Analysis on the ~(237)Np sphere surrounded by ~(235)U shells Critical Experiment

摘要

There are many reasons for the interest of Neptunium-237. Those who evaluate the very long term risks associated with the disposal of radioactive wastes have been very concerned with ~(237)Np. This concern is based on: a) its presence in high-level waste from nuclear fuel cycles, b) its very long life (2.14 x10~6 years), c) its purported gastrointestinal absorption factor and d) its critical mass. Of these issues, the great uncertainty on the critical mass of ~(237)Np in the oxide and in the metal form according to the ANSI/ANS-8.15-1981 standard is of most concern to us. In an effort to decrease the uncertainty on the critical mass of ~(237)Np for criticality safety and nuclear nonproliferation issues, a series of experiments were performed at the Los Alamos Critical Experiments Facility. The first experiment in these series consisted of surrounding a 6.07 kg solid sphere of ~(237)Np by Highly Enriched Uranium hemispheres. Neptunium is formed in considerable quantities in nuclear power reactors during the irradiation of nuclear fuel. Of the higher-mass actinides produced, only the alpha particle emitting ~(237)Np isotope has a long half-life (2.14 x10~6 years). Neptunium-237 is formed by nuclear chains initiated by (n, γ) in ~(235)U under thermal neutron flux and by (n, 2n) reactions in ~(238)U under fast neutron flux. Additionally, ~(237)Np is formed as the daughter, by alpha decay, of ~(241)Am. This summary examines the differences in the calculated and experimental keff of the 237Np/235U experiment. It also examines the systematic uncertainty in the experiment by providing a confidence level useful to users working in code qualification.