Graphical abstract Results of molecular dynamics simulations for polymorphs of silica: quartz, cristobalite, coesite, and stishovite, are compared to the experimental values of the lattice constants, density, radial and bond-angle distribution functions, equations of state, and phase transitions using different potentials. Highlights • MD simulation Potentials for modeling crystalline silica. • Results compared to experimental values, equation of state and phase transitions. • BKS potential is accurate and transferable, requiring modest computation time. Abstract This paper reviews and examines interatomic potentials or force fields for molecular dynamics (MD) simulation of crystalline silica. The investigated potentials are the BKS, Pedone, Munetoh, TTAM, and CHIK. The calculated values of the lattice constants, density, radial and bond-angle distribution functions, equations of state, and phase transitions using different potentials are compared to experimental values for polymorphs of silica: quartz, cristobalite, coesite, and stishovite. Simulation results with the BKS potential accurately predict the experimental measurements to within 2%, and converge within a reasonable timeframe on an average workstation. The Pedone potential, also parameterized for other metallic oxides, computationally is slightly more expensive and is not as accurate. The simulations with both the CHIK and TTAM potentials are less accurate than with the BKS potential for modeling silica over the entire range of the phase diagram. The simulations with the Munetoh potential are by far the cheapest in terms of the modest computational requirements, but unsuitable for modeling crystalline silica. It could not produce the nature of the α–β and I–II phase transitions in quartz or the equation of state for stishovite silica, and the predicted structural properties sometimes differ from experimental values by more than 10%.

This paper introduces a chemical kinetics model and compares its calculations with reported measurements of weight loss and total gasification rate for different NBG-18 nuclear graphite specimens in experiments performed at 876-1226 K. Results show that the gasification rate is chemical-kinetics limited at low and intermediate temperatures and diffusion-limited at high temperatures. At high temperatures, the model calculates the diffusion velocity of oxygen through the boundary layer using a developed correlation for Reynolds numbers of 0.006-1000. The agreement of the calculations with reported measurements of the total gasification rate and transient weight loss confirms the soundness of the chemical kinetics approach and validates the developed model and the multi-parameter optimization algorithm for determining the chemical kinetics parameters, based on reported measurements. These parameters are the values and Gaussian-like distributions of the specific activation energies for oxygen adsorption and desorption of CO, the specific activation energy for desorption of CO 2, the initial surface area of free active sites and the rate constants for the four elementary chemical reactions in the model. The performed parametric analyses for NBG-18 nuclear graphite specimens investigated the effects of temperature and oxygen partial pressure on total gasification rate and production rates of CO and CO 2 gases, for wide ranges of temperatures and oxygen partial pressures. [All rights reserved Elsevier].

This paper presents experimental results of saturation and subcooled boiling of FC-72 and HFE-7100 dielectric liquids on uniformly heated, 10times10mm porous graphite (PG) surfaces for potential applications to immersion cooling of high power computer chips. The experiments investigated the effects of surface inclination, from upward-facing (0deg) to downward-facing (180deg), and liquid subcooling from 0 to 30K on nucleate boiling heat transfer coefficient and critical heat flux. The presented experimental data and correlations for natural convection of dielectric liquids on PG and plane surfaces are important for cooling chips while in the standby mode when surface heat flux <20kW/m 2. The experimental curves of the nucleate boiling heat transfer coefficient for FC-72 dielectric liquid in the upward-facing orientation are used in 3-D thermal analysis for sizing and quantifying the performance of copper (Cu), PG and PG-Cu composite spreaders for removing the dissipated thermal power by an underlying 10times10mm computer chip with non-uniform heat dissipation. The 2mm-thick spreaders are cooled by either saturation or 30K subcooled nucleate boiling of FC-72 and the composite spreader consists of 0.4mm-thick surface layer of PG and 1.6mm-thick Cu substrate. [All rights reserved Elsevier].

