UNIT SHUTDOWN DUE TO DEPRESSURIZATION OF ONE FUEL CHANNEL
Monday, May 25, 1992
While in normal operation at nominal power coolant flowrate decrease in one of the fuel channels (pressure tubes) (TK 52-16) to the zero level occurred due to failure of the isolation-control valve installed in the inlet line to the above channel. This has led to fuel assembly (FA) cooling capacity degradation, TK 52-16 fuel channel depressurization and a damage to FA in this channel. As a result of coolant leak to reactor space pressure build-up occurred there, and on reaching the setpoint AZ emergency protection actuated. The reactor was scrammed and its cooldown started. The steam-water mixture from reactor space was received by the Accident Isolation System (AIS) as designed. Degradation of radiation situation occurred in the unit rooms (radioactive product spread in the ventilation and draining systems), and release of small amounts of activity from the AIS to environment. Radiation situation in the plant surveillance zone remained normal both for the overall background radiation and for specific radionuclide concentration in the atmospheric air. Raidation situation on-site in general remained within specified standards, and only near the AIS the doserate tended to increase up to 300 mcR/h. After reactor facility cooldown efforts were started to get ready for replacement of the damaged fuel channel and the unit was taken out of service for maintenance. Basis for event rating: 1. under the "off-site impacts" criterion the event is not rated. Radiation situation off-site Leningrad NPP remained normal and this event had no health effects on the public. The event resulted in the release of activity amounting to about 51 Ci for noble gases and 0.7-2.5 Ci for Iodine-131. The gamma-background levels on March 24-26, 1992 in the vicinity of Leningrad NPP were 16-20 mcR/h which is below the natural background levels. Spectrometry measurements of atmospheric air samples showed the presence of Iodine-131 at levels significantly below the permissible values. Soil examination showed no presence of I-131. 2. Under the "On-site impacts" criterion the event is rated as level 2 (in accordance with the final revision of the INES user's manual discussed at the 18-18 March 1992 meeting in IAEA). On 24.03.92 between 06:00 and 10:00 a.m. ten-fold gamma-background radiation increase in the range of 30-300 mcR/h on the plant site close to Unit 3 was detected as a result of noble gases and I-131 release. Plant personnel exposure doses during the event were below the sensitivity threshold of personal dosimeters (50 mrem) while maximum permissible personnel exposure is 5 rems. During the event, in some of Unit 3 rooms there were short-time radioactive aerosol concentration increases (by the factor of 10-50) as compared to concentrations observed at normal operation without exceeding the maximum permissible concentrations for personnel. 3. Under the "Degradation of Defence-in-depth" criterion the event is rated as level 2. In this case a real initiator occurred (fuel channel depressurization) requiring safety function implementation. This initiator as regards its frequency is assigned to the category of "possible" initiators. Safety function availability is determined as being "within the safe operation limits and conditions". Unit safety systems actuated as designed; however, the accident isolation system envisaged by the design failed to fully ensure radioactive product isolation. Under Table A2 (III.2) of the User's Manual the level of the event is 2/3; however, there are no additional factors in this case to raise the initial level (2) by 1.