Decades of experience with commercial water-cooled nuclear power reactors have demonstrated that an effective nuclear fuel combination consists of fuel pellets of sintered uranium dioxide (UO₂) and cladding made of zirconium alloy. However, the Fukushima accident in 2011 also showed that at elevated reactor temperatures following a loss of cooling water, zirconium reacts with water vapour, exacerbating the situation by releasing additional heat and hydrogen which led to explosions during the accident.

The accident has accelerated the need for the nuclear industry to develop more advanced ‘Accident Tolerant Fuels’ that may be applied to current and emerging reactor designs, and offer improved safety in severe reactor core accidents compared with the current UO₂–zirconium combination, enhanced fuel performance during, maintain cost competitiveness, and be compatible with other fuel-related activities such as storage and post-operational treatment.

Research has been ongoing into the fuel pellets, the cladding, as well as their interaction, to yield experimental data for normal and accidental reactor conditions and integrate the data into reactor safety assessment codes.

One category of activities is the development of evolutionary designs to meet near-term demands. These include introducing cladding coating material such as chromium and alumina for zirconium, and cladding of stainless steel alloy containing chromium and aluminium that both of which suppress hydrogen production, although they increase neutron absorption. New fuel pellet types are being developed, by adding materials such as chromium or molybdenum in the UO₂ to enhance heat conduction though at the expense of reduced uranium density, and therefore, shorter fuel life. Trials are currently ongoing.

Longer-term fuel concepts are also being explored. Silicon carbide is a potential cladding material since it works well at high temperatures, suppresses hydrogen production and lowers neutron absorption, though it may experience corrosion in a water environment and has low thermal conductivity that reduces heat extraction from the fuel. For the fuel pellets, uranium nitride and uranium silicide are being considered owing to their higher uranium contents and better thermal conductivity, though the nitride may react with water and hence may need ceramic encapsulation while the silicide may require the development of a less complex manufacturing route.

This article is contributed by Ir Richard Fung with the coordination of the Nuclear Division.