Although thoria-based fuels have been studied extensively in the past, namely in the 1970s, to our knowledge very little open literature is available for (Th, Pu)O2 [40, 130]. Only a few measurements of thermal conductivity have been made for ThO2-PuO2 fuel. Since CeO2 and PuO2 have similar thermodynamic and crys­tallographic properties [131], Murbayashi [114] tried to simulate the thermal conductivity as a function of temperature and CeO2 up to 10 wt% using Laser flash method. Jeffs [132, 133] determined the integral thermal conductivity of irradiated (Th1-yPuy)O2 containing 1.10, 1.75, and 2.72 wt% of PuO2 using a steady state method. The thermal conductivity of a mixture of ThO2 and 4 wt% PuO2 was also measured by Basak et al. [130] using the laser flash technique for the temperature range of 950-1,800 K. Recently, Cozzo et al. [95] reported that at 500 K the thermal diffusivity of the Th-MOX can be down to 50 % of that of its pure oxide components ThO2 and PuO2. The presence of the two different oxides inside the Th-MOX lattice, generate a high amount of phonon scattering centers. When temperature increases, the plutonium concentration affects the thermal diffusivity of the fuel to a lesser extent, because the phonon-phonon scattering mechanism increases with temperature and becomes predominant when compared to the lattice strains due to the presence of either Th or Pu atoms in the lattice [95]. However, the thermal conductivity of pure PuO2 was found to be higher than that of ThO2 for all temperatures covered by their study. This is somewhat surprising and contra­dicts the understanding that ThO2 always have a higher thermal conductivity than the other actinide oxides.

In Fig. 18, the thermal conductivity of Th-MOX with PuO2 content varying from 0 to 30 wt% are shown. At low temperature, the thermal conductivity of the Th-MOX with a PuO2 content from 0 to 30 wt% decreases with an increase of the

PuO2 content. At higher temperature (above 1,000 K), the thermal conductivity of Th-MOX with a PuO2 content from 0 to 8 wt% is almost independent from the concentration of plutonium. The conductivity of Th-MOX with 30 wt% PuO2 at high temperatures is much lower [95].

The thermal conductivity k, of (Th1-yPuy)O2 as a function of temperature and PuO2 content is reported by IAEA study [40]. Figure 19 shows a systematic decrease of thermal conductivity with increasing PuO2 content and temperature. The data are comparable with those obtained by Murabayashi [114] on simulated fuel samples of the composition ranging from 0 to 10 wt% CeO2. The best-fit equation for the thermal conductivity, k [W/m-K], of (Th1-yPuy)O2 as a function of composition, y [wt%], and temperature, T [K], was derived for the temperature range from 873 to 1,873 K [40].

k(y, T) = 1/[-0.08388 + 1.7378 — y +(2.62524 — 10-4 + 1.7405 — 10-4 — y)- T]

(63)

In order to introduce the influence of the plutonium content on parameter A, one can rely on the simplified theory of Abeles [95]. The parameter A has a second-order dependence on both the relative mass and radius differences as per the above theory. A polynomial equation of the second degree was chosen to define A(PuO2):

A(PuO2) = A0 + A1 — [PuO2] + A2 — [PuO2]2, (64)

([PuO2] = Concentration of PuO2 in wt%).

Fig. 19 Systematic decrease of thermal conductivity with increasing PuO2 content and temperature for ThO2-PuO2 system [40]. (permission from IAEA)

The values of the parameters are [95]:

A0 = 6.071 x 10-3 mKW-1, A1 = 5.72 x 10-1 mKW-1,

A2 = -5.937 x 10-1 mKW-1. B = 2.4 x 10-4 mW-1.

Figures 20 and 21 show the variation A and B parameters with PuO2 content for ThO2-PuO2 system. The parameter A increases with increase in PuO2 while the variation of B with PuO2 content was found to be random.

The experimental thermal conductivity data of high Pu bearing hypostoichio — metric and stoichiometric mixed thorium-plutonium oxide of compositions,

5.0

4.5

4.0

і

3.5

■S’

>

30

3

2.0

£

I 1.5 I-

1.0

800 1000 1200 1400 1600 1800 2000

Temperature, K

ThO2-20 % PuO2, ThO2-30 % PuO2, and ThO2-70 % PuO2 with CaO or Nb2O5 as dopant, was measured up to 1,850 K in BARC, India, by employing the ‘‘Laser — flash’’ technique and is shown in Fig. 22. As expected, ThO2-70 % PuO2 showed the least thermal conductivity among the above sample.