Experimental Studies of Microparameters of Clusters of Seven Nuclear Superheat Reactor Fuel Rods

The microparameters including the thermal-neutron disadvantage factor, DF, the epi- to sub-Cd neutron capture ratio in ²³⁸U, ρ²⁸, the epi- to sub-Cd fission ratio in ²³⁵U, δ²⁵, and the ratio of the epi-Cd ²³⁸U fission to the sub-Cd ²³⁵U fission, δ²⁸, were measured in the Ozenji Critical Facility for a seven-rod clustered nuclear superheat fuel element. The factors f, p, and ϵ were derived therefrom and the effect of ²³⁵U epithermal fissions on the neutron multiplication factor was observed to be 1.5% Δk/k. Flooding changed the individual factors f, p, and ϵ by amounts corresponding to −6.8% Δk/k, +4.7% Δk/k, and −2.9% Δk/k, respectively, yielding an overall change of −5.1% Δk/k. The maximum discrepancies between measurement and calculation are 1 to 3% for DF, ρ²⁸ , δ25, and δ28; 0.3% Δk/k for f, p, and ϵ; and 0.4% Δk/k for the infinite multiplication factor. The calculation of the effects of flooding on f, p, ϵ, and the infinite multiplication factor agrees with the experiment to within 0.3 to 0.4% Δk/k. When performing the cell calculations, care was taken to determine how to cylinderize the unit cell to perform the one-dimensional calculations with the THERMOS code, how to select the value of the L factor to be used in the JUPITER code (modified MUFT) and how to incorporate the heterogeneous effect of fast fissions.