C. Sari

A study of the ternary system U02-Pu02-Pu203

Pellets of uranium-plutonium mixed oxides with Pu/(U + Pu) ratios from 0.05 to 0.97 have been reduced in a hydrogen atmosphere at temperatures between 1400° C and 2000 °C. The results of ceramographic and X-ray diffraction analysis were used to construct the partial U-Pu-O phase diagram at room temperature for O/U + Pu) ⩽ 2.00. n nMixed oxides containing less than 20% Pu remain single phase fcc until all Pu has been reduced to the trivalent state. A large region containing two cubic phases extends from Pu/(U + Pu) − 0.20 up to the binary Pu-O system. These phases are a fcc phase with O/(U + Pu) = 1.985, in equilibrium with another fcc phase in the range 0.2 ⩽ Pu/(U + Pu) ⩽ 0.5 and with a bcc phase at higher plutonium contents. For Pu/(U + Pu) ⩾ 0.5 and low O/M values, a bcc single phase region extends up to Pu/(U + Pu) = 0.95. At Pu/(U + Pu) = 0.97 this phase is represented by a line with O/(U + Pu) − 1.51. n nA hexagonal phase of the β-Pu2O3 type was found for Pu/(U + Pu) ⩾ 0.97. The samples reduced below the line UO2.0−PuO1.5 contain a metallic phase. n nDTA measurements performed at temperatures up to 1500 °C and at different O/(U + Pu) ratios show that the region of two cubic phases has the shape of a tunnel whose top is at 620–640 °C in the binary Pu-O system and decreases progressively in the direction of increasing uranium content.

Oxygen redistribution in fast reactor oxide fuel

Abstract Out-of-pile tests were carried out in order to investigate the oxygen redistribution in uranium-plutonium mixed oxides exposed to a thermal gradient. In hypostoichiometric oxide fuel the oxygen migrates towards the low temperature region of the pellet and in hyperstoichiometric fuel the oxygen migrates in the opposite direction. The oxygen transport is explained on the basis of solid-state thermal diffusion and occurs via vacancies and interstitials. It has been shown that the heats of oxygen transport are a function of plutonium and uranium valencies for hypo- and hyperstoichiometric oxides, respectively. The experimental results allowed to construct a practical example in which oxygen profiles in fuel pins were calculated as a function of initial stoichiometry and burnup.

Phase relationships in the neptunium-oxygen system

Neptunium dioxide has been treated in reducing and oxidizing atmospheres between 570 and 2880 K. The results of ceramographic, X-ray diffraction, thermal and electron microprobe analysis were used to construct a partial neptunium-oxygen phase diagram. The highest valency of neptunium in oxides is four at temperatures above 900 K. The dioxide exhibits a hypostoichiometric homogeneity range (NpO2−x) at temperatures above 1300 K; on cooling to room temperature hypostoichiometric compositions decompose into NpO2 and metallic Np. The neptunium-oxygen system shows characteristics which are intermediate between those of the isotype uranium-oxygen and plutonium-oxygen systems.

A thermodynamic study of the uranium-americium oxide U0.5Am0.5O2 ± x

Abstract A gas equilibration method has been used to measure the oxygen potential of U 0.5 Am 0.5 O 2 ± x (1.87 ⩽ 2 ± x ⩽ 2.09) in the temperature range 873 to 1573 K. The resulting Δ G (O 2 ) data are presented as a function of temperature, oxygen content and metal valence. The partial molar enthalpy and entropy of oxygen have been calculated and compared with those of isotypic U-Pu and U-Ce oxides.

