The unit was operating at nominal power. Based on indications from the Burst Can Detection System instrumentaion personnel discovered increased steam activity at the fuel channels (FC) outlet which indicated breach of fuel assembly leak tightness. Once the affected fuel channel with a leaky fuel assembly was identified, efforts were started to extract the fuel assembly from the 47-45 FC. While extracting the fuel assembly radiation monitoring system alarm went off in the central (reactor) hall. Examination of the place of work performance revealed a fuel fragment about 5 mm in size at the receiving unit of the shipping cask containing the extracted fuel assembly and two fragments of 1.5-2 mm size on the fuel assembly plug. The fragments were removed according to the existing procedures. On completion of the above work individual dosimeter readings were checked for eight staff members who took part in the work. The check showed that two persons received external exposure doses above the annual dose limit (58.3 and 56.7 mSv respectively). The cause of fuel assembly depressurization was the rupture of the plug in one fuel rod. The cause of personnel overexposure - non compliance with the procedures.
Two events took place:
1) Depressurization of one fuel rod without degradation of the cooling system parameters.
2) Personnel exposure above the prescribed annual dose limit.
Event 1 is rated under the "degradation of defence in depth criterion" - level 1 (see section III-3.1.3 of the INES User's Manual).
Event 2 is rated under the "on-site impact" and under the "degradation of defence in depth" criteria.
Under the on-site impact criterion Level 2 was chosen according to Section II-2.2.
Under the degradation of defence in depth criterion Level 1 was chosen (see Section III-3.5). Taking into account additional factors related to safety culture deficiencies (unjustified breach of procedure) the basic rating was increased by 1.
The final rating - Level 2.
Event date: Tue, 21-11-1995