Criticality of a ²³⁷Np Sphere

A series of critical-mass experiments using a 6-kg neptunium sphere was performed on the Planet vertical-assembly machine at Los Alamos National Laboratory (LANL). The purpose of the experiments was to obtain a better estimate of the critical mass of ²³⁷Np. The configurations that were studied included surrounding the neptunium sphere with highly enriched uranium (HEU) shells as well as reflecting it with iron and polyethylene. An additional experiment using a 4.5-kg -phase plutonium sphere surrounded with HEU was performed to demonstrate how well the computer transport code and the existing cross-section data for uranium and plutonium could reproduce the experiment. For some of the configurations, the prompt-neutron decay constants at delayed critical were measured. These experiments provided an integral measurement of the cross sections for ²³⁷Np in the fast-energy and possibly in the intermediate-energy regions. The measured k_eff from these experiments was compared with the calculated k_eff from the Monte Carlo N-Particle (MCNP) transport code using ENDF/B-V and ENDF/B-VI and cross-section data evaluated by the Nuclear Theory and Applications group (T-16) at LANL. In all the neptunium experiments, the calculated k_eff values based on ENDF/B-VI data were ~1% lower than the experimental k_eff. After adjusting the cross sections for neptunium and ²³⁵U to match the bare neptunium/HEU experiment as well as Godiva k_eff criticality and spectra indexes, the MCNP code yielded a value of 57 ± 4 kg for the bare critical mass of ²³⁷Np.