O and N dissolve in monocarbide by substituting carbon or by occupying vacant C-sites in the lattice. For example, in PuC1-x, the small oxygen atoms can easily fill the vacant carbon sites, leading to a com­pound close to stoichiometric. PuC can accommo­date more oxygen (up to 78mol% PuO) than UC (<35mol% UO), probably because of the smaller size of the Pu atoms.9

Solid compact plutonium carbide has been observed to react slowly with air between room tem­perature and 573 K. However, it can burn in pure oxygen at 673 K.9,224 Pu2C3 was observed to be some­what more stable than the other Pu carbides with respect to oxidation.

The pseudobinary PuC-PuO system follows a nearly ideal solution behavior. Anselin et a/.225 measured the evolution of the PuC1-x lattice param­eter (in the presence ofmetallic Pu) with the addition of oxygen. They noticed a first rapid increase (from

496.0 to 497.3 pm) between 0 and 20 mol% PuO. This behavior was explained as resulting from a change in the actual C/Pu ratio and from lattice expansion following the occupation of vacant sites. Vegard’s law was then followed for composition richer in oxy­gen. The lattice parameter varied from 497.3 pm at 20mol% PuO to 495.6 pm at 78mol% PuO, where the solubility limit was reached (Figure 26). Extra­polated values agree with literature data on the pure compounds.

The same investigation carried out on the pseu­dobinary PuC-PuO2 showed very limited variation of the lattice parameter upon oxygen addition.225

XRD and chemical analyses of the Pu-C-O sys­tem have shown that both monocarbide and ses — quicarbide of plutonium are hypostoichiometric at low oxygen content and become stoichiometric at high oxygen content (>6000 ppm oxygen). In the biphasic mixed carbide system, MCO + MC15, cal­culations indicate that carbon activity increases with ‘O’ substitution in the monocarbide. This carbon activity increase is, however, less pronounced than it is in U-rich fuel, due to the higher tolerance of ‘O’ substitution in PuC1-x, which also implies a lower pCO in Pu-rich fuels.

PuC and PuN form solid solutions. As in the case of the Pu-C-O system, the high vacancy concentra­tion of PuC and the preferential formation of Pu2C3

lead to important deviations from Vegard’s law in the C-rich part of the PuC-PN pseudobinary system (Figure 27).9 The PuC hypostoichiometry is curtailed at high temperature by the addition of nitrogen, espe­cially near the PuN side. N addition increases the carbon activity and reduces the actinide activity in monocarbides. Moreover, nitrogen was observed to stabilize PuC2 below its decomposition temperature.9