Basic properties of the F-type centers in halides, oxides and perovskites

We present a short survey of the optical properties of primary radiation-induced point defects in alkali halides, simple oxides and some ABO₃ perovskites. We discuss in details the optical properties of single electron F and F⁺ centers in rock-salt (f.c.c.) alkali halides and oxides and show that the Mollwo-Ivey law well-known for the F-type centers in alkali halides may be extended for other rock-salt structure insulators. We also discuss the major differences in point defect production mechanisms in halides and oxides. We show that the Rabin-Klick diagram may be generalized for a whole family of alkali halides. The F-type center migration and aggregation into metal colloids in alkali halides and oxides is also discussed.     

Influence of oxide layer morphology on hydrogen concentration in tin and niobium containing zirconium alloys after high temperature steam oxidation

The influence of the oxide layer morphology on the hydrogen uptake during steam oxidation of (Zr,Sn) and Zr-Nb nuclear fuel rod cladding alloys was investigated in isothermal separate-effect tests and large-scale fuel rod bundle simulation experiments. From both it can be concluded that the concentration of hydrogen in the remaining metal strongly depends on the existence of tangential cracks in the oxide layers formed by the tetragonal - monoclinic phase transition in the oxide, known as breakaway effect. In these cracks hydrogen is strongly enriched. It results in very local high hydrogen partial pressure at the oxide/metal interface and in an increase of the hydrogen concentration in the metal at local regions where such cracks in the oxide layer exist. Due to this effect the hydrogen uptake of the remaining zirconium alloy does not depend monotonically on temperature. Differences between (Zr,Sn) and Zr-Nb alloys are caused by differences in the hydrogen production due to different oxidation kinetics and in the crack forming phase transformation in the oxides as well as in the mechanical stability of the oxides.     

Corrosion characteristics and oxide microstructures of Zircaloy-4 in aqueous alkali hydroxide solutions

The corrosion characteristics of Zircaloy-4 have been investigated in various aqueous solutions of LiOH, NaOH, KOH, RbOH and CsOH with equimolar M⁺ and OH⁻ at 350°C. The characterization of the oxides was performed using transmission electron microscope (TEM) and scanning electron microscope (SEM) on the samples which were prepared to have an equal oxide thickness in pre-transition and post-transition regimes. At a low concentration (4.3 mmol) of aqueous alkali hydroxide solutions, the corrosion rates decrease gradually as the ionic radius of cation increases. At a high concentration (32.5 mmol), the corrosion rate increases significantly in LiOH solution and slightly in NaOH solution, but in the other hydroxide solutions such as KOH, RbOH and CsOH, the corrosion rate is not accelerated. Even if the specimens have an equal oxide thickness in LiOH, NaOH and KOH solutions, the oxide microstructure formed in the LiOH solution is quite different from those formed in the NaOH or the KOH solutions. In the LiOH solution, the oxides grown in the pre-transition regime as well as in the post-transition regime have an equiaxed structure including many pores and open grain boundaries. The oxides grown in the NaOH solution have a protective columnar structure in the pre-transition regime but an equiaxed structure in the post-transition regime. On the other hand, in the KOH solution, the columnar structure is maintained from its pre-transition regime to the post-transition regime. On the basis of the above results, it can be suggested that the cation incorporation into zirconium oxide would control the oxide microstructure, the oxide growth mechanism at the metal–oxide interface and the corrosion rate in alkali hydroxide solutions.     

Over forty years of ‘Thermodynamics of Nuclear Materials’

In this paper, some perspectives of the highlights of the contributions of chemical thermodynamics to the understanding and exploitation of nuclear fuels are considered. Of particular significance are the developments in the representation of complex phase equilibria and the understanding, which has evolved of the behaviour of all fuel systems during operation and in accident conditions. Studies on metallic and ceramic fuels, actinide alloys, oxides, carbides and nitrides, will be discussed. Some directions of future endeavours are given.     

Phase diagram and thermodynamic properties of the system MnCl2-UCl4, prediction and investigation results

The MnCl₂-UCl₄ system was studied employing differential thermal analysis, common thermal analysis and high-temperature cryometry. From the results obtained, both the phase diagram and the thermodynamic characteristics of liquid phases of the system have been determined. The results were discussed in terms of regularities found in the phase equilibria of binary salt systems with common anion and compared with those for other systems of the type MCl_n-UCl₄. It appeared that the results confirmed earlier predictions on the title system.     

