[en] Various theoretical studies have shown that highly diluted plutonium solutions could have a positive temperature effect but (up to now) no experimental program has confirmed this effect. The main goal of the French Plutonium Temperature Effect Experimental Program (or PU+ in short) is to effectively show that such a positive temperature effect exists for diluted plutonium solutions. The experiments were conducted in the 'Apparatus B' facility at the CEA Valduc research centre in France and involved several sub-critical approach type of experiments using plutonium nitrate solutions with concentrations of 14.3, 15 and 20 g/l at temperatures ranging from 20 to 40 deg. C. A total number of 14 phase I experiments (consisting of independent subcritical approaches) have been performed (5 at 20 g/l, 4 at 15 g/l and 5 at 14.3 g/l) between 2006 and 2007. The impact of the uncertainties on the solution acidity and the plutonium concentration makes it difficult to clearly demonstrate the positive temperature effect, requiring an additional phase II experiment (in which the use of the same plutonium solution was ensured) from 22 to 28 deg. C performed in July 2007. This experiment has shown the existence of a positive temperature effect ∼ +2 pcm/deg. C (from 22 to 28 deg. C for a plutonium concentration of 14.3 g/l). (authors)

[en] This document gives an overview of the criticality experimental programs of interest for IRSN. These experiments, which could be carried out after June 2004 on the 'Apparatus B' CEA facility, concern: the qualification of the effect of plutonium decrease to americium and validation of the associated neutronics data, the qualification of structural material cross-sections, the determination of the temperature effect of diluted plutonium solutions, the low moderated MOX fuel qualification. The major part of the present paper is devoted to the presentation of an IRSN proposal for an international collaboration on such an experimental program for MOX fuel qualification. (author)

[en] Cofunded by Cogema, two complementary experimental programmes on burn up credit (BUC) related to fission products (FPs) are performed by CEA and IRSN at Cadarache and Valduc. After shortly recalling the main characteristics of each experiment, a first comparison of some results is presented, especially the energy range in which most part of cross section absorption are qualified. Both experiments exhibit great quality and accurate results, giving a high degree of confidence to the whole experimental French process of qualification devoted to BUC. (author)