Fukushima Daiichi NPS Prompt Report 2015


Aim of inquiry is to enhance understanding of technical issues and apply lessons learned to further enhance nuclear safety.

TOKYO, December 17, 2015-TEPCO engineers have issued their latest progress report on the continuing exploration of various technical and unsolved issues about exactly how the March 2011 accident at Fukushima Daiichi Nuclear Power Station developed.

The goal of the continuing inquiry is to completely understand exactly how various technical aspects of the accident unfolded, so that these lessons can be applied in the design of future facilities and to improve existing ones. This report is the fourth in a series and addresses six issues:

1.Safety Relief Valves: Post-accident examination of the measured plant parameters indicated that safety relief valves, designed to reduce pressure in the reactor pressure vessels, may not have functioned properly several times and efforts to reduce pressure may not have been fully successful. At Unit 2, the pressure generated inside the primary containment vessel overwhelmed the pressure of Nitrogen gas which is intended to open up the safety relief valves. There is also a possibility that the Nitrogen gas was leaking due to deteriorating sealing around the valves while in the severe environment. In the case of Unit 3, this was the result of insufficient electricity to operate the valves. A minute-by-minute review of the timeline indicates that the valves were opened and pressure dropped when demands placed on DC batteries by the high-pressure injection auxiliary pumps stopped, allowing the same batteries to then fully power the valves. The report emphasizes the importance of securing sufficient power supply, and securing proper amount of nitrogen gas pressure (including leak prevention).

2.Molten fuel relocation: The various routes the melted fuel possibly took in the complex structure inside the pressure vessels were investigated using the results of previous experiments on fuel relocation and the latest research based on the accident analysis code. The melted fuel is presumed to relocate through several identified routes to the bottom of the reactor cores but the investigation will continue to obtain a more comprehensive understanding of fuel behavior at the time of the accident.

3.Suppression Pool: At Unit 3, pressure in the primary containment vessel continued to climb at a faster rate than what had been originally estimated from the decay heat. This led to the hypothesis that steam might had leaked from the pressure vessel or thermal stratification might have occurred in the suppression pool with the effect that it failed to fulfill its condensing and cooling roles effectively. The report says it is likely that thermal stratification did occur in the suppression pool due to steam discharge to the surface water by the reactor core isolation cooling system.The matter should be examined further through a quantitative assessment of the primary containment vessel using the thermal-hydraulic analytical code.

4.Unit 1 Pipe Contamination: High dose rates of radiation have been observed near the reactor cooling water pipes of the Unit 1 reactor building and radioactive waste treatment building, places where such high dose rates should not be observed. The examination of the radiation dose rates measured inside those buildings has supported the presumption that the high level of contamination was caused as a result of the melted fuels falling down to the primary containment vessel and damaging the reactor cooling water pipes.

5.Units 2 and 3 Steam Release: A significant release of steam from the night of March 14 to March 16, 2011 is believed to have been responsible for contamination to the surrounding environment of Fukushima Daiichi. The investigation reports that the primary containment vessels in Units 2 and 3 did likely lose leakage resistant properties by March 15 and had been in a condition where radioactive materials could leak directly from them. It is therefore presumed the environmental contamination outside Fukushima Daiichi during that period was caused by steam leakage directly from the primary containment vessels and not from the vent.

6.Unit 2 Accident Progression: In the 3rd (previous) progress report, how the accident at Unit 2 unfolded was presumed from the measurement data from the CAMS installed in the primary containment vessel and the suppression pool. The presumption has now been supported by the results of the quantitative assessment of the behavior and distribution of radioactive materials. The assessment was conducted in certain assumptions where measurement data at the time of the accident could be reproduced. No relocation of radioactive materials to the suppression pool was presumed except for the venting but the leak of radioactive materials from the pressure vessel to the primary containment vessel appeared to have occurred and raised the radiation dose rate that was measured after March 15, 2011.

The overview of this progress report, can be found here (PDF 259KB).PDF

Tokyo Electric Power Company, Inc. (TEPCO) is Japan's largest power company, supplying energy to the greater Kanto area, including Japan's two most populous cities, Tokyo and Yokohama. Its 34,000 employees are committed to providing safe, reliable power to its 29.0 million customers, diversifying energy resources to ensure sustainability, and contributing to economic growth while fully meeting its responsibilities after the Fukushima Daiichi accident.
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