Japan’s Nuclear and Industrial Safety Agency (NISA) ordered nuclear power plant operators on March 28 to prepare for the possibility of earthquakes involving multiple active faults that would cause significantly bigger shocks than previously envisaged.
The Great East Japan Earthquake stunned seismologists because it overturned a previous assumption that no magnitude-9.0 class earthquake would hit the part of the Japan Trench facing the Tohoku region. The disaster saw the release of a huge amount of energy along its length involving several different segments of the trench.
NISA's order goes beyond that to raise the hypothetical possibility, not only of several earthquakes coinciding along an oceanic trench as happened in the Tohoku temblor, but of multiple quakes coinciding on apparently separate inland faults.
While there is no proof that any such event has ever occurred, scientists have been unable to rule out the possibility. With the March 11 quake proving how little seismologists know about how earthquakes work, a new hyper-cautious approach is being adopted by Japan’s regulatory authorities.
The new NISA directive, presented at a hearing of expert opinions on revising NISA’s regulations in light of the disaster, could force major changes to the assessment of the safety of Japan’s reactors.
NISA told Kansai Electric Power Co. (KEPCO), which operates the Oi plant, where the central government is pushing for a quick restart of suspended reactors, that it should plan for the possibility of a simultaneous rupture along underwater faults near the plant as well as the land-based Kumagawa fault. Those faults have a combined length of 63 kilometers.
Earlier modeling by KEPCO looked at a multiple-fault rupture in the vicinity of the Oi reactors and projected ground acceleration of 760 gal. That result, which exceeded the previous forecast of 700 gal, was endorsed by NISA, which called on KEPCO to use the new estimate to determine the anti-seismic performance of the plant's equipment.
A first-stage stress test on the Oi nuclear plant, conducted separately last autumn and endorsed by the Nuclear Safety Commission of Japan (NSC) on March 23, said core meltdowns are not likely to occur until ground motion exceeds 1,260 gal, a figure that included a safety leeway. That appeared to show the plant would be able to cope even with a catastrophic, multiple-fault seismic event.
However, the new advice will apply to eight other nuclear facilities at which multiple-fault modeling has not been completed: KEPCO’s Mihama plant; Hokkaido Electric Power Co.’s facility at Tomari; the Kashiwazaki-Kariwa nuclear plant of Tokyo Electric Power Co.; the Tokai No. 2 and Tsuruga nuclear plants run by the Japan Atomic Power Co.; the Shika nuclear plant of Hokuriku Electric Power Co.; the Shimane nuclear plant of Chugoku Electric Power Co.; and the Monju prototype fast breeder reactor.
Discussions are still under way on the possibility of multiple-fault ruptures along some active fault systems. NISA expert hearings this month have seen a number of assumptions, based on years of research by power utilities and the government, put in doubt, and some witnesses have underlined the possibility of multi-fault quakes or events involving multiple segments of a fault line.
"If we are to think about the worst case, it is all too natural to have all (active faults) break at once," said one witness.
Power utilities have maintained, as they did before the March 11 disaster, that earthquakes are not likely to involve more than one active fault, but seismologists have criticized the position, saying it has no conclusive grounds.
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