Oxygen Potentials and Defect Chemistry in Nonstoichiometric (U,Pu)O2

Report on technical activities and technical plan for university advanced manufacturing work for in-pile sensors

“…To accomplish this goal, two main tasks will need to be met: To access the micro-to millimeter length scales over which nanoparticle inks are printed during AJP, coarse grained models and hardware acceleration with graphics processing units (GPUs) will be used. GPUs are computing elements that can be added to traditional CPU-based computers that permit orders of magnitude speedup of parallelizable computations [9][10][11][12][13][14][15][16]. The barrier to accelerating molecular models with GPU hardware is implementing the parallelizable algorithms to utilize the GPU's features.…”

“…[10,[15][16][17] Cerium Oxide (CeO2) as a surrogate for plutonium oxide (PuO2) and uranium oxide (UO2) Obvious impediments to investigating and processing nuclear fuel forms involving plutonium, or its oxide, plutonia (PuO2), include the risks from toxicity, radioactivity, and proliferation concerns. [10,[16][17][18][19] Cerium oxide (CeO2) has been used historically as an inactive PuO2 surrogate, most commonly in studies of TRU/MOX fuels in the form of (U,Ce)O2 solid solutions as well as a plutonia analog in (Th,Ce)O2 systems) [10,14,[16][17][18][19][20][21][22][23][24][25][26][27][28][29]. Some of the characteristics which make CeO2 a viable surrogate for PuO2 are that Ce and Pu have similar ionic size in octahedral and cubic coordination, and their oxides in 3 + and 4 + valence states have similar melting temperatures, standard enthalpies of formation, and specific heat capacities [10,22,[30][31].…”

“…They noted that because CeO2 will readily reduce in air at typical experimental temperatures, introducing CeO2 into composites or alloys which reduces the oxygen activity will initiate vacancies that will further degrade the thermal conductivity of CeO2, thus potentially underestimating thermal transport behavior of composite systems using PuO2. [10] As noted by Kato in 2012, the variation in the stoichiometry of MOX fuels affects various properties like lattice parameter, melting temperature, and thermal conductivity resulting in several studies on the oxygen to metal (O:M) ratio dependence of oxygen potential [14,19,25,28]. These fuels can exist over a wide range of O:M ratios and the literature regarding the correlation of calculations of oxygen potential in (U,Pu)O2 to (U,Ce)O2 is limited and scattered due to the difficulty in measuring oxygen potential near the stoichiometric region [14].…”

“…[10] As noted by Kato in 2012, the variation in the stoichiometry of MOX fuels affects various properties like lattice parameter, melting temperature, and thermal conductivity resulting in several studies on the oxygen to metal (O:M) ratio dependence of oxygen potential [14,19,25,28]. These fuels can exist over a wide range of O:M ratios and the literature regarding the correlation of calculations of oxygen potential in (U,Pu)O2 to (U,Ce)O2 is limited and scattered due to the difficulty in measuring oxygen potential near the stoichiometric region [14]. The literature suggests that while similarities exist regarding the dependence of oxygen potential and lattice parameter on the O:M ratio, there are also differences, such as the oxygen potential at a given temperature being a function of plutonium valence, which will not apply to cerium [14,28].…”

“…These fuels can exist over a wide range of O:M ratios and the literature regarding the correlation of calculations of oxygen potential in (U,Pu)O2 to (U,Ce)O2 is limited and scattered due to the difficulty in measuring oxygen potential near the stoichiometric region [14]. The literature suggests that while similarities exist regarding the dependence of oxygen potential and lattice parameter on the O:M ratio, there are also differences, such as the oxygen potential at a given temperature being a function of plutonium valence, which will not apply to cerium [14,28]. Work by Byler et al investigated the effect of oxygen potential on the sinterability and thermophysical properties of ceria by tracking the O:M ratio as a function of temperature, partial pressure of oxygen (pO2), and heating rates, suggesting that strict control of pO2 is essential for controlling O:M ratios and thus identification of proper sintering paths.…”