Nuclear Fuel Cycle

A ceramic fuel is a solid inorganic material in which the interatomic bonding is predominantly ionic or covalent. The most common ceramic used for nuclear fuel is uranium dioxide (UO$_2$).

Disadvantages of Ceramic Fuel

• Hard and brittle
• Low strength
• Poor thermal conductivity — to overcome this, fuels are manufactured into small pellets (just under 1 cm diameter, slightly more than 1 cm long). The small diameter ensures that, despite the poor thermal conductivity, the outer surface of the pellet reaches a sufficient temperature for adequate heat transfer to the coolant.
• Thermal cracking — high thermal gradients during irradiation can lead to grain growth within the UO$_2$ crystal, which can cause cracking of the pellet. While this has little effect on core parameters, it can lead to the escape of fission products into the rod interior (though they remain contained within the cladding).

Advantages of Ceramic Fuel

• Very high melting point (~2800 $^\circ$C) — allows the core to operate at much higher temperatures, improving thermal efficiency
• Chemically and structurally very stable — fission products are generally retained within the UO$_2$ crystal structure
• Easy to manufacture with low fabrication costs
• Resistance to oxidation — much better than metallic fuel

Manufacture of Ceramic UO$_2$ Pellets

The production of UO$_2$ pellets follows a careful multi-step process:

1. Homogenisation: UO$_2$ powders are blended to ensure uniformity in terms of particle size distribution and specific surface area.

2. Additives: Several materials may be blended into the powder:

   • U$_3$O$_8$ — to ensure satisfactory microstructure and density
   • Gadolinium oxide (Gd$_2$O$_3$) — a burnable absorber for reactivity control
   • CONPOR — a pore-forming agent (an organic substance that vaporises during sintering, leaving pores that act as reservoirs for fission gases and provide free volume for pellet creep)
   • Niobium — a creep agent that alters the thermal expansion properties of the pellet, reducing pellet-clad interaction (PCI)
   • Lubricants for improved handling

3. Pressing: Conditioned UO$_2$ powder is fed into dies and pressed biaxially into cylindrical pellet form using a load of several hundred MPa. These are called “green” pellets. Rotary presses with 16 tool positions can produce up to 400 pellets per minute. The applied pressure is 3—4 tonnes/cm$^2$, giving green pellet densities of 5—6 g/cm$^3$.

4. Sintering: Green pellets are heated in a furnace at approximately 1750 $^\circ$C for several hours under a precisely controlled reducing atmosphere (usually argon-hydrogen). This consolidates the pellets, converting them into a true ceramic with the microstructure required for high-temperature operation. Sintering causes shrinkage and densification. The final density is approximately 10.65 g/cm$^3$ (97% of the theoretical maximum density of 10.96 g/cm$^3$).

5. Finishing: Pellets are machined to exact dimensions. Scrap material is fed back into an earlier stage of the process. Rigorous quality control ensures pellet integrity and precise dimensions.

Pellet shapes vary by reactor type:

Did you know? A single UO$_2$ fuel pellet in a typical reactor yields about the same amount of energy as one tonne of coal.