Oxygen glow discharge cleaning in nuclear fusion devices

Laboratory experiments were carried out as part of the preparations of an oxygen glow discharge cleaning experiment in ASDEX Upgrade. They aimed at evaluating the effect of mixing the oxygen with helium, the collateral damage caused by the glow discharge, as well as the influence of impurities in the films being eroded. Oxygen concentrations below 20% in helium are sufficient to achieve high erosion rates. The discharge can lead to the formation of oxide layers on surfaces which - as demonstrated for tungsten - can be rapidly reduced by post-treatment in a hydrogen discharge. For carbon, aluminum and iron the physical sputtering yield may become similar to the erosion yield of redeposited layers, but it is by more than one order of magnitude smaller for tungsten. Using a-B:C:H films with varying boron content, it was found that impurities can cause the erosion rate to drop by orders of magnitude.     

Atomic size and fast diffusion of metallic impurities in zirconium

The phenomenon of fast diffusion of metallic impurities in hcp- and bcc-Zr has been analyzed in terms of the atomic sizes of the solutes. Two different size criteria have been used, namely, the partial molar volume of the impurity at infinite dilution and the metallic radius for 12-fold coordination in the pure metallic state. The results obtained indicate that these two size criteria are complementary of each other. Also, it has been concluded that metallic impurities in hcp-Zr can be sharply classified into substitutional and interstitial ones as a function of atomic size. In turn, in the case of bcc-Zr such a sharp classification was not obtained, a situation which possibly is related to the particular softness of the bcc lattice. The solvents bcc-Ti and Nb have also been included in the discussion and found to behave in a similar way as in the case of bcc-Zr.     

用单能慢正电子束测量Si中的正电子迁移率

用单能慢正电子束流作为探针,测量了Ｐ型Ｓｉ中不同Ｂ杂质浓度的正电子湮没Ｓ参数和正电子能量的函数关系。在实验上系统地研究了正电子扩散长度和迁移率随半导体中杂质浓度的变化规律,观察到杂质的掺入不影响缺陷的开体积,但正电子在硅中的迁移率随杂质浓度的增加而减小。     

Activation Response of Martensitic Steels

A hypothetical martensitic steel has been compositionally designed in order to optimize both metallurgical and reduced activation properties. When compared with two other martensitic steels, its activation characteristics are shown to be superior for all activation indices examined. However, these excellent properties are found to be due to the assumed absence of deleterious tramp impurities. When limiting impurity concentrations are determined for the hypothetical steel, they are found to be extremely stringent, and wholly unachievable using industrial scale production methods. It is concluded that only slight improvements can be made to currently available low activation martensitic steels to reduce residual activity responses further.     

Effects of Sputtering of and Radiation by Aluminum on Magnetized Target Fusion Plasmas

We estimate numerically the rate of radiation by aluminum impurities for parameters relevant to magnetized target fusion (MTF) plasmas. We demonstrate that the coronal equilibrium is appropriate for expected MTF plasma parameters. Using the coronal equilibrium, we estimate the power radiated per impurity ion is 0.25–0.5 × 10⁻¹⁶ MW for temperatures and densities relevant to present plasma parameters taken from the FRX-L experiment at Los Alamos National Laboratory and is approximately 75.0 × 10⁻¹⁶ MW for temperatures and densities relevant to anticipated MTF plasmas. We calculate the sputtering rate of aluminum by thermal deuterium and tritium plasma ions is a few percent assuming an impact angle of 45°. Finally, we estimate that with aluminum impurity levels of a few percent, the impurity radiation power density would be approximately 25 kW/cm³ for FRX-L conditions and 2.5 GW/cm³ for anticipated conditions in a MTF plasma. While we have assumed a sputtering model of impurity generation, the results for the power density apply for impurity levels of a few percent, regardless of the generation mechanism.     

