[NukeNet] Kashiwazaki-Kariwa Seismic Design Standard

[Citizens' Nuclear Information Center]

http://www.asahi.com/english/Herald-asahi/TKY200805240056.html

EDITORIAL: Safety of nuclear plants

05/24/2008

Tokyo Electric Power Co. (TEPCO) called Thursday for new 
quake-resistant standards that are five times higher than current 
levels for its Kashiwazaki-Kariwa Nuclear Power Plant in Niigata 
Prefecture.

The Niigata Chuetsu-oki Earthquake, which struck offshore last July, 
sent fierce seismic jolts directly below the nuclear plant, causing 
extensive damage. As a result, the facility's seven reactors were shut 
down.

Using earthquake-resistance design guidelines revised in 2006, TEPCO 
factored in the new data gained from observations of that temblor to 
calculate the estimated intensity. It concluded that greater 
earthquake-proofing was needed.

Specifically, it was stated that the bedrock under Reactors 1 through 4 
must be secured against temblors 5.1 times stronger than believed safe 
to date, with the area under the No. 5 to 7 reactors to be reinforced 
by 2.6 times the present strength.

In the interests of safety, TEPCO intends to equip the facility with 
quake-resistance capabilities above and beyond these revamped 
estimates, with the reinforcement work to commence next month. While 
all Kashiwazaki-Kariwa reactors are currently shut down for inspection, 
these latest findings further complicate any plans to resume 
operations.

Why have the estimated values of the seismic shocks that these 
facilities must withstand surged to such a degree?

For one, the new guidelines were used to carry out detailed studies of 
the active faults in the vicinity of the plant.

Rigid estimates were likewise made of the scale of quakes caused by 
those faults. Precise methods for computing the swaying likely to stem 
from fault movements were also devised.

Besides that, observation data from the Niigata Chuetsu-oki Earthquake 
and other seismic activity determined that the geological strata 
beneath the nuclear plant zone also possesses characteristics that 
amplify earthquake vibrations.

Estimates of seismic waves based on the new earthquake-resistance 
design guidelines for other nuclear plants around the country were 
completed this past spring. The findings forced the operators of all 
facilities to make upward revisions in their forecasts of shocks--the 
largest being 1.6 times the previous level.

The five times greater resistance required at Kashiwazaki-Kariwa 
certainly stands out.

This series of modifications also clarified the fact that lengthy 
active faults pass directly beneath the Kansai Electric Power Co.'s 
Mihama Nuclear Power Plant and the Japan Atomic Energy Agency's Monju 
fast-breeder reactor--both located in Fukui Prefecture.

In short, the educated judgments of seismic intensity applied to the 
original construction of these and other nuclear plants have proved to 
be easygoing.

One lesson to be learned is that seismology is a science in which new 
discoveries occur daily, with rapid advances also continuing to be made 
in fault surveys and other technical fronts. The key premise, 
therefore, must be that seismic estimates will need to be updated from 
here on as well.

Power companies and others in the industry play down the fear factor, 
insisting that nuclear power plants are designed with proper leeway to 
handle earthquake activity. However, when the Kashiwazaki-Kariwa 
figures are added to the seismic estimates, it is obvious that all 
nuclear plants in Japan are frittering away whatever such leeway they 
may have had.

When thinking of earthquake-proof engineering, it is vital to realize 
that new grounds for concern will continue to emerge hand in hand with 
progress made in research.

In an earthquake-prone nation like Japan, no further time should be 
wasted in making reinforcements to restore the leeway necessary to 
ensure safe and trustworthy nuclear power generation.

--The Asahi Shimbun, May 23(IHT/Asahi: May 24,2008)