The French Nuclear Safety Authority (ASN) evaluates before being able to give its authorization the technical solution proposed by France's Electric Power Company (EDF) to recover the two fuel assemblies
On 8 September 2008 during the fuel unloading operation on the Tricastin no. 2 reactor of the EDF nuclear power plant, two fuel assemblies remained stuck to the upper internal holding structures of the reactor vessel.
ASN conducted an unscheduled inspection on 17 September 2008 on the facility in order to examine how this event was being managed by EDF, as well as the provisions applied by the operator to limit consequences of a possible fall of the assemblies. The inspection allowed examining the possible causes of this event and the envisioned feasible solutions to recover the fuel assemblies.
On the 29th and 30th of September, EDF presented to ASN on a reactor vessel model at the Center for Experimentation and Validation of Pressurized Water Nuclear Boiler Intervention Techniques (CETIC) at Chalon-sur-Saône, France, the technical solutions envisioned to recover these two assemblies.
The complete technical file of the solution retained by EDF and communicated to ASN, including a risk analysis linked to the intervention, will be assessed by the Institute for Radiation Protection and Nuclear Safety (IRSN) as requested by ASN.
According to the results of this assessment, ASN will decide on the proposed solution and, if necessary, will impose on EDF additional measures.
The replacement of fuel in a nuclear power plant comprises various steps : the reactor is shut down, the cooling circuit depressurized, and then the reactor swimming pool is filled with water containing boron and the vessel cover removed. Before unloading the assemblies, the upper internal structures placed above the fuel assemblies have to be extracted from the vessel. Then the fuel assemblies are extracted one by one from the reactor vessel and transferred under water to the fuel storage pool located outside the reactor building.
It was during the extraction of these upper internal structures placed above the fuel assemblies that the operator observed that two assemblies remained suspended.
In the current situation, a possible fall of the two assemblies would have two consequences : (1) a risk of criticality, that is, the triggering of an uncontrolled chain reaction, and (2) a risk of gaseous fission products being released inside and outside the plant.
Drawing showing the current situation of the two assemblies
During the handling of the fuel assemblies, including their possible fall, the possibility of the fuel rod protection sheaths constituting the assemblies being damaged is one of the events that ASN insists on taking into account and anticipating in a reactor's operating authorization. The containment enclosure and the ventilation system are designed to cope with these types of events. ASN verifies these provisions by means of regular inspections. To date this event has not caused any release inside or outside the reactor building's containment enclosure and the cooling of the assemblies is ensured.