Unit shutdown occurred due to the rupture of the pulse line measuring pressure in SG-4 steam header. Pulse line damage occurred because of design deficiency in the routing and fixing the pulse line during SG replacement.
While preparing the unit for restart after maintenance during primary circuit hydrotest at 180 kgf/cm2 pressure loss of leak-tightness in the inside seal of reactor main flange was found. The unit was shut down for maintenance to inspect reactor main flange seal. Loss of leak tightness was caused by the leak through the earlier plugged tube of the inter-seal space monitoring system.
BASIS FOR RATING: Initiator - rupture of SG pulse line. Criterion - degradation of defence-in-depth. Initial event - expected, real. Safety function availability - full, which corresponds to level 0/1 (A-1R). Event cause shows deficiencies in the design and operation quality surveillance program. Therefore the higher level out of the above two has been chosen.
DIFFICULTIES IN RATING:
Event date: Fri, 11-01-1991