Stress tests on reactors to look at four disaster scenarios

BY TATSUYUKI KOBORI STAFF WRITER

2011/07/17

Nuclear power companies will be required to prove their reactors' ability to withstand four disaster scenarios under new "stress tests" outlined by Japan's Nuclear and Industrial Safety Agency (NISA).

The tests, detailed in a report submitted by NISA to the Nuclear Safety Commission of Japan (NSC) on July 15, will require power companies to model their nuclear power stations' endurance in a major earthquake, a tsunami, a complete loss of power throughout a plant, and a loss of ability to release heat from the reactors.

Computer modeling will be used to identify the strengths and weaknesses of the plants, and the results will be analyzed by NISA and verified by the NSC.

Electric power companies will be required to conduct all the assessments and submit reports by the end of this year, but verification by the central government is not likely to be finished before 2012. It is unclear whether the test results will be enough to reassure local governments near the reactors.

There will be two distinct phases of testing. The first phase will be conducted on nuclear reactors that have been suspended for regular inspections but are ready for restarts as of the end of July.

That testing will be less rigorous than the second phase and will only look at key equipment in the reactors. The administration of Prime Minister Naoto Kan has said that the first-phase tests will be a precondition for the restart of nuclear reactors.

Nineteen reactors are currently suspended for regular inspections, but only about 10 reactors that were not directly affected by the Great East Japan Earthquake are expected to be subjected immediately to the first-phase tests.

Kan and three other ministers will make the final decisions on whether to authorize restarts. The NISA report did not give a timeline for the completion of the assessments.

The second-phase assessments will be targeted at all reactors under construction and all 44 existing nuclear reactors in Japan except the Fukushima No. 1 and Fukushima No. 2 nuclear plants.

The safety of the whole of the reactors, not just the key equipment covered by the first-phase tests, will be assessed and the stresses that would put the reactors into a dangerous condition will be identified. The electric power companies will be required to submit reports on the second-phase assessments by the end of the year.

NISA will issue further instructions on the tests to power companies by the end of July.