Verification and validation of a natural circulation loop model for FHRs

The University of California, Berkeley is performing integral effects experiments for thermal hydraulics code validation, as part of a project to develop the technical basis for design and licensing of fluoride-salt-cooled, high-temperature reactors (FHRs). As an initial validation case, experiments for single-phase forced/natural circulation have been performed using the Compact Integral Effects Test (CIET) Test Bay. This experimental loop has been modeled using the FHR advanced natural circulation analysis (FANCY) 1.0 code, and computational results have been compared with RELAP5-3D, Flownex, and experimental data. In this paper, the models used in FANCY 1.0 are described, and verification and validation efforts are presented. As part of code and model verification efforts, steady-state natural circulation computational results are compared to analytical calculations for a simplified case with no heat and form losses along the piping, and it is shown that results from FANCY 1.0 agree well with analytical, RELAP5-3D and Flownex solutions with certain discrepancy. Additionally, validation is performed using experimental data from the CIET Test Bay. The first data set used for validation is from natural circulation steady-state runs across a range of power inputs. Since heat losses, form losses and effects from gas entrainment in the circulating fluid are not considered in the ideal FANCY 1.0, RELAP5-3D and Flownex models, the discrepancy between these results and experimental data increases as the power input increases. The second experimental data set used for validation is for a pump coast-down transient and startup of natural circulation from isothermal conditions.