Effect of mobile helium on void nucleation in materials during irradiation

The steady-state rate of void nucleation is calculated for irradiated materials containing mobile helium. At the low displacement rates typical of a fast-breeder reactor a concentration of less than 10^?10 atom fraction helium can cause a 10²⁰ increase in nucleation rate. The helium is less effective at the high displacement rates typical of accelerator experiments, but can increase the void-nucleation rate by 10⁴ at a helium concentration of 10^?8. The calculated void-nucleation rates for low displacement rates and without helium are too low to explain the void number densities observed in breeder-reactor irradiated materials. Therefore, void nucleation in reactor environments is helium-assisted. Accelerator experiments intended to simulate void nucleation under reactor conditions must be carefully designed to observe gas-assisted rather than homogeneous void nucleation.     

A Study on Hydraulic Properties of Compacted Fe(III)-montmorillonite

Hydraulic conductivities were determined for compacted Fe(III)-montmorillonite sample. The Fe(III)-montmorillonite sample used in this study, which was prepared by the ion-exchange treatment of Namontmorillonite in FeCl₃ solution, contains less than 20% of Na⁺ ions as exchangeable cations and negligibly small amounts of iron precipitates. The measured hydraulic conductivities of compacted Fe(III)-montmorillonite were determined to be the order of 10^?9, 10^?11, and 10^?13 ms^?1 at dry densities of 0.80, 1.01, and 1.20 Mgm^?3, respectively. These values were found to be kept constant during the experimental period, suggesting no significant change in the physicochemical properties of Fe(III)-montmorillonite during the experiment. When compared with Na-montmorillonite, remarkably high values of hydraulic conductivities were found for Fe(III)-montmorillonite at the dry densities of 0.8 and 1.0Mgm^?3. On the other hand, almost the same value of hydraulic conductivity was obtained at the dry density around 1.2Mgm^?3. This different effect of exchangeable cations on the hydraulic conductivity of montmorillonite could be attributed to the different sizes of macropores in compacted montmorillonite and/or the different thicknesses of electrical double layers formed over montmorillonite sheets.     

Photoluminescence studies of neutron-transmutation-doped InP:Fe

Neutron-irradiated InP:Fe semi-insulating crystals have been investigated by photoluminescence (PL) and Hall effect. The evolution of the different PL transitions was studied as a function of the thermal neutron dose, annealing temperature and excitation power. Hall effect measurements confirm the success of the doping technique in semi-insulating InP:Fe. For tin concentration higher than 6 × 10¹⁵ cm⁻³ the InP:Fe samples annealed at 550°C are clearly n-type with a free carrier concentration slightly lower than the calculated tin concentration.     

Analysis of Adsorption Behavior of Cesium onto Quartz Using Electrical Double Layer Model

The adsorption behavior of Cs onto quartz was investigated with the use of a high performance liquid chromatograph. The surface-based distribution coefficient (K_a) of Cs onto quartz was determined in the solution of which pH ranged from 6 to 11 and ionic strength from 10^?3 M to 10^?1 M. The K_a values were found to increase with increasing pH and with decreasing ionic strength. The obtained data were successfully analyzed by an electrical double layer model and the optimum parameter values of the double layer electrostatics and adsorption reactions were obtained. The adsorption mechanisms of alkali metal ions was discussed on the basis of the present analysis.     

Electrokinetic Study of Migration of Anions,Cations, and Water in Water-Saturated Compacted Sodium Montmorillonite

