Design of an in-core fission spectrum neutron irradiation facility with pneumatic sample transfer at a research reactor
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The objective of this thesis was to provide instruction on the design, optimization and construction of a fast neutron irradiation facility with pneumatic sample transfer at the Nuclear Engineering Teaching Lab (NETL) reactor core. This facility will be used for fast neutron activation analysis and fast neutron fission in uranium and plutonium samples. Various materials and geometric designs were researched and modeled in Monte Carlo Neutral Particle (MCNP) code for the filtration of thermal neutrons from the NETL reactor core. The approach to these models were described as well as the subsequent processes in yielding the finalized design. The model geometry was recreated in SCALE to provide estimations for expected fission product activities. The design that was built was based off a layered cylindrical format that was placed inside a modified 3-element (EL) container. This container was created to fit into the 3-EL irradiation position of the core. The design balances the need to maintain high total neutron flux with the careful shaping of a neutron spectrum to be as close to the Watt fission spectrum as possible. The models indicated that the thermal+epithermal neutron flux was reduced by five orders of magnitude while leaving the fast neutron flux virtually untouched. That value was tallied after the reactor core flux is shaped by the neutron absorbing materials. The fast:(thermal+epithermal) flux ratio should exceed 100:1 with an integrated fast flux value (defined > 0.001 MeV) of at least 7.8 x 10¹¹ n cm⁻² s⁻¹. The calculated activity from the irradiation of 10 mg Pu via SCALE was determined to be 94.97 MBq.