INES-event
INES 1

DEGRADATION OF NATURAL CIRCULATION

Refuelling of Unit 4 was commenced on 18 June 1988. Operating personnel performed a planned switchover to natural circulation from loops 4 and 6 to loops 1 and 3 on 30 June. Meanwhile, the effectiveness of core cooling by natural circulation decreased sharply for about a 3 hour period. After switching over the natural circulation among the llps, the operating personnel isolated loops 4 and 6 and started to drain them. Nitrogen used to drain the water got into the loops in operation and most of the water was pushed from the SG primary side into the reactor vessel. The operator tried to stop disturbance of natural circulation by starting periodically the make-up pump to the working loops. During this injection the personnel performed venting several times to take away the gas-air mixture from the top of SG headers, Eventually, restoration of natural circulation was achieved by continuous venting of the SG headers.
BASIS FOR RATING: BIR ref. 4.2b, 4.4
DIFFICULTIES IN RATING:

Location: PAKS-4
Event date: Thu, 30-06-1988
Nuclear event report
Legenda & explanation