The Nuclear Regulation Authority (NRA), power companies and other entities have continued to use an old method of calculating the maximum level of shaking from earthquakes that could hit nuclear plants despite indications from the government's Earthquake Research Committee that this could result in underestimates, it has been learned.
The Earthquake Research Committee has been using a new formula for calculating the maximum level of shaking that would occur in an earthquake, or "standard ground motion," based on a revision in 2009. The NRA insists there is no need to review its current formula, but an expert from the Earthquake Research Committee insists, "The NRA's judgment is wrong." The situation raises questions about the nuclear watchdog's policy.
With respect to the old formula for calculating standard ground motion, Kunihiko Shimazaki, former acting chairman of the NRA, pointed out in June that the projected maximum impact of earthquakes on Kansai Electric Co.'s Oi nuclear plant and other facilities may have been underestimated and should be re-evaluated. The NRA, however, decided in July to retain the current method. The old formula has also been used for other nuclear power stations besides the Oi plant. Doubts about the formula, therefore, will likely affect safety reviews of other nuclear plants as well as decisions on whether to give the green light to restart those plants.
The Earthquake Research Committee is a government organ tasked with conducting research on earthquakes and other related matters. In 2006, it released a method of calculating ground motion -- based on the width and length of faults -- which the NRA and utilities adopted to calculate standard ground motion. However, there have been suggestions that the formula undervalues the magnitude of earthquakes, resulting in underestimation of ground motion in some cases. For this reason, the Earthquake Research Committee released a new formula in 2009, and has since been using it to calculate ground motion of earthquakes around the country.
A calculation manual devised by the Earthquake Research Committee contains both methods, but the government research panel has started considering revising the manual based on the current state of affairs.
A representative of the NRA's secretariat commented, "The 2006 formula is used based on the assumption that detailed research will be conducted. As power companies are conducting detailed research, it is a proper method to be used in evaluating nuclear plants."
Kazuki Koketsu, who heads the Earthquake Research Committee's division in charge of evaluating strong ground motion and is a professor at the University of Tokyo's Earthquake Research Institute, disagrees.
"The 2009 formula is used for making predictive calculations of ground motion triggered by active faults. That's because the 'width of faults' needed for calculations in the method adopted by the NRA cannot be ascertained even through detailed research. No matter which scholar we ask, we get the same answer that the NRA's judgment is wrong," he says.