Fukushima Daiichi NPS Prompt Report 2015

TOKYO, December 16, 2015 - An assessment of radioactive dispersion, in case of a severe accident, has demonstrated the effectiveness of its strengthened safety protections at the Kashiwazaki-Kariwa Nuclear Power Station.

The main purpose of the assessment was to determine how - if it became necessary to vent vapor and hydrogen gas from inside the facility to protect the containment vessels - the vented gases would disperse, and the likely radiation dose resulting from the dispersion.

Planning for the Unexpected

The assessment examined results depending on when the venting began following an accident: after 38 hours, 25 hours, 18 hours, and 6 hours. The 38-hour threshold is the one identified as a base point by Japan's Nuclear Regulation Authority. In general, the longer the containment vessels can retain radioactive substances inside by extending the timing of the venting, the more the radiation will be attenuated.

Comparing the scenarios with or without the use of filter vents, which can remove more than 99.9% of radioactive particles from the vented gases, they can dramatically lower radiation dispersion even in the unlikely event all other safety measures failed. The use of filter vents will result in preventing large-scale soil pollution and prolonged evacuation.

Extensive Protections Adopted Since Fukushima

The assessment will help TEPCO fulfill its responsibility to "expect the unexpected" and prepare accordingly. In fact, the likelihood of the need for venting has been substantially reduced by the variety of strengthened protections adopted in the last few years based in part on the lessons learned from the accident at Fukushima Daiichi. They include:

- Tsunami protectionc: A new 15-meter tall seawall and embankment dramatically exceeds the 6-meter maximum estimated height of a tsunami at the Kashiwazaki-Kariwa's location on the west coast of Japan.
- Flood protection: Various flood barriers, watertight doors, and waterproofing offer additional protection against tsunami and other flooding.
- Power protection: Multiple alternative sources of electricity to maintain cooling and other essential functions have been put in place. They include mobile emergency power supply vehicles, an emergency high-pressure power distribution panel at 27 meters above sea level, and air cooled gas turbine generator vehicles at 35 meters above sea level.
- Cooling water protection: To ensure the availability of water to cool the reactors, the Kashiwazaki-Kariwa has adopted "defense in depth," including:


TEPCO is expected to use the assessment results to verify the effectiveness of safety measures adopted at the facility and review opportunities for any further potential safety improvements. Further assessment with the effects of evacuation will be conducted later and the study of measures to support resident evacuations will proceed.

For more information about the assessment that was conducted, go to
http://www.tepco.co.jp/en/press/corp-com/release/betu15_e/images/151216e0102.pdf (PDF 4.53MB).

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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