In the search for a suitable fuel to replace the high enriched UAlx fuel generally used in Research and Materials Test Reactors (MTR) with a lower enrichment fuel, one viable candidate is found to be a U—Mo alloy. Mo is added to metallic uranium to extend the stability domain of the high temperature gamma phase, since this phase is stable under the required irradiation conditions in contrast to the room temperature alpha phase. 7-10 wt % Mo is sufficient to avoid transformation to the alpha phase during the production process. U(Mo) particles are used in the fabrication of so-called dispersion fuel plates or rods, in which the U(Mo) particles are mixed with a matrix material (generally Al). In case of a fuel plate, the compacted powder mixture is subsequently pressed in between two Al alloy plates, after which this sandwich structure is rolled to the required thickness. In case of fuel rods, fabrication may often be based on coextrusion methods.
A common method to manufacture the U(Mo) particles is by atomisation processes, which can best be described as a technique in which an ingot of U(Mo) alloy is molten using arc melting while it is spinning or a variation thereof. This causes molten material to be dispersed in small droplets by the centripetal forces. The droplets solidify on their way to the cooled chamber walls. The resulting spherical particles are very well suitable for the fabrication of the dispersion fuel plates or rods. Their sizes depend on the parameters used in the melting process, but are generally around 100 μm in diameter. Another production process based on grinding of ingots results in more or less irregular ground particles of similar average dimensions. Both production processes have been used in the production of test fuel plates.
Tests of U(Mo) based fuels in the reactor have revealed that the U(Mo) particles (atomised and ground) form an interaction layer with the Al matrix under irradiation. This interaction layer was demonstrated to amorphise under irradiation and shows very poor fission gas retention. This causes swelling of the fuel plates, which eventually leads to their failure by pillowing. In an attempt to remedy this behaviour, it was found that Si addition to the Al matrix results in some improvement, although it may not provide a complete solution, particularly for higher power densities. In “Dispersed (Coated particles) and monolithic (zircalloy-4 cladding) U—Mo Particles” (RERTR—2005 Meeting), Pasqualini describes CVD coating of U(Mo) particles with a silicon coating for introducing silicon as an inhibitor. Chemical vapour deposition of silicon thereby is based on silane. Furthermore, silicon is undesireable for the reprocessing of the used fuel and an alternative way, without Si, to stabilise the behaviour of the U(Mo) fuel during irradiation would be beneficial.
The incorporation of neutron poisons in nuclear fuels is a frequently used method to fine-tune the characteristics of the fuel towards its use in the reactor. Neutron poisons typically may have the purpose of lowering the reactivity at the beginning of the use of a fuel assembly in the reactor. Incorporation of neutron poisons is preferably homogeneous throughout the fuel and is often accomplished by blending neutron poison powders in the matrix. Because of several reasons, eg. the higher uranium loadings required in the LEU based fuels, this blending method is less appropriate for LEU based fuels and efforts are made to incorporate the neutron poison in the structural materials of the fuel element. In “Cd wires as burnable poison for the BR2 fuel element” by Franck et al. in Transactions of RRFM 2009, it is suggested to replace the provision of neutron poisons as powder mixed in the matrix by the provision of wires of neutron poison material in the structural materials of the fuel element.