Japan’s government formally decided Wednesday to decommission a $10 billion nuclear reactor in its west once considered a keystone in its efforts to become energy self-sufficient.

The Monju prototype fast-breeder had barely operated over the past two decades despite its envisioned role.

The decision in a ministerial meeting concluded a process that has included discussion of the country’s overall fast-reactor development policy by a government panel.

“We will decommission Monju given that it would take a considerable amount of time and expense to resume its operations,” state news agency Kyodo reported Chief Cabinet Secretary Yoshihide Suga as telling Wednesday’s meeting.

According to government forecasts, restarting operations at the plant would have cost at least 540 billion yen ($4.6 billion)

It is expected to cost at least 375 billion yen over 30 years to fully decommission the reactor. The government plans to remove its spent nuclear fuel by 2022 and finish dismantling the facility in 2047.

The reactor had been on life support for years partly out of sheer inertia and job-related politics and because it’s closure could set in motion a train of decisions effectively ending fuel recycling.

The Monju, like other breeder reactor prototypes around the world, is designed to produce more nuclear fuel than it consumes.

But such reactors are technically difficult to build and control, and the use of plutonium as fuel raises nuclear proliferation concerns.

In 1994, only a few months after first sustaining a nuclear chain reaction, Monju suffered a serious accident when liquid sodium coolant leaked and caught fire.

Sodium is used as coolant in breeder reactors despite being volatile and flammable.

The Japan Atomic Energy Agency (JAEA), which runs the Monju, was also later accused of trying to cover up the accident.

Aside from a brief operation in 2010, the Monju never operated again. During three decades it only operated for a few months at a total cost of around $10 billion.

But the Monju managed to stay alive despite a long trail of accidents, cover-ups and cost overruns, even as other foreign countries, including the United States, turned away from breeder reactor technology and closed their own prototypes.

The new Nuclear Regulation Authority (NRA), the safety agency created following the 2011 Fukushima crisis, sounded Monju’s death knell in late 2015 when it declared JAEA “not qualified as an entity to safely operate the Monju”.

It gave the Ministry of Science and Technology six months to come up with a suitable replacement and stated that any replacement must have the technical capability to perform the operations required to secure safety.

The NRA’s terms were generally considered impossible to meet since JAEA was the only entity in Japan with any practical experience in operating Monju or any breeder reactor.

Although the Monju may be essentially dead, Japan’s government under Prime Minister Shinzo Abe remains committed to using plutonium to power nuclear reactors.

NRA chairman Shunichi Tanaka, who is always precise in defining the parameters of the authority’s decisions, said his agency was questioning only Monju’s safety, not the government’s nuclear recycling policy as a whole.

But rather then creating plutonium in a reactor, it plans to take spent fuel from ordinary reactors, separate the plutonium, mix it with ordinary uranium and put it back in reactors as Mixed-Oxide (MOX) fuel.

Presently only three reactors are operating in Japan, as the NRA moves slowly and carefully in giving individual units the green light to begin operating again. One of the units, the Ikata plant on Shikoku island, uses MOX fuel.

For now, the reprocessing is done in France, but Japan has its own reprocessing plant in Rokkasho village on the northern tip of Honshu. It has its own trail of mishaps, cost overruns and delays but is currently scheduled to go into operation in 2018.

With the Monju now slated for decommissioning, and reprocessing facing an uncertain future, the attention turns to what to do with the huge oversupply of Japanese-owned plutonium stored in Europe and Japan.

There are around 48 tons in in Europe and 11 tons in Japan — enough, in theory, to make hundreds of atom bombs.

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