| Research Abstract |
Uranium dioxide doped with magnesium is expected to show better fuel performance during irradiation until high burnups. It is supposed to exhibit more preferable thermodynamic properties of less susceptibility to the accumulated fission products, the amount of which is considerable at high burnups. The dissolved magnesium would lessen the oxygen potential change by the FPs. Furthermore the FP gas release of Mg doped UO_2 fuel with larger grain size significantly reduced during irradiation in LWR. If such effect also occurs for Mg doped MOX, better fuel performance would be obtained. In the present study, the thermodynamic properties of MOX fuel can be estimated by examining quaternary solid solution, Mg_yCe_zU_<1-y-z>O_<2+x>, where the cerium solid solutions are used as a stand-in for studying the irradiation behavior of MOX fuel. The MgO-Ce_zU_<1-z>O_<2+x> phase relations as well as the solubility and crystallographic position of magnesium in the solid solution were studied. Oxygen po
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tential, (ΔGo_2) measurements were carried out and compared with those of other quaternary solid solutions. The solubility of Mg in Ce_<0.1>U_<0.9>O_<2+x> was about y = 0.05, and above this concentration MgO precipitated, forming a two-phase mixture. In Mg_yCe_zU_<1-y-z>O_<2+x> with y < 0.05, a part of the dissolved Mg atoms occupy the interstitial 4b position of fluorite structure. The ratio of the interstitial Mg to the total Mg increased from about 0.3 to 0.6 as the oxygen partial pressure decreased to 10^<-19> -10^<-21> atm. The rates of change of the lattice parameter, δa/δy, δa/δz, and δa/δx, for x greater than 0 were somewhat smaller in absolute vales than literature values. For x < 0, the rates δa/δz and δa/δx were in good agreement with the literature values. However, δa/δy was much smaller due to interstitial Mg. A plot of ΔGo_2 vs O/M ratio at 1000℃ showed a marked shift to lower x for Mg_yCe_zU_<1-y-z>O_<2+x> solid solution. The shift rate was -0.01 per 1 mol%Mg. The doing effect of Mg in Mg_yCe_zU_<1-y-z>O_<2+x> solid solution was found to be a little bit smaller than those in Mg_yEu_zU_<1-y-z>O_<2+x> and Mg_yGd_zU_<1-y-z>O_<2+x> solid solutions. Less
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