Mockup Experiments on the Thorium-Loaded Accelerator-Driven System at the Kyoto University Critical Assembly

Experimental studies on the thorium-loaded accelerator-driven system (ADS) were conducted at the Kyoto University Critical Assembly. Mockup experiments were carried out in both the critical and subcritical states to investigate the influence of different thermal neutron profiles on the thorium capture and fission reactions. Thorium plate irradiation experiments for the thorium capture and fission reactions demonstrate fission reactions in the critical state, and the calculated-to-experiment values of reaction rates show accuracy within a relative difference of ∼30%. In the ADS experiments with an external neutron source (14-MeV neutrons and 100-MeV protons), subcritical experiments were carried out in the thorium-loaded cores to investigate the influence of different thermal neutron profiles on thorium capture reaction rates by the measurement of ¹¹⁵In(n,γ)^116mIn reactions. The results reveal the difference between reaction rate distributions attributed to varying not only the neutron spectrum of the core but also the external neutron source. A comparison between the measured and calculated reaction rate distributions reflects the accuracy of reaction-rate analyses for the thorium-loaded ADS experiments with an external neutron source. Additionally, kinetic experiments were carried out to deduce the prompt neutron decay constants and subcriticality by the pulsed neutron method.