Modeling of Fluoride Molten Salt Reactor Depletion in SCALE

Molten Salt Reactors (MSRs) are Gen IV nuclear reactor designs that use fissile material dissolved in high-temperature molten salt. MSRs feature safety and economic benefits through their low operating pressure, combined fuel and coolant into one component, and high thermal efficiency. Due to the historically limited demand for modeling MSRs, there has been no established tool explicitly designed for such a task. The SCALE reactor modeling software suite, developed and maintained by Oak Ridge National Laboratory (ORNL), is selected as a tool to examine MSR modeling capabilities. SCALE has been widely used, verified, and benchmarked for modeling of light water reactors (LWRs) and it would prove beneficial to demonstrate and evaluate its use for MSRs. This thesis focuses on graphite-moderated, FLiBe-fueled MSRs. It aims to determine the impacts and trade-offs for a given SCALE MSR depletion model between the user-defined simulation parameters, accuracy of the depletion simulation results and the computational resources. Accuracy of the depletion simulations was determined through comparison of criticality estimates (keff) and isotopic compositions of key nuclides across the lifetime of the reactor simulation. The simulation parameters considered include: depletion sub-interval schemes (coarse vs fine depletion step schemes); trace-element tracking (addnux parameters 2 through 4), self-shielding methods (CENTRM vs BONAMI); cross-section libraries (ENDF/B-VII.1 252-multigroup vs continuous-energy); and specific power (10 MW/MTIHM vs 20 MW/MTIHM). All analyses were conducted up to burnups of 62 GWd/MTIHM. Additionally, the parallel performance on two supercomputing clusters (Idaho National Laboratory’s Sawtooth cluster and Georgia Institute of Technology’s PACE-Firebird cluster) was analyzed. Major findings include: • From coarser to finer depletion schemes. Maximum actinide percent differences experienced little effect (~ 0.32%), but percent difference of nuclides of significant radioactivity experienced significantly higher (~5.10%). Computational runtime was affected linearly with each additional depletion step introduced into the scheme. keff was statistically unaffected (<10 pcm difference). • From BONAMI to CENTRM self-shielding, runtime experienced relatively small change (6.81% increase), ~226 ppm average difference in keff, and ~2.51% maximum difference of actinides and nuclides of significant radioactivity. • From addnux=2 to addnux=4 nuclide tracking, runtime experienced only a 6.33% increase for MG. keff was significantly affected (167 pcm difference, growing to ~500 pcm at EOL). For CE addnux variation, the criticality and isotopic concentration differences were near indistinguishable from that of MG, but the computational runtime experienced a ~4.7x increase from addnux=2 to addnux=4. The difference between addnux=2 and addnux=3 was significantly smaller than that of addnux=3 to addnux=4, suggesting that addnux=4 contains nuclides with significantly more effect on reactivity for fluoride-fueled, graphite-moderated MSRs (potentially H-3 and He-3 stemming from larger Li-7 concentrations). • From MG to CE runtime experienced significant increase (~10x longer). Actinide maximum difference was ~7%, while the nuclides of significant radioactivity exhibited ~4% maximum difference. • Depletion at 20MW/MTIHM over 10 MW/MTIHM yielded differences that were expected due to the inherent changes in the reactor core physics, thus suggesting that analyses presented in this thesis can be scaled to other fluoride-fueled, graphite-moderated MSRs of different specific powers

Date

2023-08-28

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