Following the discovery of the unique set of properties exhibited by a group of layered ternary carbides and nitrides known as MAX phases, they have been proposed as a candidate for use in nuclear engineering. Specifically, they have been proposed as an Accident Tolerant Fuel (ATF) coating or cladding material, to provide extra protection from the high temperature oxidation conditions during a Loss of Coolant Accident (LOCA), conditions which current zirconium-based cladding has limited resistance to as seen at Fukushima in 2011. Following a survey of the literature on the properties, synthesis techniques and methods of application of MAX phases, the body of experimental work falls into three sections, which contain a series of proof-of-concept experiments relating to the use of MAX phases in ATF cladding.
The first is concerned with methods of synthesis of MAX phases. Spark Plasma Sintering (SPS) was used to synthesise Ti3SiC2, while Molten Salt Synthesis (MSS) was used to synthesise Ti3SiC2, Ti3AlC2 and Ti2AlC, the latter of which was found to be 88wt.% pure, with a synthesis temperature approximately 100°C lower than by other methods reported in the literature. Both SPS and MSS were used to conduct a series of experiments attempting to form reactor suitable hybrid MAX phases that may have been suitable as an ATF material. These attempts were sadly unsuccessful, though promising such phases have now been synthesised by hot pressing in the literature and potentially with this additional guidance on stable phases now available, future SPS or MSS synthesis may prove more fruitful.
The following section covers experiments to create a method of coating existing fuel rods with a protective MAX phase layer. Depositions were carried out of MAX phases onto flat and tubular geometry using Electrophoretic Deposition (EPD), a room temperature deposition technique that can be performed in water. Deposition was carried out onto copper, titanium, graphite and ZIRLO® (zirconium alloy) substrates. Commercially available Ti3SiC2 powder was used as a placeholder MAX phase due to its availability over other more suitable phases. Unsintered depositions of 1.89±0.26 mg/cm2 were achieved on titanium substrates in 10 minutes. These powder coatings were then rapidly densified using a Selective Laser Melting (SLM) 3D printer, and a study of laser power and focus was carried out. Dense coatings of up to 30µm thick were achieved on titanium that did not amorphise, but with a thin surface phase of TiC1-x present. Varying the laser focus parameter resulted in bands of coating, moving towards fuller coverage with a widening focus. Laser sintered samples on ZIRLO® substrates did not densify, however it is believed this is due to an unavoidable change in the SLM 3D printer used, and not an underlying issue.
The final experimental section explores the possibility of creating MAX phase cladding, by using the well-established technique slip casting to produce short lengths of tube. Ti3SiC2 slips of varying composition were analysed for their rheological properties and used to cast tubes in plaster moulds. An investigation of sintering conditions was carried out, and the resulting densified tubes were found to be ~89% of maximum density. The tubes were carefully drilled out and polished, and then subjected to destructive hoop stress tests. The highest hoop stress per unit wall thickness value for a tube was calculated at 9.1±2.2 MPa/mm.
A discussion section then considers the results found in this project and examines them in the larger context of other techniques, and their feasibility for use in the nuclear industry. The proof of concept experiments performed were moderately successful, in that a synthesis method for MAX phases was found to be effective for certain phases, a method for coating and densifying MAX phases onto substrates was found, and MAX phase tubes were cast. Work done in this project can serve as a starting point for further investigations into methods of applying MAX phases to the nuclear industry.
Glossary of acronyms
ATF – Accident Tolerant Fuel
EPD – Electrophoretic Deposition
LOCA – Loss of Coolant Accident
MSS – Molten Salt Synthesis
SLM – Selective Laser Melting
SPS – Spark Plasma Sintering