Electromigration studies of ³⁶Cl^? ions and ²²Na⁺ ions, and electro-osmosis of water traced with HTO and H₂ ¹⁸O were conducted with water-saturated compacted sodium montmorillonite having dry densities from 0.8 to 1.6 Mg/m³. The mobilities and dispersivities for each species were obtained from the apparent electromigration velocities and hydrodynamic dispersion coefficients, respectively. When corrected by water flow, the apparent diffusion coefficients of Cl^? ions obtained from the Einstein relation are in good agreement with the coefficients obtained from an earlier conventional diffusion experiment in the high dry densities (above 1.3 Mg/m³,). This result suggests that the migration pathways of Cl^? ions are nearly identical to those of water above 1.3 Mg/m³. The former diffusion coefficient is smaller than the latter in the low dry densities (below 1.3 Mg/m³), suggesting that Cl^? ions migrate in the region distant from the montmorillonite sheets. In contrast, for Na⁺ ions, the obtained D_a values without water flow correction are in good agreement with conventional values at all dry densities. This suggests that Na⁺ ions migrate mainly in the interlayer and the vicinity of montmorillonite sheets. The dispersivities increase with increasing dry densities for all species. This corresponds to a geometrical complexity that increases with increasing dry density.     

Properties of Ion Beams Generated by Nitrogen Plasma Focus

Numerical experiments have been systematically carried out using the modified Lee model code on various plasma focus devices operated with nitrogen gas. The ion beam properties (ion beam energy, ion beam flux, ion beam fluence, beam ion number, ion beam current, power flow density, and damage factor) of the plasma focus have been studied versus gas pressure for each plasma focus device. The results show that, for these studied plasma focus devices, the mean ion energies decrease with increasing gas pressure, while the beam ion number increases with higher pressure. The fluence, flux, ion current, power flow density and damage factor have maximum values at the optimum pressure. It is shown that, the maximum power flow densities range from 10¹² to 10¹⁴ W m^?2 and the damage factor values reach almost 10⁹–10¹¹ W m^?2 s^0.5. The obtained results provide much needed benchmark reference values and scaling trends for ion beams of a plasma focus operated in nitrogen gas. These results could be used as an indicator for ion properties emitted from nitrogen plasma focus for various applications including material processing.     

Fluxes of charged and neutral particles from tokamaks

The performance of present tokamak devices is significantly influenced by neutral atom and impurity ion populations originating at the wall or aperture limiter. Plasma instability and diffusion mechanisms cannot be identified until these surface-related processes are accurately described. Central neutral densities in the Oak Ridge tokamak vary from 2 × 10⁸ cm^?3 to 2 × 10⁹ cm^?3 and agree well with values predicted by our theoretical model. We extrapolate this model to the larger, denser and hotter plasmas foreseen in the next generation of experiments and beyond. We calculate the rate of impurity generation by charge exchange bombardment. Impurities will influence scientific feasibility experiments which burn D-T by raising ignition temperatures. In a version of our code which incorporates alpha heating, bremsstrahlung, synchrotron and line radiation we calculate the effects of small admixtures of high $\text{Z}$ ions on the injection power required to reach ignition and on the required pulse duration. We give the wall energy fluxes to the expected and the relative fractions of energy conduction and radiation.     

Reaction balance and efficiency analysis of a D-D fusion/fission hybrid with satellite D-3He reactors

Selected reactor physics and isotope balance characteristics of a fusion hybrid supported D-³He satellite nuclear energy system are formulated and investigated. The system consists of two types of reactors: a parent D-fueled fusion device and a number of smaller reactors optimized for D-³He fusion. The parent hybrid station breeds the helium-3 for the satellites and also breeds fissile fuel for an existing fission reactor economy. Various hybrid operational regimes are examined in order to determine favorable reactorQ values and effective fusion and fission efficiencies. A number of analytical correlations between power output, plasma energetics, blanket neutronics, breeding capacity, and energy conversion cycles are established and evaluated. Numerical examples of performance parameters such as fission-to-fusion power, overall conversion efficiency, and the ratio of satellite to parent fusion power are presented. The range of reactor efficiencies is elucidated as affected by the internal plasma power balances. As an upper bound based on optimistic injection and direct conversion efficiencies, we find the D-³He satellite system power output attaining at best 1/3 of the parent fusion power.     

