Uranium-Loaded ADS Experiments
KUCA comprises two solid polyethylene-moderated thermal cores designated A and B and one water-moderated thermal core designated C. The A-core is mainly used for experiments of ADS basic research. The three cores are operated at a low mW power in the normal operating state; the maximum power is 100 W. The constitution and the configuration of the cores can be altered easily, and the coupling with the conventional Cockcroft-Walton type accelerator and with the FFAG accelerator has allowed conducting experiments separately with the use of 14 MeV neutrons from deuteron–tritium fusion reactions and 100 MeV protons with the heavy metal target, respectively.
Fig. 9.1 Top view of the configuration of the A-core in the accelerator-driven system (ADS) experiments with 100 MeV protons
Fig. 9.2 Side view of of 3/8P0036EU fuel assembly (F, Fig. 9.1) in the A-core
The A-core (Fig. 9.1) employed in the ADS experiments was essentially a thermal neutron system composed of a highly enriched uranium fuel and the polyethylene moderator/reflector. In the fuel region, a unit cell is composed of the highly enriched uranium fuel plate 1/16 in. thick and polyethylene plates 1/4 in. and 1/8 in. thick (Fig. 9.2). The SV assembly is composed of a
5.08 x 5.08 x 5.08 cm center void region, 32 fuel unit cells, and the polyethylene blocks. In these ADS experiments, three types of fuel rods designated as the normal, partial, and special fuel SV were employed. For reasons of the safety regulations for KUCA, the heavy metal target was located not at the center of the core but outside the critical assembly. As in the previous ADS experiments with 14 MeV neutrons,
Fig. 9.3 Top view of thorium-loaded ADS core with 100 MeV protons
the introduction of a neutron guide and a beam duct is requisite to lead the highenergy neutrons generated from the heavy metal target to the center of the core as much as possible. In the uranium-loaded ADS experiments, the proton beam parameters were 100 MeV energy, 0.01 nA intensity, 30 Hz pulsed frequency, 100 ns pulsed width, and 80-mm diameter spot size at the tungsten target (100 mm diameter and 9 mm thick). The level of the neutron yield generated at the target was around 1.0 x 106 1/s by the injection of 100 MeV protons onto the tungsten target.
Thorium-Loaded ADS Benchmarks
In the ADS with 100 MeV protons (Fig. 9.3), the fuel rod was composed of a thorium metal plate and a polyethylene (PE), graphite (Gr), or beryllium (Be) moderator arranged in the A-core. Other components were selected from HEU and natural uranium (NU; 2 x 2 x 1/8 in.) plates. The cores were composed of Th-PE (Fig. 9.4), Th-Gr, Th-Be, Th-HEU-PE, and NU-PE, according to a selection of moderator materials: PE, Gr, Be, HEU-PE, and NU-PE, respectively, and spallation neutrons were generated outside the core after injection onto the tungsten target. The thorium-loaded ADS experiments were conducted especially to investigate the relative influence of different neutron profiles on capture reactions of 232Th and 238U: the reaction of 238U was taken as reference data for evaluating the validity of 232Th capture cross sections.
Fig. 9.4 Side view of Th-PE fuel assembly (TP) in thorium-loaded ADS core in Fig. 9.3
The main parameters of the proton beams were 100 MeV energy, 0.3 nA intensity, 20 Hz pulsed frequency, 100 ns pulsed width, and 40-mm-diameter spot size at the tungsten target (50 mm diameter and 9 mm thick). The level of the neutron yield generated at the target was around 1.0 x 107 1/s by the injection of 100 MeV protons onto the tungsten target.
Prompt and delayed neutron behavior was monitored by placing three 3He
detectors (20 mm diameter and 300 mm long) at three locations. Throughout the time evolution of the prompt and delayed neutrons, the prompt neutron decay constant was deduced by least-squares fitting to an exponential function over the optimal duration. Subcriticality was deduced by the extrapolated area ratio method  on the basis of prompt and delayed neutron behaviors. For 100 MeV protons, neutron detectors (3He detectors: #1, #2, and #3) were set at three locations.