About 760 tons of radioactive sodium remain in the piping and other equipment of the trouble-prone Monju prototype fast-breeder nuclear reactor, which may be ordered decommissioned, Jiji Press learned Sunday.
It has not been decided how to dispose of the radioactive sodium, said sources at the Japan Atomic Energy Agency, the operator of Monju. If the government decides to scrap the reactor, sodium disposal is expected to be a difficult challenge.
Sodium is used as a coolant at Monju, while water is used at conventional nuclear reactors. Sodium is a tricky chemical element that burns intensely if it comes into contact with air or water.
According to the agency, the Monju reactor has some 1,670 tons of sodium. Radioactive substances are contained in 760 tons of the total as it circulates inside the reactor vessel.
The Monju reactor needs to be drained of the sodium if it is to be demolished.
Radioactive and chemically active sodium has to be sealed in containers. There is no precedent of radioactive sodium disposal in Japan.
“We plan to consider the method of disposal if a decision is made to decommission it (Monju),” an official said.
Monju, located in Tsuruga, Fukui Prefecture, is a core facility in Japan’s nuclear fuel cycle policy as the reactor produces more plutonium than it consumes.
More than ¥1 trillion, mostly from state budgets, has been invested in Monju. But the 280,000-kw reactor has operated for only 250 days since it reached criticality, a self-sustained nuclear fission chain reaction, for the first time in April 1994, due to a raft of problems, including maintenance flaws, a sodium leak and fire and attempted coverup.
In November 2015, the Nuclear Regulation Authority advised the government to replace the operator of Monju. The government is carrying out a thorough review of the Monju project, including the possibility of decommissioning the reactor.
The disposal of mixed-oxide, or MOX, fuel used at Monju is another significant issue. The amount of MOX fuel, a blend of uranium and plutonium recycled from spent nuclear fuel, that needs to be disposed of is estimated at 21 tons, but Japan is not equipped to carry out disposal.
One option is to consign the disposal to a foreign country and receive the return of uranium and plutonium after the processing, along with radioactive waste.
But the agency’s cost estimate of ¥300 billion for decommissioning Monju does not include the expense of the overseas entrustment of MOX fuel disposal.
The agency aims to entrust France with the disposal of some 64 tons of MOX fuel that has been used at its Fugen advanced converter reactor, but no contract has been concluded. The Fugen reactor, also in Tsuruga, is slated to be decommissioned.
Spent MOX fuel contains larger amounts of highly toxic radioactive substances than spent uranium from conventional reactors.
The disposal of radioactive sodium and MOX fuel at Monju is emerging as additional difficult challenges for the government at a time when the final disposal site has not been decided for high-level radioactive waste from nuclear plants across Japan.