Theoretical analysis of mass transfer and reaction in a porous medium applied to the gasification of graphite by water vapor

The theories of mass transfer and reaction in porous medium were applied to investigate the gasification of graphite by water vapor in reactor. Based on several logical assumptions, the gasification of graphite by water vapor was investigated. Different shapes of graphite were analysed: semi-infinite blocks, planks and cylinders. Analysis reveals that the reaction rate varies with the location, which can explain the intense gasification that occurs at upstream during experiments. The temperature dependence of the reaction rate expressed different ranges. At low temperatures, the reaction rates depend greatly on the reaction temperature, and the influence of shape is more pronounced. At high temperatures, the dependence weakens due to the limit of gas transfer, and the influence of shape disappears. The water vapor pressure at exterior surface decreases with temperature, but the pressure gradients (both inside and outside of porous media) increases with temperature. For the platelike and cylindrical porous media, the reaction rate decreases with the thickness or radius because of the increase in specific surface. When the penetrating depth of water vapor is larger than the half-thickness or radius of porous media, the water vapor would cumulate in porous media so that the pressure of reacting gas increases.     

Candu type fuel behavior during rapid overpower transients

Results obtained in the pulse irradiation tests performed on segmented fuel elements in the Romanian Annular Core Pulse Reactor (ACPR) are discussed below. Tests included the effects of initial element internal pressure and a wide range of energy deposition on the fuel element behavior. All tests were conducted in stagnant water at room temperature and atmospheric pressure inside the capsule. The fuel elements were instrumented with thermocouples for cladding surface temperature measurement. Transient histories of reactor power, cooling water pressure, fuel element internal pressure and cladding temperature were recorded during the tests. The fuel elements were subjected to total energy depositions from 70 to 265 cal g⁻¹ UO₂. Cladding failure mechanism and the failure threshold have been established. The fuel failure mechanism is a burst type and is very similar to LOCA failure mechanism even though the rate energy deposition is higher in the ACPR tests. At higher energy deposition brittle cladding fracture near endcap weld region can be produced. The failure threshold is situated between 190 and 200 cal g⁻¹ UO₂ for standard fuel rod (0.2–0.3 MPa internal pressure) and less than 160 cal g⁻¹ UO₂ for pressurized fuel rods (internal pressure between 1 and 3.0 MPa). Pre-pressurization could be an important factor to control the failure threshold energy. The experimental program is still in progress and new experiments are foreseen to be performed in the following period.     

Impact and energy deposition of slow, highly charged ions on a solid surface

A plasma region in nanometer scale may be created by a highly charged ion impact on solid surface. The charge imbalance leads to enormous electric fields and may further induce Coulomb explosion due to electrostatic repulsion in the region. Thus, the highly charged ion is thus expected to be a powerful tool to induce surface modification in the nanometer scale. The Coulomb explosion model is applied in order to interpret the interaction mechanism and to understand the impact and energy deposition of highly charged ions on a solid surface, and to obtain the energy deposited by the ion. The energy deposition ratio is dependent on the material and charge. A high temperature and high pressure environment will be formed by the deposited energy, causing the atoms to swell up and a hillock nano-defect to be formed on surface. The height of hillock is estimated from the Coulomb explosion.     

Study on an Atmospheric Pressure Plasma Jet and its Application in Etching Photo-Resistant Materials

An atmospheric pressure radio-frequency plasma jet that can eject cold plasma has been developed. In this paper, the configuration of this type of plasma jet is illustrated and its discharge characteristics curves are studied with a current and a voltage probe. A thermal couple is used to measure the temperature distribution along the axis of the jet stream. The temperature distribution curve is generated for the He/O2 jet stream at the discharge power of 150 W. This jet can etch the photo-resistant material at an average rate of 100 nm/min on the surface of silicon wafers at a right angle.     

Surface modifications and erosion yields of silicon and titanium doped graphites due to low energy D bombardment

Influences of low energy D⁺ ion bombardment and target temperature on surface topography, surface concentration and erosion yield of carbon based binary compounds were investigated. The samples contained 10 at.% Si and 10 at.% Ti, respectively. The surface concentration was determined in situ by Auger electron spectroscopy and the topography ex situ by scanning electron microscopy. During low energy D⁺ bombardment a pronounced conelike surface developed with silicon respective titanium rich ‘caps’ protecting the underlaying carbon rich shafts from erosion. The average dopant surface concentration was up to 7 × the bulk concentration. The erosion mechanism was determined by surface concentration and chemical state of the surface: At high temperatures carbidic bindings dominated, while at room temperature a mixture of graphite and carbide covered the surface.     

