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Posted by Energetic
Seven factors influence a criticality system.
To determine whether a system containing fissile material is safe, calculations are performed using computer programmes. The analyst describes the geometry of the system and the materials, usually with conservative or pessimistic assumptions. The density and size of any neutron absorbers is minimised while the amount of fissile material is maximised. As some moderators are also absorbers, the analyst must be careful when modelling these to be pessimistic. Computer programmes allow analysts to describe a three dimensional system with boundary conditions. These boundary conditions can represent real boundaries such as concrete walls or the surface of a pond, or can be used to represent an artificial infinite system using a periodic boundary condition. These are useful when representing a large system consisting of many repeated units.Computer codes used for criticality safety analyses include MONK(UK), KENO(USA), MCNP(USA) and CRISTAL(France).
Traditional criticality analyses assume that the fissile material is in its most reactive condition, which is usually at maximum enrichment, with no irradiation. For spent nuclear fuel storage and transport, burnup credit may be used to allow fuel to be more closely packed, reducing space and allowing more fuel to be handled safely. In order to implement burnup credit, fuel is modeled as irradiated using pessimistic conditions which produce an isotopic composition representative of all irradiated fuel. Fuel irradiation produces actinides consisting of both neutron absorbers and fissionable isotopes as well as fission products which absorb neutrons.
In fuel storage pools using burnup credit, separate regions are designed for storage of fresh and irradiating fuel. In order to store fuel in the irradiating fuel store it must satisfy a loading curve which is dependent on initial enrichment and irradiation.