The Japan Atomic Energy Agency committed grave safety errors in managing the troubled Monju prototype fast-breeder reactor in Fukui Prefecture, and top officials at the Nuclear Regulation Authority said Monday they plan to make sure it stays closed.

The closure order to the government-linked JAEA will effectively dash any hope of trying to restart the reactor by year’s end, dealing another setback to Japan’s long-stalled plan to set up a nuclear fuel recycling system.

In September, the now-defunct Nuclear and Industrial Safety Agency carried out surprise inspections and determined that JAEA failed to regularly check key components of the experimental 280,000-kw reactor, as required by internal rules.

In November, the JAEA admitted that it failed to properly check nearly 10,000 pieces of equipment, including more than 50 critical “Class-1” components, including backup diesel electric generators.

The NRA, which replaced NISA last September, will also order the JAEA to rethink all its safety inspection regimens and management systems, sources said.

The NRA in December gave JAEA written instructions to investigate the causes of the improper checks and compile remedial measures.

But JAEA President Atsuyuki Suzuki reportedly insisted that the checks that were improperly carried out were mere formalities and posed no safety problems, drawing further criticism from the NRA.

“(Monju) is a very important facility in terms of safety, and it has a very strong social impact,” NRA Chairman Shunichi Tanaka told a Dec. 12 NRA session.

Tanaka said he worries that the JAEA “lacks the basic safety culture” to manage nuclear power facilities.

Monju is designed to use plutonium-uranium mixed oxide (MOX) fuel to theoretically create more fuel than it consumes through the reaction process. MOX is made with weapons-grade plutonium.

But the prototype has been effectively closed since it suffered a grave sodium coolant leak and fire in 1995 and an attempted coverup. JAEA tried to restart Monju in 2010, but it was soon halted after a heavy fuel-loading device fell into the reactor vessel.

Many experts have questioned whether Monju, which sits in the city of Tsuruga, can ever be stably run, given its serious technical problems. Nearly ¥1 trillion in taxpayer money has been spent on the project.

Outside experts also say that dangerous active faults may exist beneath the reactor’s key facilities. JAEA claims there is no evidence to support this allegation.