Little Water Found in Damaged TEPCO Reactor

The second inspection of the Primary Containment Vessel at Fukushima Daiichi Unit 2 found only about 60 centimeters of water at the bottom of the vessel. Meanwhile, an expert IAEA team delivered its review of Japan's process for assessing nuclear plants' safety.

The Tokyo Electric Power Company Inc. (TEPCO) was back in the news March 27, both because an inspection team found very little water and high radiation levels inside the Primary Containment Vessel at Unit 2 of the Fukushima Daiichi Nuclear Power Plant and also because the International Atomic Energy Agency delivered a report to Japanese authorities the previous day based on an expert team's Jan. 21-31 review of Japan's process for assessing nuclear safety at its nuclear power plants.

The report says instructions and the review process used by Japan's Nuclear and Industrial Safety Agency for Comprehensive Safety Assessments are generally consistent with IAEA standards, and it praises NISA for observing European stress tests to learn from other countries' experience with them. It includes seven recommendations for making the assessments more effective, including requiring licensees to develop comprehensive programs for managing severe accidents.

"In particular," the report states, "the feasibility and effectiveness of the following mitigative strategies should be more comprehensively covered: reliable depressurization of the reactor coolant system, long-term containment isolation, molten corium stabilization either in the reactor pressure vessel or in the containment, hydrogen mitigation inside the primary containment taking into account not only in-vessel hydrogen sources but also potential decomposition of containment materials due to molten corium attacks, and possible over pressurization of the containment by non-condensable gases. Potential for occurrence of a severe accident in the spent fuel pool and possibilities for its mitigation should be addressed as well. Although the provisions for mitigation of severe accidents are currently out of regulation scope in Japan as well as in many IAEA Member States, in view of lessons learned from the Fukushima accident, they should be considered in the assessments of coping with severe accidents."

"The mission report provides additional information regarding the team's recommendations and overall finding that NISA's instructions to power plants and its review process for the Comprehensive Safety Assessments are generally consistent with IAEA Safety standards," said team leader James Lyons, director of IAEA's Nuclear Installation Safety Division. "I hope nuclear regulators around the world use this report as a tool to evaluate their own safety assessment processes," he added. "We must learn the lessons of the Fukushima Daiichi accident so we can prevent a repeat of those terrible events a year ago."

National safety assessments that undergo peer review by IAEA are a key component of the IAEA Action Plan on Nuclear Safety, which was approved by the agency's Member States following the 2011 earthquake and tsunami that severely damaged the Fukushima Daiichi plant, cutting power to the systems that cool the three reactors in operation at the time.

TEPCO announced March 26 that its second investigation inside the Primary Containment Vessel at Unit 2 found only about 60 centimeters of water at the bottom of the vessel. TEPCO President Toshio Nishizawa announced March 25 that because of regular inspections causing the shutdown of all reactors at the company's Kashiwazaki-Kariwa Nuclear Power Station, all of the company's nuclear reactors currently are shut down. "While carrying through with the regular inspections of Kashiwazaki-Kariwa Nuclear Power Station, we will conduct a comprehensive evaluation to ensure that the safety margin against earthquakes and tsunamis exceeds the design standards. We will also implement measures to further enhance the safety and reliability against tsunamis," Nishizawa said in a statement posted on TEPCO's website. "As for the electricity supply and demand in the foreseeable future, we expect to maintain stable supply. However, we ask that you continue to make a reasonable effort to save electricity. In addition, while we have been carefully reviewing this summer's electricity supply and demand, the shut down of Unit 6 will result in a significant reduction in our electricity supply capacity. We will continue to make efforts to maintain stable operations and maintenance of the power facilities in order to secure stable power supply."

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  • OHS Magazine Digital Edition - September 2020

    September 2020

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