A team of experts appointed by the Nuclear Regulation Authority started a two-day investigation Wednesday on the activity of geologic faults running beneath the Monju prototype fast-breeder reactor in Fukui Prefecture.

The reactor belonging to the Japan Atomic Energy Agency is the fourth location visited by an NRA-led team to check whether "shattered zones," or zones of crushed rock, at the site could move in the future and undermine the safety of the facility.

The Monju reactor sits above eight small shattered zones, which may move together with an active fault running about 500 meters west of the facility.

While Monju has effectively been banned from operation because of the operator's lax safety management, reactors are not allowed to be located directly above active faults. If the shattered zones are acknowledged to be active faults, Monju would probably never be allowed to be restarted.

More than ¥1 trillion has been poured into the Monju project in the hopes it would play a key role in the country's spent-fuel recycling policy.

Instead, plagued by a sodium coolant leak and subsequent problems, the reactor has remained largely offline since it first achieved criticality in 1994.

The investigation team is led by NRA commissioner Kunihiko Shimazaki and four other experts recommended by academics. One of the outside experts will not participate in the two-day field survey and will visit the site later.

Among the four nuclear facilities that have accepted investigation teams, the NRA has reached a conclusion that reactor 2 at Japan Atomic Power Co.'s Tsuruga nuclear plant in Fukui Prefecture sits atop an active fault. But the operator disputes this assessment, which could leave the firm with no option but to scrap the unit.