2.5.1 Temperatures

In the early days of the development of fast reactors, when a high breeding ratio was thought to be of great importance, it was realised that the best fuel material from the point of view of the neutron eco­nomy would be a metal, either uranium or an alloy of uranium and plutonium. By the 1960s however it was realised that high burnup was important, and as at that time metal fuel was thought to be limited to about 3% burnup interest almost everywhere turned to oxide. The sole exception was in the USA where work continued in support of the metal-fuelled EBR-II test reactor. That work was ultimately successful in that a ternary U-Pu-Zr alloy fuel capable of up to 20% burnup was developed, and is now an alternative to oxide.

Figure 2.17 shows the thermal conductivity of solid 70U-20Pu — 10Zr (w/o), and its integral, between 500 and 900 °C. There are few data for higher temperatures. However as explained in section 2.5.2 on irradiation the metal becomes very porous, and a porosity of 30% may reduce the effective conductivity to 40% of the value for the solid material. A value of 10 Wm-1 K-1 is a reasonable conservative approximation.

The melting points of pure uranium and plutonium metals are 1135 °C and 640 °C respectively. The solidus temperatures of 20%

and 40% Pu in U are 920 °C and 780 °C respectively and the eutectic (85%Pu) is about 620 °C. The admixture of zirconium increases the solidus temperature of U-20Pu by about 13 K for each percent. Thus the solidus temperature for 70U-20Pu-10Zr is about 1050 °C.

Unlike oxide (see section 2.3.6) metal fuel is chemically compatible with sodium, so the thermal conductance between fuel and cladding can be improved by filling the gap with sodium. As a result the temper­ature difference between the coolant and the outer surface of the fuel is very small. For a linear rating of 50 kWm-1, steel cladding 0.3 mm thick and a 0.35 mm sodium-filled gap between fuel and cladding, the difference is only 53 K. At the centre of a reactor core where the power density is greatest and the coolant temperature is 500 °C the fuel surface temperature might therefore be about 550 °C. If melting is to be avoided the temperature difference between the surface and the centre of the fuel cannot be allowed to exceed 1050-550 = 500 K. Using a conservative conductivity of 10 Wm-1 K-1 the linear heat rat­ing q = 4nKf (AT/) has to be limited to ~63 kWm-1. In practice q is limited to about 50 kWm-1 to provide a margin to melting and to allow for uncertainty over the effect of porosity on the conductivity.

Thus in spite of the differences in thermal properties the linear heat rating in a metal-fuelled reactor is likely to be similar to that in one with oxide fuel. There is however a great difference in the fuel temperatures. The volumetric average fuel temperature at the centre of the reactor of the previous paragraph would be about 800 °C, whereas that for a similar oxide-fuelled reactor (with q = 50 kWm-1 and a fuel surface temperature of 1000 °C — see section 2.2.2) would be 1700 °C. This difference has a great effect on the behaviour of the fuel under irradiation.

There may be come concern about the formation of a eutectic where the fuel touches the cladding, and the possibility that the res­ulting liquid metal might damage or even penetrate it. The eutectic temperature is in the range 700-725 °C (depending on the composi­tion of the fuel). Even if the coolant temperature at the core outlet is 600 °C the maximum fuel surface temperature in an element with a peak linear rating of 50 kWm-1 (i. e. about 25 kWm-1 at the top of the core) would be around 625 °C, leaving a substantial margin to eutectic formation.