Abstract An accurate, fast-running and stable six-group, point kinetics (PK) model is developed and applied successfully to the dynamic simulation of the operation of the prismatic core, high temperature next generation nuclear plant (NGNP) reactor. The model is unrestricted by the size of the time step, which could be as much as several seconds, accounts for Doppler Broadening and the fuel and graphite temperature reactivity feedbacks, and includes an active neutron source for zero-power reactor startup. An efficient and robust numerical technique that approximates the exponential matrix using 7th order-accurate Padé(3,3) function with a discretization error on the order of (Δ t ) 3 , solves the coupled nonlinear and stiff six-groups point kinetics equations. The PK model handles reactivity insertions in excess of a prompt critical, ρ / β ¯ > $1.0, with unrestrictive time step size. Model results are successfully benchmarked using the Inhour solution for a step insertion of external reactivity. To simulate the transient response of the NGNP reactor following an external reactivity insertion and during a startup, the PK model is coupled to 84-nodes thermal-hydraulics model of the reactor, also developed in this work. With a 2 s time step, the error of predicting the reactor thermal power is ∼0.001%, increasing exponentially to ∼0.08% and ∼1.5% with increased time step size to 5 and 8 s, respectively. The present PK model has been successfully incorporated into MELCOR-H 2 nuclear reactor analysis code to simulate transient operation of Very High Temperature Reactor (VHTR) for electricity generation, using a Closed Brayton Cycle turbomachinery, and the co-generation of hydrogen using Sulfur Iodine (SI) thermochemical processes. Highlights • Developed robust approximate solution of the six-group kinetics equations. • Applied developed kinetics model to dynamic operation of NGNP reactor. • Model's non-restrictive time step and accuracy are demonstrated. • Results of dynamic simulation of NGNP are presented.

The Pellet Bed Reactor (PeBR) with an operational life of 66 full-power years is developed for lunar surface power. It has Inconel X750 structure and vessel and would be launched unfueled then loaded with spherical fuel pellets (1.0 cm dia.) on the lunar surface after being placed below grade and surrounded with regolith. The pellets, comprised of ZrC-coated UC particles (850 μm in dia.) dispersed in ZrC matrix, are delivered to the lunar surface in subcritical canisters. The canisters are designed to remain sufficiently subcritical during launch and when submerged in wet sand and flooded with seawater in the unlikely event of a launch abort accident. The PeBR power system nominally generates 100 kWe at a thermal efficiency of 21%and a reactor exit temperature of 910 K. It employs three separate closed Brayton cycle (CBC) loops each with a turbo-machine unit for energy conversion and two water heat pipes radiator panels for heat rejection. The reactor coolant and CBC working fluid is He–Xe binary gas mixture (40 g/mol). Estimates of the hot-clean excess reactivity and the full-power operation life are obtained using neutronics and fuel depletion analyses. In addition, estimates of the total radioactivity in post-operation PeBR, while being stored below grade on the lunar surface, are determined for up to 1000 years.

Saturation boiling of PF-5060 dielectric liquid on Cu micro-porous surface layers (95, 139, 171, 197 and 220-μm thick) is investigated. These layers are deposited on 10 × 10 mm Cu substrates using two-stage electrochemical process. The basic micro-structure, obtained in the first stage using current density of 3 A/cm2 for 15–44 s, depending on thickness, is strengthened by continuing electrochemical deposition using much lower current density for 10’s of minutes. For conditioned surface layers, after a few successive boiling tests, the pool boiling curves are reproducible and the temperature excursion prior to boiling incipience is either eliminated or reduced <7 K. Present nucleate boiling results are markedly better than those reported for dielectric liquids on micro- and macro-structured surfaces. Present values of CHF (22.7–27.8 W/cm2) and hMNB (2.05–13.5 W/cm2 K) are 40–70%higher than and >17 times those reported on plane surfaces (<16 W/cm2 and 0.8 W/cm2 K). Best results are those of the 171-μm thick layer: CHF of 27.8 W/cm2 occurs at ΔTsat of only 2.1 K and hMNB of 13.5 W/cm2 K occurs at ΔTsat = 2.0 K.

Saturation boiling of PF-5060 dielectric liquid on Cu micro-porous surface layers (95, 139, 171, 197 and 220-μm thick) is investigated. These layers are deposited on 10 × 10 mm Cu substrates using two-stage electrochemical process. The basic micro-structure, obtained in the first stage using current density of 3 A/cm2 for 15–44 s, depending on thickness, is strengthened by continuing electrochemical deposition using much lower current density for 10’s of minutes. For conditioned surface layers, after a few successive boiling tests, the pool boiling curves are reproducible and the temperature excursion prior to boiling incipience is either eliminated or reduced <7 K. Present nucleate boiling results are markedly better than those reported for dielectric liquids on micro- and macro-structured surfaces. Present values of CHF (22.7–27.8 W/cm2) and hMNB (2.05–13.5 W/cm2 K) are 40–70%higher than and >17 times those reported on plane surfaces (<16 W/cm2 and 0.8 W/cm2 K). Best results are those of the 171-μm thick layer: CHF of 27.8 W/cm2 occurs at ΔTsat of only 2.1 K and hMNB of 13.5 W/cm2 K occurs at ΔTsat = 2.0 K.

