Evaluation of sensitivity coefficients of effective multiplication factor with respect to prompt fission neutron spectrum

Sensitivity coefficients with respect to the fission neutron spectrum can be formulated in two ways: in terms of the unconstrained and constrained sensitivity coefficients. Differences in results obtained using the two sensitivity methods were investigated for the case of the perturbation of the fission spectrum, and for the uncertainty of the effective multiplication factor (keffkeff) with respect to the fission spectrum. It is shown analytically that both sensitivity coefficient methods ideally result in the same uncertainty predictions. However, if the unconstrained sensitivity coefficients are used, the zero-sum constraint for the absolute covariance matrix must be satisfied with high numerical accuracy.