The Jaslovské Bohunice, A1 plant was built in Jaslovské Bohunice, Slovakia region, Czechoslovakia between 1952 and 1956. The reactor in question was biult in 1956 and activated in 1972.
The 1977 accident
It had had s aeries of earlier minor accidents and a steam exploitation had killed 3 workers in 1976. The 1977 one was a Level 4 INES Scale event.
Some of the silica gel pack humidity absorbers covering the fuel rods that were being replaced were not removed, thus reducing circulation by the carbon dioxide coolant gas (CO2 gas), causing local overheating, damaging the thermally active zone (25% of the core), resulting in overheating of the fuel and the pressure channel holding it.
Heavy water got in contact with the coolant as it poured in to the damaged cooling system through leaks in the steam boilers, so both the primary and secondary gas circuits were contaminated. Fuel cladding was also corroded and a considerable amount of radioactivity was then leaked into the primary cooling circuit.
It was a Level 4 INES Scale event.