Effect of Sodium Nitrate on the Diffusion of Cl− and I− in Compacted Bentonite

For safety assessment of TRU waste disposal, the effective diffusion coefficients (D, _e ) for Cl^? and I^? in compacted bentonite (Kunigel V1) were determined as a function of NaNO₃ concentration, ranging from 0.01 to 5 mol/dm₃. The D_e values for Cl^? and I^? increased from approximately 10^?12 to 10^?11 m²/s with increasing NaNO₃ concentration. The capacity factor α, indicative of the effective porosity, also increased with increasing NaNO₃ concentration. The maximum α values of 0.21 for Cl^? and 0.24 for I^? at 5 mol/dm³ NaNO₃ concentration were lower than the total porosity of compacted bentonite (0.40), suggesting an anion exclusion effect. Therefore, the increases in the D_e values were interpreted in terms of the decrease in anion exclusion. The D_e values for Cl^? and I^?, normalized by their diffusivities in bulk water, were found to increase in proportion to the α values in a log-log plot, while the D_e values in porous media generally increase in proportion to the total porosity. This relationship between the logD_e and the log α is considered to be derived from the lower effective porosities for Cl^? and I^? than the total porosity in compacted bentonite due to anion exclusion.     

A Theoretical Study of MHD Power Generation, (I)

This paper deals with an analysis of the current and electric field distributions in the entrance and exit regions of a linear MHD power generator, for the case where the working fluid (gas) exhibits the Hall effect. For simplicity, the following assumptions are adopted:

1. The MHD power generator is of constant-velocity-type.

2. The problem is two dimensional.

3. The electrical conductivity and the Hall parameter of the fluid and the magnetic field are constant and uniform in the region of interest.

4. The seeding material is injected uniformly at the entrance plane.

Solutions are presented in closed form for several values of the Hall parameter, ωπ. They show that the electric potential in the entrance plane assumes an extreme value between the electrodes, and that the length of the “entrance region” is of the same order as the distance between the electrodes. The influence of electric field distortion at the entrance on the current and electric field distributions is negligible beyond this region. The electric field distortion occurs in the exit region also, resulting in “end loss” which is approximately proportional to σ ^eff ωτ.     

Electrical resistivity of β-zirconium-oxygen solid solutions as a function of oxygen concentration and temperature

The electrical resistivity of recrystallized, pure β-zirconium at temperatures between 900 and 1700°C as well as the resistivity of β-zirconium-oxygen solid solutions at temperatures between 1100 and 1700°C have been measured. It has been found that the resistivity increases linearly with temperature at a constant oxygen concentration and increases linearly with the oxygen concentration at a constant temperature. The supplementary resistivity $\text{Δρ/Δ?}$ caused by the dissolution of oxygen decreases linearly with increasing temperature, i.e. Matthiessen's rule is not obeyed. The temperature (T) and oxygen concentration (?) dependence of the electrical resistivity ρ of β-zirconium-oxygen solid solutions at temperatures between 1100 and 1700°C can be represented by the relation ρ = 91.9 + 2.30 × 10^?2T + 10²f(3.75 ? 1.03 × 10^?3T) with ρ in μΩ · cm, ? in atomic ratio n_O/n_Zr and T in °C. Taking these results and literature data into consideration, a survey of the electrical resistivity of zirconium-oxygen solid solutions, including the α-range, is given.     

Deuteriding of thin titanium films: The effect of carbon monoxide surface contamination

The effect of adsorbed CO on the deuteriding of thin titanium films at room temperature was measured at D₂ pressures between 10 to 25 mtorr on films contaminated with CO from exposures ranging between ～10^?8 torr-sec to 10^?4 torr-sec. In all measurements, for deuterium/ titanium atom ratios > 0.2, the deuteriding appeared to be initially limited by the sticking of D₂ on the clean or contaminated titanium deuteride surface; the effective sticking coefficient on a clean titanium deuteride surface was determined to be ～3 × 10^?3, while on a surface contaminated by an exposure of 10^?4 torr-sec to CO, the coefficient was reduced to ～2 × 10^?4. The pumping speeds of a Ti film was dramatically different when the film was evaporated over TiD₂. These changes were attributed to the presence of deuterium which diffused from the substrate film into the overlayer film.     

