Graphite-moderated reactor

Due to overheating from lack of adequate cooling, the fuel rods began to deteriorate.

After the SCRAM (AZ5) button was pressed to shut down the reactor, the control rods jammed in the middle of the core, causing a positive loop, since the nuclear fuel reacted to graphite.

Now exposed to both air and the heat from the reactor core, the graphite moderator in the reactor core caught fire, and this fire sent a plume of highly radioactive fallout into the atmosphere and over an extensive geographical area.

[4] In addition, the French Saint-Laurent Nuclear Power Plant and the Spanish Vandellòs Nuclear Power Plant – both UNGG graphite-moderated natural uranium reactors – suffered major accidents.

Particularly noteworthy is a partial core meltdown on 17 October 1969 and a heat excursion during graphite annealing on 13 March 1980 in Saint-Laurent, which were both classified as INES 4.