Redistribution behavior of various constituents in U-Pu-Zr alloy and U-Pu-Zr alloy containing minor actinides and rare earths in a temperature gradient

U-Pu-Zr based metallic fuel is one of the likely possible candidate to transmute minor actinides, such as Np, Am and Cm (MA). MA can be recovered from the high-level waste by pyroprocess, where the presence of rare earths (RE) will not be avoided. One of the most important phenomena observed on U-Pu-Zr metallic fuel type is the redistribution of the elements in a temperature gradient. In order to evaluate the redistribution, especially of MA and RE, three samples of one U-Pu-Zr alloy and two U-Pu-Zr-MA-RE alloys were heated in a temperature gradient and were analyzed by metallography, α-autoradiography and electron microprobe. The results indicated that Zr migrated to the colder region in both kinds of specimens, and that in U-Pu-Zr-MA-RE, Am and RE did not significantly move at temperatures below 1020 K, whereas Pu and Np concentrations slightly increased in the region around 900 K. By contrast, a significant depletion of Am and RE was observed in the hotter region above 970 K, while they migrated to the colder region of the sample, when the highest temperature in U-Pu-Zr-MA-RE was at above 1270 K. The transformation of the γ phase into a Zr-rich and a U-rich phases was observed in the hotter region, probably corresponding to the γ1-γ2 miscibility gap region in the U-Zr system.

Investigation of the operational limits of uranium-plutonium nitride fuels

Abstract The behaviour in a thermal gradient of uranium-plutonium nitride fuel, prepared by the carbothermic reduction process in a nitrogen atmosphere, has been investigated. Pellets with ca. 20% porosity and containing different amounts of oxygen impurity fractured during the rise in temperature, but their structure and composition were stable up to about 2000 K. Strong restructuring, plutonium depletion and pellet collapsing occurred at higher temperatures, however. Oxynitride type inclusions in nitride fuel, containing more than 1500 ppm oxygen, melted at about 2600 K.

Properties of lenticular pores in UO2, (U, Pu) O2 and PuO2

Abstract Lenticular pore migration rates in oxide muclear fuels were experimentally measured in out-of-pile heating experiments. It is deduced that those pores which are in part responsible for the formation of columnar grains, are only produced in the absence of relevant amounts of filling gas. Specimens containing important concentrations of He, produced by Pu alpha decay, show columnar grain restructuring by grain boundary migration. Some consequences are drawn concerning the possible role played by lenticular pores in the mechanisms of fission gas release from nuclear fuels.

Dense fuels in Europe

Abstract After a reduction of the activities on dense fuels in Europe during the late seventies, the interest in this fuel type was revived at the beginning of the eighties, when the economy of the closed FBR fuel cycle was reassessed. Strategies were developed to bring its economy into equilibrium with that of the LWR fuel cycle. The most important step was the decision to develop the European Fast Reactor (EFR) and to continue using oxide fuel at first. However, for economical reasons in an optimised fuel cycle a better fuel than the oxide should be employed. This fuel must avoid the deficiencies of the oxide but retain its advantages. Mixed nitride would satisfy this condition if it can be demonstrated that it can attain a burnup of at least 15 at%. With the know-how available in Europe it should be possible to achieve this goal with a rather limited research programme during the coming 10 years.

Interaction of U-Pu-Zr alloys containing minor actinides and rare earths with stainless steel

Abstract Isothermal tests and tests in a thermal gradient have shown that U-Pu-Zr alloys containing 5% of minor actinides (Np and Am) and rare earths (Nd, Ce and Y) with the Fe and Ni constituents of stainless steel interact to form compounds that melt below 1000 K. U preferentially interacts with Fe, whereas the rare earths preferentially interact with Ni. In the absence of U, Fe interacts with the rare earths and in this case the Fe and Ni interaction compounds formed melt at a temperature around 890 K. Cr does not actively participate in the formation of low melting compounds. The melting temperatures were established by dilatometry and the phases formed in the reaction layers were determined by electron probe microanalysis.

Oxygen potential of hypostoichiometric neptunium oxide between 1470 and 1850 K

Abstract The oxygen dissociation pressure of hypostoichiometric neptunium oxide was measured by the gas-equilibrium method in the 1470 to 1850 K range. The oxide was heated on a thermobalance in H 2 O H 2 gas mixtures. The partial relative molar free energy, enthalpy and entropy as a function of the O/Np-ratio are reported.