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Thermodynamic Behaviour of Hypostoichiometric UO{sub 2}; Comportement Thermodynamique de UO{sub 2} HypostoeChiometrique; Termodinamicheskoe povedenie gipostekhiometricheskoj UO{sub 2}; Comportamiento Termodinamico del UO{sub 2} Subestequiometrico

Abstract

The ability of the UO{sub 2}-type structure to accomodate excess oxygen is well known. Recent evidence has indicated that this structure is stable also in the hypostoichiometric state at high temperatures and low oxygen partial pressures, but its manifestation occurs as a uranium metal precipitate in the oxide after cooling from high temperatures. This paper presents further evidence of the existence, at high temperatures, of a stable hypostoichiometric urania and describes in part the variation in thermodynamic properties across its homogeneity range. Hypostoichiometric UO{sub 2} evaporates congruently during free vaporization in slowly flowing hydrogen (-40 Degree-Sign C dew point) at 2400 Degree-Sign C at a composition having oxygen-to-uranium ratio of 1.88. If the temperature is decreased or the moisture content (oxygen partial pressure) increased, the congruent composition increases. The water content of the hydrogen at 2400 Degree-Sign C must be at least one per cent to maintain stoichiometric uranium dioxide. When UO{sub 2} pellets are sealed in tantalum cans and heated above 1700 Degree-Sign C, the O/U ratio of the pellet changes and reaches an equilibrium value which is governed by the oxygen activity of the atmosphere surrounding the can. UO{sub 2} does not react with tantalum but, because of  More>>
Authors:
Aitken, E. A.; Brassfield, H. C.; Fryxell, R. E. [1] 
  1. General Electric Company, Nuclear Materials and Propulsion Operation, Cincinnati, OH (United States)
Publication Date:
Feb 15, 1966
Product Type:
Conference
Report Number:
IAEA-SM-66/83
Resource Relation:
Conference: Symposium on Thermodynamics with Emphasis on Nuclear Materials and Atomic Transport in Solids, Vienna (Austria), 22-27 Jul 1965; Other Information: 22 refs., 8 figs., 2 tabs.; Related Information: In: Thermodynamics. Vol. II. Proceedings of the Symposium on Thermodynamics with Emphasis on Nuclear Materials and Atomic Transport in Solids| 673 p.
Subject:
36 MATERIALS SCIENCE; ARGON; CONTROLLED ATMOSPHERES; DEW POINT; FREE ENERGY; FUEL PELLETS; HUMIDITY; HYDROGEN; INTERSTITIALS; OXYGEN; PARTIAL PRESSURE; PRECIPITATION; SELF-DIFFUSION; SOLUBILITY; STATISTICAL MODELS; STOICHIOMETRY; TANTALUM; TEMPERATURE RANGE 0400-1000 K; THORIUM OXIDES; URANIUM; URANIUM DIOXIDE; VACANCIES
OSTI ID:
22121923
Research Organizations:
International Atomic Energy Agency, Vienna (Austria); International Union of Pure and Applied Chemistry, Commission on Thermodynamics and Thermochemistry, Research Triangle Park, NC (United States)
Country of Origin:
IAEA
Language:
English
Other Identifying Numbers:
Other: ISSN 0074-1884; TRN: XA13M2419078034
Submitting Site:
INIS
Size:
page(s) 435-453
Announcement Date:
Aug 27, 2013

Citation Formats

Aitken, E. A., Brassfield, H. C., and Fryxell, R. E. Thermodynamic Behaviour of Hypostoichiometric UO{sub 2}; Comportement Thermodynamique de UO{sub 2} HypostoeChiometrique; Termodinamicheskoe povedenie gipostekhiometricheskoj UO{sub 2}; Comportamiento Termodinamico del UO{sub 2} Subestequiometrico. IAEA: N. p., 1966. Web.
Aitken, E. A., Brassfield, H. C., & Fryxell, R. E. Thermodynamic Behaviour of Hypostoichiometric UO{sub 2}; Comportement Thermodynamique de UO{sub 2} HypostoeChiometrique; Termodinamicheskoe povedenie gipostekhiometricheskoj UO{sub 2}; Comportamiento Termodinamico del UO{sub 2} Subestequiometrico. IAEA.
