The 280MWe unit has been laid up since a secondary sodium leak in December 1995, having first supplied electricity to the grid in August that year. The fast breeder reactor programme is a key part of Japanese policy for greater energy independence.
The Kanazawa branch of the Nagoya high court ruled in favour of a case brought by 32 residents living near the site, who, in September 1985, filed against the project on safety grounds.
The court said that approval given in December by the Ministry of Economy, Trade and Industry (METI) to the Japan Nuclear Cycle Development Institute (JNC) to install sodium leakage counter-measures was annulled because the safety case for its operation was inadequate. JNC had planned to start the modification work next month.
The decision also overturns a March 2000 ruling of a lower court in Fukui, which determined that, apart from the 1995 sodium accident, there were no safety deficiencies at Monju.
In last month's high court ruling, several specific safety concerns were highlighted. Judge Kazuo Kawasaki said Monju "lacked preventative measures against a sodium leak," and that it "presented a risk of core decay."
The residents that brought the case claimed that "significant knowledge, including that sodium might react with the iron floor liner," was not sufficiently addressed. It was also suggested that high-temperature ruptures could be caused by contact in steam generators between water leaking from pipes and sodium. The court also questioned the demonstration of recriticality safety in an accident.
In response to the ruling, METI said: "Nullification of a past administrative disposition requires a serious, clear contravention. This has been made clear in supreme court precedents and the decision deviates from this."
On 31 January, the government launched its appeal against the decision.