金属Ce表面氧化物的真空稳定性

为探讨金属铈在超高真空条件下作为钚模拟材料的可行性，采用X射线光电子能谱（XPS）分析技术研究了超高真空条件下金属Ce表面氧化物在不同温度时的真空稳定性。实验结果表明，不同温度和超高真空条件下，金属Ce表面氧化物的真空稳定性各不相同。在室温、超高真空条件下没有观察到金属Ce表面氧化物发生变化；但在873 K、超高真空条件时，金属Ce表面氧化物发生还原反应，样品表面生成Ce₂O₃。根据热力学计算，推断了高温、超高真空条件下发生还原反应的过程，金属Ce表面氧化物的真空特性与金属Pu存在一定差异。     

On the Cs,Te fission product-induced attack and embrittlement of stainless steel cladding in oxide fuel pins

Since Cs and Te fission products are both implicated as causative agents in FCCI (fission product-assisted inner surface attack of stainless steel cladding) and in FPLME (fission product-assisted liquid metal embrittlement of AISI-Type 316), attempts are made to rationalize the observed out-of-pile Cs: Te dependences of FCCI and FPLME incidence and severity, and their particular Cs, Te synergisms, in terms of Cs-Te thermochemistry and phase equilibria. Successful rationalization in the case of FPLME is taken to point up the critical importance of Te activity and Cs-Te physical state in the FPLME mechanism. A similar conclusion is reached for CCCT, the nonoxidative mode of FCCI, however oxidative modes of FCCI are concluded to rely more on the physical or catalytic properties of Cs-Te mixtures than on specific thermodynamic properties such as Te or Cs activities. The possibility of synergistic coupling between oxidative FCCI and FPLME in irradiated fuel pins is also examined, and it is concluded that although the available evidence does not support such coupling under monotonie loading, it is suggested — as intergranular notch-sensitivity in FPLME — under cyclic loading conditions.     

A fabrication technique for a UO2 pellet consisting of UO2 grains and a continuous W channel on the grain boundary

A new fabrication process of UO₂-W composite fuel has been studied in order to improve the thermal conductivity of the UO₂ pellet by the addition of a small amount of W. A fabrication process was designed from the phase equilibria among tungsten, tungsten oxides and UO₂. The conventionally sintered UO₂ pellet which contains W particles is heat-treated in an oxidizing gas and then in a reducing gas. In the oxidizing heat-treatment W particles are oxidized and liquid tungsten oxide penetrates within the UO₂ grain boundary, and in the reducing heat-treatment liquid oxide is transformed to solid tungsten which forms a continuous channel along the UO₂ grain boundary. This developed technique can provide a continuous W channel covering UO₂ grains for a UO₂-W composite fuel even with a small amount of a metal phase - below 6 vol.%. The thermal diffusivity of the UO₂-6 vol.%W cermet composite increases by about 80% when compared with that of a pure UO₂ pellet.     

Effect of the Concentration of Metal Ions on their Adsorption on Various Hydrous Iron and Aluminum Oxides

The adsorption of Eu(III) and Co(II) on laboratory-prepared amorphous hydrous iron and aluminum oxides, goethite and gibbsite, was studied as a function of pH over a wide range of metal ion concentrations, in order to see the effect of the change in the degree of site occupation by metal ions. When the metal ion concentrations were low, a simple Langmuir equation with a single adsorption constant was able to describe adsorption on these hydrous oxides. At high metal ion concentrations, adsorption on goethite and gibbsite exhibited clear saturation maxima expected from a simple one-to-one interaction between the metal ion and the adsorption site, while adsorption on laboratory-prepared hydrous oxides continued to increase gradually even at high metal ion concentrations.     

Lithium reduction of americium dioxide to generate americium metal

The lithium reduction process has been developed for application to a pyrochemical recycle process for oxide fuels. This process uses lithium metal as a reductant to convert oxides of actinide elements to metal. Lithium oxide generated as a product of the reduction would be dissolved in a molten lithium chloride bath to enhance reduction. In this work, the reduction of americium dioxide to its metal was experimentally confirmed. At the end of the reduction, more than 99.9% of the Am was recovered from the salt phase to a solid Am phase. It was also shown that the lithium oxide concentration in lithium chloride is required to be kept under 5.1 wt%. When the concentration of lithium oxide was between 5.1 and 6.3 wt%, americium formed monoxide.     

