In this section, the vapor pressure of a metal gas over a solid actinide nitride is summarized.

aUnder 1 atm of nitrogen. bNot available.

cCongruent melting under 1 atm of nitrogen.

The major vapor species observed over UN are nitrogen gas, N2(g), and mono-atomic uranium gas, U(g). UN(g) can also be detected in addition to U(g) and N2(g),60 but its pressure is three orders of mag­nitude lower than that of U(g); therefore, the contri­bution of UN(g) can be ignored in practice. Some data on the pressures of N2(g) and U(g) over solid UN(s) are shown in Figure 14.

Hayes et a/.46 have derived equations for the pres­sures of N2(g) and U(g), which were developed by fitting the data from eight experimental investigations. According to that paper, the reported data on N2(g) agree with each other, but that of U(g) over UN(s) vary somewhat. The U(g) pressure obtained by Suzuki et a/61 is a little higher than that predicted from the equation developed by Hayes et a/., but is in agreement with values given by Alexander et a/.62 It is well known that the evaporation of UN is accompanied by the precipitation of a liquid phase, where UN(s) = U(1) + 1/2N2(g) and U(1) = U(g). The reported vapor pressure of U(g) over UN(s) is close to or a little lower than that over metal U63 It is suggested that the dissolution of nitrogen and/or impurity metal from crucibles into the liquid phase could affect the observed partial pressure. Some scat­tering of the previously reported data on U(g) over UN(s) may be also caused by a reaction of the liquid phase in UN with the crucible material. From this viewpoint, it appears that the partial pressure of U(g)

Figure 14 Partial pressure of N2(g) and U(g) over UN (s) as a function of temperature. Adapted from Hayes, S. L.; Thomas, J. K.; Peddicord, K. L. J. Nucl. Mater. 1990, 171, 300-318; Suzuki, Y.; Maeda, A.; Arai, Y.; Ohmichi, T. J. Nucl. Mater. 1992, 188, 239-243;

Alexander, C. A.; Ogden, J. S.; Pardue, W. H. J. Nucl. Mater. 1969, 31, 13-24.

over UN(s) should be a little higher than that pro­posed by Hayes et a/.

The vapor species over PuN are nitrogen gas N2(g) and mono-atomic plutonium gas Pu(g). PuN(g) is not detected because PuN is more unstable than UN.64 The N2 pressure over PuN has been reported by Alexander et a/.,65 Olson and Mulford,42

Pardue eta/.,66 and Campbell and Leary.67 These data seem to agree with each other.

The vapor pressure of Pu(g) over PuN(s), as a function of temperature, is shown in Figure 15. The values reported in the different studies almost completely agree with each other.61,68,69

According to Alexander et a/.,65 the ratio Pu(g)/ N2(g) is 5.8 throughout the investigated temperature range of 1400-2400 K, which suggests that PuN eva­porates congruently; that is, PuN(s) = Pu(g) + 1/2N2(g). However, Suzuki eta/, have reported that Pu(g) over PuN(s), at temperatures lower than 1600 K, is a little higher than the values extrapolated from the high-temperature data, and that it approaches that over Pu metal with further decrease in tempera­ture. There is some possibility that a liquid phase forms at the surface of the sample during the cooling stages of the mass-spectrometric measurements, because PuN has a nonstoichiometric composition range at elevated temperatures, while it is a line compound at low temperatures.

The vapor pressure of U(g) and Pu(g) over UN(s), (U, Pu)N(s), and PuN(s) are shown in Figure 16. Table 3 gives the vapor pressures of U(g) and Pu(g) which are represented in Figure 16 in the form of logarithmic temperature coefficients. It is noteworthy that the vapor pressure of Pu(g) over mixed nitride was observed to increase with an increase in the PuN content.

Nakajima eta/.70 have measured the vapor pressure of Np(g) over NpN(s) in the temperature range of 1690-2030 K by using the Knudsen-cell effusion mass spectrometry. This data is plotted in Figure 17 as a function of temperature. The partial pressure of Np (g) can be expressed using the following equation:

logp Np(g)(Pa) = 10.26 — 22 200/T [7]

The vapor pressures of Np(g) over NpN(s) obtained by Nakajima et a/. are similar to those of Np(g) over

104/T (K-1)

liquid Np metal found by Ackermann and Rauh71; these all are shown in Figure 17. Therefore, the decomposition mechanism is considered to be the following reaction: NpN(s) = Np(1) + 1/2N2(g), Np(1) = Np(g).

Takano et al.72 have estimated the vapor pressure of Am(g) over AmN by using values of the Gibbs free energy of formation available in literature.34 The evaporation of AmN obeys the following reac­tion: AmN(s) = Am(g) + 1/2N2(g). The estimated vapor pressure of Am over AmN, expressed as a function of temperature, is

logpAm(g)(Pa) = 12.913 — 20197/T

(1623 <T(K) < 1733) [8]

The calculated vapor pressures of Am over AmN are plotted in Figure 17 as a function of temperature. The vapor pressure of Am over AmN is higher than those of other actinide vapor species over their respective nitrides.