An evaluation of low temperature radiation embrittlement mechanisms in ferritic alloys

Investigations underway at Oak Ridge National Laboratory (ORNL) into reasons for the accelerated embrittlement of surveillance specimens of ferritic steels irradiated at 50°C at the High Flux Isotope Reactor (HFIR) pressure vessel are described. Originally, the major suspects for the precocious embrittlement were a highly thermalized neutron spectrum, a low displacement rate, and the impurities boron and copper. Each of these possibilities has been eliminated. A dosimetry experiment made at one of the major surveillance sites shows that the spectrum at that site is not thermalized. A new model of matrix hardening due to point defect clusters indicates little effect of displacement rate at low irradiation temperature. Boron levels are measured at 1 wppm or less, which is inadequate for embrittlement. Copper and nickel impurities are shown to promote radiation strengthening at high doses but not at the low doses pertinent to the surveillance data. It is shown that a copper embrittlement scenario has other drawbacks, and it is argued that copper impurity is not responsible for the accelerated embrittlement of the HFIR surveillance specimens. The dosimetry experiment revealed unexpectedly high levels of reaction products in some of the fast flux monitors, which are found to be caused by an exceptionally high ratio of gamma ray flux to fast neutron flux at the pressure vessel. Gamma rays can also induce atomic displacements, leading to the suggestion that the accelerated embrittlement may be provoked by gamma irradiation.     

Influence of cascade micropores on diffusion fluxes of point defects in reactor vessels materials

The main mechanism determining radiation embrittlement of the materials of reactor vessels are considered to be a change in the cohesive strength of the grain boundaries resulting from the segregation of surface-active impurities (mainly phosphorous), hardening of the material by precipitations of a second phase, segregation of impurities onto interphase surfaces of the precipitate-matrix interface and an associated increase in the hardening effect of the radiation precipitations. The materials of reactor vessels are irradiated at a temperature of 250–300°C which is insufficiently high for the thermal activation of diffusion processes. When describing the process of an accelerated diffusion of copper and phosphorous atoms by irradiation, a model has so far normally been used which neglects the cascade mechanism for generating defects and also the formation and evolution of point defect complexes. Individual studies taking account of the reduction in the efficiency of generating point defects in cascades due to the formation of micropores contain certain deficiencies. The present work is devoted to developing and analyzing a satisfactory model of radiation damage which is to a considerable extent free of these deficiencies. Translated from Atomnaya énergiya, Vol. 86, No. 5, pp. 370–383, May, 1999.     

Mechanism of formation of zirconium sponge in zirconium production by the magnesothermic process

The mechanism of formation of zirconium sponge in the production of zirconium by the magnesothermic process has been investigated by introducing into the reducing agent a very soluble but not very volatile additive (aluminum, tin) acting as marked atoms. It was found that the growth of the zirconium sponge on the wall of the reaction crucible above the level of the molten bath is the result of the reaction of zirconium chloride vapor with molten magnesium as the latter ascends by capillary action through the previously formed sponge. The amount of zirconium chloride reacting with magnesium in unit time depends more upon the perimeter of the crucible than upon the surface area of the bath. To intensify the reduction of zirconium chloride by magnesium, it is recommended that partitions be placed in the crucible to act practically in the same way as the crucible wall.     

Impurity Measurements in Plasma Boundary of JIPP T-II Tokamak/Stellarator

Impurity transport in the scrape-off layer of the JIPP T-H Tokamak/Stellarator hybrid torus machine in Institute of Plasma Physics, Nagoya University has been investigated by a probe measurement. Silicon and Cu probes were inserted into the boundary plasma during discharges. The surface of the probes was analyzed by several analytical techniques including AES, XPS, RBS and PIXE. Main metallic impurities deposited on the probes were Mo and Fe which originated from the limiters and the inner wall of the vacuum chamber. Depth concentration profile of deposited impurities was measured by AES along with sputter-etching of the surface with Ar ions. Deposited Fe impurity had the maximum concentration at the top surface of the probe, while the depth profile of Mo showed the maximum concentration at around 8 A from the top surface. Transportation energy of these impurities are discussed on the basis of the projected range data.     

