A combined calorimetric and computational study of the energetics of rare earth substituted UO2 systems

Abstract

The energetics of rare earth substituted UO 2 solid solutions (U 1-x Ln x O 2-0.5x+y , where Ln = La, Y, and Nd) are investigated employing a combination of calorimetric measurements and density functional theory based computations. Calculated and measured formation enthalpies agree within 10 kJ/mol for stoichiometric oxygen/metal compositions. To better understand the factors governing the stability and defect binding in rare earth substituted urania solid solutions, systematic trends in the energetics are investigated based on the present results and previous computational and experimental thermochemical studies of rare earth substituted fluorite oxides (A 1-x Ln x O 2-0.5x , where A = Hf, Zr, Ce, and Th). A consistent trend towards increased energetic stability with larger size mismatch between the smaller host tetravalent cation and the larger rare earth trivalent cation is found for both actinide and non-actinide fluorite oxide systems where aliovalent substitution of Ln cations is compensated by oxygen vacancies. However, the large exothermic oxidation enthalpy in the UO 2 based systems favors oxygen rich compositions where charge compensation occurs through the formation of uranium cations with higher oxidation states.

Thermodynamics of Fluorite-Structured Oxides Relevant to Nuclear Energy: A Review

“…Charge balance can also occur by host cation oxidation (U 4+ −U 5+ −U 6+ ). 141,142 Higher valence doping is not common although there are studies on doping of urania with Nb 2 O 5 to improve creep behavior. 143 In this case, it is likely that extra oxygen atoms go into interstitial positions or the system can be stabilized by localizing electrons on cation sites.…”

“…Mazeina et al 226 have shown for the UO 2 −CaO and UO 2 −YO 1.5 systems that partial oxidation of U to form phases in the UO 2 −UO 3 −CaO and UO 2 −UO 3 −YO 1.5 ternary systems significantly stabilizes the solid solutions, as demonstrated by the negative enthalpies of formation. Experiments by Zhang et al 56,141 later demonstrated that the uranium oxidation state can also be 5+ as an intermediate between UO 2 and UO 3 4.2.3. Multicomponent Systems.…”

“…However, when the dopant is of lower valence (as in Y 2 O 3 –ZrO 2 (YSZ) or Ln 2 O–CeO 2 systems , ), additional oxygen vacancies are introduced into the structure as a charge compensating mechanism. Charge balance can also occur by host cation oxidation (U 4+ –U 5+ –U 6+ ). , Higher valence doping is not common although there are studies on doping of urania with Nb 2 O 5 to improve creep behavior . In this case, it is likely that extra oxygen atoms go into interstitial positions or the system can be stabilized by localizing electrons on cation sites.…”

“…In urania systems, low valence ions are more flexible in size mismatch due to stabilization mechanisms such as the introduction and ordering of vacancies and the oxidation of U 4+ . The substitution of two U 4+ by two trivalent ions can be charge compensated by one vacancy, or in more oxidizing environments charge compensation can occur by oxidation of U 4+ to U 5+ for each trivalent substituent , (see Figure ).…”

Phase Segregation, Transition, or New Phase Formation of Plutonium Dioxide: The Roles of Transition Metals

“…For instance, H tends to form [O 2– H + ] − in PuO 2 by binding with lattice O whereas interstitial H + is involved in UO 2 . This difference results from the dual nature of 5f electrons: Pu 5f electrons in PuO 2 are more localized and less favorable to participate in further chemical bonding or vice versa for UO 2 . ,− In contrast, the substitutional metal site in fluorite-type oxides such as UO 2 , ThO 2 , and CeO 2 is determined to be the exclusive site for low-concentration transition-metal (TM) impurities. ,− This conclusion is proposed to hold for PuO 2 despite the fact that a comprehensive study on TM behaviors in PuO 2 is lacking; the only exception is the DFT energetics and structures of Fe in PuO 2 , which partially supports the conclusion …”