QUANTIFYING THE PASSIVE GAMMA SIGNAL FROM SPENT NUCLEAR FUEL IN SUPPORT OF DETERMINING THE PLUTONIUM CONTENT IN SPENT NUCLEAR FUEL WITH NONDESTRUCTIVE ASSAY

摘要

The objective of safeguarding nuclear material is to deter diversions of significant quantities of nuclear materials by timely monitoring and detection. There are a variety of motivations for quantifying plutonium in spent fuel (SF), by means of nondestructive assay (NDA), in order to meet this goal. These motivations include the following: strengthening the capabilities of the International Atomic Energy Agencies ability to safeguard nuclear facilities, shipper/receiver difference, input accountability at reprocessing facilities and burnup credit at repositories. Many NDA techniques exist for measuring signatures from SF; however, no single NDA technique can, in isolation, quantify elemental plutonium in SF. A study has been undertaken to determine the best integrated combination of 13 NDA techniques for characterizing Pu mass in spent fuel. This paper focuses on the development of a passive gamma measurement system in support the spent fuel assay system. Gamma ray detection for fresh nuclear fuel focuses on gamma ray emissions that directly coincide with the actinides of interest to the assay. For example, the 186-keV gamma ray is generally used for 235U assay and the 384-keV complex is generally used for assaying plutonium. In spent nuclear fuel, these signatures cannot be detected as the Compton continuum created from the fission products dominates the signal in this energy range. For SF, the measured gamma signatures from key fission products (~(134)Cs, ~(137)Cs, ~(154)Eu) are used to ascertain burnup, cooling time, and fissile content information. In this paper the Monte Carlo modeling set-up for a passive gamma spent fuel assay system will be described. The set-up of the system includes a germanium detector and an ion chamber, and will be used to gain passive gamma information that will be integrated into a system for determining Pu in SF. The passive gamma signal will be determined from a library of –100 assemblies that have been created to examine the capability of all 13 NDA techniques. Presented in this paper is a description of the passive gamma monitoring instrument, explanation of the work completed thus far involving the source set up methodology and the design optimization process, details of key fission product ratios of interest, limitations and key strengths of the measurement technique, and considerations for integrating this technique with other NDA techniques in order to develop a complete spent fuel assay strategy .