Aitken, E. A., Brassfield, H. C., and Fryxell, R. E. 1966. "Thermodynamic Behaviour of Hypostoichiometric UO{sub 2}; Comportement Thermodynamique de UO{sub 2} HypostoeChiometrique; Termodinamicheskoe povedenie gipostekhiometricheskoj UO{sub 2}; Comportamiento Termodinamico del UO{sub 2} Subestequiometrico." IAEA.
@misc{etde_22121923,
title = {Thermodynamic Behaviour of Hypostoichiometric UO{sub 2}; Comportement Thermodynamique de UO{sub 2} HypostoeChiometrique; Termodinamicheskoe povedenie gipostekhiometricheskoj UO{sub 2}; Comportamiento Termodinamico del UO{sub 2} Subestequiometrico}
author = {Aitken, E. A., Brassfield, H. C., and Fryxell, R. E.}
abstractNote = {The ability of the UO{sub 2}-type structure to accomodate excess oxygen is well known. Recent evidence has indicated that this structure is stable also in the hypostoichiometric state at high temperatures and low oxygen partial pressures, but its manifestation occurs as a uranium metal precipitate in the oxide after cooling from high temperatures. This paper presents further evidence of the existence, at high temperatures, of a stable hypostoichiometric urania and describes in part the variation in thermodynamic properties across its homogeneity range. Hypostoichiometric UO{sub 2} evaporates congruently during free vaporization in slowly flowing hydrogen (-40 Degree-Sign C dew point) at 2400 Degree-Sign C at a composition having oxygen-to-uranium ratio of 1.88. If the temperature is decreased or the moisture content (oxygen partial pressure) increased, the congruent composition increases. The water content of the hydrogen at 2400 Degree-Sign C must be at least one per cent to maintain stoichiometric uranium dioxide. When UO{sub 2} pellets are sealed in tantalum cans and heated above 1700 Degree-Sign C, the O/U ratio of the pellet changes and reaches an equilibrium value which is governed by the oxygen activity of the atmosphere surrounding the can. UO{sub 2} does not react with tantalum but, because of the high solubility of oxygen in tantalum, the latter functions as a membrane. Using the data from congruent evaporation, and tantalum capsule tests, conducted in various argon-hydrogen mixtures, the oxygen activity in urania as a function of stoichiometry has been determined. The partial molar free energy of oxygen, G(O{sub 2} ), increases almost linearly on the oxygen deficient side with increasing oxygen-to-uranium ratio. Near the stoichiometric composition G(O{sub 2}) rises steeply. Using these results together with estimated G(O{sub 2}) values on the oxygen excess side obtained from the literature, it is shown that the data at a given temperature are consistent with a statistical model. The model assumes that departure from the stoichiometric composition is accommodated by an excess of oxygen atoms occupying interstitial positions or by a deficiency of oxygen on the oxygen sub-lattice. Disordering occurs with the oxygen interstitials and oxygen vacancies. The Frenkel energy of the oxygen is 60 kcal/g-atom which agrees favourably with previous estimates from oxygen self-diffusion data. The relationship between G(O{sub 2}) and the partial pressure of the gaseous oxides, UO and UO{sub 2} , is discussed and some general comments are made on results obtained, using these experimental methods, on other compounds such as ThO{sub 2}. (author) [French] La possibilite pour une structure de type UO{sub 2} d'admettre un exces d'oxygene est bien connue. Des observations recentes indiquent que cette structure est stable egalement a l'etat hypostoechiometrique a haute temperature et sous basse pression partielle d'oxygene, mais elle se manifeste sous forme de precipites d'uranium metallique dans l'oxyde apres refroidissement. Les auteurs donnent de nouvelles preuves de l'existence a haute temperature d'une urane hypostoechiometrique stable et decrivent en partie les variations des proprietes thermodynamiques dans l'intervalle d'homogeneie. UO{sub 2} hypostoechiometrique s'evapore de facon congruente au cours d'une vaporisation libre dans un courant lent d'hydrogene (point de rosee -40 Degree-Sign C) a une temperature de 2400 