Fukushima Daiichi NPS Prompt Report 2015

Continuing inquiry's third progress report finds support for measures being employed at Kashiwazaki-Kariwa Nuclear Power Station to strengthen facility's defenses against accident

TOKYO, May 20 -Engineers at the Tokyo Electric Power Co. have issued their latest report on the findings of their continuing inquiry into some of the important technical questions that remain about exactly how the March 2011 accident at Fukushima Daiichi unfolded at each of the affected reactors.

The findings help validate the measures taken to strengthen the Kashiwazaki-Kariwa Nuclear Power Station[KK], for which the company is seeking restart approval from the government.

Cause of Dose Increase on March 20, 2011

A significant finding of this third progress report is that there was no surge in the release of radioactivity from Unit 3 on or after March 20. A March 20, 2011 spike in radioactivity dose rates at the plant site, together with reports of iodine in tap water in Tokyo around that time, had raised questions about whether there had been a separate venting episode, or some other unexplained or overlooked event. But the report says the increase in monitoring post measurement was most likely attributable to a shift in prevailing winds and that the emission of radioactivity from Unit 1 to 3 had remained constant.

In the course of the inquiry, a review of accident handling records, wind measurements, and other data was performed, including an inquiry into the source of black smoke that was seen emanating from the southeast side of the Unit 3 reactor building at around 4 pm on March 21, and which the report believes was likely the ignition of oil used in PLR pump speed controller. There is no evidence of a surge in release of radioactive substances the report says, and "the site dose increase measured around March 20 was most likely due to a change in wind direction that blew the constantly escaping radioactive substances the opposite way."

This portion of the inquiry indicates the various countermeasures being employed at KK to keep the integrity of the primary containment vessel are valid. TEPCO also believed it was important to address public concerns and provide a better understanding about the dose increase around March 20.

Venting System

Yet another portion of the inquiry is looking into 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 is designed to vent steam only when necessary to prevent a more serious accident that could result from excessive pressure in the containment. To do this, the vent line incorporates (among other devices) a "rupture disk" which is set to burst open when pressure in the venting line reaches emergency levels, allowing pressure to be reduced to safer levels.

The inquiry is focusing on whether that disk worked properly, and the report says results remain inconclusive. Engineers are examining various portions of the venting and pressure management system to determine, largely from circumstantial evidence, how pressure built and was relieved in various parts of the Unit. The level of contamination on the exit side of the rupture disk, when compared with levels at other components of the air and steam management systems, suggest that the more likely scenario is that the rupture disk did not burst when it should have, but that gases escaping from Unit 1 backflowed into the Unit 2 system. The venting systems for the two units are joined at the exhaust stack.

These findings regarding the rupture disk are not definitive and the inquiry will continue until to be found the clear evidence, the report says.

Progression of Accident at Unit 1

Another focus of inquiry has been the progression of the accident within Unit 1, and specifically determination of where the reactor pressure vessel (which sits inside the primary containment vessel) leaked steam.

The engineers were able to analyze data, particularly differential water pressure from the two reactor water level gages, which had not indicated correct values, located inside the PCV, to infer that the PCV became extremely hot near its top rather than at the bottom. This suggests that the steam leak from the pressure vessel into the containment vessel occurred at the top, the report says.

This finding, the report says, supports the validity of previous findings that were inferred from plant operating data and previous experience. Its significance is to validate the countermeasures being employed at KK, including cooling devices that would spray cold water onto the top of the containment vessel to prevent overheating and subsequent damage.

The full report, which includes data tables and technical drawings, may be seen at http://www.tepco.co.jp/en/press/corp-com/release/betu15_e/images/150520e0101.pdf. Its formal title is "Report on the Investigation and Study of Unconfirmed/Unclear Matters in the Fukushima Nuclear Accident Progress Report No.3"

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 KK and at Fukushima Daiichi as well.

About TEPCO

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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