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 <i>Nature</i> 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.