Degree-Sign C, sa composition correspondant a un rapport entre l'oxygene et l'uranium egal a 1,88. Si l'on diminue la temperature ou que l'on augmente la teneur en eau (pression partielle de l'oxygene), la composition congruente augmente. La teneur en eau de l'hydrogene a 2400 Degree-Sign C doit etre d'au moins 1% opour maintenir le bioxyde d'uranium a l'etat stoechiometrique. Lorsqu'on scelle des pastilles d'UC dans des capsules en tantale et qu'on les chauffe au-dessus de 1700 Degree-Sign C, le rapport O/U des pastilles varie et atteint une valeur d'equilibre qui depend de l'activite de l'oxygene dans l'atmosphere qui entoure la capsule. UO{sub 2} ne reagit pas avec le tantale, mais a cause de la forte solubilite de l'oxygene dans ce material, ce dernier joue le role d'une membrane. En utilisant les donnees provenant de l'evaporation congruente et en procedant a des essais dans des capsules de tantale avec divers melanges argon-hydrogene, les auteurs ont determine l'activite de l'oxygene dans l'urane en fonction de la stoechiometrie. L'energie libre molaire partielle de l'oxygene, G(O{sub 2}), augmente d'une maniere presque lineaire du cote pauvre en oxygene lorsqu'on augmente le rapport O/U. Au voisinage de la composition stoechiometrique, G(O{sub 2} ) augmente brusquement. En utilisant ces resultats ainsi que les valeurs estimatives de G(O{sub 2}) deja publiees pour le cote riche en oxygene, on montre que les donnees pour une temperature donnee sont en bon accord avec le modele statistique. Le modele implique l'hypothese que l'ecart par rapport a la composition stoechiometrique est cause par un exces d'atomes d'oxygene occupant des positions interstitielles ou par un defaut d'oxygene dans le sous-reseau de cet element. Il se produit un desordre des atomes interstitiels d'oxygene et des lacunes d'oxygene. L'energie de Frenkel de l'oxygene est de 60 kcal/at.-g ce qui concorde bien avec les estimations precedentes obtenues a partir de donnees sur l'autodiffusion de l'oxygene. Les auteurs discutent la relation entre G(O{sub 2}) et la pression partielle des oxydes gazeux UO et UO{sub 2}. Ils formulent enfin quelques observations d'ordre general sur les resultats qu'ils ont obtenus en utilisant ces methodes experimentales avec d'autres composes tels que ThO{sub 2}. (author) [Spanish] Es bien sabido que las estructuras tipo UO{sub 2} son capaces de admitir un exceso de oxigeno. Las pruebas recientes indican que esta estructura es tambien estable en estados subestequiometricos a elevadas temperaturas y bajas presiones parciales de oxigeno, pero se manifiesta como precipitado de uranio metalico en el oxido despues del enfriamiento. Los autores presentan en la memoria nuevas pruebas de la existencia de un oxido de uranio subestequiometrico estable a altas temperaturas y describen la variacion de las propiedades termodinamicas en su intervalo de homogeneidad. El UO{sub 2} subestequiometrico se evapora de manera congruente durante la vaporizacion libre en una lenta corriente de nitrogeno (punto de rocio -40 Degree-Sign C) a 2400 Degree-Sign C, con una composicion cuya relacion oxigeno/uranio ascienda a 1,88. Al disminuir la temperatura o crecer el contenido de humedad (presion parcial de oxigeno), aumenta la composicion congruente. Para mantener el dioxido de uranio en condiciones subestequiometricas, el contenido de agua del hidrogeno a 2400 Degree-Sign C no debe ser inferior a 1%. Cuando se colocan pastillas de UO{sub 2} en recipientes hermeticos de tantalo y se las calienta por encima de 1700 Degree-Sign C, la relacion O/U de las pastillas cambia y alcanza un valor de equilibrio que depende de la actividad de oxigeno de la atmosfera circundante. El UO{sub 2} no reacciona con el tantalo, pero debido a la alta solubilidad del oxigeno, el metal trabaja como una membrana. Usando los datos provenientes de la evaporacion congruente y de ensayos en capsulas de tantalo, realizados con diversas mezclas argon-hidrogeno, los autores determinaron la actividad de oxigeno en oxidos de uranio, en funcion de la estequiometria. La energia libre molar parcial del oxigeno, G(O{sub 2}), aumenta en el