The cleaning system had been installed to remove dirt and corrosion from fuel elements and control rods during shutdown, as there had previously been problems with magnetite corrosion products from the steam generators being deposited on the fuel elements which affected the flow of coolant. The sixth set of thirty partially spent elements were in the tank having been cleaned, the cleaning having finished at 16:00. At 21:50, radiation alarms mounted on the cleaning system detected a sudden increase in the amount of krypton-85. The suspicion was that one of the fuel rod assemblies was leaking. At 22:30, the reactor hall was evacuated because of elevated radiation levels both there and in the ventilation stack.