Thermal Hydraulics System-Level Code Validation and Transient Analyses for Fluoride Salt-Cooled High-Temperature Reactors

Abstract

Verification and validation (V&V) of thermal hydraulics analysis codes for fluoride salt-cooled high-temperature reactors (FHRs) is identified as one of the key tasks that need to be addressed before FHRs can be licensed and deployed. System-level code validation of thermal hydraulics modeling in support of FHR development and licensing is the main objective of this study. The advanced high-temperature reactor (AHTR), one of the available FHR pre-conceptual designs, is the main focus of this research. FHRs feature passive heat removal capability using Direct Reactor Auxiliary Cooling Systems (DRACS) to remove decay heat during transients and accidents via natural convection/circulation flows. Due to the importance of natural circulation flows to FHR decay heat removal, the key objective of this study is to perform validation of system-level analysis codes on heat transfer performance evaluation for natural circulation flows.

Two system-level analysis codes, namely, RELAP5 SCDAPSIM/MOD 4.0 and System Analysis Module (SAM), are selected for this code validation study. Experimental data from a Purdue University natural circulation water loop and a low-temperature DRACS test facility (LTDF) using water as a surrogate for molten salts at the Ohio State University are utilized for RELAP5 and SAM code validation. An extensive test matrix is developed for the LTDF tests, including DRACS startup and pump trip scenarios. The code simulation results from RELAP5 and SAM show good agreement for fluid temperatures and mass flow rates with the experimental data.

For code validation for molten salt applications, steady-state experimental data obtained from the FLiBe natural circulation loop at the University of Wisconsin is utilized. It was found that the flow resistance in the loop is under-estimated by the SAM model. With higher flow resistance applied in the SAM model for the six tests simulated, the simulation results of the salt temperature differences across a cooler are within 27% compared to the experimental data. The correlated flow resistance is applied to this model due to potential pipe corrosion and salt freezing films near the outlet of the air cooler in the experiment. This research also identifies salt freezing model as an additional need in modeling FHRs with current system codes.

An uncertainty analysis is performed for the SAM code by investigating the effect of the uncertainties in molten salt thermophysical properties on the uncertainties of the predicted quantities of interest. From the sensitivity analysis for the high-temperature fluoride salt test facility (HT-FSTF), which adopts FLiNaK as the primary coolant, it is found that the FLiNaK viscosity and thermal conductivity have a higher influence on the salt temperature while the viscosity and specific heat capacity of FLiNaK can significantly affect the natural circulation flow velocity.

Furthermore, an AHTR reactor model is developed using the similar modeling approach in RELAP5 with a fluted-tube DRACS heat exchanger and a fluted-tube natural draft DRACS heat exchanger. Reactor normal operation and two accident scenarios, namely, station blackout (SBO) and loss of multiple DRACS loops, are analyzed. During SBO, DRACS provides sufficient decay heat removal capability, which leads to sufficient temperature margins from fuel damage and salt boiling. Overall, the simulation results show that during both transients, the reactor decay heat can be sufficiently removed by the ambient air, fully relying on passive natural circulation/convection with the proposed DRACS design in the AHTR.