Characteristics of upper-bound fuel rods for criticality studies of fresh and spent fuel assembly transport casks

The reactivity of light water reactor fresh and spent fuel assemblies depends greatly on the geometrical characteristics of the fuel rods of which they consist, as well as on their moderation. As concerns the geometrical characteristics, each type of fuel rod can be accurately identified on the basis of the diameter of the pellet, the thickness of the cladding and the outside diameter of the cladding. The values of these three parameters do, however, vary slightly relative to their nominal values, as a result of not only dimensional tolerance limits or changes caused by irradiation. These tolerance limits are generally mentioned explicitly or implicitly in the descriptions of the permitted contents in the certificates of approval, whereas the criticality studies on which they are based frequently cover only a very small number of fuel rods whose geometrical parameters do not vary. This means that these parameters constitute upper bounds relative to those of the certificates in terms of reactivity, hence the importance of a general method for establishing the set of upper bound parameters. (authors)