26.08.2015   An Introduction to Nuclear Materials

Uranium Nitride

In uranium-nitrogen system, ceramic compounds such as UN, U2N3, and UN3, with uranium mononitride (UN), are the most stable and the only compound with properties of a nuclear reactor fuel. UN has a NaCl-type (interpenetrating FCC) crystal structure. UN has a theoretical density of 14.32 gcm~3 under normal condi­tions. UN maintains its stoichiometry up to high temperatures, and becomes nonstoichiometric at >1500 °C. Melting temperature of UN is about 2650 °C. UN does not melt congruently and begins to dissociate into free uranium and gaseous nitrogen at a temperature that is a function of the system nitrogen overpressure, which can be analytically described by Eq. (7.14), where Tm represents the melting

354 I 7 Nuclear Fuels

Table 7.5 A comparative summary of a few characteristics of UO2, UC, and UN Ref. [2, 5].

Fuel

Lattice

structure

Dimensions

(A)

Melting point (°C)

U

content

(wt%)

Macroscopic cross section (fission) (cm1)

Absorption

cross

section

(cm1)

Fast fission neutrons/thermal neutrons (g)

UO2

Fluorite

a = 5.469

2760±30

88.15

0.102

0.187

1.34

UC

Rock

salt

a = 4.961

~2300

95.19

0.137

0.252

1.34

UN

Rock

salt

a = 4.880

~2650

94.44

0.143

0.327

1.08

temperature in K and PN2 represents the nitrogen partial pressure in the unit of atm (1atm ffi 101.3 kPa) Ref. [18].

Tm = 3035 (Pn2 )°’°2832. (7.14)

The thermal conductivity and specific heat of UN have already been shown in Figures 7.20 and 7.21.

Here we summarize various features of different uranium ceramic fuels in a tabular form (Table 7.5).

7.3.4

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