Mechanical response of LMFBR and P1 cladding tubes under transient heating

Type 316 stainless steel tubing specimens comparable to LMFBR cladding were burst tested with relatively constant internal pressure in the 219–836 psi range and with increasing temperature. Continuous measurements of diameter change, temperature, and pressure were recorded as the samples were heated to temperatures near the melting point at rates from 10–1800°F/sec. The effects of varying initial wall thickness, cold work level, length, and thermal experience were explored. Ductile failures were observed at 10°F/sec, and stable strains at time of failure were greater than those reported by HEDL. At 200°F/sec the initially 20% cold-worked, 15-mil wall tubing produced brittle failures; while initially 40% cold worked, 10-mil wall samples displayed a mixture of ductile and brittle features. At 1000°F/sec the behavior of the latter material was prodominately brittle, although stable strains as large as 6% were observed. Failure temperatures were generally above 2000°F. When substantial ductility was displayed, an exponentially increasing stable strain was recorded as temperature and time progressed: from such curves temperatures corresponding to 1% strain were derived. Factors controlling the mechanical response appear to be separable by analysis based on the recorded data and the variety of materials and conditions of the tests.     

In situ investigation of U(IV)-oxide surface dissolution and remineralization by electrochemical AFM

Surface chemical processes of UO₂ are investigated on a nanoscopic scale by electrochemical atomic force microscopy (ECAFM) using a home-developed electrochemical cell. Dissolution reactions of the solid surface and subsequent remineralization are observed at the solid-water interface under different redox conditions and carbonate concentrations. The local dissolution rates vary between different grain faces, grain boundaries and etch pits. A correlation between dissolution rates and the grain orientations relative to the specimen surface can be demonstrated by electron backscatter diffraction (EBSD). Remineralization under oxidizing conditions occurs mainly at grain faces with higher dissolution rates. The remineralized products are particles of 200-900 nm in diameter and exhibit a tabular morphology. Profound knowledge of the UO₂ surface chemistry on a nanoscale may help to clarify the related mechanisms explaining the macroscopically observed dissolution rates.     

Substrate temperature effect on F etching of SiC: Molecular dynamics simulation

In this study, we performed molecular dynamics simulations to investigate F⁺ continuously bombarding SiC surfaces at temperatures of 100, 400, 600 and 800 K with the energy of 150 eV. The simulation results show that the etch rate of Si atoms is more than that of C atoms. With increasing temperature, the deposition yield of F atoms decreases, while the etch yields of C and Si atoms increase. In etching products, SiF, SiF₂ and CF species are dominant. Their yields increase with increasing temperature.     

Experimental study of the fracture kinetics of a tubular 16MnNiMo5 steel specimen under biaxial loading at 900 and 1000 °C. Application to the rupture of a vessel bottom head during a core meltdown accident in a pressurized water reactor

This study proposes an experimental analysis of the creep, crack initiation and crack propagation phases in a 16MnNiMo5 steel subjected to thermomechanical loading representing a core meltdown accident in a pressurized water reactor involving the transfer of a molten corium bath to the bottom head. The experimental setup enabled a biaxial mechanical loading (internal pressure + tension) to be applied to a tubular specimen at 900 and 1000 °C. In addition to the usual temperature, load, displacement and pressure measurements, the specimen was observed by two high-speed numerical cameras and an infrared camera. The crack's initiation and propagation conditions and the depressurization law were inferred from these measurements. At such temperatures, creep induces very large strains prior to the occurrence of the cracks which, in the worst-case scenario, can propagate at velocities as high as several meters per second. The design of the experiment enabled us to study the influence of the temperature (magnitude and hoop distribution), of the toughness of the steel (two grades were studied) and of the volume of pressurized gas. The results show that creep and crack propagation are highly dependent on temperature, and also that crack initiation and propagation are highly dependent on the degree of heterogeneity which is responsible for the localized initiation of the crack.     

Deformation and Fracture Behavior of Zircaloy-2 Deformed at Constant Strain Rate in Iodine Environment, (I)

The relation between strain rate and iodine stress corrosion cracking (SCC) was studied on Zircaloy-2 subjected to uniaxial stress under constant extension rate in an iodine partial pressure of 4 Torr and at a temperature of 350°C. The specimens were machined from actual fuel cladding tube. In iodine environment, the tangentially directed specimens registered sharp decreases in stress beyond maximum point; this is attributed to crack initiation and propagation. Fracture ductility diminished with decreasing strain rate. Severe SCC was observed at strain rates below 2×10^?3min^?1. At high strain rates, the mechanism governing the rate of SCC appeared to be the time-dependent corrosion process. The axially directed specimens showed no signs of embrittlement due to gaseous iodine; this is attributed to the particular texture of the Zircaloy. Fracture surface observations indicated that transgranular cleavage fracture along the basal plane appeared to play a significant role in the stages of SCC initiation and propagation.     

