Criticality of Plutonium Nitrate Solutions in Slab Geometry

A series of criticality experiments was performed on plutonium nitrate solutions in slab geometry. The solutions contained plutonium at concentrations ranging between 58 and 412 g Pu/liter for material with three different isotopic contents: 4.6, 18.4, and 23.2 wt% ²⁴⁰Pu. Acid molarities varied from 1.6 to 5.0. The experiments were performed with a variable thickness slab-type vessel of 42-in. height and width, whose thickness could be adjusted throughout a range of 3 to 9 in. The experimental vessel was used with and without a water reflector and also with a 1-in.-thick Plexiglas reflector. The critical experiment data from the finite slabs were corrected to yield values of critical thicknesses for one-dimensional infinite slabs, i.e., slabs of finite thickness but of infinite height and width. Analytical corrections, based on experimental data, were subsequently used to correct the critical infinite slab thicknesses for materials extraneous to the plutonium solutions, such as the effect of the stainless-steel vessel walls and room return neutrons. The analysis provided values for clean one-dimensional assemblies that were then used as an integral check of calculational methods using cross sections from the ENDF/B-II data file. The computed values of k_eff for these “clean assemblies” ranged between 0.988 and 1.040; the values increased somewhat with increasing concentration.