The sintering of uranium mononitride (UN) depends on temperature and the N, pressure maintained over the nitride during heat treatment. At a given temperature, an N, pressure that maintained the UN in the single-phase region slightly above the phase boundary where the reaction UN-U + sN2(g) occurred was most effective in accelerating the sintering of singlephase UN. For example, specimens sintered at 1600°C under N, pressures of either 1140 or 1.7~10-~ torr had essentially identical compositions, but the density of the former was 10.78 g/cm3 (75% of theoretical), whereas that of the latter was 12.20 g/cm' (W% of theoretical). Results were similar at temperatures up to 2100°C. The X-ray lattice constant of UN sintered at reduced N, pressures was slightly larger than that of UN sintered in 1140 torr of N,. The observed constants ranged from 4.88904 to 4.88991 A; the combined O + C content varied from 400 to 900 ppm.

Air oxidation of (U, Pu)O2 solid solutions having Pu/(U+Pu) ratios of 0.2 and 0.25 was studied using a recording thermal balance. For Pu/(U+Pu)=0.2, the original fluorite MO2 phase oxidized first to tetragonal M3O7; further oxidation at 450°C

A radial heat flow technique was used to measure the thermal conductivity, k, of polycrystalline UO2in the range ‐57° to 1100°C. The technique yielded results with a probable accuracy of ±1.5% and a precision of ±0.1% in the range 50° to 1100°C. Meaningful measurements were limited to 1100°C by Pt‐90 Pt10Rh thermocouple instability, although the apparatus was structurally sound to 1400°C. The thermal conductivity data up to 1000°C could be explained on the basis of heat transport by phonons. The thermal resistance, l/k, exhibits a linear temperature dependence from 200° to 100°C, which is expected for an insulator well above the Debye temperature. The slope of the l/k‐temperature plot is 0.0223 cm w−1 which is independent of impurity content and is associated with three phonon umklapp processes. The intercept is sensitive to impurity content as indicated by the fact that it was decreased by a decrease in the oxygen/uranium ratio. Between 1000° and 1100°C, there is a slight departure of l/k from linearity which may be due to the onset of an electronic contribution. Near room temperature, UO2 has a maximum in k which is apparently caused by the rapid decrease in specific heat below this temperature.

A description of a n improved r a d i a l h e a t flow technique for measuring t h e thermal conductivity of s o l i d s in the range-57 t o 1 1 0 0~~ is presented. T h e technique yielded results with a probable accuracy of f1.5% and a reproducibility of f0.1% in t h i s range.

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