Development of a Portable-Neutron Detector for Verification of Spent Fuel in Sealed Casks

Final disposal of spent reactor fuel presents new challenges to the safeguards community. Spent fuel assemblies intended for permanent disposal are placed into sealed casks that are never intended to be reopened. Once sealed, a cask may be subject to periodic inspections by safeguards authorities. An inspector needs to verify that nuclear material, such as fuel assembly, has not been diverted from the cask. Because casks should not be reopened, inspectors must rely on nondestructive assay (NDA) verification methods. High radiation background levels, Shielding by the cask, cumbersome equipment, and the disparate burn up and age of fuel assemblies make \"through-the-wall\" verification measurements difficult with traditional NDA techniques. A new prototype detector was developed at Las Alamos National LAboratory to address the measurements needs of spent fuel cask inspectors. This highly portable system consists of two passive neutron counters placed back to back, separated by a lated of cadmium. Passive neutron totals count is performed at several positions along the outside surface of the cask. Mont Carlo simulations have indicated that this system could detect the diversion of one or more assemblies from a 21-assembly storage cask. The development and field testing of this prototype counter by Los Alamos Laboratory is described.