[ RadSafe ] Japanese nuclear plant in quake risk

[Boby Mathew]

http://www.nature.com/news/2008/080604/pdf/453704a.pdf
 
A nuclear reprocessing plant in northern Japan
is sited directly above an active geological fault
line that could produce a magnitude 8 earthquake,
some earth scientists say.
The massive Rokkasho plant for uranium
enrichment, spent fuel reprocessing and nuclearwaste
storage is built on an uplifted marine terrace
of sloping sedimentary rock layers on the
northeast coast of the island of Honshu. According
to Mitsuhisa Watanabe, an earth scientist at
Toyo University in Tokyo, there
is an active fault lying directly
under the plant. Watanabe presented
his findings on 27 May at
the annual meeting of the Japan
Geoscience Union in Chiba.
But Japan Nuclear Fuel Limited (JNFL), which
runs the plant and is based in Aomori City, disagrees,
saying that Watanabe’s announcement
has “unnecessarily sparked fear in people”. JNFL
says that seismic reflection profiling shows that
no part of the fault line described by Watanabe
has seen any action for 1 million years, and
that the fault doesn’t extend beneath the plant.
National guidelines issued in 2006 state that
only faults with movement within the previous
120,000 to 130,000 years need be considered
active when evaluating earthquake resistance of
nuclear facilities. The JNFL survey concluded
that there was no reason to fear an earthquake of
more than magnitude 6.5 at the site, and that the
plant could withstand a 6.9 quake nearby.
Last July, Tokyo Electric Power Company’s
Kashiwazaki-Kariwa nuclear power plant —
which was designed to withstand a magnitude
6.5 earthquake — was unexpectedly rocked by
one of 6.8 (see Nature 448, 392–393; 2007).
Watanabe analysed JNFL’s seismic reflection
profiles of the Rokkasho site in addition to his
own earth-deformation surveys based on aerial
shots taken between February
and early May this year. He says
that the uplifted structure created
some 120,000 years ago
shows many signs of deformation
since then — characteristic
of land sitting over what is called a reverse fault,
which he estimates at about 15 kilometres long.
“There is definitely a fault there that has been
active until recently,” Watanabe says. He adds
that the fault might link up with an undersea
fracture to create a 100-kilometre-long fault
capable of pounding the Rokkasho plant with
a magnitude 8 earthquake.
Seismology and earthquake-safety specialist
Katsuhiko Ishibashi, emeritus professor at
Kobe University, agrees with Watanabe that
there is probably a 15-kilometre fault directly
below the plant. The idea of a longer fault
needs further investigation, he says. Either way,
Ishibashi worries that an earthquake larger than
expected could inflict serious damage on the
plant. “In the worst-case scenario, the whole
of northern Japan and even as far as the wider
Tokyo area could suffer a serious radiation disaster,”
he says.
Jim Mori of Kyoto University’s Disaster Prevention
Research Institute says that Watanabe’s
conclusions are reasonable, but the data could
interpreted in other ways. He recommends
further study, including higher-resolution
seismic surveys, bore holes drilled into the
fault — which would be possible, but probably
too costly — and more geological work at sites
along its length.
JNFL submitted its seismic report in November
2007 to the Nuclear and Industrial Safety
Agency, which is now reviewing it. An official
there told Nature that Watanabe’s critique would
be taken into account, but he did not say what
measures would be taken if the possibility of a
larger earthquake was borne out.
The Rokkasho plant is at the heart of Japan’s
plan to reprocess spent fuel for plutonium that
can be mixed with fresh uranium. This has met
with resistance and the country has yet to decide
on a site where it could build a power plant to
burn the reprocessed mixed oxide fuel. The current
debate is likely to complicate issues. ■
David Cyranoski
 
NEWS NATURE|Vol 453|5 June 2008