Tepco sees progress in understanding of Fukushima accident

Tokyo Electric Power Company (Tepco) today released the latest findings of an ongoing investigation into how the accident at the Fukushima Daiichi nuclear power plant unfolded. Tepco said the findings support measures being taken at the Kashiwazaki Kariwa plant, which it hopes to restart.

Tepco engineers have been investigating "some of the important technical questions" that remain about how the March 2011 accident unfolded at each of the affected reactors at the Fukushima Daiichi plant in Japan.

Their latest document - Report on the Investigation and Study of Unconfirmed/Unclear Matters in the Fukushima Nuclear Accident: Progress Report No.3 - follows those published in December 2013 and August 2014. The first looked at conditions before and after the accident, while the second looked at organizational and technical causes of the accident.

Some 52 issues were identified as being "unsolved events related to the detailed development of the incident following the accident", Tepco said.

"Overall, the report represents progress in understanding at the most detailed and technical level possible exactly how the accident progressed."

Tepco

A significant finding of the report, the company said, is that there was no increase in the release of radioactivity from unit 3 on or after 20 March. A spike in dose rates at the plant had led to speculation of a separate venting episode or some other "unexplained or overlooked event". However, Tepco now believes that the increased measurements were due to a shift in prevailing winds and that the release of radioactivity from units 1-3 had in fact remained constant.

The investigation also looked at whether unit 2's emergency system to vent steam into the atmosphere in order to release pressure in the containment vessel worked properly. The system incorporates a "rupture disk" which is set to burst when pressure in the unit's venting line reaches a certain level.

The report says the level of contamination of the exit side of the rupture disk suggests that the device did not burst when it should have and that gases escaping from unit 1 backflowed into the unit 2 venting system. The venting systems of the two units are connected at the exhaust stack.

However, Tepco says these results "remain inconclusive" and that the investigation will continue.

The report also focused on where the reactor pressure vessel of unit 1 leaked steam. The investigators said data suggests that the primary containment vessel, in which the reactor pressure vessel sits, became very hot near its top rather than its bottom. This implies that the steam leaked from the top of the pressure vessel, they said.

Tepco says the significance of this finding "is to validate the countermeasures being employed at Kashiwazaki Kariwa, including cooling devices that would spray cold water onto the top of the containment vessel to prevent overheating and subsequent damage."

Tepco said, "Understanding the unsolved issues of details of how the incident developed after the initial accident is not only the responsibility of the parties involved in the accident." It is also important in order to help in decommissioning activities at Fukushima Daiichi; for improving accident simulation models; and improving nuclear power plant safety technology, the company said.

"Overall, the report represents progress in understanding at the most detailed and technical level possible exactly how the accident progressed and helps validate various assumptions that have driven the adoption of countermeasures at Kashiwazaki Kariwa and at Fukushima Daiichi as well," Tepco said.

The company has applied to restart units 6 and 7 of its Kashiwazaki Kariwa plant. The Advanced Boiling Water Reactor units were built in Niigata prefecture in the late 1990s. Tepco has submitted information on safety upgrades across the site and at those two reactor units that it claims meet new regulatory requirements. Although it has done work at the other units at the site, Tepco is concentrating its resources on units 6 and 7 while it deals with the clean-up at Fukushima Daiichi.

Researched and written
by World Nuclear News