It was built in an Alpine cave in Lucens, Vaud, during 1962. It was a small, civil pilot nuclear reactor, that was destroyed by an partial meltdown accident during 1969.
The 1969 accidentEdit
Some water condensation had formed on some of the magnesium alloy fuel element components during the previous shutdown and corroded them. It subsequently accumulated in and blocked some of the 73 vertical fuel channels, thus impeding the flow of carbon dioxide coolant. A coolant leak, rector overheating incident and fuel rod fire then occurred, leading to a partial meltdown. The pressure tube surrounding the fuel channel split because of the overheating and bowing of the burning fuel assembly, releasing the irradiated CO2 coolant gas. The radiation release and subsequent massive radioactive contamination was confined to the cave's interior, which was then sealed off until it was cleaned out several years later.
It has no official INES level, but was probably a Level 5 INES Scale event.