Potential risk of a criticality event during refuelling

V. Laurioux, P. Deschamps, Eurosafe, Berlin, 4-5 novembre 2002, Rapport IRSN-DES/548

Following a core unloading due to an unscheduled shutdown during the cycle of Dampierre unit 4, the plant operator had to operate the refuelling in its previous configuration. During this operation, the fuel assembly concerned by loading step n° 25 was left in the fuel building and the assembly for the following step, n° 26, was placed in the reactor vessel instead of the previous one, causing an irregularity in core pattern only detected at step no. 139. At that point, the refuelling machine operator realized that he was handling an assembly equipped with its rod cluster, whereas according to his handling sheet, the assembly should not be equipped with. Thanks to the favourable conditions (primary system boron concentration = 2345 ppm and cycle burn-up = 2 GWj/tU), the core remained subcritical. However, the incident analysis revealed a potential criticality risk under less favourable circumstances, i.e. refuelling with fresh 1st cycle fuel assemblies and a primary system boron concentration at the lower limit of the range allowed by the Technical Operating Specifications (2000 ppm). Moreover, further studies have shown that the two neutron source range channels (SRC) are able to provide a count rate (reflecting neutron flux) but would not detect a local increase in reactivity under refuelling conditions (normal or defective), unless a reactive pattern was formed in the immediate vicinity of one of the two channels. However, if the criticality is reached, the SRCs are able to diagnose the critical state from the significant power level of 0.1% rated power. The human error that had led to the administrative validation of a fuel handling step that had not been physically carried out highlighted both the fragility of organizational lines of defence and the inadequacy of I&C systems with regard to technical in-depth defence lines. Following these observations, and beyond the preventive measures supposed to limit the occurence of a divergence during refuelling operations, the IRSN analysis has concluded to the necessity to implement technical devices allowing, as the reactive pattern is in progress during refuelling, a neutron flux measurement connected to the actual reactivity level.