Large discrepancy in amount of plutonium at shutdown French MOX plant

The French Nuclear Safety Authority (ASN) issued on 14 October 2009 a formal decision to halt decommissioning operations at a former MOX fuel fabrication plant in Cadarache in the South-East of France. The announcement followed the declaration of significant inconsistencies in the estimate of plutonium quantities held up in glove boxes.

The Atelier de Technologie Plutonium, or ATPu is a MOX fuel fabrication plant operated between 1961 and 2004 to produce plutonium fuel for fast breeder (FBR) and light water (LWR) reactor programs. ATPu ceased commercial production in 2003 - with the exception of the fabrication in 2004 of fuel using weapon-grade US plutonium (EUROFAB) for the manufacturing of four Lead Test Assemblies (LTAs) in the framework of the US plutonium disposition program.

The plutonium hold-up turns out much higher than estimated during operation. While former operator AREVA estimated the hold-up to amount to 8 kg of plutonium, owner CEA, the French Atomic Energy Commission, has already collected 22 kg, and forecasts to collect as much as 39 kg in total. This translates into an unexpected plutonium hold-up of 30 kg and more.

The findings have serious implications both in the fields of nuclear safety and security. On the safety side, the deposit of such quantities considerably increases the risk of a criticality accident. On the security side, the fact that such high hold-up figures went undisputed illustrates the failure of an accountancy system supposed to guarantee that no diversion of a significant quantity of plutonium goes unnoticed.

ASN rated the incident Level 2 on the International Nuclear Event Scale (INES), but the seriousness of the situation lead the ASN to halt operations and to declare the operator in breach of rules. For the first time ASN used for that matter of the right to call for "grave and imminent" risk, introduced by the French law on nuclear transparency and security in June 2006. ASN stated that the failure to detect the potential level of plutonium hold-up during the operation of the facility and the delays in reporting the event to ASN, constitute deficiencies in the safety culture of the licensee CEA and of the commercial operator AREVA.

On 3 November 2009 ASN authorized the resumption of decommissioning of 22 glove boxes probably containing less than 200 grams of fissile material. Decommissioning of the remaining 198 glove boxes likely to exceed that amount of fissile material remains subject to specific licensing.

(wit Yves Marignac)