Radiation induced surface segregation of impurities in boron carbide

Surface segregation of impurities and formation of a compound has been observed on heating pellets of boron carbide which were previously irradiated with 100 keV helium ions. This phenomenon is attributed to oxidation.     

Oxidation and vitrification of aluminum with lead borate glass for low level radioactive waste treatment

ABSTRACT

For the direct vitirification of low level radioactive wastes containing metallic aluminum, the oxidation and vitirification of aluminum was carried out using lead borate glass as a flux. Among the six fluxes employed in this study, 0.11Na₂O ?3PbO ?B₂O₃ was the most suitable for the oxidation and vitirification of aluminum based on the initial oxidation rate and alumina solubility. The increase of the lead fraction in the flux increased the oxidation rate, while the sodium fraction affected the alumina solubility resulting in the product maintaining its transparent appearance. The agglomerate product by the oxidation was confirmed metallic lead by X-ray diffraction analysis, and its impurities, aluminum, boron and sodium, were lower than 0.2 wt% by the quantitative analysis.     

Strong segregation gettering of transition metals by implantation-formed cavities and boron-silicide precipitates in silicon

We have mechanistically and quantitatively characterized the binding of transition-metal impurities in Si to cavities formed by He implantation and to B---Si precipitates resulting from B implantation. Both sinks are inferred to act by the segregation of metal atoms to pre-existing low-energy sites, namely surface chemisorption sites in the case of cavities and bulk solution sites in the case of the B---Si phase. These gettering processes exhibit large binding energies, and they are predicted to remain active for arbitrarily small initial impurity concentrations as a result of the segregation mechanisms. Both appear promising for gettering in Si devices.     

Depth profiling a near surface carbon contaminant in implanted first wall materials

Current attempts to generate stable plasmas in CTR devices are encountering severe material problems. Plasma species, interacting with the surface of the containment vessel, can desorb surface species which can contaminate the plasma or, interacting with surface impurities, can change the properties of the near surface region. Although methods to minimize these synergistic effects have been suggested, there exists only minimal information related to the fundamental surface and near surface (< 100 Å) processes involved. One process of interest, the interaction of energetic plasma species with adsorbed hydrocarbon contaminants has been examined using Imaging, Field-Desorption Mass Spectrometry. Angstrom resolved depth profiles of a carbon contaminant produced by ion-beam decomposition of an adsorbed surface hydrocarbon layer have been measured for tungsten, molybdenum and stainless steel specimens. The observed, 30 Angstrom penetration of carbon into the near surface region suggests that conventional first-wall cleaning techniques may be ineffective in completely removing this impurity.     

Impurity Transport in a Simulated Gas Target Divertor

Future generation fusion reactors and tokamaks will require dissipative divertors to handle the high particle and heat loads leaving the core plasma (100–400 MW/m² in ITER). A radiative divertor is proposed as a possible scenario, utilizing a hydrogen target gas to disperse the plasma momentum and trace impurity radiation to dissipate the plasma heat flux. Introducing an impurity into the target hydrogen gas enhances the radiative power loss but may lead to a significant impurity backflow to the main plasma. Thus, impurity flow control represents a crucial design concern. Such impurity flows are studied experimentally in this thesis. The PISCES-A linear plasma device (n 3 × 10¹⁹ m^–3, kT _e 20 eV) has been used to simulate a gas target divertor. To study the transport of impurities, a trace amount of impurity gas (i.e., neon and argon) is puffed near the target plate along with the hydrogen gas. Varying the hydrogen gas puffing rate permits us to study the effects of various background plasma conditions on the transport of impurities. A 1-1/2-D fluid code has been developed to solve the continuity and momentum equations for a neutral and singly ionized impurity in a hydrogen background plasma. The results indicate an axial reduction in the impurity concentration upstream from the impurity puffing source. Impurity entrainment is more effective for higher hydrogen target pressures (and for higher hydrogen plasma densities). However, if there is a reversal of the background plasma flow, impurity particles can propagate past the plasma flow reversal point and are then no longer entrained.     

