Abstract and subjects
Accurate modeling of subcritical multiplication is needed for accelerator-driven subcritical reactors, reactor startups, critical experiment approaches to criticality, and detection of nuclear materials. Such calculations are multiplied fixed-source calculations, not eigenvalue calculations. The traditional approach to obtaining the subcritical multiplication factor M, however, is based on the k(eff) eigenvalue [1]: =(S+Q/Q)=1/(1-k(eff)) M(eff) (1) where k(eff) is determined by a standard criticality eigenvalue calculation, Q is the fixed source intensity, and S is the steady state fission production. This formulation is valid only if the fixed source distribution q is distributed in space and energy as the fundamental critical eigenmode. Generally, this is not the case, and in systems, such as those driven by 14 MeV DT fusion neutrons, the actual multiplication may be significantly different. Multiplication factors that treat the fixed source and fission distribution separately in calculating the subcritical multiplication were developed in [2] and provide a more realistic way to model subcritical multiplication. This summary provides a brief review of subcritical multiplication factors based on [2], prescribes a means of calculating them using Monte Carlo methods, and provides numerical results to validate the approach. Comparison of the numerical results to Eq. (1) is also provided to illustrate the inaccuracies that may arise from Eq. (1).