Influence de la dose de neutrons rapides sur la formation des cavites dans l'aluminium

The results of neutron irradiation of aluminium at 55°C under a fast flux of $\text{(1.4 ± 0.2) 10}{}^{14}\text{n · cm}{}^{\mathrm{?2}}\text{· sec}{}^{\mathrm{?1}}$ to doses ranging between 10¹⁹ and $\text{3 × 10}{}^{21}\text{n · cm}{}^{\mathrm{?2}}$ are presented. Experimental observations suggest that dislocations play an important role in void nucleation and growth. Monte-Carlo calculations of void growth in the presence of a dislocation describe reasonably well the experimental results.     

Influence de l'ecrouissage sur la formation des cavites d'irradiation dans l'aluminium

Aluminium samples, either annealed or in the asdeformed condition, were subjected to irradiation doses ranging from 8 × 10¹⁹ to 3.1 × 10²¹ n/cm². Initial deformation raises the degree of swelling associated with voids in aluminium irradiated at 55 °C, but reduces it at 220 °C. In annealed aluminium the loops are sometimes interstitial-type, sometimes vacancy-type; the appearance of voids is linked to an increase in the density of dislocations. Examination of samples stretched by different amounts shows that at a fixed temperature swelling is a function of the dislocation density introduced initially.     

Etude de la formation des cavites d'irradiation dans le cuivre I-irradiation aux electrons de 1 MeV

We have studied the lormation of voids and interstitial loops in thin copper foils irradiated by 1 MeV electrons as a function of temperature and of gas content. We have observed a close relationship between voids and the dislocation network. By irradiating specimens degassed by annealing, we were able to show that for irradiation temperatures ? 100°C, it is the gas dissolved in the matrix which is responsible for nucleating both loops and voids. It follows that the upper temperature limit and the temperature of maximum swelling are displaced to higher temperatures when the gas content of the material increases. The temperature range of swelling and the position of peak swelling are therefore linked above all to the nucleation conditions of the defect accumulations.     

Etude de la formation des cavites d'irradiation dans le cuivre II-irradiation aux ions cuivre de 500 keV — effet de la dose

The development of microstructure as a function of the irradiation dose has been studied for thick foils of copper irradiated by 500 keV copper ions. A comparison between the results obtained with degassed samples and those for non-degas-sed samples has allowed us to show that the gases dissolved in the metal aided the nucleation of interstitial loops. The relationships between the interstitial loops and voids were studied, and some conclusions drawn concerning the growth of voids and the effect of free surfaces.     

Heterodiffusion de l'or,de l'argent et du cuivre dans les phases cubiques δ et ε du plutonium

The solute diffusion at infinite dilution of ¹⁹⁸Au and ^110mAg in cubic phases of Pu has been studied using the serial sectroning method. The solute diffusion coefficients in the b.c.c. ? phase can be expressed by: $\text{D}{}^{\mathrm{Au}}{}_{\mathrm{?Pu}}\text{= 5,7 × 10}{}^{\mathrm{?5}}$ exp(?10300/RT) cm²/s and $\text{D}{}^{\mathrm{Ag}}{}_{\mathrm{?Pu}}\text{= 4,9 × 10}{}^{\mathrm{?5}}\text{exp(?9600/RT) cm}{}^2\text{/s}$. The solute diffusion mechanism is interstitial of the dissociative type in both cases. These experiments confirm the activated interstitial model which has been proposed for self diffusion of ?Pu. Indeed the solute diffusion coefficients of Au and Ag are near of the self diffusion coefficients of Pu. The mechanisms are therefore interstitial in both cases. In the f.c.c. δ phase of Pu where self diffusion takes place by a vacancy mechanism, the solute diffusion coefficients of Au and Ag are near of the self diffusion coefficients of δ Pu. Solute diffusion takes place also by a vacancy mechanism. On the other hand, the extrapolation at infinite dilution of experiments of solute diffusion of Cu in ?Pu (Matano-Wagner coupling) gives the following results: $\text{D}{}^{\mathrm{Cu}}{}_{\mathrm{?Pu}}\text{= 1 × 10}{}^{\mathrm{?3}}\text{exp(?12300/RT)}$ cm²/s. The solute diffusion mechanism is interstitial of the dissociative type. In the ? phase the smaller the atomic radius the faster the migration: r_Co < r_Cu < r_?Pu < r_Ag = r_Au, and D^Co_?Pu > D^Cu_?Pu >D^Pu_?PU > D^Ag_?Pu ≈ D^Au_?Pu.     

