Most PWRs use uranium fuel, which is in the form of uranium dioxide. Uranium dioxide is a black semiconducting solid with very low thermal conductivity. On the other hand, uranium dioxide has a very high melting point and has well-known behavior. The UO2 is pressed into pellets. These pellets are then sintered into the solid cylinder (with a height and diameter of about 1 centimeter, the size being greater than the diameter). The dimensions of the fuel pellets and other components of the fuel assembly are precisely controlled to ensure consistency in the characteristics of the fuel. These pellets are then loaded and encapsulated within a fuel rod (a metallic cladding tube), made of zirconium alloys due to its very low absorption cross-section (unlike stainless steel). The surface of the tube, which covers the pellets, is called fuel cladding. Fuel rods are the base element of a fuel assembly. Fuel rods have the purpose of containing fission products, ensuring mechanical support for the pellets, and allowing the heat removal to the coolant fluid of the heat generated by nuclear reactions. A typical fuel rod has a 4 m, with a diameter of around 1 cm. An 1100 MWe (3300 MWth) nuclear core may contain 157 fuel assemblies composed of over 45,000 fuel rods and some 15 million fuel pellets.
Advanced Fuel Pellets
According to the NEA report, the fuel designs covered by the Task Force on Advanced Fuel Designs consist of three different concepts:
- Improved UO2 fuel. Regarding the improved UO2 fuel, this particular design was divided into two sub-concepts: oxide-doped UO2 and high-thermal conductivity UO2 (designed by adding metallic or ceramic dopants).
- High-density fuel.
- Encapsulated fuel (TRISO-SiC-composite pellets).
Special Reference: Nuclear Energy Agency, State-of-the-Art Report on Light Water Reactor Accident-Tolerant Fuel. NEA No.7317, OECD, 2018.
Improved UO2 fuel
Most PWRs use uranium fuel, which is in the form of uranium dioxide. Uranium dioxide is a black semiconducting solid with very low thermal conductivity. On the other hand, uranium dioxide has a very high melting point and has well-known behavior. The UO2 is pressed into pellets. These pellets are then sintered into the solid cylinder (with a height and diameter of about 1 centimeter, the height being greater than the diameter).
Regarding the improved UO2 fuel, this particular design was divided into two sub-concepts, such as:
- Doped UO2. Desirable attributes for accident-tolerant fuel (ATF) pellets include enhancing the retention of fission products (FPs) and minimizing pellet-cladding interaction. According to Westinghouse proposals, chromium (Cr2O3) and aluminum (Al2O3) doped UO2 pellet, known as our ADOPT pellet, achieves greater uranium efficiency through:
- Increased density of fissile material
- A higher creep rate than standard UO2 at high temperatures
- A higher thermal stability
- Reduced wash-out in the event of a fuel rod leaker
- Reduction of fission gas release in a transient scenario
- High-thermal conductivity UO2 (designed by adding metallic or ceramic dopant). Uranium dioxide is a black semiconducting solid with very low thermal conductivity. Thermal conductivity is one of the parameters which determine the fuel centerline temperature. This low thermal conductivity can result in localized overheating in the fuel centerline; therefore, this overheating must be avoided. The concept of cermet (ceramic-metallic) fuel for LWRs is considered. With a low volume fraction of highly conductive metallic additive, the CERMET fuel pellets present a higher conductivity than UO2 standard pellets, lowering the fuel temperature in normal operating conditions and increasing the margins with respect to fuel melting in case of an accident.
