American National Standard ANSI/ANS-8.6: Safety in conducting subcritical neutron multiplication measurements in-situ

Safe and economical operations with fissile materials require knowledge of the subcriticality of configurations that arise in material processing, storage, and transportation. Data from critical experiments have been a principal source of information with which to establish safety margins. However, the lower cost and the expediency of performing confirmatory subcritical measurements on the process floor or in the storage vault resulted in much of the early criticality safety guidance being based on subcritical in situ experiments. Two applications of in situ measurements and the ANS-8.6 standard at (LANL) Los Alamos National Laboratory in the past decade come to mind. The first is a one-of-a-kind operation calculated to be subcritical by ∼10% when fully assembled, which included thick reflection. The second application involves inverse multiplication measurements on boiling water reactor and pressurized water reactor assemblies with loadings and enrichments typical of commercial reactors. As US Department of Energy environmental restoration activities escalate, situations will undoubtedly arise in which the degree of subcriticality of buried material will be desired knowledge prior to its disturbance. The advancement of technologies to handle such situations is certainly a necessary item, but adherence to the in situ standard and other professional practices will also provide the necessary confidence that margin of subcriticality are maintained acceptably large