A Powerful Remote Source of O Atoms for the Removal of Hydrogenated Carbon Deposits

We describe a system to deliver a large flux of O atoms for the removal of hydrogenated carbon films from surfaces in remote areas of tokamaks with carbon divertors. The oxygen plasma is generated via electrode-less radiofrequency discharge in a discharge chamber connected to a remote chamber by a 2 m long complex-shaped glass tube 4 cm in diameter. The density of O atoms in the remote chamber was measured with a nickel catalytic probe and its variation with discharge power obtained. The density was close to the detection limit of the probe (around 1 × 10¹⁹ m^?3) as long as the vacuum system was pumped with a rotary pump at a nominal pumping speed of 80 m³ h^?1. The density increased well over 10²⁰ m^?3 when a Roots pump was added. The effective pumping speed at the current setup was up to 200 m³ h^?1. At such conditions, the maximal O-atom density at 2 m from the source was up to 3 × 10²⁰ m^?3. The density depended on the pressure as well as the discharge power. The behavior of O-atom density far away from the source was explained by gas phase and surface phenomena. The effective pumping speed was found to be of crucial importance. The setup was used for removal of model hydrogenated carbon films. Experiments were performed at sample temperatures up to 600 K and etching rates up to 50 nm/s were obtained. We found that the experimental setup is suitable for removal of hydrogenated atoms on a large scale.     

Vibration Characteristic of Heated Rod Induced by Subcooled Flow Boiling

A subcooled boiling loop with annular flow channel on an electrically heated rod at the channel axial center was used to make an experimental approach to investigate the vibration characteristic induced by subcooled flow boiling. The experiments were carried out to measure vibration induced in the rod under different conditions of incoming coolant subcooling temperature of 10–80 K, coolant flow rates of 0.1–0.35 × 10^?3 m³/s and imposed linear power densities of 17–600 W/cm. Distilled water was used as coolant in the loop. The vibration behavior of the rod was measured by using an accelerometer. Also, a high-speed motion analyzer system was used to analyze the bubble behaviors in the different subcooled boiling conditions.

The results show the intensive SBIV (subcooled boiling-induced vibration) which highly depends on dynamic force generated by rapid growth and collapse of vapor bubbles under high heat flux loading whilst they still attach to the heating surface. These behaviors were influenced strongly by the conditions of subcooling temperature, linear power density and flow rate.     

Threshold energy surface and frenkel pair resistivity for Cu

In-situ electrical resistivity damage-rate measurements in the high voltage electron microscope have been used to study electron irradiation induced defect production in copper single crystals at $\text{T < 10 K}$. Analysis of the directional and energy dependence yields a threshold energy surface that is significantly different from those of previous investigations: two pockets of low threshold energy centered at 〈100〉 and 〈110〉 surrounded by regions of much higher threshold energy. The corresponding damage function exhibits a plateau at 0.6 Frenkel pairs. The present results imply a Frenkel pair resistivity for Cu of (2.75 ^+0.6_?0.2) × 10^?4 Ω cm.     

Analysis of Tritium Behavior in Very High Temperature Gas-Cooled Reactor Coupled with Thermochemical Iodine-Sulfur Process for Hydrogen Production

