GOTHIC™ has been used to simulate the Experimental Breeder Reactor-II (EBR-II) Shutdown Heat Removal Test 17 and Shutdown Heat Removal Test 45R (SHRT-45R), which correspond to protected and unprotected loss-of-flow events, respectively. GOTHIC is a versatile generalpurpose, thermal-hydraulic software package that is a hybrid between traditional system thermal-hydraulic and computational fluid dynamics codes. It is a practical engineering tool that has been used for the design and licensing of existing plants, small modular reactors (SMRs), and next-generation plant designs. Historically, the software has been applied for containment analysis and operability assessments for light water reactors (LWRs), but the recent improvements included in GOTHIC 8.3(QA) allow for the software to be used to simulate advanced, non-LWR concepts currently being developed such as sodium, molten salt, lead, and gas-cooled designs.It will be demonstrated in this paper that GOTHIC includes both the required attributes to model EBR-II and the appropriate physics to accurately simulate the steady-state operating conditions as well as SHRT-17 and SHRT-45R. The GOTHIC model of EBR-II was developed using only publicly available information. The nodalization was selected not only to capture the important phenomena but also to remain computationally efficient. The GOTHIC results show good agreement in both magnitude and trend with the experimental data. Differences are within the bounds of experimental uncertainty and required engineering assumptions applied in the model to fill in gaps in information, particularly for the various leakage paths that existed throughout the primary side of EBR-II, and were not well characterized during the tests.
GOTHIC 8.3(QA) includes capabilities for modeling advanced, non-light water cooled reactors. Important capabilities introduced in GOTHIC 8.3(QA) include fluid property tables for various molten salts, an enhancement to the tracer tracking module to allow radioactive decay energy to be released locally in the carrier fluid and other improvements to the neutron kinetics module. With these new capabilities in place, GOTHIC is used to benchmark steady-state and transient conditions in the Molten Salt Reactor Experiment (MSRE), which operated at Oak Ridge National Laboratory from 1965 to 1969. In this experimental reactor, UF4 fuel was dissolved in molten fluoride salt, and criticality could be achieved only in the graphite moderated core. An air-cooled radiator transferred fission and decay heat to the environment. The design thermal output of the MSRE was 10 MWt, but the radiator design limited the output to 8 MWt. The original design parameters neglected the impact of decay heat on system temperatures. GOTHIC is used to benchmark system operating parameters at both the 10 MWt design condition and the 8 MWt operating condition, both with and without decay heat. The cases that include decay heat apply 7% of the nominal thermal output using the eleven decay heat precursors from ASB 9-2 as tracers. The results of the benchmark exhibit good agreement with design and operating data and demonstrate heat-up due to decay heat in the fuel salt outside the core. In the MSRE, delayed neutron precursors are not confined to the core because the fuel and fission products flow through the system. As a result, there are different values for (effective) delayed neutron fraction with and without flow, and the decay of delayed neutron precursors outside the core under full-flow conditions reduces reactivity by 0.212 % δk/k. Zero power physics testing included fuel salt pump start-up and coast-down transients with a control rod automatically moving to maintain criticality. The control rod motion calculated by GOTHIC is a reasonable match to measured data from these transients. Low power testing included a natural convection transient with no control rod motion such that reactor power was responding to heat load demand from the radiator. The reactor power and fuel salt and coolant salt temperatures calculated by GOTHIC exhibit good agreement with measured data.
The Molten Salt Reactor Experiment (MSRE), which operated at Oak Ridge National Laboratory from 1965 to 1969, was an experimental reactor that used UF 4 fuel dissolved in molten fluoride salt. Criticality was achieved when the fuel salt mixture passed through the graphite-moderated core region. Therefore, because the fuel and fission products flowed through the system, delayed neutron precursors were not confined to the core, and decay heat was released outside the core, which is a unique challenge relative to more traditional reactor designs with solid fuel. Therefore, research and demonstration reactors such as MSRE have become a valuable source of information for benchmarking modeling and simulation tools for advanced reactor designs. One such tool being considered is GOTHIC, which is a coarse-grid computational fluid dynamics multiphysics software package. GOTHIC includes attributes and physical phenomena needed for modeling these advanced, non-light water reactor designs. For example, GOTHIC includes fluid property tables for various molten salts; a tracer-tracking module for modeling fission products and the radioactive decay and heat release by delayed neutron precursors locally in the fluid outside the core; and other necessary capabilities for modeling molten salt reactor (MSR) designs, including the ability to model dissolved gases. GOTHIC is used to benchmark steady-state and transient conditions from the MSRE. Zero-power physics testing included fuel salt pump start-up and coast-down transients with a control rod automatically moving to maintain criticality. The control rod motion calculated by GOTHIC is a reasonable match to measured data from these transients. Further, low-power testing included a natural convection transient with no control rod motion such that reactor power was responding to heat load demand from the radiator. The reactor power and fuel salt and coolant salt temperatures calculated by GOTHIC exhibit good agreement with measured data. These results confirm GOTHIC capabilities for modeling MSR designs with circulating fuel.
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