Degradation of UN and UN–U3Si2 pellets in steam environment

In this work, a systematic study of the degradation of UN pellets (density range 96%–99.9% and grain size of 6–24 µm) and UN-10%U₃Si₂ (wt%) composite in a steam environment is presented. Static steam autoclave tests were performed at 300 °C and 9 MPa for period of 0.5–1.5 hours. Microstructural analyses of UN pellets show that, in a high-pressure atmosphere, the fuel collapses principally by intergranular cracking generated by the precipitation of an oxide phase in the grain boundaries. This mechanism leads to a premature mechanical collapse of the fuel pellet, exposing fresh surfaces to steam, and ultimately accelerating the oxidation process. Increasing density (specifically eliminating open porosity) was found to delay the oxidation process, while increasing grain size was found to accelerate the degradation process due to a greater susceptibility to mechanical fracture by way of intergranular oxidation. The performance of the UN-10%U₃Si₂ composite proved to be better when compared to UN. The U₃Si₂ phase served to stabilize the UN grain boundary interface and reacted preferentially with the steam, thereby altering the failure mechanism. In this composite material, the cracking was predominantly intra-granular and the exposure of fresh surfaces was limited, resulting in a slower degradation process.     

Oxydation menagee du chromite de fer stoechiometrique a basse température,réduction du composé obtenu

The influence of oxygen on stoichiometric, finely crystallized iron chromite gives a compound with formula FeCr₂O_4,5. The overall rate of the transformation is controlled by the diffusion of iron cations in the spinel lattice and the simultaneous formation of iron vacancies. Oxidation is followed by means of thermogravimetric, radiocrystallographic, E.P.R. and electrical conductivity analysis. The system can be brought back to the initial state by reduction of the oxidized product with hydrogen. This reverse reaction occurs with the same experimental energy of activation and the same rate pressure law. The data obtained are explained by a vacancy Fe³⁺ ion association mechanism.     

Creosote Bush (Larrea tridentata) Resin Increases Water Demands and Reduces Energy Availability in Desert Woodrats (Neotoma lepida)

Although many plant secondary compounds are known to have serious consequences for herbivores, the costs of processing them are generally unknown. Two potential costs of ingestion and detoxification of secondary compounds are elevation of the minimum drinking water requirement and excretion of energetically expensive metabolites (i.e., glucuronides) in the urine. To address these impacts, we studied the costs of ingestion of resin from creosote bush (Larrea tridentata) on desert woodrats (Neotoma lepida). The following hypotheses were tested: ingestion of creosote resin by woodrats (1) increases minimum water requirement and (2) reduces energy available by increasing fecal and urinary energy losses. We tested the first hypothesis, by measuring the minimum water requirement of woodrats fed a control diet with and without creosote resin. Drinking water was given in decreasing amounts until woodrats could no longer maintain constant body mass. In two separate experiments, the minimum drinking water requirement of woodrats fed resin was higher than that of controls by 18-30% (about 1-1.7 ml/d). We tested several potential mechanisms of increased water loss associated with the increase in water requirement. The rate of fecal water loss was higher in woodrats consuming resin. Neither urinary water nor evaporative water loss was affected by ingestion of resin. Hypothesis 2 was tested by measuring energy fluxes of woodrats consuming control vs. resin-treated diets. Woodrats on a resin diet had higher urinary energy losses and, thus, metabolized a lower proportion of the dietary energy than did woodrats on control diet. Fecal energy excretion was not affected by resin. The excretion of glucuronic acid represented almost half of the energy lost as a consequence of resin ingestion. The increased water requirement and energy losses of woodrats consuming a diet with resin could have notable ecological consequences.     

Comportement electrochimique de la galene (PbS) dans les conditions de pH proches de celles de la flottation

The electrochemical behaviour of lead sulphide mineral (galena) has been investigated by cyclic voltammetry and controlled potential electrolyses. In the pH range 5–9, the oxidation leads—at + 0.5 V vs sce—to the formation of elemental sulphur and lead species: Pb²⁺, Pb(OH)₂ or Pb(OH)Cl depending on the pH, and on the nature of the electrolyte.

The reactions involved in the overall anodic/cathodic cycle have been interpreted; at pH 9, galena is reduced at − 1.3 V, to lead metal and HS⁻ ions.     

Effect of time of application of fertiliser nitrogen on the distribution and identity of the nitrogenous constituents of young apple trees

The initial response to an application of fertiliser N appeared always as a rise in the level of asparagine. Following spring and summer N applications this high level of asparagine later decreased as those of arginine and the insoluble N increased, but after autumn N application both asparagine and arginine accumulated, especially in the roots. Leaves were exceptional both in containing a high proportion of insoluble N at all times and in containing a large amount of soluble glutamine soon after spring N application, in addition to the arginine and asparagine. Direct analyses have been transformed on to a weight basis and the results are given diagrammatically as the total weight of N present in each tissue of the tree as each of the four components: insoluble N, soluble arginine, soluble asparagine, or the total of all other soluble compounds. This has enabled the movement of N within the whole tree and the interchange between compounds in response to season and to fertiliser application to be recognised more readily. The possible significance of the relative mobilities of arginine and asparagine is also discussed, together with the possible merits of sampling leaves or other tissues, for example the roots, for predicting the future crop potential of apple trees.     

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