Fast and robust methods for interrogation of special nuclear material (SNM) are of interest to manyagencies and institutions in the United States. It is well known that passive interrogation methodsare typically sufficient for plutonium identification because of a relatively high neutron productionrate from 240Pu [1]. On the other hand, identification of shielded uranium requires active methodsusing neutron or photon sources [2]. D-D (2.45 MeV) and D-T (14.1 MeV) neutron-generatorsources have been previously tested and proved to be relatively reliable instruments for activeinterrogation of nuclear materials [3, 4]. In addition, the newest generators of this type are smallenough for applications requiring portable interrogation systems.Active interrogation techniques using high-energy neutrons are being investigated as a method todetect hidden SNM in shielded containers [4]. Due to the thickness of some containers, penetratingradiation such as high-energy neutrons can provide a potential means of probing shielded SNM. Inan effort to develop the capability to assess the signal seen from various forms of shielded nuclearmaterials, the University of Michigan Neutron Science Laboratory’s D-T neutron generator and itsshielding were accurately modeled in MCNP. The generator, while operating at nominal power,produces approximately 1 × 10~(10) neutrons/s, a source intensity which requires a large amount ofshielding to minimize the dose rates around the generator. For this reason the existing shieldingcompletely encompasses the generator and does not include beam ports. Therefore, several MCNPsimulations were performed to estimate the yield of uncollided 14.1-MeV neutrons from thegenerator for active interrogation experiments and to determine the expected dose with a port in thebiological shielding. Beam port diameters of 5, 10, 15, 20, and 25 cm were modeled to assess theresulting neutron fluxes and doses were calculated for a 10-cm port. The neutron flux outside thebeam ports was estimated to be approximately 2 × 10~4 n/cm~2-s and the dose rate in the same regionexceeded 1 rem/hr.
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