Coated nuclear fuel
Eddie López-Honorato and Ping Xiao from the School of Materials at The University of Manchester, UK, describe how surface engineering developments can help nuclear energy generation.
Many countries are examining the possibility of building new nuclear power plants due to increased demand for electricity, concerns over climate change and worries over future security of energy supply. One design that has attracted particular attention is the high/very high temperature reactor (V/HTR) because of its added capacity to produce hydrogen through a thermochemical process, at a high output temperature of 900-1,200ºC. An important safety feature of the V/HTR is the type of nuclear fuel it uses. Instead of placing fuel pellets in metallic rods as current reactors do, small fuel kernels, 500µm in diameter, are coated with four layers of ceramics to create a miniature fission product containment vessel.
This type of fuel, known as TRISO (tristructural isotropic) coated fuel particle, is made of a 90µm buffer layer of low density pyrolytic carbon, inner pyrolytic carbon (IPyC) at 40µm, 35µm of silicon carbide (SiC) and an outer pyrolytic carbon coating (OPyC) of 40µm (see image below). These are all deposited by fluidised bed chemical vapour deposition, and their properties are vital for technology safety and development.
Development of V/HTR technology started in the late 1960s, but the characteristics of the PyC and SiC coatings have not changed considerably. The coated fuel produced in the 1980s in Germany is still considered the best TRISO fuel ever produced.
To improve rentability of nuclear technology, future V/HTRs will need to operate at higher temperatures and burn-ups than those used in the past. This new set of conditions is expected to result in greater damage and stresses that could render the old fuel unsuitable for such harsh conditions. Silicon carbide and carbon coatings with improved microstructure and mechanical properties are needed to ensure the safe enclosure of all important radio nuclei, suitable characterisation techniques are also needed to classify these products and elucidate the changes in these materials at high temperatures and under irradiation.
Different approaches have been considered. One involves the introduction of extra layers of PyC/SiC or a burnable poison layer, to mitigate palladium corrosion in SiC or control excess reactivity in the kernel. Another is the continuous reduction of grain size in the SiC to produce nanostructured SiC coatings. Reports have already stressed the improvements in mechanical properties and diffusion of fission products by reducing grain size – some results are suggesting a considerable improvement in radiation damage. By carefully controlling the chemistry of the deposition process, SiC coatings are produced with 100-200nm grain sizes. The use of functionally graded coatings is also being studied as a possible way to control crack formation and propagation in SiC.
Raman spectroscopy, nuclear magnetic resonance, X-ray microtomography and nanoindentation are some of the characterisation techniques that are being applied for the characterisation of PyC and SiC coatings.
Raman spectroscopy in particular has proven to be an invaluable tool for these coatings as it can provide information on stoichiometry, defects, microstructure and texture. Coupled with a high sensitivity for C, Si and SiC, fast acquisition times and small spatial resolution of one to two micrometres makes this technique suitable. Nanoindentation is one of few proven characterisation techniques capable of providing reliable mechanical properties of such small and complex systems. It has been used to study the factors controlling Young’s modulus, hardness and fracture behaviour of SiC. Additionally, 3D tomography images with a resolution up to 50nm can be obtained that could help elucidate the radiation damage in this fuel, as well as crack formation and propagation in SiC.
Overall, after four decades in the development of the V/HTR, advances in characterisation and a better understanding of materials’ properties are leading to a new era of fuel production, where tailored SiC and PyC coatings can be produced. This will ensure maximum performance and guarantee the safety of this nuclear technology, especially under accident conditions.
Further information: School of Materials, The University of Manchester