22.08.2015   Comprehensive nuclear materials

Thermal Properties

2.03.3.1 Melting or Decomposition

In this section, the melting points and decomposition temperatures of actinide mononitrides are discussed in conjunction with the nitrogen pressures because this behavior depends on the nitrogen partial pres­sure of the system. The vapor pressure of a metal gas over the solid nitride is discussed in the next section as ‘vaporization behavior.’

The liquid mononitride MN (liq.) can be observed when congruent melting occurs under a pressurized nitrogen atmosphere; otherwise the solid mononi­tride MN (s) decomposes into nitrogen gas and liquid metal that is saturated with nitrogen, according to the following reaction,

MN(s) = 1/2N2 + M(liq, sat. with N) [1]

Olson and Mulford have determined the decomposi­tion temperatures of ThN,41 UN,6 NpN,25 and PuN42 by the optical observation of the nitride granules when they were heated under controlled nitrogen pressures. Figure 11 shows the relationship between the nitrogen pressure p (atm) in logarithmic scale and the reciprocal decomposition temperature 1/ T(K — ). The solid curves show the following equations:

ThN : logp(atm) = 8.086 — 33224/T + 0.958 x 10-17 T5

(2689 < T(K) < 3063) [2]

UN : logp(atm) = 8.193 — 29540/T + 5.57 x 10-18T5 (2773 < T(K) < 3123) [3]

NpN: logp(atm) = 8.193 — 29540/T + 7.87 x 10-18 T5 (2483 < T(K)<3103) [4]

T (K)

3200 3000 2800 2600

image84

Figure 11 Decomposition pressures of ThN, UN, NpN, and PuN as a function of reciprocal temperature above 2500 K reported by Olson and Mulford.6’25’41’42

PuN : logp(atm) = 8.193 — 29540/T + 11.28 x 10-18 T5

(2563 < T(K) < 3043) [5]

The temperature at which the vertical rise in nitrogen pressure is observed for ThN, UN, and NpN corre­sponds to the congruent melting point, and is 3063 ± 30K for ThN (p> 0.7 atm), 3123 ± 30 K for UN (p> 2.5 atm), and 3103 ± 30K for NpN (p> 10 atm). The congruent melting for PuN was not achieved in the nitrogen pressure range up to 24.5 atm.

The presence of an oxide phase, as an impurity, seems to lower the melting point and decomposition temperature. In the case of ThN mentioned above, the melting point and decomposition temperature of a specimen containing 0.6 wt% oxygen fell by ^ 130 K from those of the oxygen-free specimens (^0.04 wt% oxygen). A similar experiment conducted by Eron’yan etal.43 with ZrN, a transition metal nitride that has the same crystal structure, has revealed a decrease in the melting point by 200-300 K when the oxygen content increased from 0.15 to 0.5-1.0wt%.

Some data sets on the equilibrium nitrogen pres­sure, in eqn [1] for UN and uranium carbonitride

image157

3600 3400

T (K)

3200 3000

2800

1 1

1 1

1

Timofeeva

Brundiers

TPRC

Smirnov

Houska

Hayes

да Takano и Aldred © Benedict ► Kruger

AmN-

 

10-3

 

16

 

14

 

10-4

 

12

 

10-5

 

(O

10

Ш

I—

О

8

 

4

 

Ф

CL

 

9

 

1273 K^ ■

 

10-6

 

x’ PuN

 

293 K NpN

293K’ UN, — ‘ о

TiN

 

10-

 

6

 

. ‘ ZrN HfN

 
image158

4.6 4.8 5

10000/ T (K-1)

 

4

2.6 2.8 3 3.2 3.4 3.6 3.8

10000/T (K-1)

Figure 13 Coefficients of linear thermal expansion at 293 (open symbols) and 1273 K (closed symbols) for some transition metal nitrides and actinide nitrides plotted against reciprocal decomposition temperature under 1 atm of nitrogen. For references see Table 2.

 

5.2 5.4 5.6

 

Figure 12 Decomposition pressures of U(C, N) as a function of reciprocal temperature below 2400 K. Solid lines by Ikeda et al.44 and broken lines by Prins et a/.45 Dotted line for UN reviewed by Hayes et a/.46

 

U(C, N), as measured by the Knudsen-cell and mass — spectroscopic technique at lower temperatures, are available and are shown in Figure 12. The dotted curve represents the correlation for UN developed by Hayes et a/.46 using eight data sets available in literature. The nitrogen pressure is

given as:

logp(atm) = 1.8216 + 1.882 x 10-3T — 23543.4/ T (1400 < T(K) < 3170) [6]

The N2 pressure for decomposition of UCi-xNx, as measured by Ikeda et a/44 and Prins et a/.45, decreases with a decrease in x, together with a lowering in the activity of UN in UC1-xNx. The nitrogen pressure over UC0.5N0.5, at a certain temperature in the graph, is approximately one-fifth of that of UN. When con­sidering a nitride or carbide as nuclear fuel for fast reactors, it should be noted that the decomposition pressure of nitrogen can be lowered and that the reactivity of carbide with moisture can be moderated by employing the carbonitride instead of the nitride or carbide.

No experimental data on the melting behavior of transplutonium nitrides such as AmN and CmN have

 

been reported. Takano et a/.22 have examined the relationship between the decomposition temperature and the instantaneous coefficients of linear thermal expansion (CTE) and used it to predict the decom­position temperature of AmN. Figure 13 shows the CTE at 293 and 1273 K plotted against reciprocal decomposition temperature under 1 atm of nitrogen for some transition metal nitrides (TiN, ZrN, HfN) and actinide nitrides (UN, NpN, PuN). The data used for this is summarized in Table 26,22,25,41-43,50-59 with references. Except for the large CTE value for PuN at 293 K, a reasonable linear relationship is shown by the agreement of the broken lines. From the CTE values for AmN, determined by the high-temperature X-ray diffraction technique, the decomposition temperature of AmN under 1 atm of nitrogen was roughly pre­dicted to be 2700 K, which is much lower than that of PuN.