Subcritical multiplication factor and source efficiency in accelerator-driven system

The subcritical multiplication factor ks   and the external neutron source efficiency φ∗φ∗ are important parameters in the accelerator-driven system (ADS) performance assessment. The theoretical relation between ks and the effective multiplication factor keff in a subcritical system is discussed in different cases of subcritical system. On the basis of the theoretical background, the dependence of ks   and φ∗φ∗ on subcriticality, source position, and energy is numerically investigated using a simple thermal subcritical model. For the sake of experimental evaluation of ks   and φ∗φ∗, the ADS experiments have been carried out in the subcritical systems combined with 14 MeV pulsed neutrons of the Kyoto University Critical Assembly (KUCA). The ks   and φ∗φ∗ parameters are successfully measured by utilizing the reaction rate distribution obtained by the optical fiber detectors in the subcritical system, within a relative difference of less than 7% and 12% for ks   and φ∗φ∗, respectively, between measured and calculated values for most studied cases.