Characterization of a 6-kg Neptunium Metal Sphere

The Np sphere is a 6070.4-g sphere of neptunium metal cast by Los Alamos National Laboratory in 2001. Neptunium is of interest to the nuclear community for applications such as space exploration and reactor fuel. The Np sphere is composed mostly of 237Np, which is a threshold fissioner. It requires an approximately 700-keV neutron to fission and therefore can go critical with only fast neutrons. The objective behind casting the sphere was to calculate the critical mass of neptunium and further understand neptunium nuclear data. This objective was partially met with critical benchmarks; however, an attempt to maximize sensitivity to 237Np led to the design and execution of a subcritical experiment. When analyzing data from this experiment, there were questions about the sphere’s composition, contaminant distribution, and hot spot location. This work begins by summarizing what is currently known about the fabrication and material composition of the sphere. Two material analyses have been completed on pieces of neptunium metal from the fabrication of the sphere that indicate the existence of 241Am, 243Am, and 244Cm. Thereafter, new neutron and gamma data revealed information about the hot spot distribution and its location. The hot spot, thought to be fixed at a mark made on the cladding during fabrication at the sprue location, now appears to move. Although lack of information and movement of the hot spot complicate its ability to be included in benchmark experiments, the need for additional neptunium nuclear data validation data still exists. The results detailed in this work will be vital in the design and planning of future Np sphere experiments.