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 nuclear-waste 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.
"There is definitely a fault there that
has been active until recently."
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
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.
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* Seismic activities appear to be in the increase
in the past decade. There had been several this year in Indonesia alone,
not forgetting the horrendous Sichuan earthquake that wiped off some 80,000
Chinese last month. It would be advisable for Japan Nuclear Fuel Limited
to take heed of the earth scientist Watanabeâ€™s finding; whether
the fault line runs directly below the nuclear reprocessing plant or not
should not be the main contention. A construction that can withstand a
magnitude 6.9 earthquake is no criterion for complacency. The possibility
of a 7.0+ quake is always high. The Kashiwazaki-Kariwa incident last year
already signaled a serious warning. Japan has been prone to high magnitude
earthquakes throughout its history. Unless and until the plant can be built
to be quake proof, JNFL ought to give the whole project (especially its
location) a very careful second thought. (Tan Boon Tee)
04 Jun, 2008
Posted by: B T Tan