Molecular Dynamics of Supercritical Water for Nuclear Data Development

The plant system of a supercritical-water-cooled reactor is the once-through direct-cycle type, where steam-water separators and coolant recirculation systems are not necessary. It is different from those of a boiling water reactor (BWR) and a pressurized water reactor. The supercritical-water-cooled reactor is sensitive to perturbations of the feedwater flow rate because all of the core coolant, driven by the feedwater pumps, flows to the turbines without recirculating core flow. The axial coolant density change is three times larger than that of a BWR. It is necessary to analyze the controllability of the reactor against coolant flow and pressure perturbations to assess the technical feasibility of the reactor. The behaviors of a fast reactor cooled by supercritical water are analyzed for three principal perturbations: change of the control rod position, the feedwater flow rate, and the turbine control valve opening. Based on the step responses to the perturbations, the reactor control system is designed such that the pressure is controlled by the turbine control valves, the main steam temperature is controlled by the feedwater flow rate, and the core power is controlled by the control rods. It is not appropriate to control the pressure by the feedwater flow rate like in a supercritical fossil-fired power plant because of the nuclear thermal-hydraulic coupling. Parameters of the control system are selected by the test calculations to satisfy both fast convergence and stability criteria. Reactor behaviors with the designed control system are stable against the perturbations, although because the plant is the once-through direct-cycle type, the coolant inventory is small. Reactors cooled by supercritical light water are controllable with the described control system.

Characterization of phase structures of a supercritical water/supercritical carbon dioxide/heavy oil system with molecular dynamics simulations

The purpose of the study is to analyze the breeding ratio of a supercritical pressure light water cooled fast reactor (SCFR) and to design a breeding core of SCFR. The sensitivities of core parameters to the breeding ratio are analyzed. The core is designed by coupling two-dimensional R-Z neutronics and a multi-channel thermal-hydraulic calculation. The parameters which have high sensitivities to the breeding ratio are the diameter of the fuel rods and the diameter of the coolant tubes of the briquet blanket. The briquet fuel assembly means that the coolant flows in tubes and the fuel is contained outside of the tubes, “a tube in shell” fuel assembly. For increasing heavy metal fraction, briquet blanket is considered. The positions of the fuel and the coolant are exchanged for increasing heavy metal fraction in briquet blanket. The breeding ratio of SCFR is 1.021 with fuel rod type blanket and 1.034 with the briquet blanket. When both seed and blanket are composed of briquet type fuel elements, the breeding ratio reaches 1.046 because of the high fuel volume fraction. The reactor power also increases with the briquet core. But SCFR can be a breeding reactor even if both seed and blanket consist of rod type fuels.

Molecular dynamics (MD) calculations have been performed to determine equilibrium structure and properties of systems modeling supercritical (SC) water and SC aqueous solutions at two states near the critical point using the simple point charge (SPC) potential model of Berendsen et al. for water. Both thermodynamic and dielectric properties from the simulations for pure water are accurate in comparison with experimental results even though the SPC model parameters were fitted to properties of ambient water. Details of the near-critical clustering in SC water have been predicted which have not been measured to date. MD studies have also been undertaken of systems that model sodium and chloride ions and neutral argon in SC water at the same states. The first solvation shell in SC water is observed to be similar to that in ambient water, and long-range solvation structures in SC water are similar to those observed for simple SC solvents. An excess of water molecules is observed clustering around ionic solutes which behave attractively and a deficit is observed around neutral atomic solutes which behave repulsively. These results should be helpful in developing a qualitative understanding of important processes that occur in SC water.

Density dependence of reorientational dynamics in supercritical water

Core design of a high-temperature fast reactor cooled by supercritical light water

Subchannel analysis of a fast reactor cooled by supercritical light water

The nuclear isotropic shielding constants sigma((17)O) and sigma((13)C) of the carbonyl bond of acetone in water at supercritical (P=340.2 atm and T=673 K) and normal water conditions have been studied theoretically using Monte Carlo simulation and quantum mechanics calculations based on the B3LYP6-311++G(2d,2p) method. Statistically uncorrelated configurations have been obtained from Monte Carlo simulations with unpolarized and in-solution polarized solute. The results show that solvent effects on the shielding constants have a significant contribution of the electrostatic interactions and that quantitative estimates for solvent shifts of shielding constants can be obtained modeling the water molecules by point charges (electrostatic embedding). In supercritical water, there is a decrease in the magnitude of sigma((13)C) but a sizable increase in the magnitude of sigma((17)O) when compared with the results obtained in normal water. It is found that the influence of the solute polarization is mild in the supercritical regime but it is particularly important for sigma((17)O) in normal water and its shielding effect reflects the increase in the average number of hydrogen bonds between acetone and water. Changing the solvent environment from normal to supercritical water condition, the B3LYP6-311++G(2d,2p) calculations on the statistically uncorrelated configurations sampled from the Monte Carlo simulation give a (13)C chemical shift of 11.7+/-0.6 ppm for polarized acetone in good agreement with the experimentally inferred result of 9-11 ppm.

