Reproducible fabrication of PuO2 fuel

PuO₂ fuel pellets with stable microstructures at 80 to 90% theoretical density for use at temperatures up to 1400°C can be fabricated by a granulation and hot pressing process. Three interdependent factors which must be controlled for reproducible fabrication of microstructure and density to ±0.5% are: particle size and morphology of plutonium oxalate precipitate, particle size of calcined PuO₂ powder after ball milling, and temperature at which granules made from milled powder are presintered prior to hot pressing. Relationships are presented between each of these factors and their effect on the density and integrity of the fuel form.     

Radiation-Chemical Behavior of Plutonium in the Heterogeneous System PuO2-Ground Water

Radiation-chemical behavior of plutonium in the heterogeneous system PuO₂-ground water from the Red forest in the vicinity of Chernobyl NPP was studied. It was found that, in nonirradiated and irradiated ground water, Pu(IV) passes into the aqueous phase with subsequent transformation into Pu(V) and Pu(III). The degree of plutonium leaching is governed by the amount of organic substances, particularly fulvic and mellitic acids, in the ground water, and the degree of conversion of Pu(IV) to Pu(III) and Pu(V), by the ionizing radiation dose. The physicochemical and radiation-chemical behavior of plutonium in the systems was judged from the spectral characteristics of the irradiated and nonirradiated solutions.     

Differential equation for the melting temperature of small-size particles

A differential equation for determining the melting temperature of small-size particles has been derived within the framework of a thermodynamic approach. The equation has a rather general form that admits the dependence of physical quantities such as the heat of melting, surface tension, and molar volumes in the solid and liquid state on the particles radius. The result of the integration of the obtained equation in particular cases leads to the well-known formulas.     

Low temperature melting curve of He3

Measurements of the melting curve of He³ have been made from 3 m° K to 40 m° K, after cooling the liquid-solid He³ mixture by the adiabatic solidification method. The slope of the melting curve shows that at the lowest temperature there is a substantial departure of the solid He³ entropy fromR ln 2. This effect is due to nuclear spin ordering. Temperatures below 3 m° K were obtained during compression. By theoretically estimating the melting curve below 3 m° K it is possible to estimate temperatures from pressure measurements. By such means it is estimated that Kelvin temperatures of 1.5 to 2 m° K were actually achieved.This work has been supported by the U.S. Atomic Energy Commission under Contract AT(11-1)-34, P.A. 143.In partial fulfillment of the requirements for a Ph.D. degree at the University of Illinois.     

On the temperature of melting of nanoparticles and nanocrystalline substances

A new expression for the temperature of melting of a nanoparticle, which agrees with the well-known result of the Debye model, has been obtained within the framework of a rather strict thermodynamic approach. It is shown that the characteristic Debye temperature and the melting temperature can depend in a critical manner on the fractal structure of the nanocrystalline solid.     

Relationship between crystallization temperature and melting temperature in crystalline materials

The ratio of the absolute temperature at which the homogeneous nucleation rate or the crystal growth rate is maximum (T _cmax) to the absolute melting temperature (T _m) is analysed by two parameters: one is the ratio of the activation energy for migration (E) and the heat of fusion (H _m), the other is the ratio of mean molar surface energy and H _m. In analysing the crystallization data for a large number of crystalline materials such as metals and inorganic, organic and polymeric materials, the ratios and E/H _m show roughly constant values for a given type of material. The constancy of the two parameters will then give rise to a constant value ofT _cmax/T _m.     

Material surfaces and nanosystems close to the melting temperature

This is a concise review of some facts and theory work on the equilibrium behavior of solid surfaces and nanosystems close to the bulk melting point. We focus in particular on computer simulation results obtained for metals and ionic insulators.     

Physicochemical characteristics and radiation-chemical processes in the system PuO2-sorbed water

The experimental data on formation of H₂ and O₂ during storage of PuO₂ samples containing sorbed water were analyzed. It was shown that the rates of formation of these gases, all other factors being the same, are governed by the procedure of water sorption: in sorption from the liquid phase these rates are significantly higher than those in sorption from the gas phase. To elucidate the conditions of safe storage of PuO₂, it is necessary to obtain systematic quantitative data on the kinetics of H₂ and O₂ formation in the system consisting of PuO₂ and water sorbed from moist air. The optimized mathematical model of sorbed water radiolysis adequately describing the experimental data on the formation of H₂ and O₂ at room temperature in the system consisting of PuO₂ and water sorbed from the liquid phase is presented. The rate constant of H₂ and O₂ recombination in the presence of PuO₂ containing 2–3% water was found to be ≤ 1 × 10^?5 mol^?1 s^?1. With the knowledge of the reaction rate constants, the model allows calculation of the amounts of H₂ and O₂ and the pressure in the storage vessel depending on the amount of sorbed water, radiation dose rate, and storage duration.     

