Celikten, O.
, Weiss, A.
and Sahin, D.
(2024),
Preconceptual Design Activities of the NIST Neutron Source: Preliminary Neutronics Assessments, Technical Note (NIST TN), National Institute of Standards and Technology, Gaithersburg, MD, [online], https://doi.org/10.6028/NIST.TN.2316, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=957588
(Accessed December 8, 2024)
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Preconceptual Design Activities of the NIST Neutron Source: Preliminary Neutronics Assessments
Published
Author(s)
Osman Celikten, ,
Abstract
The NIST Center for Neutron Research (NCNR) hosts the National Bureau of Standards Reactor (NBSR), which has been operational since 1967. The NBSR is aging, and current NCNR facilities fall short of accommodating scientific demand. Therefore, NIST is pursuing a replacement reactor, namely the NIST Neutron Source, or NNS. The NNS is proposed as a 20 Megawatt thermal (MWth) compact-core-reactor, cooled and moderated by light water (H2O). The core is fueled by nine high-assay low-enriched uranium-molybdenum (U-10Mo) monolithic curved plate fuel assemblies in a three-row by three-column square lattice. The NNS core is encased in a chimney surrounded by a heavy water (D2O) filled reflector tank where two liquid deuterium (LD2) cold neutron sources lie. This work discusses the neutronics of the NNS's pre-conceptual equilibrium core design, which includes its safety aspects. Descriptions of the design targets as well as anticipated reactor materials used, are included alongside the Monte Carlo N-Particle® (MCNP) model setup. A list of assumptions and model development details are shown to clearly illustrate the limitations of the analyses in this work. The equilibrium core computation process is shown alongside the converged fuel management scheme, which predicts a cycle length of 40 days with three fresh fuel assemblies each cycle. Neutron flux distributions illustrate the high leakage of thermal neutrons out of the core, which is desirable. Two critical parameters describe reactor utilization and fuel performance, namely the maximum power density and the maximum local integral fission density, or in short fission density. For the NNS, the maximum power density is estimated to be 18 kW/cm3, and the maximum local fission density is about 4.5 x 1021 fissions/cm3. The same parameters are 4.88 kW/cm3 and 2.0 x 1021 fissions/cm3 for the existing NBSR. Please note that NBSR currently utilizes high-enriched U3O8 fuel and plans to use low-enriched U-10Mo fuel by 2030. For comparison, the maximum power density and the maximum local fission density values for the NBSR utilizing U-10Mo fuel are estimated as 12.6 kW/cm3 and 6.2 x 1021 fissions/cm3, respectively. Criticality safety assessments demonstrate negative reactivity coefficients for the moderator temperature, void, and H2O to D2O mixing from accidents involving the reflector tank. This technical note demonstrates that the current pre-conceptual NNS design is feasible and safe from a neutronics perspective.Citation
Technical Note (NIST TN) - 2316
Report Number
2316
NIST Pub Series
Pub Type
NIST Pubs
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Keywords
NNS, Neutronics, MCNP, Criticality, Safety, Nuclear, NCNR
Citation
Issues
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Created November 26, 2024