9.3.2.1 Static Experiments

the profile of neutron flux for the 232Th capture reactions was estimated through the horizontal measurement of 115In(n, y)116mIn reaction rate distribution, as well as described in Sect. 9.3.1.1. The wire was set in an aluminum guide tube, from the tungsten target to the center of the fuel region [from the position of (13, 14 — A0) to that of (13, 14 -1); Fig. 9.3], at the middle height of the fuel assembly. The absolute values of the measured reaction rates (Fig. 9.7) revealed differently the variation of

reaction rates attributed to varying the neutron spectrum in the core, when the spallation neutrons generated by 100 MeV protons were injected into the core. The moderating effect of the high-energy neutrons in some cores (Th-PE, Th-HEU-PE, and NU-PE: keff = 0.00613, 0.58754, and 0.50867, respectively) was observed around the boundary between the core and polyethylene regions. The 115In(n, y)116mIn reaction rates in the NU-PE core were higher than in other cores, demon­strating, that the reaction rates of 238U in the NU-PE core were larger than those of 232Th in the thorium cores with the use of 100 MeV protons. Additionally, the effect of the neutron spectrum on the reaction rates was observed with 100 MeV protons by comparing the measured results of reaction rates shown in Fig. 9.5. Thus, an expected physical effect was indeed observed as a result of the neutron spectrum change obtained by varying the moderator materials in the fuel assembly. Addi­tionally, the accuracy [5] of experimental and numerical analyses was compared successfully with the ratio (C/E) of calculations and experiments around the relative difference of 10 %, through the subcritical parameter of neutron multiplication M.