INES-event
INES 3

UNAVAILABILITY OF LOW POWER EMERGENCY PROTECTION (LPEP) DUE TO LOSS OF IONIZATION CHAMBER POWER SUPPLY

After unit shutdown for preventive maintenance during the routine test of reactor overpower emergency protection in the low power range (LPEP) the failure of ionization chamber power supply cabinets of 3 trains out of 4 due to insulation damage of the +500 V power cable was found. The damage (melting) of cable insulation occurred due to high temperature in the room, where the cable route passes. Operating personnel activated only AZM protection (power level emergency protection) and started to perform refueling (unloading of the spent fuel assembly, installation of additional absorber) without activating the LPEP protection, which is forbidden by the procedure. In fact, because of the effect of RPS normal interlocking the LPEP protection could not be activated at any position of RPS control rods.
BASIS FOR RATING: Initial event - deenergization of ionization chamber power supply cabinets. Event - expected, postulated. Safety function availability - insufficient (since no protection existed for the 0%-10% Nnom range), which corresponds to Level 2/3 (D-2P). The event revealed operation deficiencies (contradictions between the items of the technological procedure and reactor operating instruction) as well as safety culture deficiency (item 4.2,b). Therefore the maximum level has been chosen.
DIFFICULTIES IN RATING:

Location: CHERNOBYL-1
Event date: Wed, 10-04-1991
Nuclear event report
Legenda & explanation