Uncertainty due to Nuclear Data for an MTR Fuel Assembly

For a research reactor, and in particular for the High Flux Reactor (HFR) in Petten, every cycle has a distinct core loading. This regards both the fuel assemblies loading and mainly the loading of the experiment rigs. These experiments can be of the most diverse types, comprising strong absorbing materials as well as fissile isotopes. This poses some challenges at every cycle because of the possible influence on the power profile and neutron flux distribution, which are of great importance for the isotopic production facilities loaded at every cycle in the case of the HFR. The isotope production facilities require predictable and preferably as constant as possible irradiation conditions (neutron flux levels, mainly). Another point of interest is the cycle length, which has to be guaranteed beforehand by taking into account in the assessment analysis the expected on-power core loading changes. These changes are caused by regular loading/unloading of some of the isotope production rigs. For optimization reasons, there is also a tendency to increase the cycle lengths and at the same time to minimize the costs associated to the loading of fresh fuel and control rod assemblies. To justify a further increase in cycle length, a quantification of the uncertainties on the reactivity curve is required, so that the adopted design margins can be decreased in a controlled and responsible manner. The uncertainties in the reactivity curve can be associated to several sources, ranging from uncertainties in engineering and operational parameters to uncertainties in basic nuclear data. This paper includes the quantification of uncertainty on the reactivity curve associated to a typical HFR fuel assembly as result of uncertainties in the cross section data of the main fuel, structural and coolant isotopes. (authors)