 INES 3 |
COMPLETE STATION BLACK OUT DUE TO FIRE IN TURBINE BUILDING |
Unit 1 was operating at 185 MWe. At 0330 hours in the morning a fire occurred in the turbine building. The reactor was tripped manually. Crash cooldown of the reactor was initiated. Secondary shutdown system got actuated on initiation of crash cooldown as per logic. There was a complete loss of power supplies in unit 1 including loss of classs I & II supplies. Borated heavy water was added to the moderator through Gravity Addition of Boron System (GRAB) to maintain the reactor in a subcritical state at all stages subsequently.
TC set and its accessories, bus ducts and excitation cubicles suffered extensive damage. Many cables were damaged because of fire which resulted in complete station black out. Core decay heat removal was through forced circulation till the time PHT circulation pumps tripped, subsequent to that, heat removal was by flow coast down and natural circulation (thermosyphoning). Water was fed to the secondary side of steam generators using diesel fire pump.
There was no loss of life or injury to personnel. There was no radioloical impact due to the incident either on the plant or in the public domain. In accordance with the specified safety procedures an emergency confined to the plant only was declared which was lifted about 19 hours after the incident had occurred.
The event is rated at level 3 as per table II of User's Manual.
Justification of the Rating:
1. Turbine generator trip was followed by manual reactor trip and total loss of power and complete station black out. Loss of onsite electrical power (class IV, III, II or I) is a possible initiator (Ref. Part III.A.3.2, Page 53 of User's Manual) which occurred actually (i.e. real).
2. The on site and off site criteria are not relevent for rating. Only criterion for degradation of defence in depth is applicable.
3. There are three safety functions for any reactor (Ref. part II - 1.1 Page 19 of user's manual)
(i) Control of the reactivity
(ii) Adequate cooling of the radioactive material
(iii) Confinement of the radioactive material.
The reactor was tripped and borated heavy water was added to the moderator to maintain the reactor in safe shutdown state with adequate margin of subcriticality.
Core cooling was done by thermosyphoning and injection of water to steam generators.
There was no release of radioactive material from the PHT system and as such radioactive materials were confined.
Though all the safety functions were adequate the incident was rated at level 3, because the whole system though available was degraded by loss of many indications.
Event date: Wed, 31-03-1993
