Tokyo Electric Power Co has published an English-language version of summary of an interim internal analysis of the Fukushima incident.
The original interim analysis, dated 2 December, reviewed the main facts of the 11 March earthquake and tsunami, and its effects on nuclear power plants Fukushima Daiichi and Daiini.
It said that the tsunami height was 13m at Fukushima Daiichi and 9m at Fukushima Daiini, and these heights had not been predicted previously based on historical and geographical models.
The report confirmed that several design changes to the Fukushima Daiichi and Daiini reactor plant systems had been made, starting in the 1990s:
-At unit 1, piping and motor-operated valves were installed to enable water injection into the reactor from the existing make-up water condensate system and the fire protection system via the core spray system
-At units 2-6, and Fukushima Daiini 1-4, piping and motor-operated valves were installed to enable water injection into the reactor from the existing make-up water condensate system and the fire protection system via the residual heat removal system.
-Hardened vents able to withstand high pressures were attached to the primary containment vessels in case of high pressures from failed heat removal in the PCV. These vents could allow operators to release pressure in the PCV from the main control room.
-Cross-links between adjacent units were installed in case of total loss of backup emergency diesel generators and other DC power sources.
-Manuals and worker instructions were revised to implement accident management measures, and new standard operational procedures were devised for severe accidents. Plant operators and emergency response team members periodically received training on these procedures.
The report said:
"In the Fukushima accident, the destruction caused by the tsunami resulted in the loss of almost all equipment and power source functions expected to be activated in case of accidents, including those for AM measures prepared together with the government. As a result, workers on the site were forced to
adapt to sudden change of circumstances such as injecting water into the reactors using fire engines, and the accident management became extremely difficult. The situation on the site was far beyond the originally estimated accident management conditions, and as a result, the expansion of the accident could not be prevented under the framework of the prepared safety measures."
The report said that visual inspections were carried out after the earthquake and before the tsunami. In the case of the unit 1 isolation condenser (which failed after the tsunami for reasons that remain unclear) inspections found no damage to vessels or piping, and no evidence of high-pressure steam leaks. Interviews with staff members who checked on the unit 3 high-pressure coolant injection system suggest that pipe ruptures were unlikely.
Most electrical switchgears were flooded. Even though supply cars could be prepared to provide temporary external electricity, there were almost no operable switchgears to connect them to the stations' power grid.
The report said that the necessity of venting the unit 1 PCV was recognized early, although staff waited until local personnel were evacuated before venting the unit manually using temporary equipment (14:30 on 12 March, an hour before the unit 1 explosion). Venting at units 2 and 3 were carried out under harsh conditions after the unit 1 explosion.
The explosions were probably caused by hydrogen generated from air reacting with exposed zirconium cladding. Although it is not sure, TEPCO suspects that the hydrogen gas leaked out of the PCV head via penetrations and hatches, whose silicone rubber seals would have deteriorated due to high temperatures.
At Unit 3, measures to prevent hydrogen explosions were being considered, but were not actually implemented because of the high possibility of inducing explosion by spark discharge. (Arrangements were made to prepare tools for boring a hole through the wall of the reactor building using a water jet machining device. However, the tools did not arrive before the explosion of Unit 3.)
The report estimates that unit 1 core damage happened quickly: on 12 March at 15:00 the reactor vessel pressure decreased, despite no intervention, while the PCV pressure increased. It concludes from this that the pressure in the reactor pressure vessel could not be maintained due to the damage to the vessel, and that core damage had advanced considerably since the 30 hours since the earthquake.
The report has outlined eight countermeasures to prevent such an incident happening again.
1. Flood protection. Installation of tidal embankment, boards, and wall and flood protection of door and penetrations.
2. High-pressure cooling water injection facilities (required within one hour). High pressure injection is initially required due to high reactor pressure in the case that the plant experiences an abnormal shutdown. During the accident, some motor-driven equipments were inoperable due to the station black out (SBO). Hence, a steam-driven high pressure facility is the key issue. Furthermore, when choosing motor-driven high pressure cooling water injection facilities, it is important to select equipment with minimum operating requirements.
3. Depressurizing equipment (within 4-8 hours). Depressurization of the rector pressure vessel is
essential in order to remove heat and bring it to a
cooling stage. During the Fukushima accident, the DC power
necessary to operate the main steam safety relief valve for depressurizing was insufficient. In addition to securing N2 for valve operations, securing a power source is necessary.
4. Low-pressure water injection facilities (within 4-8 hours). Low-pressure cooling water injection equipment consists of an emergency system, a make-up water condensate system (MUWC) and a fire protection system. In the case of the SBO, only the diesel-driven fire pumps (DDFP) of the FP
will be operable. Preparing reliable low-pressure injection equipment is important, including the fire engine used.
5. Heat removal/cooling facilities
5a. PCV venting (Within 1-2 days). In case seawater cannot be used as a cooling source, suppression chamber venting that utilizes air as a cooling source is necessary. In order to conduct suppression chamber venting, opening motor-operated (MO) valves as well as air-operated (AO) valves are necessary.
5b. Heat removal via shutdown cooling mode (within 3-7 days). Shutdown cooling mode procedures by residual
heat removal system (RHR) that utilizes sea water as a cooling source is necessary. Thus, in addition to ensuring a power source, restoring the seawater system utilized as the ultimate heat sink for preparing alternative pumps, or motor repairs is necessary.
5c. Heat removal from spent fuel pool (Within 7-10 days: depending on decay heat from spent fuels). Spent fuel pool cooling and cleanup system (FPC) is basically tsunami-resistant since it is located inside the reactor building.
Hence it is important to maintain the power source. Furthermore, in light of having a sufficient a amount of time to respond, monitoring with instruments is important.
6. Ensuring power supply to the monitoring instruments (required within 1 hour). During the Fukushima accident, the monitoring instruments were rendered inoperable and restoring power to the instruments took time. Thus, ensuring immediate power supply for instruments (through flood protection of battery rooms, and provision of portable backups) is important.
7. Mitigation measures following reactor core damage. During the accident, not only was the containment function lost, but also restoration efforts were seriously hampered due to the hydrogen explosion caused by the possible leak of hydrogen from the primary containment vessel to the
building. Countermeasures include preparation to drill holes in the roof or opening blow-out panels to prevent the accumulation of hydrogen. In light of defense in depth, it is important to establish countermeasures in the case of the reactor core damage, which happened at Fukushima Daiichi (including provision of outside power source, debris removal equipment, securing communication tools, securing lighting equipment, provision of health protection equipment).
Other mid-term and long-term technical issues should be considered:
a. Isolation signal review. During the Fukushima accident, the loss of the isolation condenser cooling function was caused by the valve closing up due to loss of DC power. Hence, the concept concerning the isolation signal should be reviewed.
b. Venting line improvement. In order to improve venting that is able to significantly filter out radioactive materials, measures such as the aggressive activation of the rupture disk will be looked into while taking the accidental release of radioactive materials into consideration.
c. Mitigation measures for radioactive material release during venting. The design of a filter vent to mitigate the release of radioactive materials will be considered.
d. Surveillance instrument improvements. Given that the water level gauge measurements deviated greatly from the actual value at the power station, research and development in this area will be developed.
The investigation continues, and TEPCO said that further reports will be issued on the release of radioactive materials, radiation control, human resources, material procurement and disclosure of information.
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