| dc.description.abstract | An externally driven subcritical molten FLiBe-Thorium LiF-BeF2-ThF4 (52.8-27.2-20 mol%) salt assembly is being constructed at Texas A&M Nuclear Engineering and Science Center (NESC). The experiment is called the Thorium Engineering and Science Assembly (ThESA). The objective of ThESA was to be a separate effects test to isolate the phenomenon of thorium-232 fissioning in a FLiBe medium. ThESA was chosen to be driven with a deuteron-deuteron generator to ensure an unobscured fission neutron flux above the 3 MeV threshold. It was the objective of this thesis to support the design of ThESA to ensure the maximization of thorium fissioning in a FLiBe medium.
The thesis work was initiated with a comparison study between differing pin and pool assembly types. Multiple pins and pool type assemblies were analyzed using MCNP with either FLiBe-Th or a combination of FLiBe and ThO2 to measure the effective neutron multiplication factor (k-inf), neutron flux, and fission rates. The comparison study included each design's practicality, such as fuel heating, transportation, and glovebox physical restraints, as well as neutronics results. The results of the comparison study indicated that a pool-type would be the superior choice in producing the most thorium fissions in FLiBe. The pool-type assembly was also the most experimentally practical considering design and operational constraints. Following the final assembly choice, a parametric reflector study was performed on the pool type assembly. The reflector study results indicated that the ThESA objective would not benefit from a reflector, considering a large and heavy reflector's cost and complexity. After the mechanical team matured the design, a safety analysis was performed that included dose rates, criticality simulations, and an assessment to ensure no proliferation threats. The safety analyses revealed no significant hazards from ThESA. Then the addition of steel inserts in the pool intended for data gathering equipment was included in the MCNP model. The inclusion of the steel inserts was done to ensure that there would not be a significant reduction in the thorium fission rate. The analysis revealed that steel inserts would not induce a meaningful reduction in the assembly's fission rate. The final study goal was to capture the energy dependent neutron flux spectrum to capture the fraction of fission neutrons that, with certainty, would be differentiable from the source neutrons. The flux was subdivided into energy groups to observe the neutrons born from fission with energies above 3 MeV. The 3 MeV threshold was chosen due to the lack of source neutrons above this energy range. The total neutrons produced from fission above 3 MeV was 2.87E5 +/- 5.75E+01 neutrons per second. The work performed in this study did reveal that there was a substantial fission neutron population that could be differentiated from the source neutrons to be studied in an isolated effects test. | en |