Molecular dynamics simulation on the diffusion coefficients of the carbon and hydrocarbon radicals in the hydrogen production process in supercritical water gasification

The extent, distribution, and temporal and structural aspects of hydrogen bonding in supercritical water were investigated through molecular dynamics simulations at 773 K and densities between 115 and 659 kg/m3 using a flexible water potential. Pair energy distributions in supercritical water do not have the bimodal nature of liquid water distributions. Supercritical water molecular pairs are energetically disposed towards parallel alignment of dipoles in the first solvation shell whereas liquid water pairs favor a near-orthogonal orientation. An energetic criterion was used to identify hydrogen bonded molecular pairs. The number of hydrogen bonds per water molecule in supercritical water is one-sixth to one-half of that present in ambient water. Unlike in ambient water in which almost all the molecules belong to a hydrogen bonded gel, hydrogen bonded clusters in supercritical water typically consist of fewer than five members. The persistence time constant for hydrogen bonds is 0.1 ps in supercritical water in contrast to 0.6 ps in ambient water.

All atom molecular dynamics simulation study of polyethylene polymer in supercritical water, supercritical ethanol and supercritical methanol

Core and plant design of the power reactor cooled and moderated by supercritical light water with single tube water rods

Solvation of asphaltenes in supercritical water: A molecular dynamics study

We report results of molecular dynamics (MD) simulations of the limiting conductances of MgCl2 and CaCl2 in supercritical water as a function of water density using the SPC/E model for water. The limiting conductances of Mg2+, Ca2+ and Cl- over the whole range of water density considered exhibits a linear dependence of the limiting conductance on the water density. In the cases of Mg2+ and Ca2+, a solventberg picture for the behavior of small divalent cation emerges from our studies. From the view of the solventberg picture, the ion and its shell moving together as an entity interacts with the second hydration shell water molecules, and its mobility is restricted mostly by the number of the second hydration shell water which is proportional to the water density of the whole system. In the case of Cl-, the range of water density considered in this study belongs to the higher-density region (above 0.45 g/cm3) in which the effect of the number of hydration water molecules around ions dominated. As the water density increases, the water molecules of the first hydration shell restrict the mobility of Cl- and the limiting conductance of Cl- decreases nearly linearly. Significant different dependence on the water density is observed between the calculated limiting conductances of MgCl2 and CaCl2 at 673 K and the experimental results over the water density of 0.60–0.90 g/cm3. Possible limitation of the extended simple point charge (SPC/E) model with regard to this difference should be pointed out and the use of a more precise model like the revised polarizable (RPOL) model is indispensable for a further MD study to gain a complete picture of the chemical circumstance around the ions.

We propose in this paper a theoretical model for fluid state thermodynamics based on modeling the fluctuation distributions and, hence, the corresponding moment generating functions providing the free energy of the system. Using the relatively simple and physically coherent gamma model for the fluctuation distributions, we obtain a complete theoretical equation of state, also giving insight into the statistical/molecular organization and phase or pseudo-phase transitions occurring under the sub- and super-critical conditions, respectively. Application to sub- and super-critical fluid water and a comparison with the experimental data show that this model provides an accurate description of fluid water thermodynamics, except close to the critical point region where limited but significant deviations from the experimental data occur. We obtain quantitative evidence of the correspondence between the sub- and super-critical thermodynamic behaviors, with the super-critical water pseudo-liquid and pseudo-gas phases being the evolution of the sub-critical water liquid and gas phases, respectively. Remarkably, according to our model, we find that for fluid water the minimal subsystem corresponding to either the liquid-like or the gas-like condition includes an infinite number of molecules in the sub-critical regime (providing the expected singularities due to macroscopic phase transitions) but only five molecules in the super-critical regime (coinciding with the minimal possible hydrogen-bonding cluster), thus suggesting that the super-critical regime be characterized by the coexistence of nanoscopic subsystems in either the pseudo-liquid or the pseudo-gas phase with each subsystem fluctuating between forming and disrupting the minimal hydrogen-bonding network.