The unit was operating at rated power. Routine inspection of the control rod drive mechanism housings was in progress. Boron traces and small steaming were discovered in the flange joint of control rod housing 12-43 (located in central position) with nozzle of the reactor vessel head. The reactor was normally unloaded and then cooled down according to the operating procedure. After disassembling of the control rod housing erosion wear of base metal in the flange (on mating surface) was identified. Previous inspection on 27.02.94 did not detect any indications of the defect.
Direct cause of the event was manufacturing deficiency of the nickel-made sealing gasket. Root cause - deficiency of the quality control during manufacturing and low effectiveness of the designed system for control of tightness in flange joint.
Basis for rating:
1. No off-cite impacts
2. No on-site impacts.
3. This event is rated under the "degradation of defence-in depth" criterion according to procedure based on initiators and using guidance for structural defects.
In fact, the surveillance programme identified the defect in the flange joint before the size became unacceptable. In such a case a level 0 would be appropriate.
This defect has led to a real initiator "reactor coolant system leakage that would not prevent a controlled reactor shutdown and coooldown" (initiator frequency - high/expected).
Safety function availability - full.
For this initiator in question the basic rating according to Table II of the INES user's manual (1992 revision) is 0. There were no additional factors that required uprating. Therefore level 0 has been chosen.
Event date: Sun, 06-03-1994