The commitment to Monju is rooted in the way Japan has sold its nuclear program to local communities, experts say. In persuading towns and villages to provide land for nuclear power stations, Japan has promised that the spent nuclear fuel -- which remains highly radioactive for years -- will not be stored permanently on site, but used as fresh fuel for the nuclear fuel cycle.

Giving up on any part of the fuel cycle would mean the government would have to find communities willing to become the final resting ground for the spent fuel.

"Of course, no community would accept that, and suddenly Japan's entire nuclear program would become unviable," said Keiji Kobayashi, a retired fast-breeder reactor expert formerly at the Kyoto University Research Reactor Institute.