Preparation of Massive Plutonium Metal Directly from Its Oxides

This report details a technique for the rapid production of massive Pu metal directly from its oxides. Salient features of this single unit process are as follows: the reduction of plutonium oxides with Ca metal; the extraction, by dissolution, of the calcium oxide reaction product with molten calcium chloride; and a quantitative separation of the two immiscible molten metal phases by the immiscible salt solution.

Reduction, separation and massive metal yield efficiencies are in excess of 99.96%. The purity of the metal produced is equal to that of the initial feed material. The batch cycle time is 15 min. The single waste product is far below recovery level and is discardable as a small volume solid waste.     

Adiabatic elastic constants of uranium monocarbide

Using the phase-comparison technique, the room temperature adiabatic elastic constants of UC single crystals grown by the radio-frequency, induction heated floating-zone technique have been measured as a function of carbon composition (4.52–4.96 wt% C). The temperature dependence of the moduli was measured from 80 to 900 °K in a hypostoichiometric sample. There is a systematic increase of c₄₄ with increasing carbon concentration. The elastic-moduli values are compared with published values for some alkali halides and refractory metal monocarbides. An increase in temperature results in a linear decrease in c₁₁ and c, but c₄₄ increases with temperature to a maximum at 80°C. The temperature coefficients of the moduli at T ? 80 °C are comparable to those observed in the alkali halides. These results indicate that the ionic crystal character of the interatomic bonds increases with temperature and that the carbon concentration in the UC phase can deviate from stoichiometric composition.     

The effect of oxides on PIXE measurements

The presence of oxides on metallic samples used for PIXE analysis affects quantitative measurements. This effect has been calculated based on thick target analysis on layers of different compositions. The ratio of oxidized metal yield to clean metal yield is seen to depend on proton energy and on oxide thickness. Calculations are presented for oxides of Al, Si, Fe, and Cu, and applied to experimental data on Si. The method may be applied to measuring thicknesses of oxides of known stoichiometry.     

Literature review of thermal and radiation performance parameters for high-temperature, uranium dioxide fueled cermet materials

High-temperature fissile-fueled cermet literature was reviewed. Data are presented primarily for the W-UO₂ as this was the system most frequently studied; other reviewed systems include cermets with Mo, Re, or alloys as a matrix. Failure mechanisms for the cermets are typically degradation of mechanical integrity and loss of fuel. Mechanical failure can occur through stresses produced from dissimilar expansion coefficients, voids created from diffusion of dissimilar materials or formation of metal hydride and subsequent volume expansion. Fuel loss failure can occur by high temperature surface vaporization or by vaporization after loss of mechanical integrity. Techniques found to aid in retaining fuel include the use of coatings around UO₂ fuel particles, use of oxide stabilizers in the UO₂, minimizing grain sizes in the metal matrix, minimizing impurities, controlling the cermet sintering atmosphere, and cladding around the cermet.     

Development of thermodynamic databases in the system U–Zr–Ce–Cs–Fe–B–C–I–O–H for application to simulating phase equilibria in severe nuclear accidents

In case of severe nuclear accidents involving melt down of nuclear fuels at high temperatures, it is of considerable importance to accurately evaluate the highly-volatizing behavior of fission products (FPs) over multicomponent debris. Particularly, cesium (Cs)- and iodine (I)- bearing chemical species are regarded as notable FPs. In the present work, the authors have generated original thermodynamic databases for the system U–Zr–Ce–Cs–Fe–B–C–I–O–H featuring Cs- as well as I-bearing subsystems, which are contained in oxide, iodide, and metal (including borides and carbides) sub-databases. It has been confirmed that the phase diagrams calculated by the present set of the databases reproduce the corresponding literature data well in various kinds of subsystems of the above multicomponent system. The present set of databases has subsequently been applied to simulate phase equilibria and volatizing behavior of Cs- and I-including species, respectively, in multicomponent debris under specific temperature and atmospheric conditions corresponding to severe nuclear accidents.     

