Metal borides are a class of refractory materials with a set of thermomechanical properties that make them well-suited for applications in extreme environments, most notably their high thermal conductivities and their high melting points. In addition to this, the unique nuclear properties of boron make metal borides interesting materials for the nuclear industry as well: boron has two naturally occurring isotopes, with one being a strong neutron absorber (and widely used in this role in the nuclear industry) and the other being substantially neutron-transparent, and with well-established supply chains for its isotopic separation.
Thus, metal borides represent a class of materials with adjustable neutron opacity and which are able to conduct and dissipate heat, including that which is generated inside their bulk by nuclear reactions, with potential applications as fuels or fuel additives for fission reactors, and for shielding materials for fusion reactors.
However, the most established route of preparation of many metal borides is still the direct reactive melting of boron and the respective base metal, in a process that – while conceptually simple – is usually energy-intensive and challenging to scale-up.
In this work, uranium and tungsten borides are discussed, respectively, for use in fission and fusion reactors, in particular uranium diboride (UB2) and ditungsten pentaboride (W2B5). Uranium diboride is envisioned as a component for a composite Advanced Technology Fuel (ATF) concept in which UB2 particles are dispersed in a UO2 matrix to improve its thermal conductivity and its uranium density and to act as intrinsic burnable absorbers. Ditungsten pentaboride is envisioned as a potential shielding material for compact fusion reactors of the spherical tokamak-type.
New synthetic routes to uranium and tungsten borides have been tested and optimised with the support of thermodynamic modelling. All known borides of uranium (UB2, UB4, UB12) have been prepared from the borocarbothermic reduction of uranium dioxide (UO2) with boron carbide (B4C), carbon (C), and diboron trioxide (B2O3). The synthesis route has been shown to be capable of reliably producing UB2 with purities between 90% and 95% (on a heavy metal basis). An analogous process has been used to produce three different borides of tungsten (W2B, WB, W2B5) from
2
tungsten trioxide (WO3), obtaining products with purities greater than 99% (on a heavy metal basis) in the case of WB and W2B5.
Several thermodynamic models have been used to predict and describe the chemical interactions of UB2. In particular, contact interactions of UB2 with materials relevant to the nuclear industry have been assessed and experimental data on the oxidation of UB2 have been successfully interpreted.
A preliminary assessment of the burnup chemistry of UB2 was performed using data from neutronic simulations, describing the possible behaviours of major irradiation products and suggesting that excessive fragmentation of boron may challenge the stability of the UB2 matrix.
Solid solutions of UB2 and ZrB2 have been prepared as surrogate materials for UB2 doped with Zr produced by fission events, measuring the lattice distortion associated with the inclusion of Zr in the crystal lattice.
Finally, a possible path to the preparation and testing of the ATF composite fuel concept is presented, outlining the extant research needs.