushima Still in the realm of the deterministic DEC and strengthened D-i-D 4 th layer (software) Inevitable preparation for the unforeseen events and a disaster that exceeds all worst-case scenarios; Expect and prepare for the unexpected but how? Example: Tsunami disaster education for the children in Kamaishi, a small coastal town (pop: ~ 40,000) in Iwate Prefecture END Responses taken to fight the nuclear accident and to mitigate the consequences Hisashi NINOKATA Professor, Politecnico di Milano Department of Energy CeSNEF-Nuclear Engineering Division Nuclear Reactors Group Professor Emeritus, Tokyo Institute of Technology