Active faults - 50,000 years in 1987 - 400,000 years now

December 7, 2012

Dangerous fault alleged 25 years ago at Shika nuclear plant site

http://mainichi.jp/english/english/newsselect/news/20121207p2g00m0dm074000c.html

TOKYO (Kyodo) -- Experts had said there was a dangerous fault at the planned site of the Shika nuclear plant in Ishikawa Prefecture in 1987, well before a similar suspicion arose earlier this year, according to government documents given Friday.

The previous claim had been left unaddressed until the government stated in July that a fault beneath the first reactor of the two-reactor nuclear plant was active and dangerous, triggering a detailed survey by the plant's operator, Hokuriku Electric Power Co.

In Japan, nuclear reactors are not allowed to be built on any site that includes an active fault.

If survey determines that the fault is active, the power utility could be forced to decommission the reactor, currently offline, and reconsider its safety check standards for nuclear plant sites.

In an interim survey report given to the Nuclear Regulation Authority on Friday, Hokuriku Electric said it found no data indicating the fault underneath the reactor is active. Its final survey report is planned to come by the end of June.

The documents released by the NRA said that experts serving as advisers to the government checked the site and found a shear alleged to have been caused by an active fault in May-June 1987, before the construction of the reactor began in 1988.

A Hokuriku Electric official said the shear was then concluded to have been caused by tidal erosion and posed no problem.

The shear, which was found in a layer that was 120,000 to 130,000 years old, might have failed to attract attention because safety check standards in 1987 had defined a dangerous fault as having been active up to 50,000 years ago.

The standards were revised in 2006 to define it as having done so up to 130,000 years ago. A dangerous fault is now defined as having been active up to 400,000 years ago.