Numerical simulation and sensitivity analysis of the nitrate natural circulation loop

<bold>Background</bold>The nitrate natural circulation loop (NNCL) serves as a crucial experimental platform for studying the natural circulation characteristics of molten salts. Numerical simulation and sensitivity analysis of parameters in the NNCL deepen our understanding of molten salt natural circulation, thereby accumulating experience for the design and validation of passive residual heat removal systems in molten salt reactors.<bold>Purpose</bold>This study aims to validate the accuracy of numerical simulations and analyze uncertainty using experimental data, and study the impact of input parameters on natural circulation flow rate through sensitivity analysis.<bold>Methods</bold>The modified RELAP5/MOD4.0 code was employed to establish a nitrate natural circulation analysis model. New models related to molten salt such as molten salt fluids (nitrate, FLiNaK, <italic>etc</italic>.), heat transfer correlations, liquid-fueled reactor models, <italic>etc</italic>. were added to RELAP5/MOD4.0. The key inserted models had been validated by experiments. And the NNCL system model consists of three coupled parts, including the primary circuit, molten salt pool and air cooling system modules. The propagation of input uncertainty approach, which was based on Monte Carlo methods, was used to calculate uncertainty bounds of numerical analysis. 14 uncertain parameters for NNCL transient were selected through previous studies, experimental data and expert judgment, and a list of input parameters along with their associated density functions was adopted by using a probabilistic methodology. Wilks' formula was used to quantify the minimum number of code runs, and the upper tolerance limit's percentile and confidence were set to the standard 95%/95%. Once the code run number and sets of uncertain input parameters were established, input uncertainty was propagated through the modified RELAP5/MOD4.0 code, and then the upper and lower uncertainty bands were obtained. The sensibility of input parameters was analyzed by performing Multiple Linear Regression (MLR) method which utilized a straight line to describe the relationship between variables, and the F-test was used to assess whether the MLR models comply with statistical laws. The standardized regression coefficient (SRC) was used to rank the importance of input parameters affecting natural circulation.<bold>Results</bold>Numerical analysis results indicate that variations in NNCL cold-end and hot-end temperatures, as well as main loop nitrate flow rates, align with experimental trends, and the maximum error between the upper and lower limits of uncertainty analysis relative to experimental data is -5.7% to 4.2% for the DRACS (Direct Reactor Auxiliary Cooling System) Heat Exchanger (DHX) inlet temperature, -3.5% to 4.0% for the DHX outlet temperature, and -12% to 12% for the NNCL primary loop nitrate flow rate. The uncertainty analysis's upper and lower bounds effectively encompass experimental data and experimental errors. The calculated F-value is 2 682.0, which is greater than 2.32, this indicates that at the <italic>α</italic>=0.01 significance level, there is a significant linear relationship between the nitrate flow rate of the NNCL system and the 14 input parameters.<bold>Conclusion</bold>The incorporation of uncertainty analysis with the modified RELAP5/MOD4.0 code provides a good prediction of NNCL system operational characteristics. Through sensitivity analysis, three most sensitive factors affecting natural circulation are list as follows: heat transfer coefficient of air side, local resistance coefficient of primary loop, and density of nitrate.