Measurement of k_eff with an improved neutron source multiplication method based on numerical analysis

In this work, we developed a numerical analysis-associated experiment method to determine the effective multiplication factor k_eff, which is difficult to obtain directly from conventional neutron source multiplication (NSM) method. The method is based on the relationship between k_eff, subcritical multiplication factor k_s and external neutron source efficiency Φ^* in the subcritical system. On basis of the theoretical analysis, the dependence of k_s and Φ^* on subcriticality and source position was investigated at the Chinese Fast Burst Reactor-II (CFBR-II). A series of k_s were measured by NSM experiments at four subcritical states (k_eff=0.996, 0.994, 0.991 and 0.986) with the ²⁵²Cf neutron source located at different positions (from the system center to outside) at each subcritical states. The Φ^* was obtained by Monte-Carlo simulation for each condition. With the measured k_s and calculated Φ^*, k_eff of the subcritical system was evaluated with a relative difference of <1% between values obtained by the improved method and by positive period method. Especially, the relative difference of <0.18% with the source located at the system center. (authors)