lado pobre en oxigeno en forma proporcional a la relacion oxigeno/uranio. Al approximatse a la composicion estequiometrica, G(O{sub 2}) aumenta bruscamente. Usando estos resultados junto con los valores estimados de G(O{sub 2}) que se encuentran en la literatura tecnica para el lado rico en oxigeno, se demuestra que los datos correspondientes a una temperatura determinada concuerdan con un modelo estadistico. El modelo supone que la desviacion respecto de la composicion estequiometrica se produce por un exceso de atomos de oxigeno en posiciones intersticiales, o bion por un defecto de oxigeno en el subreticulo de este elemento. El orden de los atomos intersticiales y de los huecos de oxigeno se altera. La energia de Frenkel es de 60 kcal/atomo gramo para el oxigeno, lo que concuerda satisfactoriamente con evaluaciones anteriores basadas en datos de autodifusion del oxigeno. Los autores analizan en la memoria la relacion existente entre G(O{sub 2}) y la presion parcial de los oxidos gaseosos UO y UO{sub 2}, y formulan algunas observaciones generales sobre los resultados obtenidos al aplicar estos metodos experimentales a otros compuestos, tales como el ThO{sub 2}. (author) [Russian] Sposob- nost' struktury tipa UO{sub 2} razmeshhat' izbytok kisloroda horosho izvestna. Nedavnij opyt pokazal, chto jeta struktura stabil'na takzhe v stehiometricheskom sostojanii pri vysokih temperaturah i parcial'nyh nizkih davlenijah kisloroda, no ona projavljaet sebja, kogda metal- licheskij uran osedaet v okisi posle ostyvanija ot vysokih temperatur, Jeta rabota opisy- vaet dal'nejshie dokazatel'stva sushhestvovanija pri vysokih temperaturah stabil'noj gipo- stehiometricheskoj okisi urana i chastichno opisyvaet izmenenie termodinamicheskih svojstv v oblasti ee gomogennosti. Gipostehiometricheskaja UO{sub 2} kongrujentno vyparivaetsja v hode svobodnogo isparenija v medlenno tekushhem vodorode (tochka rosy - 40 Degree-Sign S) pri 2 400 Degree-Sign S pri sosta- ve s otnosheniem kislorod-uran, ravnym l;sli temperatura umen'shaetsja ili soderzhanie vlagi (chastichnoe davlenie kisloroda) uvelichivaetsja, to uvelichivaetsja i kongrujentnyj sostav. Soderzhanie vody v vodorode pri 2 400 Degree-Sign S dolzhno sostavljat' po men'shej mere odin procent dlja podderzhanija stehiometricheskoj dvuokisi urana. Kogda tabletki UO{sub 2} germetizirujutsja v tantalovyh obolochkah i nagrevajutsja do temperatury svyshe 1 700 Degree-Sign S, otnoshenie o/i tab- letki menjaetsja i dostigaet ravnovesija, opredeljaemogo aktivnost'ju kisloroda atmosfery, okruzhajushhej obolochku. UO{sub 2} ne reagiruet s tantalom, noiz-zavysokoj rastvorimosti kislo- roda v tantale poslednij igraet rol' membrany. Ispol'zuja dannye kongrujentnogo vypari- vanija i opytov s tantalovoj obolochkoj, provedennyh v razlichnyh smesjah argon - vodorod, byla opredelena aktivnost' kisloroda v okisi urana kak funkcija stehiometrii. Chastichnaja moljarnaja svobodnaja jenergija kisloroda G(O{sub 2}) vozrastaet pochti linejno pri nedostatke kislo- roda s vozrastaniem otnoshenija kislorod-uran. Kogda sostav priblizhaetsja k stehiometri- cheskomu, velichina G(O{sub 2}) rastet ochen' rezko. Ispol'zuja jeti rezul'taty vmeste s ocenochny- mi velichinami G(O{sub 2}) pri izbytke kisloroda, vzjatymi iz literatury, pokazano, chto dannye pri opredelennoj temperature soglasujutsja so staticheskoj model'ju. Jeta model' dopuskaet, chto otklonenie ot stehiometricheskogo sostava proishodit i z-zaizbytka atomov kisloroda, zanimajushhih mezhdouzlija, ili i z - z a nedostatka kisloroda na kislorodnoj podreshetke. Razu- porjadochenie proishodit s vnutrennimi atomami kisloroda i s kislorodnymi vakansijami. Jenergiju kisloroda po Frenkelju sostavljaet 60 kkal/gramm-atom, chto horosho soglasuetsja s ocenkami, poluchennymi iz dannyh kislorodnoj samodiffuzii. Rassmatrivaetsja svjaz' mezhdu G(O{sub 2}) I chastichnym davleniem gazoobraznyh okisej, UO i UO{sub 2} i v obshhem plane kommentirujut- sja rezul'taty opytov s drugimi soedinenijami, takimi kak ThO{sub 2} provedennyh s ispol'zova- niem teh zhe jeksperimental'nyh metodov. (author)}
place = {IAEA}
year = {1966}
month = {Feb}
}