The prepared UO2 powder is pressed into green pel­lets in reciprocal or rotary presses at 150-500 MPa. The density of green pellets reaches 50-60% TD. Pellets are normally fabricated with dished ends and/or chamfered edges. The dishes compensate for radial variation in thermal expansion in-reactor, and the chamfers reduce the pellet-cladding mechanical interaction (PCMI). In VVERs and some fast reactors, pellets are made with a central hole to reduce fuel centerline temperatures. The pressing actions of the dies and the punches are carefully controlled to obtain a homogeneous local density distribution in the green pellet and to prevent defects in the green pellet. Two typical LWR UO2 pellets are shown in Figure 11.

2.15.4.2.2 Dewaxing and sintering

The volatile additives such as binders, lubricants and pore formers (if used) are removed from the green pellets by heating at 600-800 °C in a furnace for several hours. The additives will decompose into harmless gases at low temperature. This dewaxing process is generally done as the first step of the sintering process. The green pellets are then sintered in a reduction atmosphere at 1600-1800 °C for times that are based on control samples from previous batches, but are typically 3-10 h. U3O8 powder, mixed with the

(a) (b)

original UO2 powder, can also be used to control the final product density.39

The properties of UO2 fuel pellets such as thermal conductivity, gas bubble mobility, and creep rate influence fuel performance in-reactor. These proper­ties are affected by the grain size and the porosity distribution of the pellets. Early LWR fuel pellets had a small grain size (2-3 pm), but the requirement for greater fission gas retention by large grain fuel has led to the current use of 10-20 pm grain size material. As higher burn-ups become required, greater fission gas retention in the fuel pellets may be expected in the future. The grain size of UO2 pellets can be increased by controlling the sintering conditions or by using sintering additives such as Al2O3, SiO2, TiO2, Nb2O5, or Cr2O3.40-42

2.15.4.2.3 Finishing and inspecting

As-sintered pellets have an hour-glass shape because of the internal density distribution generated during pressing, and the diameter of the pellet must be accu­rate at 10 pm. Also, from the viewpoint of gap conduc­tance, the pellet surface must be smooth. Therefore, pellets are ground by a centerless grinding machine.

After grinding, pellets are inspected to check their diameter, length, density, and appearance; inspec­tions are almost completely automated except for appearance. Analyses for their uranium enrichment, impurities, and microstructures are also done.