A Mathematical Model of Radiolysis of Water Sorbed on PuO2

A mathematical model of radiolysis of water sorbed on PuO₂ was developed. The model is basedon the postulate that the kinetic characteristics of formation of the initial products in -radiolysis of sorbedwater (H₂, O₂, H₂O, e^- _{e q}, OH) depend on its content. To explain experimental data showing that the steady-state amounts of H₂ are practically equal to the initial amount of sorbed water, the reaction betweenPuO₂ and H₂O₂ yielding H₂O and superstoichiometric plutonium dioxide was postulated. The minimum value of the rate constant of this reaction was found (2 ×10^{- 3} mol^{- 1} s^{- 1}). The proposed mathematical model adequately describes the experimental data on formation of hydrogen and oxygen released from water in contact with PuO₂ at its prolonged storage.     

Formation of H2 and O2 in Radiolysis of Water Sorbed on PuO2

Samples of PuO₂ containing sorbed water, when stored at room temperature, release hydrogen and oxygen into the gas phase. At a given content of water, the rates of formation of H₂ and O₂ are in direct proportion to the dose rate of plutonium radiation. With increasing concentration of water from 0.3 to 3% the rates of formation of H₂ and O₂ increase by a factor of 60-100, which is caused by increase in both the energy of -radiation absorbed by water and radiation-chemical yields of formation of H₂ and O₂. The radiation-chemical yields of formation of oxygen and hydrogen at the 0.3% content of sorbed water are lower, and at the water content exceeding 1%, higher than the yields of these products in radiolysis of free water. On storage of powder-like and pelletized samples of PuO₂ of power reactor origin with a 3% water content for 150-200 days, the stationary concentration of hydrogen approximately corresponding to the initial concentration of sorbed water is established.     

PuO2含量对MOX燃料组件物理特性的影响

当超临界水冷堆组件采用铀与钚的氧化物混合而成的MOX燃料时,与普通UO_2燃料相比其中子学特性存在差异。为了研究PuO_2含量对组件物理特性的影响,采用蒙特卡罗方法计算得到了不同PuO_2含量时MOX燃料物理特性参数,包括俘获裂变比、总微观裂变截面、燃料每次裂变平均释放的能量、每次裂变释放的中子数及有效裂变中子数。进一步得到了装载不同含量PuO_2时MOX燃料组件特性参数,包括能谱、Keff、相对功率、相对功率峰值和缓发中子份额。研究结果对MOX燃料组件的设计有一定参考价值。     

Determining the temperature field and the melting rate of an electrode in the multiphase system of the electric-slag melting process

The problem of melting metals by the electric-slag method is formulated mathematically and some numerical results are shown. The parameters which need to be determined include the rate of electrode melting as well as the melting and crystallization isotherms of the electrode and the ingot.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 20, No. 1, pp. 87–95, January, 1971.     

Correlation between glass transition temperature and melting temperature in metallic glasses

We report that the glass transition temperature (T_g) of a variety of metallic glasses (MGs) correlates with the eutectic or peritectic temperature of two main components corresponding stoichiometric proportion in their binary phase diagram. The correlation suggests that the T_g of MGs is mainly determined by their solvent of two base components, which have composition close to the eutectic and peritectic points in the binary phase diagram and the weakest link in amorphous structure. The results have implications for understanding the structure and glass transition in MGs and for predicting and designing metallic glasses with a desirable T_g.     

Effect of nanoparticle size on the onset temperature of surface melting

In this letter the onset temperature of surface premelting, T_sm, is determined in terms of the mean-field approximation. There are established relationships, describing the dependence of T_sm on a characteristic size of nanoparticles. It is shown that if a radius of a particle, r, exceeds 10 nm approximately, this dependence is weak. On the contrary, it is strong if the radius of a particle is less than 5 nm. The transition of surface layers into the quasi-liquid and pseudo-crystalline states is under consideration, too. A comparison of the proposed model with available experimental data and with molecular dynamics simulations is carried out.