Assessing the Uncertainty of Reactivity Worth Scale Measurements

The characteristics of any nuclear power plant should be determined according to well-defined calculation conditions and accuracies. The neutronic tools should go under a series of verification, validation and uncertainty quantification processes. The proof of the best estimate values and uncertainties of the neutronic tools should be achieved using measurements in critical facilities. Concerning the sodium void reactivity worth in sodium fast reactor cores (SFR), measurements can take the usual form of successive substitutions of components with different materials (voided rodlets replacing Na rodlets for instance) and different sizes in order to vary the relative importance of central and leakage components. The reactivity variations induced by these changes are compensated by a change in the position of the shim rod if the reactivity variation is smaller than half a beta(eff) and if larger by the addition or the removal of peripheral sub-assemblies. The calibration of the shim rod is performed by measuring a fission chamber response when dropping a control rod and solving the Nordheim equation. This gives an S curve where the position of a control rod is associated to a reactivity expressed in $. In this study, the uncertainties associated to this calibration are revisited, not only due to the uncertainties in the beta(eff) value (similar to 2%) but also to the uncertainties in the decay time constants associated to each family (similar to 6%) of delayed neutrons, these being quite different from one nuclear data evaluation to another.