[en] Zirconium alloys are used as cladding materials in Pressurized Water Reactors (PWR). As they are submitted to very extreme conditions, it is necessary to check their behaviour and especially to make sure they meet the safety criteria. They are therefore studied under typical in service-loadings but also under accidental loadings. In one of these accidental scenarios, called Loss of Coolant Accident (LOCA) the cladding temperature may increase above 800 C, in a steam environment, and decrease before a final quench of the cladding. During this temperature transient, the cladding is heavily oxidised, and the metallurgical changes lead to a decrease of the post quench mechanical properties. It is then necessary to correlate this drop in residual ductility to the metallurgical evolutions. This is the problem we want to address in this study: the oxidation of PWR cladding materials at high temperature in a steam environment and its consequences on post quench mechanical properties. As oxygen goes massively into the metallic part - a zirconia layer grows at the same time - during the high temperature oxidation, the claddings tubes microstructure shows three different phases that are the outer oxide layer (zirconia) and the inner metallic phases (α(O) and 'ex β') - with various mechanical properties. In order to reproduce the behaviour of this multilayered material, the first part of this study consisted in creating samples with different - but homogeneous in thickness - oxygen contents, similar to those observed in the different phases of the real cladding. The study was especially focused on the β-->α phase transformation upon cooling and on the resulting microstructures. A mechanism was proposed to describe this phase transformation. For instance, we conclude that for our oxygen enriched samples, the phase transformation kinetics upon cooling are ruled by the oxygen partitioning between the two allotropic phases. Then, these materials were mechanically tested at various temperatures and for various mean oxygen contents. This allowed us, first, to establish ductile to brittle transition temperatures (for a given oxygen content), and second, to establish behaviour laws for those materials (function of oxygen content and temperature). It was thus possible to start some preliminary finite element calculations to describe the cladding behaviour under the standard technological ring compression test. (author)

No abstract available

[en] We construct dual Lagrangians for G/H models in two space-time dimensions for arbitrary Lie Groups G and H is a subset of G. Our approach does not require choosing coordinates on ((G)/(H)), and allows for a natural generalization to Poisson-Lie T-duality. For the case where the target metric on G/H is induced from the invariant group metric on G, the dual system is a gauged Higgs model, with a non-constant metric and a coupling to an antisymmetric tensor. The dynamics for the gauge connection is governed by a BF-term. Poisson-Lie T-duality is relevant once we allow for a more general class of target metrics, as well as for couplings to an antisymmetric tensor, in the primary theory. Then the dual theory is written on a group G-bar dual to G, and the gauge group H (which, in general, is not a subgroup of G-bar) acts non-linearly on G-bar. The dual system therefore gives a non-linear realization of a gauge theory. All dual descriptions are shown to be canonically equivalent to the corresponding primary descriptions, at least at the level of the current algebra

[en] We exploit the reparametrization symmetry of a relativistic free particle to impose a gauge condition which upon quantization implies space-time noncommutativity. We show that there is an algebraic map from this gauge back to the standard 'commuting' gauge. Therefore the Poisson algebra, and the resulting quantum theory, are identical in the two gauges. The only difference is in the interpretation of space-time coordinates. The procedure is repeated for the case of a coupling with a constant electromagnetic field, where the reparametrization symmetry is preserved. For more arbitrary interactions, we show that standard dynamical system can be rendered noncommutative in space and time by a simple change of variables

[en] A technique for the weld-sealing of lids in containers for Pu²³⁸O₂ used as a heat source cell in cardiac pacemakers is in the first stage of development. This stage includes the manufacture of the welding box by the TIG method, tests with containers of stainless steel and pure tantalum and the development of optimal conditions for sealing the inner containers made of Ta/10% W. The control during all the staaes is also discussed. (author)

[en] Upper limits are estimated to the number density of extra-solar Oort clouds (ESOC) through which the solar system might pass and to the probable number of attendant planetary impacts by comets. All stars are assumed to have Oort clouds. The model is based on the observed stellar spatial density and the ratio of the total number density to the observed number density. It is estimated that 486 close stellar passages and 12,160 ESOC encounters may have occurred. Each encounter would have produced a shower of hyperbolic comets, with the results of 1-3 ESOC impacts with the earth. It is concluded that the great majority of terrestrial cratering events by comets have and will come from solar Oort cloud comets. 19 references

[en] Ultrasonic attenuation was measured as a function of temperature in two bcc metals (niobium and tantalum) and in two hcp metals (zirconium and titanium) in order to determine the dislocations relaxation peak temperatures. The measurements were carried out by the hitherto little used pulse-echo method, using longitudinal ultrasonic waves of a frequency of about 10 MHz, on specimens that underwent annealing heat-treatment and plastic deformation of about 30%. Relaxation peaks were found in the above four metals at temperatures close to room temperature. The results of this research were compared to those given in literature by plotting the frequency as a function of the peak inverse temperature. A straight line was obtained which represents the results of experiments given in literature and those given here, the points falling close to the straight line. The present work constitutes the first stage of an extensive research with the object of measuring dislocations relaxation peaks under high hydrostatic pressure

[en] We review the Skyrme model and discuss a method for incorporating quark degrees of freedom into the model. In addition, by generalizing the Skyrme/quark model to three flavors and taking into account the Wess-Zumino term, we obtain a condition on the SU(3) charges in the quark sector of the theory

[en] We find a spherically symmetric solution to the gauged SU(2)/sub L/ x SU(2)/sub R/ chiral model. It corresponds to a new classical solution to the Weinberg-Salam model in the limit of infinite Higgs mass and sin²θ/sub W/ - 0. It has an energy of 11.6 TeV and is classically unstable under small perturbations of the fields. Quantum corrections may stabilize the solution via the introduction of higher order terms in the effective action, and would change its energy

[en] The effect of electrode vertex angle on penetration depth and weld bead width, in automatic tungsten-inert-gas (TIG) dcsp bead-on-plate welding with different currents, has been studied for stainless steel 304L plates 1.5 mm and 8 mm thick. It has been found that for thin plates, wider and deeper welds are obtained when using sharper electrodes while, for thick plates, narrower and deeper welds are produced when blunt electrodes (vertex angle 180 deg) are used. An explanation of the results, based on a literature survey, is included