A review of refractory metal alloys and mechanically alloyed-oxide dispersion strengthened steels for space nuclear power systems

Mechanical and thermo-physical properties of refractory metal alloys and mechanically alloyed (MA)-oxide dispersion strengthened (ODS) steels are reviewed and their potential for use in space nuclear reactors is examined. Preferable refractory alloys for use in liquid metal and gas-cooled space reactors include Nb-1%Zr, PWC-11, Mo-TZM, Mo-xRe where x varies from 7% to 44.5%, T-111 and ASTAR-811C. These alloys are heavy, difficult to fabricate, and are not readily available. The advantages of the MA-ODS alloys are: (a) their strength at high temperatures (>1000 K), which decreases slower with temperature than those of niobium and molybdenum alloys; (b) relatively lightweight and less expensive; (c) low swelling and no embrittlement with exposure to high-energy neutrons (>0.1 MeV) up to 10²⁷ n/m²; and (d) high resistance to oxidation and nitration. The few data available on compatibility of MA-ODS alloys with alkali liquid metals up to 1100 K are encouraging, however, additional tests at typical temperatures (1000-1400 K) in space nuclear reactors are needed. The anisotropy of MA-ODS alloys when cold worked, and particularly rolled into tubes, should not hinder their use in space nuclear power systems, in which operation pressure is either near atmospheric or as high as 2 MPa, but joints weldability is an issue.     

Synthesis of simulated cruds for development of decontaminating agents

The Chalk River Unidentified Deposit (cruds) were classified into Types 1 and 2 based on the analyses of actual cruds from target objects such as steam generator (SG) of Korea and Japan Pressurized Water Reactor (PWR). A metal oxide or metal hydroxide was used to synthesize simulated cruds having chemical compositions and crystalline structure similar to the actual one by 12 different methods. CRUD 4 (metal oxides in the autoclave vessel) and CRUD 10 (metal oxides in a crucible after hydrazine pretreatment) were chosen as the best method for Types 1 and 2, respectively. These simulated cruds showed reasonable dissolubilities by dissolution tests using well-known decontaminating agents. As these cruds can be synthesized easily without using any specialized equipment or reagents in a short time and in large quantities, they are expected to stimulate the development of decontaminating agents and processes.     

探索碱金属阳离子在钍基硫酸盐配合物自组装过程中的作用

配体通常会直接影响钍配合物的自组装模型,但是碱金属阳离子在其自组装过程中的潜在作用还有待进一步研究。本工作中,通过添加不同碱金属阳离子,可以获得一系列四价钍的硫酸盐结构配合物,说明了碱金属阳离子对于整个拓扑结构以及四价钍水解聚合方式的影响。四价钍硫酸盐的结构分析表明,引入K⁺、Cs⁺可以改变晶体骨架的维度,形成一维链状、二维层状或三维框架的钍硫酸盐结构,较大的Cs⁺可促进二维层状低聚物的形成,同时形成了罕见的［Th₃(μ₃-O)］¹⁰⁺三聚体中心。过去认为抗衡离子在配合物形成过程中扮演无关紧要的角色,而这一工作表明碱金属抗衡离子影响了结构的整体堆积方式。     

Basic thermodynamic data in nuclear technology: New developments,old problems

Current trends in the determination of the basic thermodynamic data in nuclear technology are reviewed. Examples are given to illustrate the need for accurate and precise measurements of energy and entropy in the calculation of chemical equilibria and phase diagrams.     

Generalization of Solovev's approach to finding equilibrium solutions for axisymmetric plasmas with flow

Solovev's approach of finding equilibrium solutions was found to be extremely useful for generating a library of linear-superposable equilibria for the purpose of shaping studies.This set of solutions was subsequently expanded to include the vacuum solutions of Zheng,Wootton and Solano,resulting in a set of functions [SOLOVEV_ZWS] that were usually used for all toroidally symmetric plasmas,commonly recognized as being able to accommodate any desired plasma shapes (complete-shaping capability).The possibility of extending the Solovev approach to toroidal equilibria with a general plasma flow is examined theoretically.We found that the only meaningful extension is to plasmas with a pure toroidal rotation and with a constant Mach number.We also show that the simplification ansatz made to the current profiles,which was the basis of the Solovev approach,should be applied more systematically to include an internal boundary condition at the magnetic axis;resulting in a modified and more useful set [SOLOVEV_ZWSm].Explicit expressions of functions in this set are given for equilibria with a quasi-constant current density profile,with a toroidal flow at a constant Mach number and with specific heat capacity 1.The properties of [SOLOVEV_ZWSm] are studied analytically.Numerical examples of achievable equilibria are demonstrated.Although the shaping capability of the set [SOLOVE_ZWSm] is quite extensive,it nevertheless still does not have complete shaping capability,particularly for plasmas with negative curvature points on the plasmaboundary such as the doublets or indented bean shaped tokamaks.