Gasification of nuclear graphite in the unlikely event of massive air ingress in High-Temperature and Very-High Temperature gas-cooled Reactors is a safety concern, requiring accurate and reliable predictions of the erosion rate of the external surface and within volume pores. At low temperature, gasification occurs within the open pores gradually degrading the mechanical strength of graphite components. Gasification shifts gradually to the external surface with increasing temperature. At high temperatures, although the rates of chemical reactions increase exponentially with temperature, they are limited by the oxygen diffusion to the external surface. A semi-empirical Sh correlation is developed to calculate the oxygen diffusion velocity. It is based on an extensive database of reported measurements of the convective heat transfer coefficient for heated wires and cylinders in air, water and paraffin oil flows at 0.006 les Re les 2.42 times 10 5 and 0.068 les Pr les 35.2 and the mass transfer coefficient at 4.8 les Re les 104 and Sc = 0.609 and 1300-2000. The database also includes reported values of the averaged Sh for gasification of a cylinder of V483T nuclear grade graphite (300 mm long and 200 mm in dia.) at 1141-1393 K in ascending cross-flow of nitrogen gas containing 5 vol.% oxygen at 533 les Re les 1660. The Sh correlation is within +or-8% of the compiled 807 data points and applicable to both internal and external parallel and cross-flow conditions. When implemented in a chemical-reaction kinetics model, the calculated gasification rates are consistent with reported measurements for different size specimens of nuclear graphite grades NBG-18, NBG-25, IG-11, IG-110 and IG-430 at intermediate and high temperatures in atmospheric air (0.08 les Re les 30). [All rights reserved Elsevier].

Methods and apparatuses are provided for the removal and transportation of thermal energy from a heat source to a distant complex for use in thermochemical cycles or other processes. In one embodiment, an apparatus includes a hybrid heat pipes/thermosyphon intermediate heat exchanger (HPTIHX) system that is divided into three distinct sections, namely: an evaporation chamber, a condensation chamber, and a working fluid transport section of liquid and vapor counter-current flows.

Investigated numerically are the effects of slip and viscous heat dissipation on the friction number of thermally developing laminar water flow in micro-tubes. Also investigated are the effects of the micro-tube diameter, D, aspect ratio, L/D, and Reynolds number on the friction number. The decrease in the dynamic viscosity slightly decreases the friction number below that of the Hagen–Poiseuille, determined from the solution of the Navier–Stokes equations assuming thermally and hydro-dynamically fully developed flow and constant fluid properties. A slip at the wall decreases the friction number with increased L/D and decreased D. Results confirmed that the flow in micro-tubes is never thermally developed, significantly increasing the friction number with increased Reynolds number beyond critical values. Below the critical Reynolds numbers, the effect of the thermally developing flow is negligible and the friction numbers in micro-tubes is almost identical to those of the Hagen–Poiseuille, with and without slip at the wall. The comparison with reported experimental data confirmed the results of the present analysis.

Investigated numerically are the effects of slip and viscous heat dissipation on the friction number of thermally developing laminar water flow in micro-tubes. Also investigated are the effects of the micro-tube diameter, D, aspect ratio, L/D, and Reynolds number on the friction number. The decrease in the dynamic viscosity slightly decreases the friction number below that of the Hagen-Poiseuille, determined from the solution of the Navier-Stokes equations assuming thermally and hydro-dynamically fully developed flow and constant fluid properties. A slip at the wall decreases the friction number with increased L/D and decreased D. Results confirmed that the flow in micro-tubes is never thermally developed, significantly increasing the friction number with increased Reynolds number beyond critical values. Below the critical Reynolds numbers, the effect of the thermally developing flow is negligible and the friction numbers in micro-tubes is almost identical to those of the Hagen-Poiseuille, with and without slip at the wall. The comparison with reported experimental data confirmed the results of the present analysis. [All rights reserved Elsevier].

The history of the deployment of nuclear reactors in Earth orbits is reviewed with emphases on lessons learned and the operation and safety experiences. The former Soviet Union's “BUK” power systems, with SiGe thermoelectric conversion and fast neutron energy spectrum reactors, powered a total of 31 Radar Ocean Reconnaissance Satellites (RORSATs) from 1970 to 1988 in 260 km orbit. Two of the former Soviet Union's TOPAZ reactors, with in-core thermionic conversion and epithermal neutron energy spectrum, powered two Cosmos missions launched in 1987 in 800 km orbit. The US’ SNAP-10A system, with SiGe energy conversion and a thermal neutron energy spectrum reactor, was launched in 1965 in 1300 km orbit. The three reactor systems used liquid NaK-78 coolant, stainless steel structure and highly enriched uranium fuel (90–96 wt%) and operated at a reactor exit temperature of 833–973 K. The BUK reactors used U-Mo fuel rods, TOPAZ used UO2 fuel rods and four ZrH moderator disks, and the SNAP-10A used moderated U-ZrH fuel rods. These low power space reactor systems were designed for short missions (0.5 kWe and 1 year for SNAP-10A, <3.0 kWe and <6 months for BUK, and 5.5 kWe and up to 1 year for TOPAZ). The deactivated BUK reactors at the end of mission, which varied in duration from a few hours to 4.5 months, were boosted into 800 km storage orbit with a decay life of more than 600 year. The ejection of the last 16 BUK reactor fuel cores caused significant contamination of Earth orbits with NaK droplets that varied in sizes from a few microns to 5 cm. Power systems to enhance or enable future interplanetary exploration, in-situ resources utilization on Mars and the Moon, and civilian missions in 1000–3000 km orbits would generate significantly more power of 10's to 100's kWe for 5–10 years, or even longer. A number of design options to enhance the operation reliability and safety of these high power space reactor power systems are presented and discussed.

