[en] This paper gives an exhaustive inventory of criticality accidents that occurred in nuclear facilities of USA, UK and USSR. The paper starts with the description and the analysis of the Oak Ridge Y-12 plant criticality accident from June 16, 1958, and the general material, human and environmental consequences of criticality accidents. The safety rules followed in CEA sites and COGEMA fuel cycle facilities to prevent criticality accidents are briefly described. (J.S.). 2 tabs., 1 photo

[en] The operation of the LWRs requires the storage of irradiated fuel elements in cooling pools, to which it is possible to access directly from the reactor core. After transportation to the reprocessing plants, the storage must be continued in storage pools located at the entry of the plant. The requirements retained in order that no safety problems arise during the overall storage time have been defined from the acquired experience : they are relative to all functions having an effect on the normal operating conditions : cooling, containment, shielding, handling, controls, waste and effluents processing, etc. The requirements have been defined so that the storages cannot be the cause of accidents either resulting in an impact on the environment or being the consequences of the environment conditions : criticality, loss of water, missile impact, earthquakes, etc. They need permanent presence of a qualified staff. (author)

[en] Storage conditions for the cooling of fast neutron spent fuel assembly, before reprocessing, will be adapted specifically to this fuel cycle, but in a preliminary testing period it is better to take advantage of the industrial storage conditions already used for spent fuel of light water reactors, even if no extrapolation is possible in the future. In this aim fuel assemblies are dismantled, as soon as thermal conditions allow it, and pins are gathered in leak proof containers in a gas atmosphere and put in a cooling pool. This solution gives good results but studies and experiments are resumed, for future processing, for safe, easy and economical transport and storage conditions

[en] In this paper, the authors present the philosophy which have conducted them at the realization of this book, the critical values retained and the future developments

[en] The operation of the LWRs requires the storage of irradiated fuel elements in cooling pools, to which it is possible to access directly from the reactor core. After transportation to the reprocessing plants, the storage must be continued in storage pools located at the entry of the plant. The requirements retained in order that no safety problems arise during the overall storage time have been defined from the acquired experience; they are relative to all functions having an effect on the normal operating conditions: cooling, containment, shielding, handling, controls, waste and effluents processing, etc. The requirements have been defined so that the storages cannot be the cause of accidents either resulting in an impact on the environment or being the consequences of the environment conditions: criticality, loss of water, missile impact, earthquakes, etc... They need the permanent presence of a qualified staff

[en] In France, all the calculations for preventing criticality are based on uranium oxyde with a maximal enrichment of 3.5% in 235 U. Because the electricity producers use now a more enrichment fuel (until 4.5%, in weight), it is necessary to take in account the burnup for studies of criticality in containers or in reprocessing plants. In this paper the author shows the main problems for taking in account the burnup

[en] After briefly mentioning processes, the principal measures used to guaranty the safety-criticality as well as the way to apply them are reviewed

[en] Fast neutron reactors fuels have a high proportion of plutonium and undergo severe irradiation. Risks during spent fuel reprocessing and subsequent fabrication will depend on isotopic composition, fission product content, physico-chemical form of products, quantities handled. These risks (criticality, contamination, irradiation) are listed for the different steps of the cycle and methods used to control the risks (chemical reaction yields, equipment reliability, intervention, conditions...) are indicated. Problem arising from wastes and effluents produced at each step are briefly given

[en] An examination is made of the various parameters acting on the neutron balance: mass, moderation, density, geometry, interactions, absorption and poisoning. The monitoring methods are deduced from this. Using two examples, it is shown how the various parameters act together and in different ways so that it is difficult to predict the results and that it is necessary to have calculation codes well qualified on experience

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