 INES 2 |
Discovery of a crack in boiler superheater penetration |
While undertaking a boiler tube seal weld repair on a superheater on Reactor 21 a crack of significant extent was discovered in weld "A" adjacent to the tube plate. The mechanism for the crack is not yet determined and the potential cannot be discounted at this stage for similar cracks to occur on Reactor 22 which would be outside the operational safety case. The conservative decision has been taken to affect a controlled shutdown of Reactor 22.
Arising from the discovery a Project Team has been set up under the chairmanship of a company Engineering Division senior manager. The team will have responsibility for managing all aspects of the project including investigation of the causes, additional inspections, repairs and production of the safety case for return to service.
Justification of Rating:
Using Part III-2.8 on page 30 of the INES User's Manual for assessing structural defects indicates that for a defective component whose failure could lead to an initiator use of Table I is appropriate to give an upper value to the basic rating. Using this approach for an unlikely potential initiator with full safety function availability gives a basic rating upper limit of 2. The guidance then allows for adjustment dependent upon the likelihood that the defect would have led to component failure.
The mechanism of component degradation is not yet fully understood, but the extent of the cracking is such that there appears to be little margin to failure remaining. Additionally, it is clear that the failure of this component was considered in the safety case to be unlikely, and therefore no routine surveillance programmes were in place to monitor the weld.
Given the above situation, INES level 2 appears appropriate based on the current level of understanding. This is confirmed by verification as an incident with significant failures in safety provisions.
Event date: Fri, 19-11-1999
