|Budget Amount *help
¥1,400,000 (Direct Cost : ¥1,400,000)
Fiscal Year 1996 : ¥1,400,000 (Direct Cost : ¥1,400,000)
Regarding to advanced type oxide fuels for light water reactor, a study was made to develop the solid solution (M_y, U_<1-y>O_<2+X>) fuel in a metal M of lower valency than uranium in UO_2 is contained. Such fuels are expected to give better fuel performance enabling their use until high burn-ups with their characteristic properties that they have a wide range of nonstoichiometry in the x<O region (in contrary to UO_<2+X>) and the oxygen potential increase with increasing burnup is markedly low in this region as well as FP gas release rate. In this fiscal year, the study was focused to the solubility, impurity phases and p (O_2) for Mg_yNb_zU_<1-y-z>O_<2+x> and Mg_yEu_zU_<1-y-z>O_<2+x> solid solutions. For Mg_yNb_zU_<1-y-z>O_<2+x> it was found that Nb atoms were precipitated as NbO_2 or Mg_2NbO_6 at p(O_2)=10^<-13> and p(O_2)<10^<-19> atm with a very small solubility into UO_2 at 1200 ﾟC,suggesting that the improvement of the thermadynamic properties of fuel is difficult to be attained by the addition of Nb. On the other hand below p(O_2)<10^<-19> atm, an unreported phase was found to be formed, which turned out to be a new oxide with hexagonal crystal structure. Measurement of p(O_2) for Mg_yEu_zU_<1-y-z>O_<2+x> solid solution revealed that the O/(Eu+U) ratio, which gives the steepest change in p(O_2), was shifted to the x < O region. This phenomenon is supposed to take place for the solid solutions with devalent metals. Therefore, it is considered that Eu in the Eu_y, U_<1-y>O_<2+x> partly takes on divalent oxidation state. This shift was much larger for Mg_yEu_zU_<1-y-z>O_<2+x>, the effect of Mg being synergistic. However, the shift was approximately in proportion to the Mg concentration at 1200 ﾟC.