If uncertainties regarding the behavior of An car­bides, mostly linked to metastability and uncontrol­lable oxygen and nitrogen impurities, still represent an obstacle to the fabrication and employment of these materials as an alternative nuclear fuel to oxi­des, their higher fissile density constitutes a big advantage. Moreover, the metallic thermal conduc­tivity (Figure 3) and high melting temperature of An carbides ensure a higher conductivity integral margin to melting (CIM), defined by eqn [1], for these mate­rials with respect to the traditional UO2, UO2-PuO2, and ThO2 fuels:

Tm

1(T )dT

Top

Here, Top is the reactor operational temperature at the fuel-cladding interface (around 500 K for light water reactor (LWR), and up to 1500 K for the Generation IV very high-temperature reactors, VHTRs) and Tm is the fuel melting temperature. The better compatibility of carbides with liquid metal coolants compared to oxides is a further rea­son for making them good alternative candidates for high burnup and/or high temperature nuclear fuel.

Uranium carbide was traditionally used as fuel kernel for the US version of pebble bed reactors as opposed to the German version based on uranium dioxide.8 Among the Generation IV nuclear systems, mixed uranium-plutonium carbides (U, Pu)C consti­tute the primary option for the gas fast reactors (GFRs) and UCO is the first candidate for the VHTR.1 In the former case, the fuel high actinide density and thermal conductivity are exploited in view of high burnup performance. In the latter, UCO is a good compro­mise between oxides and carbides both in terms of thermal conductivity and fissile density. However, in the American VHTR design, the fuel is a 3:1 ratio of UO2:UC2 for one essential reason, explained by Olander.32 During burnup, pure UO2 fuel tends to oxidize to UO2+x UO2+x reacts with the pyrocarbon coating layer according to the equilibrium:

UO2+x + xC ! UO2 + xCO [IV]

The production of CO constitutes an issue in the VHTR because the carbon monoxide accumulates in the porosity of the buffer layer. The CO pressure in this volume can attain large values and, along with the released fission gas pressure, it can compro­mise the integrity of the coating layers and contribute to the kernel migration in the fuel particle (‘amoeba effect’). In the presence of UC2, the following reaction occurs rather than reaction [IV] in the hyperstoichio­metric oxide fuel:

UO2+X + XUC2 ! (1 + x)UO2 + 2xC [V]

Because no CO is produced in reaction [V], the latter is more desirable than [IV] in view of the fuel integrity.

Thanks to its fast neutron spectrum, the GFR can suit a 232Th-233U fuel concept, in the chemical form of (Th, U)C2 mixed carbides.33,34 However, the tho­rium cycle is at the moment not envisaged in Gener­ation IV systems.

The use of Pu-rich mixed carbide fuel has recently been proposed for the Indian Fast Breeder Test Reactor.35 However, pure plutonium carbides present a low solidus temperature and low thermal conduc­tivity, which are important drawbacks, with respect to pure U — or mixed carbides, for a nuclear fuel.

More details about the use and behavior of uranium carbides as nuclear fuel can be found in Chapter 3.03, Carbide Fuel.