A Monte Carlo technique for sensitivity analysis of alpha-eigenvalue with the differential operator sampling method

A method for Monte Carlo sensitivity analyses of α-eigenvalue (prompt neutron time decay constant) in a subcritical system is developed using the first-order differential operator sampling (DOS) method. The first-order derivative of α-eigenvalue with respect to nuclear data is calculated using the DOS method that includes the capability of calculating perturbed source effect. This paper is an extension of the author’s previous work for development of the sensitivity analysis method for keff-eigenvalue. Unlike the conventional Monte Carlo method for α-eigenvalue calculation that uses the power iteration of fission sources, this paper introduces a recently developed “time source method”. The “time source method” has a weakness for a void-containing subcritical system, which is overcome by assigning a virtual total cross section in the void region. The perturbed source effect, which is caused by the change of nuclear data in a subcritical system, can be calculated by two methods, the source perturbation iteration method and the superhistory method. The source perturbation iteration method is superior in terms of computation efficiency, but a huge computer memory is required. The superhistory method dramatically reduces the memory requirement, although it worsens the variance of the sensitivity coefficients. The method developed in this paper is applied to some numerical tests that use multi-group constants, and it is verified by comparing to the results obtained by a deterministic perturbation theory