ASN Classifies Melox Incident as INES Level 2

The French nuclear safety authority (Autorité de Sûreté Nucléaire, ASN) has classified an incident at Areva NC’s Melox mixed-oxide fuel fabrication plant at Marcoule in southern France as level 2 on the International Nuclear Event Scale (INES).

The event, on 3 March 2009, occurred when Areva NC received a non-routine delivery of fissile material at the plant. The material was a mix of uranium and plutonium oxide from an external installation not belonging to the Melox plant.

ASN said receipt of the delivery resulted in a slight violation of the criticality security level because the reception procedure that was followed was inadequate and no formal procedure was in place for this kind of delivery. Human error was the root cause factor, ASN said. Initial analysis of the incident by ASN showed there was enough safety margin left that the violation of the criticality security level would not have had any criticality consequences.

Criticality security levels are part of the technical specifications of the plant. They ensure that there is never a mass of fissile material larger than half of the critical mass coming together in any plant sector. The critical mass is the smallest amount of fissile material needed for a self-sustained chain reaction.

ASN said the operator should make sure criticality incidents do not happen under any circumstances and take preventive measures, in particular introducing supplementary procedures for non-routine deliveries and making sure a system is in place to quickly detect any events.