Experimental Study on Heat Transfer Characteristics of Vertical 5×5 Heated Rod Bundles around Critical Pressure with R-134a

With the aim of researching the heat transfer characteristics around the critical pressure in rod bundle geometry, the behaviors of the heating surface temperatures at the vertical 5×5 heated rod bundles with R-134a were experimentally investigated under both pressurizing and depressurizing processes, where the pressure constantly decreases from supercritical to subcritical and constantly increases from subcritical to supercritical under constant inlet thermal conditions. As the pressure approaches the critical pressure, a DNB type or a dryout type of CHF was induced, and then the post-CHF was kept until the vicinity of the critical pressure. Under the supercritical pressure, the post-CHF disappeared due to the extinction of boiling phenomena, but the heat transfer deterioration was induced in a certain pressure range. In the depressurizing process, the heating surface temperatures followed the trajectory of temperature behaviors as in the pressurizing process except for the quenching point. The quenching point from the post-CHF moved to the lower pressure side than the pressure as for the CHF in the pressurizing process. This phenomenon was considered as the hysteresis phenomenon frequently observed in pool boiling curve characteristics.     

Systematics of annealing of tracks of relativistic nuclei in phosphate glass detectors

We have measured track etch rates for relativistic uranium, gold and lanthanum ions in two types of phosphate glasses as a function of annealing time and temperature. All the data can be represented by a single equation in which the fractional thermal track-fading rate is proportional to an inverse power of annealing time and to a Boltzmann factor of temperature. The results enable one to predict the long-term stability of latent tracks for ions of arbitrary ionization rate.     

Packaging and security of radioactive material

Abstract

Elastomers are widely used as the main sealing materials for containers for low and intermediate level radioactive waste and as an additional component to metal seals in spent fuel and high active waste containers. The safe encapsulation of the radioactive container inventory has to be guaranteed according to regulation and appropriate guidelines for long term storage periods as well as for temperatures as low as ?40°C during transport. Therefore, the understanding of failure mechanisms that lead to leakage at low temperatures is of high importance. It is known that the material properties of elastomers are strongly temperature dependent. At low temperatures, this is caused by the rubber–glass transition (abbreviated: glass transition). During continuous cooling, the material changes from a rubber-like entropy elastic to a stiff energy elastic behaviour, which allows nearly no strain or retraction. Hence, rubbers are normally used above their glass transition, but the minimum working temperature limit is not defined precisely; this can cause problems during the above noted applications. Therefore, the lower operation temperature limit of elastomer seals must be determined in dependence of the material properties. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) are combined with standardised measurements to determine the compression set according to ISO 815. To reduce the test time of the standard tests, a faster technique than normally used was developed. Additionally, the breakdown temperature of the sealing function of complete O ring seals was measured in a component test set-up to compare it with the results of the other tests. The experimental set-up is capable of measuring the leakage rate at low temperatures by the pressure rise method. A fluorocarbon rubber (FKM) was selected for this investigation as it is often used for radioactive waste containers. The materials (seals and test sheets) were purchased from a commercial seal producer.     

The effect of deformation temperature on the microstructure evolution of Inconel 625 superalloy

Hot compression tests of Inconel 625 superalloy were conducted using a Gleeble-1500 simulator between 900 °C and 1200 °C with different true strains and a strain rate of 0.1 s⁻¹. Scanning electron microscope (SEM) and electron backscatter diffraction technique (EBSD) were employed to investigate the effect of deformation temperature on the microstructure evolution and nucleation mechanisms of dynamic recrystallization (DRX). It is found that the relationship between the DRX grain size and the peak stress can be expressed by a power law function. Significant influence of deformation temperatures on the nucleation mechanisms of DRX are observed at different deformation stages. At lower deformation temperatures, continuous dynamic recrystallization (CDRX) characterized by progressive subgrain rotation is considered as the main mechanism of DRX at the early deformation stage. However, discontinuous dynamic recrystallization (DDRX) with bulging of the original grain boundaries becomes the operating mechanism of DRX at the later deformation stage. At higher deformation temperatures, DDRX is the primary mechanism of DRX, while CDRX can only be considered as an assistant mechanism at the early deformation stage. Nucleation of DRX can also be activated by the twinning formation. With increasing the deformation temperature, the effect of DDRX accompanied with twinning formation grows stronger, while the effect of CDRX grows weaker. Meanwhile, the position of subgrain formation shifts gradually from the interior of original grains to the vicinity of the original boundaries.     

