The fissile material in today’s nuclear power stations is predominantly uranium and plutonium as oxides, pressed and sintered to pellets, which are filled into tubes and arranged in assemblies (Sections 9.2 and 9.4). Other material forms have been tried in the past or are being considered, but none of them is applied on a commercial scale in power reactors.

9.5.1 Oxide pellet design and manufacture

Pellet fabrication for light water reactors starts with uranium hexafluoride (UF6) enriched to the required concentration of U-235 in an enrichment plant (Chapter 7). The UF6 is received in solid form in containers, heated to gaseous form and chemically converted to uranium dioxide (UO2) powder. Several conversion processes are available (Assmann, 1982), e. g.:

• Ammonium diuranate (ADU) wet conversion where UF6 is hydrolised in an aqueous ammonia solution from which ammonium diuranate is precipitated and then calcinated to UO2.

• Ammonium-uranyl carbonate (AUC) wet conversion where uranyl — tricarbonate is calcinated to produce UO2 powder.

• A dry process where UF6 is converted to UO2 powder by mixing it with steam and hydrogen in a kiln.

The dry process is a more environmentally friendly conversion technique than ADU and AUC since the clean waste, HF, can be disposed of more easily than the uranium-contaminated waste from the wet ADU and AUC processes. The dry process is therefore the preferred one today.

Doping agents are mechanically blended to the powder via a pre-blend, which ensures good homogeneity. The pre-blend is a mix of about 5% of the UO2 powder lot and the doping agents. The UO2 powder is further conditioned by adding pore former to obtain the desired pore distribution in the sintered pellet.

The powder is pressed to so-called green pellets with a pressure of 400-500 MPa obtaining about 50% of the theoretical density of UO2. The green pellets are sintered in a continuous, electrically heated production furnace in a reducing hydrogen atmosphere. The sintering takes 4-5 hours at a temperature of 1700-1800 °C. The density is increased to about 95% of the theoretical density or 10.5 g/cm3. Variants of this basic process are used to reduce the time and energy requirements and to influence the microstructure (e. g. grain size) of the ceramic pellets (Harada, 1997). In any case, the final product must have an oxygen-to-uranium ratio very close to 2 since deviations from stoichiometry will influence properties such as the fission gas diffusion coefficient and the thermal conductivity. Excess oxygen will also corrode the cladding from inside.

Fuel pellets have precise dimensions. The nominal pellet diameter, which in most designs is in the range 8-10 mm, must be met within a tight tolerance of ±12 pm in order to obtain an accurately known gap between the pellets and the cladding. However, due to friction forces between the powder and the die, the green pellets have density variations leading to uneven sintering shrinkage. The pellets are therefore ground to a cylinder with precise dimensions. In addition, a chamfer is ground to the end faces to reduce chipping, which is a cause of fuel failure. An end face dishing is formed to accommodate the larger thermal expansion in the centre of a fuel pellet. The overall shape of the final product is shown in Fig. 9.10.

In the past, moisture in pellets caused cladding corrosion from inside, hydrogen embrittlement and fuel failure. Before insertion into the cladding tube, the pellets are therefore dried in a vacuum at 120-150 °C to reduce absorbed water on the surface to less than 10 ppm.