Monju (もんじゅ) was a Japanese sodium-cooled fast reactor, located near the Tsuruga Nuclear Power Plant, Fukui Prefecture.

Monju was a sodium cooled, MOX-fueled, loop-type reactor with three primary coolant loops, designed to produce 280 MWe from 714 MWt.

However, there was massive public outrage in Japan when it was revealed that Power Reactor and Nuclear Fuel Development Corporation (PNC), the semi-governmental agency then in charge of Monju, had tried to cover up the extent of the accident and resulting damage.

[18] In late February, JAEA requested Fukui Prefecture and Tsuruga City for deliberations aimed at resuming test operation.

Having obtained the go ahead from both entities, JAEA started criticality testing, after which it took some months before commercial operation could resume – as for any new nuclear plant.

Test runs were to continue until 2013, at which point the reactor could have started to feed power into the electric grid, beginning "full fledged" operation.

[25] On 31 May 2013 science and technology minister Hakubun Shimomura announced that Shojiro Matsuura, (77 years) the former chairman of the Nuclear Safety Commission, would be the next president of JAEA on Monday 3 June.

At that moment it was not possible to restore the connection, because the reactor site in Tsuruga was inaccessible due to mudslides and fallen trees caused by the typhoon.

[33] On August 3, 2016, it was discovered that an alert triggered on November 19, 2015, when the quality of the water in a spent nuclear fuel rod pool deteriorated, was ignored until April 2016 and rectified only the next month.

On September 30, officials of the Science and Technology ministry explained their decision not to start the test run at meetings in the city of Tsuruga and Fukui Prefecture.

[37] After it was revealed in November 2012, that regular safety checks had been omitted, the Nuclear Regulation Authority ordered JAEA to change its maintenance rules and inspection plans.

[citation needed] Although the resignation was accepted by the government, the move was a surprise, because on May 16 Susuki had spoken on a meeting in the Japanese parliament, the Diet, and to the NRA secretariat and had pleaded to restore the public's trust in the JAEC.

On 20 November a seven-member Japanese government commission decided that the future of the Monju reactor should be thoroughly reviewed before a decision could be made for this 2012 budget.

Some members of the commission thought that there would be little public support for restarting the fast breeder project, and that it was uncertain that the reactor could be taken into commercial service in 2050 as originally planned.

Decisions about the 2012 budget would be taken after the discussions in a panel of cabinet members about the nuclear policy of Japan, including the fast breeder reactor project, would be complete.

[41] Reports in 2012 indicated that plans to generate electricity at Monju would be abandoned, and the plant repurposed into a research centre for handling spent nuclear fuel.

The NRA said that before it could plan a restart of the reactor, JAEA must allocate appropriate funds and human resources to rebuild a maintenance and management system to prevent the recurrence of coolant leakages and other problems.

[43] On 2 March 2015 Noboru Hirose, a senior NRA official, told NHK[44] at the beginning of a 3-week regular safety check that he could not say when test runs would be permitted to start.

On top of this, the presence of the oceanic faults were not taken into account by NISA and JAP in the assessment of the safety of the Tsuruga nuclear power plant.

Because the Mihama Nuclear Power Plant and the Monju fast breeder reactor could also be affected by a possible earthquake caused by the Urazoko fault.

[46] On 17 July 2013 a commission of five experts led by NRA commissioner Kunihiko Shimazaki started the investigations on the geological activity of 8 zones of crushed rock under the reactor.

Another acoustic survey of the grounds was planned by Japan Atomic Energy Agency and a geological examination to determine the age of the clay and stones in the faults.

[49] On 21 October 2011 the Japanese government appointed a commission to study ways to cut wasteful expenditures, one possibility being decommissioning the Monju prototype fast breeder reactor.

[50] On 27 November, after a visit to the plant, nuclear disaster minister Goshi Hosono said that scrapping the Monju-fast-breeder reactor was an option that would be given serious thought.

Politicians and private sector experts of the ruling Democratic Party of Japan made proposals for a thorough operational and budgetary review in the government's energy policy screening session earlier in the week before his visit.

The phases of the plan are:[54] Despite its intention to close the Monju facility, the Cabinet appeared to reaffirm its commitment to a fast breeder program of some kind, essential if Japan's stockpile of some 50 tonnes of plutonium is to be disposed of.

The successor to Monju was expected to be a larger demonstration plant to be completed around 2025, built by the newly formed Mitsubishi FBR Systems company.