Amelioration de la precision du dosage de l'oxygene dans le sodium par la methode d'amalgamation

The difficulties in analyzing oxygen present in trace amounts in liquid sodium by the amalgamation method stem from the possibility of contamination of sodium during the manipulations and transfers. The precision has been improved by developing a method of working which made it possible to estimate the contamination during each analysis. The results obtained are compared to the values obtained by the classical amalgamation method and also to those quoted by Noden, obtained by vacuum distillation or by use of the vanadium-wire and the electrolytic oxygen meter.     

Role de l'iode sur le developpement de la reaction entre le combustible et sa gaine in acier inoxydable (type 316) dans les reacteurs

During the operation of a reactor, the fuel $\text{(U, Pu}{}_{\mathrm{2-x}}\text{)}$ reacts with its cladding (stainless steel). The role of iodine, a fission product, in this reaction has been examined. Out-of-pile experiments have been done to study the compatibility of stainless steel with iodine, in open or closed systems, isothennally or in a temperature gradient, with or without the presence of fuel, using metallography, X-ray diffraction and X-ray microprobe analysis. Under isothermal conditions, iodine reacts directly with 316 stainless steel. Taking into account its composition and thermodynamie variables, iodides form in the following order:$\text{Mnl}{}_2\text{, Mnl}{}_2\text{+ Crl}{}_2\text{, Mnl}{}_2\text{+ Crl}{}_2\text{+ Fel}{}_2\text{, Mnl}{}_2\text{+ Crl}{}_2\text{+ Fel}{}_2\text{+ Nil}{}_2$In a thermal gradient, spectacular corrosion of the steel at low temperature is accompanied by substantial loss of chromium. In the hot zones, iron and chrome precipitate in equivalent amounts. The presence of $\text{(U, Pu)O}{}_{\mathrm{-x}}$in no way alters these observed phenomena. Thus the corrosive action of iodine on steel does not require the presence of oxygen. In irradiated fuel, free iodine (or iodine liberated by the decomposition of caesium iodide, a compound of fission products) can take part in the development of the reaction between fuel and cladding by demanding the latter of chromium, which reduces its resistance to oxidation by $\text{(U, Pu)O}{}_{\mathrm{2-x}}$.     

Etude de la formation des cavites dtrradiation dans le cuivre IV-etude des mecanismes

The analysis of experimental results concerning the formation of irradiation-voids in copper bombarded by electrons, by copper ions and by neutrons has been performed by means of a computer simulation model by the Monte Carlo method. The computations show that the nucleation of voids results from competition between the growth of vacancy aggregates formed in displacement spikes, the formation of vacancy aggregates by precipitation of mobile vacancies and the diffusion of gas towards vacancy aggregates already formed or in course of formation. Moreover, the theoretical curves representing the variation of swelling with temperature, for irradiation with electrons, ions and neutrons, are in excellent agreement with experimental results.     

Influence des conditions de trempe sur le gonflement par irradiation aux neutrons d'un acier austenitique au niobium

Steels of type AISI 316 and 318 which had been subjected to different heat-treatments were neutron-irradiated at 450 °C to a fluence of 5 × 10²¹ n/cm². Density measurements after irradiation reveal a swelling of 0.15% for AISI 316 and 318 steels quenched from 1050 °C and tempered at 700 °C. A homogeneous dispersion of niobium carbide does not appear to influence the amount of swelling. On the other hand, the AISI 318 steel hyperquenched from 1270 °C and tempered at 700 °C swells to an extent of only 0.025%. Examination by electron microscopy showed that this reduction in swelling is associated with the dispersion of the fine niobium carbide precipitates on extrinsic stacking faults, as a consequence of the heat-treatment. It was noted that voids were absent in the neighbourhood of these stacking faults with their associated precipitates.     

Calcul d'energie de fautes d'empilement dans le dioxyde d'uranium

Energy computations of some (100), (110) and (111), planar defects were performed using an ionic bond model for stoichiometric uranium dioxyde. The repulsive contribution to the fault was estimated in two different ways, i.e. using the Born-Mayer classical treatment, or potentials derived from shell model calculations. The stability of the various defect configurations has been studied; on the basis of our numerical values, we may conclude that dislocation dissociation is unlikely in stoichiometric uranium dioxyde.     

