On 21 December 2010, the French Atomic Energy Commission (CEA) declared an "anomaly" discovered at the shut-down MOX fuel fabrication facility ATPu (Atelier de Technologie Plutonium) at Cadarache in the South of France. A CEA press release states that the fissile material content in four waste drums would "exceed the authorized limit, but remain well within safety margin". There are no figures in the press release. An AREVA spokesperson stated in a phone interview that the content would be "several tens of grams" per drum before admitting that two drums were "just exceeding 200 grams". The CEA has suggested a level 1 INES rating for the event.

The French Nuclear Safety Authority (ASN) is more precise in its follow-up letter to an inspection carried out on 22 December 2010. The four drums significantly exceeded the 100 g authorized limit with respectively 253 g, 211 g, 103 g and 103 g. The measured fissile content is up to 2.7 times higher than indicated in the inventory (94 g, 86 g, 65 g, 63 g). ASN has requested CEA, within one month, to carry out a safety analysis of all 99 waste drums at ATPu and the 34 at LPC (Laboratoire de Purification Chimique) that was associated with the operation of ATPu. ASN also wants to be "kept informed in real-time" about eventual discoveries of additional non-conform waste drums and measures taken to prevent criticality risks.

In addition to the off-limit waste drums, 750 g of fissile material have been identified in a nitrogen cooler between glove-boxes. ASN stated that the cooler "had not been considered a retention point" and requested the operator to declare an event.

This is not the first time that Cadarache experiences higher than expected concentrations of fissile material and plutonium in particular. In October 2009, ASN had suspended decommissioning operations at ATPu following the discovery of higher than expected retained plutonium quantities in the facility's glove-boxes.

UPDATE 01/27/2011: The reference values as indicated by the French Nuclear Safety Authority ASN at the request of the author, are 590 g of plutonium for a critical mass and 460 g as "safe mass". The reference isotopic composition for "fissile environment" underlying the calculations that form the basis of the safety report for the decommissioning operations is: 91.45% Pu-239, 5% Pu-240, 3.25% Pu-241, 0.3% Pu-242 with a density of 11.47 at optimum moderation with a reflection by 20 cm water. The regulatory limits are 100 g per waste drum in the case of drums that can be stacked and 200 g per waste drum if they are stored in a sub-critical environment (in particular with a 60 cm distance all around and without stacking).