Uranium dioxide is the material employed for power generation by the majority of commercial nuclear power reactors today. The oxide is sintered into short cylindrical pellets that are contained within sealed metal cladding tubes which are collected into clusters to form fuel assemblies loaded in the reactor. In a pressurised water reactor which is the predominant reactor type, a fuel pellet is just under 10 mm in diameter and over 10 mm high. The reactor typically contains over 100 fuel assemblies, each of which generally holds over 200 cladding tubes, providing a large contact surface with the reactor coolant for effective heat transfer.

The earliest nuclear fuel made use of uranium metal, as found in some prototype reactors built in the 1960s. However, the metal experienced significant changes in dimensions following thermal cycling and irradiation damage during reactor operation, imposing limits on the extent of fuel usage. In contrast, uranium alloys that were explored were found to absorb too many neutrons, making the nuclear reaction less efficient.

Uranium dioxide has the advantage of a high melting point (2860°C), limited dimensional change following irradiation, good containment of nuclear fission products, and minimal chemical reaction with its cladding material or the reactor coolant, enabling fuel usage at about an order of magnitude higher than for uranium metal.

Nevertheless, the oxide pellet may experience some swelling during reactor operation, caused by radiationinduced changes to the pellet microstructure and the built-up of gaseous and solid fission products. This swelling effect can be alleviated by reducing the voids in the sintered pellet, which now commonly reaches 95% of its theoretical density.

However, the low thermal conductivity of uranium oxide implies a high internal pellet temperature. In the case of a pressurised water reactor with a nominal reactor coolant temperature of around 300°C, the fuel pellet may reach 500°C at its surface and 2200°C at its centre. Such high temperatures lead to high thermal stresses within the pellet, and increase the need for reliable cooling to provide for the high thermal capacity and heat storage in the nuclear fuel.

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