Pyrophoric behaviour of uranium hydride and uranium powders

Thermal stability and spontaneous ignition conditions of uranium hydride and uranium metal fine powders have been studied and observed in an original and dedicated experimental device placed inside a glove box under flowing pure argon. Pure uranium hydride powder with low amount of oxide (<0.5 wt.%) was obtained by heat treatment at low temperature in flowing Ar/5%H₂. Pure uranium powder was obtained by dehydration in flowing pure argon. Those fine powders showed spontaneous ignition at room temperature in air. An in situ CCD-camera displayed ignition associated with powder temperature measurement. Characterization of powders before and after ignition was performed by XRD measurements and SEM observations. Oxidation mechanisms are proposed.     

The oxidation of polycrystalline uranium dioxide in air at room temperature

The nature of the interaction between interstitial impurities and substitutional solutes in vanadium is investigated experimentally by measuring the internal friction. The nature of the interaction can be explained in terms of the chemical affinity. Substitutional atoms, whose oxide or nitride formation energy is greater than that of vanadium act as attractive centers to oxygen or nitrogen. On the other hand, substitutional atoms with formation energy smaller than that of vanadium act as non-interactive or repulsive centers. Iron, copper, nickel, chromium, and manganese are non-interactive or repulsive to oxygen; iron, nickel, beryllium, copper, chromium, and manganese are non-interactive or repulsive to nitrogen. The region around the substitutional iron forbidden to oxygen is estimated to extend to the second nearest neighbour sites. Titanium, aluminum, and beryllium are attractive to oxygen and aluminum and titanium to nitrogen. Binding energies estimated are 0.26 ± 0.4 eV for titanium and oxygen, 0.14 eV for aluminum and oxygen, and 0.27 eV for beryllium and oxygen.     

Types of hydride phase development in bulk uranium and holmium

The characteristics of hydride phase development in bulk uranium and holmium were studied utilizing metallographic examinations of partially hydrided samples. It has been found that the precipitation of the hydride phase during the reaction with gaseous hydrogen occurs, in the case of uranium, only at the hydride-metal interface while, for holmium, bulk precipitation (of the dihydride) also contributes significantly. The holmium-hydrogen reaction starts with a dissolution stage which proceeds until bulk saturation is reached. Then, intergranular hydride precipitation takes place throughout the bulk with some preference for surface precipitation. The difference in the development of the hydride phase between uranium and holmium was attributed to strain effects associated with the volume expansion which accompanies the hydride formation. Cooling a hydrogen-saturated solid solution of holmium results in the precipitation of an intragranular lamellar form of the hydride.     

The effect of surface oxidation of uranium carbide powder on its compatibility behaviour with stainless steels

Vacuum-bonded and sodium-bonded compatibility tests between stainless steels 316, 321 and 347, and both stoichiometric and hyperstoichiometric UC powders which had been surface-oxidised, were carried out for 1000h at 700° C. Results prove that the free carbon liberated during surface oxidation of UC powders is an important source of carbon during carburisation of the cladding, and can cause substantial embrittlement of the cladding even in a stoichiometric carbide fuel powder in both vacuum-bonded and sodium-bonded tests. A model is proposed for the transfer of carbon from an oxidised fuel powder of this kind, to the cladding in a vacuum-bonded test in which a suitable CO/CO₂ and/or CH₄/H₂ gas atmosphere is established and acts as a transfer medium for the carbon between the fuel and the cladding. Comparison of this model with the experimental results for a stoichiometric fuel powder yielded reasonable agreement.     

The oxidation behaviour of highly irradiated uranium in dry carbon dioxide at 375–500°C and in dry air at 200–300°C

The influence of irradiation of adjusted uranium, to a burn-up of 6400 MWd/t, has been studied upon its reactivity out-of-reactor in carbon dioxide at 375–500° C and in air at 200–300° C. In the absence of appreciable swelling, the irradiation increased the attack in carbon dioxide and air by factors of only 1–8 and 1–2 respectively. With swollen metal, the attack increased with swelling, without there being any notable dependence of the enhancement factor on the environment. Thus, the enhancement factor was ≈100 with a swelling of 50% and had a maximum value of 1000–2000 for 120% swollen material. The pore structure of swollen uranium was established by BET surface area and density measurements to be primarily closed. Consequently, the enhanced reactivity was not due to access of the gas to the internal surface area and is attributed to break-up of the metal surface during oxidation.     

Influence of fission products on ruthenium oxidation and transport in air ingress nuclear accidents

In separate effect tests at 1000–1200 °C Ru oxidation rate and content of Ru in escaping air flow have been studied with special emphasis on effects of other fission product elements on the Ru oxidation and transport. The results showed that in the decreasing temperature section (1100–600 °C) most of the RuO₃ and RuO₄ (≈95%) decomposed and formed RuO₂ crystals; while the partial pressure of RuO₄ in the escaping air was in the range of 10^?6 bar. The re-evaporation of deposited RuO₂ resulted in about 10^?6 bar partial pressure in the outlet gas as well. Measurements demonstrated the importance of surface quality in the decreasing temperature area on the heterogeneous phase decomposition of ruthenium oxides to RuO₂. On the other hand water or molybdenum oxide vapour in air appears to decrease the surface catalyzed decomposition of RuO_x to RuO₂ and increases RuO₄ concentration in the escaping air. High temperature reaction with caesium changed the form of the released ruthenium and caused a time delay in appearance of maximum concentration of ruthenium oxides in the ambient temperature escaping gas, while reaction with barium and rare earth oxides extended Ru escape from the high temperature area.     

