VAPOR-PRESSURES OF U(G) AND CE(G) OVER (U((1-Y))CE-Y)C (Y=0.10, 0.21,0.24) DETERMINED BY KNUDSEN EFFUSION MASS-SPECTROMETRY

The vapour pressures of Ce(g) and U(g) over mixed uranium and cerium c arbides were determined by measuring the partial pressures of Ce(g) an d U(g) species in the temperature range 1491-2233 K. The Ce content va ried from 11 to 24 mol%. As the samples contained different quantities of non-metals such as O and C, the compositions of mixed carbides bel onged to different phase fields. On the basis of vapour pressures and composition, the dependence on the partial pressures in different phas e fields were analysed. The partial pressures of Ce(g) and U(g) for di fferent sample specifications are given by the following equations (U0 .895Ce0.105)C0.908N0.072O0.017 log p(Pa)(U)+/-0.04=11.60-28122/T(K) (1 799-2220K) log p(Pa)(Ce)+/-0.05=9.67-22592/T(K) (1593-2233K) (U0.791Ce 0.209)C0.991N0.00O0.002 log p(Pa)(U)+/-0.06=10.76-26421/T(K) (1730-206 4 K) log p(Pa)(Ce)+/-0.04=9.28-19974/T(K) (1491-2022 K) (U0.756Ce0.244 )C1.045N0.0241O0.013 log p(Pa)(U)+/-0.05=10.57-26259/T(K) (1871-2113 K ) log p(Pa)(Ce)+/-0.03=9.80-19264/T(K) (1712-2100 K) The chemical acti vities of U a(U) and Ce a(Ce) were evaluated. Values of a(U) at 2100 K for the three samples respectively are 0.32, 0.29 and 0.23, whereas t he values of a(Ce) at 2100 K are 0.018, 0.127 and 0.91. The high a(Ce) value for 24 at.% Ce seems to indicate the presence of almost pure li quid Ce metal phase.