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Event Report Forms (ERFs) are filed to the IAEA. Sometimes there are multiple event reports, updating the rating, the event description. Here you only see the latest event report

ERF send date: Fri, 25-06-1993
ERF description: The unit was at a thermal power 3% of rated (Nnom) power with TG-2 at coast down and TG-1 under maintenance. A short circuit occurred in the armature winding of the reversible motor generator (RMG-1) connected to the unit DC panel (UDCP-1). Automatic breaker which disconnects UDCP-1 from RMG-1 failed to open, and the DC grid continued to receive voltage which significantly differed from the nominal. The resulting current oscillations have led to the opening of the automatic breaker which supplies power to RPS emergency protections panels from UDCP-1. The automatic breaker of the same panels fed from the common unit DC panel opened on unexplained cause. Because of loss of voltage on the RPS panel AZ1 protection actuated and the reactor was scrammed. The signal for diesel generators (DG) start was generated. The prestart oil circulation pumps (POCP) of these diesels fed from UDCP-1 started. Because of the total short circuit current in RMG-1 generator and the load current including the POCP starting currents, the automatic breaker connecting UDCP-1 to the battery opened. The automatic breaker feeding UDCP from RMG-1 was opened manually. As a result of UDCP deenergization operating DC supply to control circuits of some mechanisms without standby sypply from other DC panels was lost. This resulted in the failure to start two service DGs and one standby DG, loss of remote control function of safety injection pumps, emergency feedwater pumps, SG safety valve and normal operation systems mechanisms. 20 mins. later the power to UDCP-1 was supplied through the mutual redundancy grid from the common unit DC panel. Reactor parameters remained under control and within safe operation limits.
Basis for rating:
1. No off-site impacts.
2. No on-site impacts.
3. This event is classified under the "degradation of defence-in-depth" criterion.
Two real initiators were observed:
a) Loss of DC power sources (initiator frequency - high/expected).
Safety function availability - adequate.
b) Reactor scram (initiator frequency - high/expected).
For each of the initiators in question level 2/3 is appropriate according to Table II of the INES User's Manual (rev. 1992).
While assessing the extent of safety function availability it was taken into account that the functions of reactor scram and fuel cooling were performed by normal operation features without deviations from safe operation limits and conditions. Reactor parameter monitoring features functioned as designed. Normal DC power supply was restored within 20 minutes.
The loss of DC power occurred on common cause. This was accounted for while determining the extent of safety function availability and is not considered as a basis for choosing a higher level.
In view of the above level 2 has been chosen.
ERF rating date: 01-01-1993
ERF contact person: I. Jouk
ERF address: (VNIIAES) FERGANSKAYA 25, MOSKOW 109507, Russian Federation
ERF affiliation: Ministry of the Russian Federation for, Atomic Energy, All Russian Research Inst. for Nuclear P

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