On May 13, during Chinon B1 shut down for inspection and refuelling, the operator discovered while inspecting the civil engineering that the tie rods of the reactor pit lateral stops had loosened.
The reactor pit is a concrete structure which supports the reactor vessel. It lies on the basemat and bears at the sides on the concrete which covers the basemat through 18 stops. Each stop is attached to the concrete with eight prestressed anchoring bars. This device is intended to absorb the horizontal stresses which could be generated during an earthquake, thus protecting the reactor vessel.
At Chinon B1, several of these bars were found to be loose and corroded, calling the reactor vessel pit's earthquake resistance into question.
Upon DSIN request, a similar control was performed at eight other 900 MWe reactors (before 15 July 1996). The same fault was found on all of them, and as a whole, 24 reactors might be affected. EDF started analysis to identify the causes and set repair procedures. These analyses are being performed. In the meanwhile, before the eight above mentioned reactors start again, EDF carries out provisional repairs, deemed acceptable for safety.
This incident was first rated INES level 1, but due to equipment degradation questioning the seismic resistance requirements stated in the Safety Analysis Report, associated with lack in the surveillance programme, it was uprated to level 2 by the Director of Nuclear Installations Safety.
following this incident, the Nuclear Installations Safety Directorate required EDF to:
1) send it the programme and the schedule for action undertaken by EDF to identify the cause of the anomalies to analyse the safety aspects involved, and to specify the tests and corrective actions to be implemented.
2) specify its strategy with regard to the resistance of reactor pits to design earthquake,
3) check all reactors potentially affected during their next outage.
4). check the Tricastin 1 and 4 reactors during the summer as their next scheduled outage is too far off.
Event date: Mon, 13-05-1996