Heterogeneous character of supercritical water at 400 °C and different densities unveiled by simulation

Abstract

Molecular dynamics simulations are used to examine the molecular microstructures and the “clustering” behavior of supercritical water at 400 °C and different densities.

Supercritical Water: A Simulation Study to Unravel the Heterogeneity of Its Molecular Structures

“…To explore the molecular heterogeneity of SCW at 400 °C and the optimal pressure of 31 MPa, we analyzed the radial distribution function, g ( r ), with respect to the center-of-mass (COM) of water molecules. This function is defined as follows [ 28 , 29 ]: where represents the number of molecules within a spherical shell spanning from radius r to , essentially within a volume of . The notation signifies that this average is computed over a duration of 10 ns in the MD simulation.…”

“…As the distance r increases beyond this point, g ( r ) shows a noticeable dip, implying the formation of void spaces. The secondary peak at ~5.5 Å indicates the presence of a neighboring molecular cluster, likely comprising 4.5 water molecules [ 29 ]. As the distance further increases, the interactions among water molecules reach a state of equilibrium, indicative of a balancing of molecular forces.…”

“…It also reproduces, with good agreement, the liquid–vapor coexistence curve for water and predicts critical parameters, which compare favorably to the experimental values [ 51 ]. Furthermore, it has also demonstrated proficiency in accurately characterizing the structures and properties of water at high temperature and pressure conditions up to the supercritical state [ 26 , 28 , 29 , 39 , 40 , 41 , 42 ].…”

“…This knowledge is especially relevant for developing Monte Carlo track-structure chemistry simulations, which rely on a precise representation of the molecular nature of the medium-all current models of radiation tracks employ a continuum approach to water structure, thereby neglecting the actual molecular structure [27]. An accurate description of this structure is key to advancing radiolysis studies, as it enables more reliable predictions of the yields of radiolytic species in nuclear reactor coolants [26,28,29].…”

“…This study builds upon prior research, where we probed the heterogeneous properties of SCW at diverse densities at 400 • C [28,29]. We now aim to employ MD simulations to conduct a detailed analysis of the structural characteristics of SCW over a temperature range of 400 to 800 • C while maintaining a constant pressure of 31 MPa at 400 • C and 25 MPa between 500 and 800 • C. These particular conditions closely correspond with the operational parameters established for SCWRs and their SCW-SMR variants [7,12,14].…”