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.

1/8”Th (1/8”NU) Unit cell (3.18 mm) (15.88 mm) Al plate (20.00mm)

/_________

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 [13] 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.