Studies of two-region subcritical uranium heavy water lattices

Abstract

Reactor physics parameters were measured in eleven two-region subcritical assemblies moderated by heavy water. The regions of the assemblies consisted of nine different lattices of various fuel rod size, U235 enrichment, and spacing. The following parameters were measured in the assemblies: bare and cadmium-covered gold foil radial traverses; bare gold foil axial traverses; the ratio of epicadmium to subcadmium capture r tes in U238 (P28); and the ratio of fissions in U238 to fissions in U233(628)- From analysis of axial traverses at various radial positions, it was determined that the axial buckling was independent of radial position in the assemblies. A method was developed to apply the age equation to the experimental gold foil traverses. This analysis yielded the quantity [ ... ] for each region of the assemblies. Calculated values of [ ... ] were used to obtain values of the infinite multiplication factor from this parameter.

For assemblies of sufficiently large inner regions, the values of km so found agreed within experimental uncertainty with independent determinations. The slowing-down spectra. arising from the age theory analysis were used to extrapolate two-region assembly measurements of P28 to critical assembly values. General agreement was found between these extrapolated values and the results of measurements made in full, single region lattices. The heterogeneous expressions for uncollided flux derived by Pilat were extended to two-region assemblies and used to determine single rod values of 628 from two-region assembly measurements. The theory was also used to predict values of 628 for each of the lattices composing the two-region assemblies. Both the single rod values and the full lattice predictions agreed within experimental error with previously reported results. The determination of material buckling from two-region subcritical assemblies is also discussed.!

Because of the nature of the assemblies investigated, satisfactory measurements of the material buckling could not be made.