If the core had the same composition throughout the power density would be nearly proportional to the flux and would be distributed across the core as shown on the left of Figure 1.12. This would be most undesirable from a thermodynamic point of view because if the coolant flowed at the same rate through all parts of the core it would emerge at different temperatures. When cold coolant from the periphery of the core mixed with hot coolant from the centre there would be a gain of entropy and a consequent loss of work output, and there would be large temperature fluctuations in the mixing region that might damage the structure (see Chapter 3). Alternatively if the flow in the outer part of the core were restricted to equalise the outlet temperatures pumping work would be wasted as the coolant flowed through the restrictions. For this reason the core is normally made in two or more radial zones, with higher fuel enrichment in the outer zones. The effect of this is to increase the power density in the outer region as shown on

the right of Figure 1.12, thus reducing the coolant outlet temperature differences.

Figure 1.13 shows the radial distribution of neutron flux and power density in a small breeder reactor having two core zones, of roughly equal volume, with enrichments of 22% and 28%, surrounded by a breeder. If the core were uniform the enrichment would be about 24%. The peak power densities at the centre of the core and the inside of the outer zone are roughly the same and the radial power peaking factor, Pmax/Pave, where Pmax is the power generated in the most highly rated channel and Pave is the average power per channel, is reduced from about 1.35 in a single-zone core to 1.21 in the two-zone core.

An important point to notice in Figure 1.13 is the change in the power density at the inside of the breeder with time. As fissile material is generated in the breeder, predominantly at its inside edge, it in turn undergoes fission and generates power. Over the life of the inner fuel elements of the radial breeder (which would stay in the reactor much longer than the core fuel) the power density rises considerably. In this particular reactor at the start of its life the power density at the centre of an inner radial breeder fuel element is 60 MW m-3 when the reactor power is operating at 600 MW (heat), but this rises to 220 MW m-3 after 1.6 years at power.