The cause was a corrosion-induced loss of heat dispersal leading to the destruction of a pressure tube which caused an adjacent pressure tube to fail, and partial meltdown of the core, resulting in radioactive contamination of the cavern.

[1][5][6][7] The heavy-water moderated, carbon dioxide gas-cooled reactor was built in a cavern.

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

[11] The accident was caused by water condensation forming on some magnesium alloy fuel element components during shutdown and corroding them.

One of the vertical fuel channels was sufficiently blocked by it to impede the flow of carbon dioxide coolant, causing the magnesium alloy cladding to melt and further block the channel.