Molecular Dynamics Simulation of Sub-Critical and Supercritical Water at Different Densities

Abstract

Interest in both the fundamental and applied chemistry of supercritical water (SCW) has recently increased, especially in environmental science but also because of the possibility in developing a new generation of water-cooled nuclear reactors operating with the coolant at supercritical temperatures.  Such a state corresponds to water above its critical temperature: T_c = 373.95 °C, P_c = 22.06 MPa, r_c = 0.322 g/cm³.  Controlling the water chemistry of an SCW reactor requires the ability to understand and mitigate the effects of water radiolysis.  As experiments at very high temperatures and pressures, and especially beyond the critical point of water, are difficult to perform, computer simulation becomes a very valuable alternative tool of investigation.  Preliminary results suggest that a key requirement for predicting water radiolysis in an SCW reactor is to have access to a detailed picture of the heterogeneous molecular structure of SCW, and to see how this influences radiation energy deposition and the subsequent radiolysis reactions.  Local density and molecular configurational fluctuations (associated with criticality) are also believed to play a pivotal role in the understanding of the detailed atomistic picture underlying the mechanism of localization of excess electrons in sub-critical water and SCW.  Relevant information concerning the microscopic aspects of this particularly interesting molecular system can be gained from all-atomistic computer simulations.  In this work, molecular dynamics (MD) simulations with a full description of the atomic interactions through the use of a force field have been carried out on water systems at different densities (0.17, 0.31, and 0.55 g/cm³) and different temperatures (360 and 400 °C).  Having access to a complete molecular description of those systems, radial distribution functions (RDF) can then be computed.  Detailed analysis of these RDF reveals the formation of water clusters whose behavior as a function of temperature and density is in agreement with experimental data.