Helium solubility and behaviour in uranium dioxide

A set of devices was developed in order to infuse UO₂ disks with helium, at high temperature and pressure, to measure the helium infused quantity and from these data to calculate the helium solubility in the UO₂ matrix. Samples of UO₂ single crystal and UO₂ polycrystal were infused at a temperature of 1473 and 1743 K in a helium atmosphere ranging between 50 and 100 MPa. These samples were then annealed and the helium released was measured with a mass spectrometer. From the obtained spectra it was possible to give an interpretation of the helium release mechanism and to calculate its solubility in the UO₂ lattice in these specific thermodynamic conditions. Additionally to the helium solubility measurement from infused samples, a 37 years old sample of ²³⁸PuO₂, retrieved from an old ²⁴²Cm radioisotope thermoelectric generator (RTG), containing radiogenic helium, was also measured to widen perspectives of this kind of measurements to damaged sample more representative of spent fuel.     

铀矿床氧化带中铀矿物形成的条件

对铀矿床氧化带的资料进行了总结,提出热液铀矿床氧化带的分类。综合了独联体和保加利亚境内40个不同规模和不同工业类型铀矿床的观察结果,并且通过已有的资料分析了涉及表生带中氧化带和铀矿物富集带形成的原因。在观察和文献资料的基础上,对外生铀矿床富集体总系统中的氧化带亦进行了分类。     

Investigations on swelling and fission gas behaviour in uranium dioxide

UO₂ irradiated at temperatures between 1000 and 2100 K was investigated with respect to fission gas behaviour and swelling. The amount of fission gas was measured in three steps as released fission gas, fission gas retained in bubbles and pores, and fission gas in the fuel matrix. The retained fission gas reaches concentrations up to $\text{1.6 × 10}{}^{\mathrm{?2}}$ gas atoms per uranium atom at temperatures below 1250 K and decreases with increasing temperature. The swelling was evaluated by measuring the volume changes and by immersion density measurements. The maximum fission gas swelling without extensive bubble migration is about 20% at 2000 K. It diminishes to about 5% at 1250 K.     

The influence of precipitation conditions on the properties of ammonium diuranate and uranium dioxide powders

Continuous precipitation in one and two stages was investigated to determine the effect of precipi nation variables on the properties of ADU precipitates and the sinterability of subsequent UO₂ powders in pellet fabrication. The pH at which precipitation occurred was the most important parameter in determining the size of ADU agglomerates and the settling rate and filterability of the slurry. In two-stage precipitation, the ADU properties were determined by the proportion of uranium precipitated at different pH values. Washing nitrate from ADU appeared not to affect the properties of the subsequent UO₂ pswder but a significant decrease in filterability occurred when the ADU was extensively washed. When ADU was reduced to UO₂ at 600 °C the agglomerate structure of the ADU was retained. The larger the agglomerates in the UO₂ powder (lower pH of precipitation of precursor ADU) the less sinterable it was; pseudomorphs of large agglomerates were still discernible in the sintered pellets. Thus settling rate of the ADU slurry gave an early indication of the likely sinterability of the resultant UO₂ powder since both were functions of agglomerate size.     

The effect of certain compounds on the oxidation of uranium in acid media

In the hydrometallurgical treatment of ores the main losses of uranium are due to the leaching process. It is therefore of interest to study substances which catalyze the transfer of uranium to solution. Studies into the effect of variable-valency elements on the degree of solution of uranium showed that additions of vanadium, copper, and iron make it possible to completely oxidize uranium in fairly low concentrations of sulfuric acid. In the case where copper and iron are contained in difficultly soluble minerals, these elements can be added in the form of soluble salts.     

地浸采铀浸出液氧化沉淀法除铁研究

在氧化沉淀法去除铁的过程中,氧化反应分别在自然曝气、充气和除钙的条件下进行,对照结果进行分析,得出最佳的除铁配方,同时还着重对氧化反应速率及其影响因素(pH,Eh等)进行了探索性研究,为以后试验提供了一定的依据。     

萨瓦甫齐铀矿床层间氧化带地球化学特征

萨瓦甫齐铀矿床不同地段层间氧化带的氧化还原程度是不同的。利用诸如w(Fe3+)/w(Fe2+)、w(Ra)/w(U)和有机炭等地球化学参数探讨了萨瓦甫齐铀矿床层间氧化带的地球化学特征,即矿床西南部层间氧化带氧化程度较弱,表现为全硫(∑S)、总铁(TFe)和矿石的铀质量分数低;而矿床中部及东北部层间氧化带氧化程度较强,尤其以东北部突出,它们的全硫、总铁和矿石的铀质量分数均高。铀矿化强弱明显与层间氧化带的氧化还原程度有关,层间氧化带的氧化程度越强,铀矿化则愈强;反之,则弱。