Experimental Study on Neutron Energy Spectra during Moderation in the Thermal Reactor

Effects of neutron leakage and absorption on neutron energy spectra during moderation were studied experimentally by using resonance foils of In, Au, Co and Mn and also by using the information obtained with BF3 and 238U fission counters.The influence of neutron leakage and absorption on the neutron energy spectrum were measured separately by activation measurements with these foils at the center of critical systems using UO2SO4 and D2O as fuel and moderator respectively, and it was indicated that the neutron spectrum φ (u) had a break point in the vicinity of 100 eV due to the combination of these two effects, especially in the systems of high fuel concentration.The space dependence of intermediate neutron energy spectra in the reflected systems was also investigated. A considerably wide 1/E neutron spectrum was observed at a small distance outside the core-reflector interface and a peak of neutron leakage was also observed at a small distance inside the same interface. In the reflected system, the characteristics of the intermediate neutron energy spectrum as a function of “space” could be understood fairly well.