Fissile materials (²³⁵U et ²³⁹Pu, for instance) in nuclear fuel facilities (laboratories and factories) and transportation leads to a specific risk called criticality. It' the risk to trigger starting and maintenance conditions of a fission chain reaction.
The criticality state depends of several features, in particular :

• The mass of fissile material included in each device and of their geometrical design,
• The concentration of uranium or plutonium in the solutions,
• The presence, within the fissile field, of absorbing and leaking neutrons.

In order to give a range, the criticality risk occurs while manipulating over 60 kg of enriched uranium at 3,5 % of ²³⁵U and over 500 g of ²³⁹Pu. In each nuclear facility where the criticality risk exists, control modes of criticality are defined, implying limitations for each features, in order to maintain an under - criticality state for each equipment.

A criticality accident, even if all the mechanical consequences are seldom prejudicial, would have, in most cases, severe radiological consequences for workers. Such an accident would cause radioactive material ejections from the facility and a strong psychological impact on workers and on the public opinion minds.

In France, this risk has been studied from the very beginning of research and production activities involving nuclear materials, and thanks to preventing measures undertaken, no accident occurred. In others parts of the world, excepting nuclear power plants and military activities, 20 accidents have occurred making several death by external radiation. The first way to prevent criticality risk is to evaluate, though calculations, the criticality conditions of any equipment that might contain fissile material. Prevention also relies on the assessment, by nuclear operators, of the working conditions of these equipments

By the middle of the nineteen nineties, in the field of criticality, calculation tools used datas and softwares developed in the nineteen eighties, such as the CEA86 library and APOLLO1 calculation software. These tools were not accurate enough for future needs. Their perenity was not guaranteed and their field of application was far too restricted to deal with the new needs such as MOX fuels, high burnup rate fuels and those generated by downstream cycle researches. This situation led to the decision in 1995, to develop a brand new safety-criticality package called " CRISTAL " for safety assessment studies.

CEA, COGEMA and IRSN

The CRISTAL package is developed and qualified within the framework of a collaboration between the IRSN (Institute of Protection against radiation and Nuclear Safety), COGEMA (general Company of the nuclear matters) and the Management of the nuclear energy (DEN) of the CEA, by taking account of the needs of the possible applications. After four years of development, validation and qualification, the first version (V0.1) of the form has been delivered to users since November 1999.

This useful collaboration between the IRSN, COGEMA and the CEA/DEN was designed to last until 2005 with the development and the qualification of CRISTAL V1 version. CRISTAL V1's essential goal is to model, for the criticality risk assessment, the burnup (destruction of the cores of uranium and plutonium) and thus the appearance of new actinides and fission products. Indeed, within the framework of their respective activities of design or exploitation of nuclear facilities, EDF, FRAMATOME and COGEMA have studied the possibilities of taking into account the "Burn-Up Credit" which is the margin of reactivity due to materials obtained (actinides and fission products) during the nuclear fuel irradiation, and the IRSN must for its part have tools to evaluate the rightness of the assumptions retained by the owners. Also, it was decided to develop and qualify a computation chain "evolution of the assessments materials and criticality" based on coupling the forms dedicated to calculations of assessments matters of the nuclides (DARWIN and CESAR) and CRISTAL. The DARWIN and CESAR packages come upstream of CRISTAL package, and help to calculate the evolution of the physical dimensions of interest, while taking into account the detailed history of the assembly (in the reactor and during cooling), in particular the abundance of each nuclide during fuel cycle, on any reactor type (PWR, BWR, LMFBR, ...).

Many industrial plants, downstream of the fuel cycle, regarding to the prevention of the criticality risks, were originally dimensioned for enriched uranium reference fuels, 235 U enrichment lying between 3,5 % and 4 %. This is common for storage and irradiated fuel reprocessing (3,75 % for the factory the most recent UP2/800) facilities as well as spent fuels transport packaging coming from the ordinary water reactors. In most cases, significant margins appeared in the assumption retained by the operators in their criticality studies concerning these new fuels.

The increase in initial enrichment in ²³⁵U of fuels in UO₂ and the mixed fuel arrival based on plutonium and uranium oxide made the revaluation necessary. To keep on working with these new conditions, this revaluation should concern the conditions of prevention of the risks of criticality . Taking into account the Burn-Up Credit is particularly usefull in fields such as transportation of irradiated assemblies, the design of new and more powerful transports packages, storage in swimming pool and the reprocessing of irradiated assemblies.

"Burn-Up Credit" matter in criticality studies requires having a calculation code qualified for abundances of the neutrons (actinides and fission products) in irradiated fuels and for the negative reactivity brought by these nuclides in various configurations. This calculation code is based on the coupling of the evolution packages (DARWIN and CESAR) and of the criticality package : CRISTAL.