Atomistic study of the thermodynamic equilibrium of nano-sized helium cavities in βSiC

The estimation of the number of inert gas atoms contained at equilibrium in microscale bubbles in a solid usually relies on a well-known formula equilibrating the internal pressure of He to the surface energy of the bubble. This approach evidences a strong variation with temperature of He content for a given bubble. At the opposite, at the Angstrom scale, ab initio calculations for He contained in vacancy assemblies neglect temperature effects. In this work, empirical potential molecular dynamics simulations are used to study, in the case of helium inserted in cubic silicon carbide, the variation of the He content of sub-nanoscale cavities with temperature. To do so free energy for He atoms inserted in cavities made of a few vacancies (up to 29) are calculated. One then evidences the existence of a sub-surface segregation in interstitial sites close to the surface of the cavity. The variation of the He content with temperature is observed to be negligible at the nanoscale, thus validating the ab initio approach.     

Analysis of optical emission spectroscopy data during silicon etching in SF6/O2/Ar plasma

Silicon etching is an essential process in various applications,and a major challenge for etching process is anisotropic high aspect ratio etching characteristics.The etch profile is determined by the plasma parameters and process parameters.In this study,the plasma state with each process parameters were analyzed through the optical emission spectroscopy(OES)plasma diagnostic sensor by both chemical and physical approaches.Electron temperature and electron density were additionally acquired using the corona model with OES data that provides chemical species information,and the etch profile was evaluated through scanning electron microscope measurement data.The results include changes in profile with gas ratio,bias power,and pressure.We figure out that factors like ion energy and ion angular distribution as well as chemical reaction affect the anisotropic profile.     

Investigations of the origins of deposited carbon species in gaps between divertor tiles using a kinetic code system

In order to investigate the origins of deposited carbon species in gaps, the simulations have been performed using a kinetic code system. At low plasma temperatures, the deposited carbon species mainly originate from top surfaces of the tile, while at high plasma temperatures the deposited carbon species are basically derived from side surfaces of the tile. A substantial variation of the deposition rate of carbon species originating from side surfaces is obtained due to physical sputtering and topography advantage. The deposited carbon species derived from top surfaces and side surfaces demonstrate different deposition characteristics for physical sputtering and chemical erosion: (i) for deposited carbon species from top surfaces, the deposition ratio for physical sputtering increases evidently and deposition rate virtually increases by one order of magnitude with increasing plasma temperature; and the deposition ratio for chemical erosion reduces correspondingly and deposition rate decreases gradually; (ii) for deposited carbon species from side surfaces, the deposited carbon species principally arise from physical sputtering; the deposition rates for chemical erosion are of the order of magnitude of 10¹⁵ m^?2 s^?1 for all studied plasma temperatures, and the deposition rates for physical sputtering can be two to three orders of magnitude greater than that for chemical erosion.     

High Temperature Corrosion of Iron Chromium Alloys by Tellurium

The reaction rates between iron-chromium alloys (1.17, 5.65 and 11.96 a/0Cr) and tellurium were measured in the temperature range of 873–1,023 K at 350 Pa of tellurium vapor pressure, and in the tellurium vapor pressure range of 66.7–600 Pa at 923 K. The electron probe microanalysis, marker experiment and X-ray diffractometry were employed to clarify the mechanism of the telluride scale growth.

The reaction rates between iron-chromium alloys and tellurium obeyed the parabolic rate law. It was observed that the telluride scale consists of the inner, the intermediate and the outer layers; chromium is concentrated in the inner layer which may grow by the inward diffusion of tellurium; the intermediate layer consists of mainly β-iron telluride, and the outer layer consists of both δ- and δ'-iron tellurides below 980 K, and δ-iron telluride alone above 980 K. The intermediate and outer layers grow by the outward diffusion of iron in iron tellurides.

The protection effect by chromium may be explained by the fact that chromium telluride covered over the surface of iron-chromium alloy interrupts the outward diffusion of iron.

The reaction between iron-chromium alloy and tellurium can be classified into three regions from the view point of the activation energy. From the comparison of the activation energy of the reaction between iron-chromium alloy and tellurium with that for the diffusion of iron in iron tellurides, the rate-determining step for the scale formation for each region was discussed.     

Radiation loads on the ITER first wall during massive gas injection

Unmitigated disruptions in ITER can produce strong localized surface damage on the first wall (FW). Massive gas injection (MGI) systems are being designed to dissipate a large fraction of the plasma stored energy at the disruption thermal quench (TQ) and hence reduce the consequences for FW components. The stored energies can be high enough, however, for there to be potential for the photon flash at the MGI TQ to drive local melting of beryllium FW components. To estimate the poloidal distribution of FW surface temperatures, the MGI process is being simulated using the 2D code TOKES, assuming toroidal symmetry. High pressure neon injection, assimilation and transport of injected impurities through the entire plasma volume are modelled. The output of these simulations is used by the melt motion code MEMOS to assess the resulting maximum surface temperature and the regimes with melting on the FW surface.