Effect of neutron spectrum on subcritical multiplication factor in accelerator-driven system

A subcritical multiplication factor k(s) is the key parameter to determine the neutron multiplication in accelerator-driven system (ADS). The k(s) deduction method in experiments has been developed on the basis of the previous experimental results with soft spectrum core but the applicability of the previous deduction method to hard neutron spectrum core is not obvious. The present study performs the ADS experiments composed of uranium-lead (U-Pb) zoned subcritical core and the spallation neutron source at Kyoto University Critical Assembly, and the reaction rates distributions are measured in the hard neutron spectrum core. The previous k(s) deduction method is applied to the experimental data and the calculation results obtained by the combined use of MVP and PHITS, and the calculated k(s) values do not agree with the experimental data due to the hard neutron spectrum originating from the U-Pb zoned fuel region. By taking account of the hard spectrum effect in the U-Pb zone into the previous k(s) deduction method, the revised k(s) values in the experiments agree well with the calculated ones. Thus, the hard neutron spectrum is concluded to have a significant impact on the k(s) deduction method, and should be taken into account to deduce k(s) appropriately.