The movement of the bedrock under the Fukushima No.1 nuclear power plant during the Great East Japan Earthquake was larger than pre-quake estimates used by the plant’s operator in its disaster planning, according to government simulations.

Tokyo Electric Power Co. (TEPCO) maintained before the March 11 catastrophe that bedrock motion would exceed its “standard seismic motion” only once in 10,000 to 1 million years and used the figure to evaluate whether its buildings and equipment would withstand a quake.

But a new government analysis, announced Dec. 9 by the Nuclear and Industrial Safety Agency (NISA), suggests that the actual shaking on March 11 was 675 gal, in excess of the “standard seismic motion” of 600 gal at a depth of 196 meters beneath the ground posited in TEPCO’s planning. A gal is a unit of acceleration.

TEPCO still denies that the March 11 quake, rather than the subsequent tsunami, caused damage to key equipment, and maintains that the shaking at the plant was within what it was designed to withstand.

High radiation levels at the Fukushima No.1 site have made it impossible to verify the utility’s claims through on-site inspections, but the issue of whether damage was caused by the quake rather the tsunami is a key point of contention because it might require the redrawing of safety guidelines across the nuclear industry.

The "standard seismic motion" is a concept used in the seismic design of all nuclear power plants in Japan and measures the motion in the bedrock that the buildings and equipment at nuclear facilities are designed to withstand. It is supposed to build in a large safety leeway over actual seismic activity by hypothesizing, in the case of the Fukushima No. 1 plant, a huge magnitude-7.9 event involving more than one offshore seismic source.

NISA said the actual bedrock motion could have been predicted only if a different combination of seismic source areas had been used to constitute that hypothetical event.

At Tohoku Electric Power Co.’s Onagawa nuclear plant in Miyagi Prefecture, the actual motion of the bedrock 8.6 meters beneath the ground on March 11 is estimated at 636 gal, well in excess of that plant’s standard seismic motion of 580 gal.

The actual motion at Onagawa also exceeded the standard seismic motion during the magnitude-7.2 aftershock of April 7, although Tohoku Electric had anticipated an earthquake of a similar type. That could indicate fundamental problems with the model being used.

Bedrock motion is also known to have exceeded expectations at the Tokai No. 2 nuclear power plant in Ibaraki Prefecture operated by Japan Atomic Power Co.

The government's Regulatory Guide for Reviewing Seismic Design of Nuclear Power Reactor Facilities was revised in 2006, and the anticipated motion levels for all of Japan’s nuclear plants were revised at that time. The government endorsed the revisions.

The 2006 guidelines were supposed to have incorporated the latest seismological knowledge and built in greater leeway.

Seismic safety is a key part of the stress tests that have been imposed by the government as a precondition for restarting suspended nuclear reactors. NISA has already received stress test reports on four nuclear reactors.

The impact of seismic motion is also a focus of investigations into the accident at the Fukushima No. 1 plant, where re-evaluation and anti-seismic reinforcement based on the 2006 guidelines had not been finished when the disaster hit.

(By Eisuke Sasaki, The Asahi Shimbun, 10/12/2011)