2.4.1 Recrystallisation

During irradiation the crystal structure of the fuel is changed almost completely. Figure 2.7 shows a polished and etched cross-section of typical pellet fuel after irradiation and Figure 2.8 shows a similar axial cross-section of vipac fuel. Throughout most of the cross-section the

original particulate structure of the vipac fuel has completely disap­peared.

The most striking change is that a hole appears in the centre. It is surrounded by a region of high density in which the grains are

Un restructured Equiaxial Columnar

grains grains grains

long and narrow and lie along radii of the cylinder. This is known as the “columnar grain” region. Outside it is a region where the grains are larger than in the original material but are oriented at random. This is called the “equiaxial grain” region. Finally, in the outermost part of the fuel it retains its original structure, and this is called the “unrestructured” region. These three distinct regions are shown in Figure 2.9.

Observations made after various periods of irradiation show that this basic structure is set up very quickly — within an hour of achiev­ing the full power density. Thereafter the pattern of equiaxial and columnar grains spreads outwards from the centre but at a rate that decreases very rapidly. The higher the linear rating the greater the radii of the restructured regions, and for most of the life of the fuel it is a fair approximation to regard the outer edge of the columnar grain region as being close to the 1800 °C isotherm, and that of the equiaxial region close to the 1600 °C isotherm.

As far as is known the restructuring takes place as follows. At intermediate temperatures the sintering that started during manufac­ture continues. The pores between the grains tend to coalesce and the individual grains grow to form the equiaxial grain region.

At higher temperatures the pores become mobile and move up the temperature gradient. There are two mechanisms for this process: surface diffusion whereby atoms of fuel move round the pore from the hot to the cold side, and volume diffusion whereby atoms evap­orate from the hot side and condense on the cold. The result is that the pore moves towards the region of higher temperature and at the same time the fuel is recrystallised as the pore moves through it. The atoms deposited on the cold side of the pore form a single new crystal, relatively free from imperfections and occupying the volume swept out by the pore as it moves.

The pores move to the centre — the hottest part — of the fuel and there form the central void, while the density of the columnar grain region increases to something close to the theoretical value, which is that of a single crystal. The speed with which the pores move depends strongly on temperature so the outer boundary of the columnar region moves quickly to start with and then very slowly. Initially it is the pores incorporated in the material when it is manufactured that move in this way but later it is the bubbles formed by the accumulation of fission — product gas on the grain boundaries (section 2.3.1). As a result the columnar grain region is continually being recrystallised.