Implementation of Solar Salt as Fluid in ASYST4.1 and Validation for a Natural Circulation Loop

Abstract Various small modular reactor (SMR) designs such as Micro Modular Reactor (MMR), Stable Salt Reactor - Wasteburner (SSR-W) and the Natrium reactor utilize an intermediate molten salt loop for heat transport and thermal energy storage (TES). This technology is well proven for solar thermal plants and uses solar salt (NaNO3-KNO3 in 60:40 ratio by weight) as a heat transfer and heat storage material. The primary advantages are its thermal stability, large temperature range, low cost, high density, and low vapor pressure. For example, MMR is a prismatic block design which has emerged as one preferred concept of gas cooled reactors (GCRs) and uses helium as the primary coolant and the solar salt as intermediate fluid for transporting the heat to TES system. The current thermal hydraulic system codes can model helium as a dry non-condensable gas but in most cases solar salt properties are not available in these codes. Hence there is a lack of integrated analysis capability across the MMR-type system. An effort is made to collect a robust set of thermodynamic and transport properties and heat transfer correlations for solar salt and incorporate them into the code ASYST4.1. It will allow for the simulation of design basis accident which include the dynamics of the primary system, intermediate loop, and TES. Reliable computational tools are required for thermal hydraulics design and accident analysis of Small Modular Reactors (SMR) including associated experimental validation. One emerging tool is ASYST (Adaptive SYStem Thermal-hydraulics) - ISA (Integral Simulation and Analysis). It is a new code which combines the capabilities of SCDAPSIM/MOD4 and SAMPSON and is being developed jointly by Innovative Systems Software (ISS), USA and Institute of Applied Energy (IAE) of Japan. The thermal hydraulic module, ASYST-THA, replaces the original US NRC-developed RELAP5 code used in RELAP/SCDAPSIM/MOD3.x and the THA code used in SAMPSON, with new system level hydrodynamic options that include multidimensional, multi-fluid models originally developed by ISS and IAE. ASYST represents an internationallevel collaboration on system code development and benefits from the expertise and experiments available world-wide. This work presents the details of solar salt implementation into ASYST as well as a benchmarking study on Molten Salt Nitrate Natural Circulation Loop (MSNCL). ASYST calculated results are found to closely match the experimental data. This work has been further extended to quantify uncertainties in the calculated heater outlet/inlet temperature and the flow rate for 1600 kW case using Integrated Uncertainty Analysis (IUA) package of ASYST4.1. The margin in the temperatures is about 35 K which confirms that the ASYST can both overpredict or underpredict the experimental temperatures.