气冷微堆燃料设计的中子学特性影响研究

为分析气冷微堆燃料设计的中子学特性影响,基于方形燃料组件模型,利用蒙特卡罗程序RMC研究了TRISO颗粒、燃料芯块在燃料设计中的主要参量对组件中子学特性的影响。研究结果表明,燃料颗粒体积占比和包覆层厚度不变时,组件寿期随燃料核芯直径的增大先显著增大,而后趋于平稳;燃料颗粒体积占比和燃料核芯直径不变时,组件寿期随包覆层厚度的增大而减小;燃料装载量不变时,芯块直径增大,组件寿期显著增大,而芯块高度影响较小;无燃料区厚度的增加对组件中子学特性有明显的负面影响,基体材料密度、基体杂质硼当量对组件中子学特性的影响较小。研究结果将为后续气冷微堆包覆颗粒弥散燃料的设计优化提供指导。     

Impurity effects on reduced-activation ferritic steels developed for fusion applications

Reduced-activation steels are being developed for fusion applications by restricting alloying elements that produce long-lived radioactive isotopes when irradiated in the fusion neutron environment. Another source of long-lived isotopes is the impurities in the steel. To examine this, three heats of reduced-activation martensitic steel were analyzed by inductively coupled plasma mass spectrometry for low-level impurities that compromise the reduced-activation characteristics: a 5-ton heat of modified F82H (F82H-Mod) for which an effort was made during production to reduce detrimental impurities, a 1-ton heat of JLF-1, and an 18-kg heat of ORNL 9Cr–2WVTa. Specimens from commercial heats of modified 9Cr–1Mo and Sandvik HT9 were also analyzed. The objective was to determine the difference in the impurity levels in the F82H-Mod and steels for which less effort was used to ensure purity. Silver, molybdenum, and niobium were found to be the tramp impurities of most importance. The F82H-Mod had the lowest levels, but in some cases the levels were not much different from the other heats. The impurity levels in the F82H-Mod produced with present technology did not achieve the low-activation limits for either shallow land burial or recycling. The results indicate the progress that has been made and what still must be done before the reduced-activation criteria can be achieved.     

电子枪加热坩埚熔池的传热研究

提高电子枪高温铀蒸发效率是ＡＶＬＩＳ铀循环的重要环节。本工作从铀循环系统的实际出发，为减轻由高温铀热腐蚀造的杂质污染和对坩埚的侵蚀作用，运用动量及能量方程研究了坩埚内带有固－液界面的熔池传热特性，着重研究了固－液界面的形状、坩埚的厚度、热导率以及电子枪参数对熔池传统热特性的影响。这些研究为坩埚设计提供了参数依据。     

六氟化铀中杂质元素热中子吸收的硼当量的测定和评价

一般杂质元素对热中子的吸收,以相当于硼的量来表示,称为热中子吸收的硼当量。美国材料试验学会(ASTM)的规格中规定六氟化铀中总杂质元素吸收热中子的硼当量不得大于8 ppm,因为硼当量过大会无效地消耗中子,影响核燃料元件的反应性。设杂质元素与硼的吸收热中子等效,则根据两者吸收中子的反应速率(R)相等,可得     

Influence of Impurities on Carrier Removal and Annealing in Neutron-Irradiated Silicon

The influence of impurities on carrier removal and annealing has been investigated in neutron-irradiated silicon in the resistivity range from 0.5 to 50 ohm-cm. Carrier removal rates in n-type material are strongly dependent upon the crystal growth method and are lower in Czochralski-grown (oxygen containing) material than in material grown by the vacuum-float-zone or LOPEX techniques. A slight dependence of the removal rate on the dopant impurity is observed in vacuum-float-zone material but not in Czochralski-grown material. The annealing behavior of n-type material is also very crystal growth dependent. An annealing stage located between approximately 144°C and 170° C is observed in vacuum-float-zone and LOPEX-grown material but not in material grown by the Czochralski method. The location of the stage is dependent upon the dopant impurity. Carrier removal at room temperature in p-type material is not influenced by the growth method or the dopant impurity. However, dopant effects are observed upon annealing.