At the Kyoto University Research Reactor Institute, the world’s first injection of spallation neutrons generated by high-energy proton beams into a reactor core was successfully accomplished on March 4, 2009. By combining the fixed field alternating gradient (FFAG) accelerator with the A-core (Fig. 1) of the Kyoto University Critical Assembly (KUCA), a series of accelerator-driven system (ADS) experiments were carried out by supplying spallation neutrons to a subcritical core through the injection of 100MeV protons onto a tungsten target of 80mm diameter and 10mm thickness. In these experiments, the proton beams from the FFAG accelerator were generated at 30Hz repetition rate and 10 pA current. The neutron intensity generated at the tungsten target was around 1 10 s . The objective of these experiments was to conduct a feasibility study on ADS from the viewpoint of reactor physics, in order to develop an innovative nuclear reactor for a high-performance transmutation system with a capability of power generation or for a new neutron source for scientific research. The A-core employed in the ADS experiments was essentially a thermal neutron system composed of a highly enriched uranium fuel and a polyethylene moderator/reflector. In the fuel region, a unit cell is composed of a 93% enriched uranium fuel plate 1/1600 thick and polyethylene plates 1/400 and 1/800 thick. In these ADS experiments, three types of fuel rods designated as the normal, partial, and special fuels were employed. From the reason of the safety regulation for KUCA, the tungsten target was located not at the center of the core but outside the critical assembly, and the outside location was similar to that in previous experiments using 14MeV neutrons. As in previous ADS experiments with 14MeV neutrons, the introduction of a neutron guide and a beam duct is requisite to lead the high-energy neutrons generated from the tungsten target to the center of the core as much as possible. The detailed composition of the normal, partial, and special fuel rods, the polyethylene rod, the neutron guide, and the beam duct was described in Refs. 3–5). To obtain the information on the detector position dependence of the prompt neutron decay measurement, neutron detectors were set at three positions shown in Fig. 1: near the tungsten target (position (17, D); 1=200 BF3 detector) and around the core (positions (18, M) and (17, R); 100 He detectors). The prompt and delayed neutron behaviors (Fig. 2), which were an exponential decay behavior and a slowly decreasing behavior, respectively, were experimentally confirmed by observing the time evolution of neutron density in ADS. These behaviors clearly indicated that neutron multiplication was caused by an external source: sustainable nuclear chain reactions were induced in the subcritical core by spallation neutrons through the interaction of the tungsten target and the proton beams from the FFAG accelerator. In these kinetic experiments, subcriticality was deduced from the prompt neutron decay constant by the extrapolated area ratio method. The difference between the measured results of 0.74% k=k and 0.61% k=k at the positions (17, R) and (18, M) in Fig. 1, respectively, from the experimental evaluation of 0.77% k=k, which was deduced from the combination of both the control rod worth by the rod drop method and its calibration curve by the positive period method, was within 20%. Note that the subcritical state was attained by the full insertion of C1, C2, and C3 control rods into the core. The thermal neutron flux distribution was estimated through the horizontal measurement of the In(n, )In Atomic Energy Society of Japan Corresponding author, E-mail: pyeon@rri.kyoto-u.ac.jp Present address: SR Center, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu-shi, Shiga 527-8577, Japan Present address: Institute of Nuclear Safety System, Incorporated, 64, Sata, Mihama-cho, Mikata-gun, Fukui 919-1205, Japan Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 46, No. 12, p. 1091–1093 (2009)