The history of the deployment of nuclear reactors in Earth orbits is reviewed with emphases on lessons learned and the operation and safety experiences. The former Soviet Union's “BUK” power systems, with SiGe thermoelectric conversion and fast neutron energy spectrum reactors, powered a total of 31 Radar Ocean Reconnaissance Satellites (RORSATs) from 1970 to 1988 in 260 km orbit. Two of the former Soviet Union's TOPAZ reactors, with in-core thermionic conversion and epithermal neutron energy spectrum, powered two Cosmos missions launched in 1987 in 800 km orbit. The US’ SNAP-10A system, with SiGe energy conversion and a thermal neutron energy spectrum reactor, was launched in 1965 in 1300 km orbit. The three reactor systems used liquid NaK-78 coolant, stainless steel structure and highly enriched uranium fuel (90–96 wt%) and operated at a reactor exit temperature of 833–973 K. The BUK reactors used U-Mo fuel rods, TOPAZ used UO2 fuel rods and four ZrH moderator disks, and the SNAP-10A used moderated U-ZrH fuel rods. These low power space reactor systems were designed for short missions (0.5 kWe and 1 year for SNAP-10A, <3.0 kWe and <6 months for BUK, and 5.5 kWe and up to 1 year for TOPAZ). The deactivated BUK reactors at the end of mission, which varied in duration from a few hours to 4.5 months, were boosted into 800 km storage orbit with a decay life of more than 600 year. The ejection of the last 16 BUK reactor fuel cores caused significant contamination of Earth orbits with NaK droplets that varied in sizes from a few microns to 5 cm. Power systems to enhance or enable future interplanetary exploration, in-situ resources utilization on Mars and the Moon, and civilian missions in 1000–3000 km orbits would generate significantly more power of 10's to 100's kWe for 5–10 years, or even longer. A number of design options to enhance the operation reliability and safety of these high power space reactor power systems are presented and discussed.

This paper examines the effects of using noble gases and binary mixtures as reactor coolants and direct closed Brayton cycle (CBC) working fluids on the performance of terrestrial nuclear power plants and the size of the turbo-machines. While pure helium has the best transport properties and lowest pumping power requirement of all noble gases and binary mixtures, its low molecular weight increases the number of stages of the turbo-machines. The heat transfer coefficient for a He-Xe binary mixture having a molecular weight of 15g/mole is 7% higher than that of helium, and the number of stages in the turbo-machines is 24-30% of those for He working fluid. However, for the same piping and heat exchange components design, the loop pressure losses with He-Xe are ~3 times those with He. Consequently, for the same reactor exit temperature and pressure losses in piping and heat exchange components, the higher pressure losses in the nuclear reactor decrease the net peak efficiency of the plant with He-Xe working fluid (15 g/mole) by a little more than ~2% points, at higher cycle compression ratio than with He working fluid. [All rights reserved Elsevier].

This paper examines the effects of using noble gases and binary mixtures as reactor coolants and direct closed Brayton cycle (CBC) working fluids on the performance of terrestrial nuclear power plants and the size of the turbo-machines. While pure helium has the best transport properties and lowest pumping power requirement of all noble gases and binary mixtures, its low molecular weight increases the number of stages of the turbo-machines. The heat transfer coefficient for a He–Xe binary mixture having a molecular weight of 15 g/mole is 7%higher than that of helium, and the number of stages in the turbo-machines is 24–30%of those for He working fluid. However, for the same piping and heat exchange components design, the loop pressure losses with He–Xe are 3 times those with He. Consequently, for the same reactor exit temperature and pressure losses in piping and heat exchange components, the higher pressure losses in the nuclear reactor decrease the net peak efficiency of the plant with He–Xe working fluid (15 g/mole) by a little more than 2%points, at higher cycle compression ratio than with He working fluid.