An important choice facing the designer of sealed fuel elements is whether to place the gas storage volume, or plenum as it is usually called, above or below the core. If it is below, surrounded by cooler inlet coolant, it can be smaller than if it is above, where it is immersed in the hotter outlet coolant. But if for some reason the plenum should burst or leak the gas would be released and pass upwards, displacing coolant from the core and possibly causing a positive reactivity transi­ent (see Figure 1.26).

For a breeder reactor there is usually an axial breeder region con­taining natural or depleted UO2 between the core fuel and the plenum. Provision has to be made to allow the gas from the core to pass through the breeder to the plenum. The axial breeder on the other side of the core (i. e. above the core if the plenum is below) can be incorporated in the same fuel elements with the core. Alternatively it can be made in the form of separate fuel elements that, because of the low power density in the breeder, can have a larger diameter than the core elements. The advantage of the latter arrangement is that the resistance to coolant flow is reduced, but it increases the complexity.

Figure 2.6 shows a typical core fuel element that incorporates both axial breeders, has the fuel in the form of pellets, and has the plenum below the core. The retainers are necessary only to keep the fuel pellets in position during manufacture, transport and loading into the reactor.

Retainer Upper Breeder

Figure 2.6 A typical fuel element.

Almost as soon as the power is raised the pellets swell to press against the cladding (see section 2.4.4) and become jammed.

The diameter of the fuel element is determined by heat transfer and manufacturing cost considerations (section 3.2.1). The thickness of the cladding has to be sufficient after allowing for corrosion both on the outside (sections 3.3.4 and 3.3.5) and the inside (section 2.4.7) to withstand the stresses due to fuel swelling and fission-product gas. Its thickness is usually about 0.3 or 0.4 mm.

Radial breeder fuel elements are usually similar in form to those of the core but with larger diameter. Even at the end of several years of irradiation the power density in the breeder adjacent to the core is only 20% or so of that at the core centre. The breeder elements can therefore have more than twice the diameter of the core elements before the limiting linear heat rating is reached.