Basic experiments on the accelerator-driven system (ADS) at the Kyoto University Critical Assembly are carried out by combining a solid-moderated and -reflected core with the fixed-field alternating gradient accelerator. The reaction rates are measured by the foil activation method to obtain the subcritical multiplication parameters. The numerical calculations are conducted with the use of MCNPX and JENDL/HE-2007 to evaluate the reaction rates of activation foils set in the core region and at the location of the target. Here, a comparison between the measured and calculated eigenvalues reveals a relative difference of around 10% in C/E values. A special mention is made of the fact that the reaction rate analyses in the subcritical systems demonstrate apparently the actual effect of moving the tungsten target into the core on neutron multiplication. A series of further ADS experiments with 100？MeV protons needs to be carried out to evaluate the accuracy of subcritical multiplication parameters. 1. Introduction The accelerator-driven system (ADS) is a hybrid technique combining a reactor core and an accelerator, which has been used worldwide in research and development of nuclear transmutation of minor actinides (MAs), long-lived fission products (LLFPs), and next-generation neutron sources. In ADS, a large number of high-energy neutrons are generated directly at a heavy metal target when high-energy protons produced by the accelerator are injected onto the target. The high-energy neutrons can be utilized for maintaining nuclear fission reactions in the reactor core and achieving the purposes of the introduction of ADS. The current research on ADS involved mainly an experimental feasibility study with the use of critical assemblies and test facilities: MASURCA [1–3], YALINA-booster and -thermal [4–6], VENUS-1 [7], and the Kyoto University Critical Assembly (KUCA) [8–14]. Moreover, numerical simulations [15–20] were executed by the deterministic and stochastic approaches for the evaluation of MAs and LLFPs in ADS. The new ADS test facility of GUINEVERE [21] is being commissioned to start actual operation in subcritical states after the first critical experiments. The Kyoto University Research Reactor Institute is pursuing an innovative research program (Kart & Lab.: Kumatori Accelerator-Driven Reactor Test Facility & Innovation Research Laboratory) to develop the fixed-field alternating gradient (FFAG) [22–24] accelerator and to establish a new neutron source by ADS in combination with KUCA and the FFAG accelerator. With the coupling of the KUCA core and
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