 INES 2 |
SG blow down pipe rupture at Kozloduy NPP unit 1 during refuelling outage |
At 19 May 1998 when Unit 1 was at refuelling outage, the fuel loading of the reactor was completed and the concentration of H3BO3 in the primary circuit was 12g/l. During the decontamination of secondary side of SG 3 by chemical solution a rupture of SG blow-down pipe occurred and part of the solution was spilled in the SG compartment. The solution then reached the emergency boron tank and polluted it with chemical products.
On 20-21 May the tank was drained, cleaned and filled again with concentration of H3BO3 equal to 12 g/l. During the drainage and cleaning of the tank three additional lines were assured for delivery of boron solution to the primary circuit and to the spent fuel pool, if necessary. Also measures were taken to prevent possible penetration of clean water to the reactor and spent fuel pool. Nevertheless the operation was outside operational limits and conditions because the emergency boron tank was not available.
During the week 22 (25-29 May 1998) the event was investigated by the Bulgarian Nuclear Safety Authority (BNSA). The cases for the SG blow-down pipe rupture are still not clearly identified. Additional control of all similar pipes of all SG's of WWER-440 units at Kozloduy NPP was required by the BNSA.
The event was preliminarily rated at level 1 according to the Section III-3.2 of the INES user's manual. After the event investigation the INES level was increased by BNSA to level 2 due to deficiencies of safety culture violation of procedures without justification (Section III-5.3).
Provisional INES rating level 2.
Event date: Tue, 19-05-1998