The tritium concentration in the hydrogen product in Japan's future very high temperature gas-cooled reactor (VHTR) system coupled with a thermochemical water-splitting iodine-sulfur (IS) process (VHTRIS system), named GTHTR300C, was estimated by numerical analysis. The tritium concentration in the hydrogen product significantly depended on undetermined parameters, i.e., the permeabilities of a SO₃ decomposer and a H₂SO₄vaporizer made of SiC. Thus, the estimated tritium concentration in the hydrogen product for the conservative analytical condition ranged from 3.4 × 10^?3 Bq/cm³ at STP (38 Bq/g-H₂) to 0.18 Bq/cm³ at STP (2,000 Bq/g-H₂). By considering the tritium retained by core graphite and the reduction in permeation rate by an oxide film on the heat transfer tube of the IHX and the HI decomposer, the tritium concentration in the hydrogen product decreased to the range from 3.3 × 10^?5 Bq/cm³ at STP (0.36 Bq/g-H₂) to 5.6 × 10^?3 Bq/cm³ at STP (63 Bq/g-H₂), which were smaller than those for the conservative analytical condition by factors of about 3.2 × 10^?2 and 9.6 × 10^?3, respectively. The effectof the helium flow rate in the helium purification system on the tritium concentration in the hydrogen product was also evaluated.     

Irradiation-enhanced precipitation in Al-Ge alloys

Ion irradiation of a supersatured solid solution of Ge in Al gives striking increases of the homogenous nucleation rate of semi-incoherent Ge precipitates within the Al matrix, particularly at high dose rates and elevated temperatures. Electron-transparent and 250 μm foils were irradiated at 25°C and 125°C to doses from 12 to 120 dpa at dose rates from $\text{3.7 × 10}{}^{\mathrm{?3}}\text{dpa/s}$ to $\text{3.7 × 10}{}^{\mathrm{?1}}\text{dpa/s}$. Irradiation produced very high (>10¹⁵/cm³) precipitate densities under conditions where thermal ageing would have produced few, if any, precipitates. The high precipitate densities were produced most readily at 125° C, at high dose rates and in thicker or cold-worked foils. These results are interpreted in terms of theories of phase stability and precipitate nucleation. The influence of dose rate, foil thickness, and cold work are explained in terms of their effect on point defect concentrations and the relative arrival rates of vacancies and interstitials. Except for the observation of more copious precipitation at the higher temperatures, theory and experiment are in agreement.     

Analysis of Two-Phase Liquid Metal MHD Induction Converter

An analysis is made on the performance characteristics of a liquid-metal MHD induction converter with liquid-gas two-phase mixture as working fluid. The equivalent electrical conductivity and the velocity vary along the generator channel in this kind of induction converter.

Two important parameters which represent the variations of the equivalent electrical conductivity and the velocity respectively are defined. With these parameters the induction equation is analytically solved with the perturbation technique.

Quantities representing generator performance, such as power densities and generator efficiency, are obtained from the perturbed magnetic field and the parameters mentioned above.

Suitable combination of values for these parameters will tend to let the effects brought by the variations of electrical conductivity and of velocity cancel each other, and the relation between these parameters is analytically derived that assures the non- perturbation of the magnetic field and of the gross output power density. In this condition of non-perturbation, the generator efficiency approaches that for the unperturbed case when the velocity variation and the inlet slip ratio are small.     

The effects of gas transport properties on blister formation in He+-implanted glass

Blister formation in He⁺-implanted glasses is correlated with the measured helium gas diffusivity. A scries of glasses with diffusivities from ～3 × 10^?7 to ～5 × 10^?12cm²sec^?1 was implanted under nearly identical conditions with 150 keV He⁺ ions at a flux of 15 μA cm^?2 and a nominal sample temperature of 110°C. Glasses with D less than ～1 × 10^?9cm²sec^?1 were fully blistered, whereas those with D greater than ～3 × 10^?8cm²sec^?1 showed no surface deformation. Glasses with diffusivities between ～3 × 10^?8 and ～1 × 10^?9cm²sec^?1 had local regions with low density coverage of relatively large blisters. The critical concentration of implanted helium, estimated by comparing experimental data with results from a simple theoretical model, is ^{～1 × 1019 cm?3}, consistent with high pressure solubility measurements. Reemission data at low fluence are qualitatively in agreement with analytical calculations. Implications for CTR technology are discussed.