JAPAN鈥檚 showcase fast-breeder nuclear reactor was shut down last weekend when the secondary cooling system sprang a leak. Engineers found that 鈥渓arge amounts鈥� of liquid sodium had escaped from a suspected welding crack, and toxic sodium compounds covered the floor and part of a wall in a room containing the coolant pipes.
Liquid sodium is used to cool the 280-megawatt fast-breeder reactor because it runs at such high temperatures. A fast-breeder is fuelled by plutonium instead of uranium, and Japan is the only country to be energetically promoting fast-breeder technology. But critics have long warned of the dangers of liquid sodium.
The Monju reactor in western Japan has already suffered one problem with its cooling system, when the length of the cooling pipes was miscalculated. The anomaly was picked up in 1991 before Monju鈥檚 start-up, and was corrected the following year when extra lengths of pipe were welded on.
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Officials at Japan鈥檚 Power Reactor and Nuclear Fuel Development Corporation (DONEN), which operates Monju, now suspect that the welding was badly done. When engineers entered the room where the leak happened, they found a cake of sodium debris on the wall nearest one of the welds, suggesting that sodium had spewed out from that point. Wearing protective masks and carrying oxygen tanks, they also found a 鈥渢hick layer鈥� of the leaked material on the floor, and the air was filled with dust from toxic sodium compounds.
Over the weekend, engineers began removing the 250 tonnes of sodium from the cooling system, and the leak stopped. The biggest problem now is how to clear the highly toxic dust. DONEN officials do not want to operate the ventilation system for fear of spreading the dust even further.
The leak is a major setback for Monju, which is Japan鈥檚 first large-scale prototype fast-breeder reactor. Monju went critical last year, and produced its first electricity in August this year. The programme has been plagued by delays, technical problems and cost overruns. Critics say the fast-breeder reactor is dangerous and uneconomic at a time when nuclear fuel is both abundant and cheap, thanks to the end of the Cold War.
Although the Superph茅nix fastbreeder reactor in France has also leaked some of its liquid sodium coolant, this is the first accident in Japan. DONEN officials say Monju will be shut down for at least three months while engineers repair the system.