Lucens reactor

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The Lucens reactor at Lucens, Vaud, Switzerland, was a nuclear reactor destroyed by a serious accident in 1969.

In 1962 the construction started of a Swiss-designed pilot nuclear power plant. The heavy-water moderated, carbon dioxide cooled, reactor was built in an underground cavern ^[1] and produced 30 megawatts of heat (which was used to generate 8.3 megawatts of electricity).^[2] It became critical in 1966 and the plant was commissioned in 1968. It was fueled by 0.96% enriched uranium alloyed with chromium cased in zircalloy (magnesium with 0.6% Zirconium) inserted into a graphite matrix. Carbon dioxide gas was pumped into the top of the channels at 6.28 MPa and 223°C and exited the channels at a pressure of 5.79 MPa and at a temperature of 378°C.

It was intended to operate it until the end of 1969, but on January 21, 1969 during a startup it suffered a loss of coolant accident, leading to a partial core meltdown and massive radioactive contamination of the cavern, which was then sealed.

The accident was caused by water condensation during shutdown forming on some of the magnesium alloy fuel element components and corroding them. The corrosion products from this accumulated in some of the fuel channels. One of the 73 vertical fuel channels was sufficiently blocked by it to impede the flow of carbon dioxide coolant so that the magnesium alloy cladding melted and further blocked the channel. The increase in temperature and exposure of the uranium metal fuel to the coolant eventually led to the fuel catching fire in the carbon dioxide coolant atmosphere. The pressure tube surrounding the fuel channel split because of overheating and the burning fuel assembly bowing and the carbon dioxide coolant leaked out of the reactor.^[3]^[4]

No irradiation of workers or the population occurred, though the cavern containing the reactor was seriously contaminated. The cavern was decontaminated and the reactor dismantled over the next few years.