 INES 2 |
Leakage on a piping connected to the primary circuit |
A leakage has been detected on a piping connected to the primary at Dampierre-1 on December 14, 1996. The leak rate increased and led the operator to shutdown the reactor in order to localize the leak before reaching the safety threshold of 230 l/hour on December 21, 1996.
The operator localized a non isolable primary water leak on a piping connecting the safety injection system to the plant primary system. The defected part of the pipe has been replaced during a one month outage.
The damaged pipe length has been examined at the irradiated material workshop at Chinon and the preliminary expertise concluded to a through-wall crack located on a portion of straight piping and not on specific areas such as bends and welds as it could be expected. The most probable hypothesis is that the origin of this crack is due to a thermal fatigue due to a variation of temperature produced by cold water coming from non-tight valves located upstream.
The operator, EDF, decided to make controls on straight parts of the piping during scheduled outages on 900 MW plants and, in some cases, decided specific outages.
Other defects have been found on two pipes at Dampierre 3 in March 1997 and on Fessenheim 2 in April 1997 (so that other portions of pipes have been replaced and will be expertised).
EDF has provided DSIN with a study on the evolution of the phenomenon. It appears that on a particular pipe small non-detectable defects can potentially lead to a through-wall crack within less of a cycle if these defects are initiated and stand small cold leaks from upstream valves.
The analysis of this study by DSIN has led to re-rate the incident level 2.
Still, EDF will give its strategy to solve this problem on the mean term by the end of June. Until the implementation of this strategy EDF will have to proceed to specific additional controls on these pipes.
Event date: Sat, 14-12-1996
