An Equation of State for Liquid Metals for Use in Nuclear System Thermal-Hydraulic Codes: Formulation and Code Verification in RELAP5

Liquid metals are enabling working fluids for several advanced nuclear systems, including fast reactors, accelerator-driven systems, and fusion blankets. System thermal-hydraulic (STH) codes require thermodynamically consistent property tables over pressure-temperature domains, whereas most liquid-metal correlations are available only as functions of temperature at a reference pressure. This paper presents the formulation and the code verification of four liquid-metal working fluids in RELAP5/Mod3.3: lead (Pb), lead-bismuth eutectic (LBE, denoted PbBi), lead-lithium alloy (PbLi, here Pb-17Li at.%), and sodium (Na). The liquid branch is reconstructed from reference correlations for specific volume, sound speed, and isobaric specific heat through a linearized pressure model, whose correction remains below 0.2% for the heavy liquid metals and below 1% for sodium over the whole tabulated pressure range. The reference pressure is set to the saturation pressure at the maximum tabulated temperature, which maximizes the admissible liquid domain, and a van der Waals equation of state closes the vapor branch. Liquid transport properties and selectable low-Prandtl-number heat-transfer correlations are implemented in the Fortran source code. Verification comprises property comparisons and two non-regression tests, a U-tube manometer, and a natural-circulation loop. The vapor model is a table-completion closure and must not be used for boiling-dominated transients.