Etude de la reaction du magnesium massif avec l'air

The reaction of magnesium with air follows very irregular kinetics; periods of slow and rapid growth alternate in random fashion. We show that initially the metal reacts simultaneously with nitrogen and oxygen. A nitride layer is formed next to the metal and an oxide layer next to the gas. This double layer becomes temporarily protective, which leads to a substantial slowing down of the rate of weight increase. Sudden cracking entails a rapid acceleration of both oxydation and nitridation, giving rise to two surface layers. An irregular sequence of several fast and slow stages leads to the development of a sharply layered film. The cracks formed when the oxide layer is thicker are gradually sealed by the products of vapour-phase reactions. The total layer then acquires a distinct protective power, and the gases can no longer reach the metal surface. The reaction then starts again by the oxidation of the strata of nitride formed earlier and proceeds at a slow but regular pace.     

Contribution a l'etude des cinetiques de l'oxydation a l'air du hafnium

The oxidation kinetics in air of Van Arkel hafnium were studied between 750 and 950°C. Comparison of the results obtained by thermogravimetry and microhardness measurements has allowed us to characterise the overall oxidation kinetics which are of parabolic type and also the two separate kinetics, likewise parabolic, of growth of the oxide film and of dissolution of oxygen in the metal underlying the oxide. For each of these three kinetics the rate constants are respectively given by the following relations: $\text{K}{}_{\mathrm{p}}\text{= 3.1 × 10}{}^8\text{exp (?46 000/RT)}$, $\text{K}{}_1\text{= 1.4 × 10}{}^6\text{exp (?36 000/RT)}$, $\text{K}{}_2\text{= 3.4 × 10}{}^{10}\text{exp (?59 000/RT)}$.These results are in agreement with a classical diffusion model of oxygen, both in the oxide and in the metal, set up on the basis of Wagner's general theory. The activation energy of diffusion of oxygen in hafnium is $\text{53.0 ± 2}$ kcal/mole.     

Pic de relaxation associe aux interstitiels intrinseques dans le zirconium deforme a basse temperature

Internal-friction measurements carried out at three widely different frequencies (≈1, 16 and 350 Hz) have allowed the detailed study of a damping peak in zirconium deformed at low temperature and not allowed to warm up beyond ?68°C. The values obtained for the activation energy and the frequency factor (respectively $\text{0,27 ± 0.02}$ eV and $\text{10}{}^{\mathrm{13\pm 0.8}}\text{sec}{}^{\mathrm{?1}}$), the narrowness of the peak — which allows it to be defined in terms of two simple relaxation times — and the fact that disappearance of this peak at ?68°C is not directly consistent with a dislocation-pinning mechanism, lead to the postulate of a relaxation mechanism involving point defects of a paired oxygen self-interstitial type formed during low-temperature deformation.     

Etude de la formation des cavités dans le cuivre irradie aux ions cuivre de 500 keV

We have studied the formation of voids in thin foils of copper irradiated by 500 keV Cu⁺ ions, as a function of temperature, irradiation intensity and the dissolved gas content of the samples. A theoretical examination of the phenomena gives a good account of the effects of temperature, the rate of defect creation and the foil thickness, but it fails to account for the fact that at 400° C voids are formed for a dose of 3 × 10^?4 dpa/s but not for 3 × 10^?3 dpa/s. This is tentatively explained in terms of the influence of the gas content. The effect of gas content on the formation of voids and dislocation loops has moreover been demonstrated by degassing thin foils by annealing in ultrahigh vacuum, or by implanting helium before irradiation.     

Techniques de mesure des coefficients d'heterodiffusion dans le plutonium. Application a la diffusion du cobalt dans le plutonium ϵ(cc)

The diffusion of ⁶⁰Co in bec ? plutonium has been studied by the sectioning method, and the following results have been obtained: D = 1.4 × 10^?3 exp (?9900/RT) cm²/sec over the temperature range 484–621°C. Cobalt diffuses rapidly in ? Pu. Since the diffusion coefficient does not change across the phase transition ? Pu (fcc) → ? (bcc), the diffusion mechanism must be dissociative in the two phases.     

Etude de la forgeabilite et de la resistance a la deformation de l'alliage U-0,2% V par essais de torsion entre 400 et 800°C

We have measured by hot torsion the resistance to deformation and the ductility of U/0.2wt% V alloy between 400 and 800°C for strain rates in the range 10^?2 ?3.6 sec^?1. In this temperature range, the alloy shows three successive distinct phases — orthorhombic α, tetragonal β, cubic γ — which behave differently. We found two useful ranges for hot-working of this alloy: in the α range below 600°C and in the γ range. By means of plasticity theory we have established the equivalence of hot torsion and hot tension and simulated a first rolling pass. Some tensile tests between 600 and 800°C and a rolling test at 600°C have shown good justification for the transition from torsion to other modes of deformation. A metallographic study after fracture showed the existence of recrystallisation in the α phase above 600°C and also in the γ phase, which explains the good hot workability.