As the fuel is irradiated in the reactor its isotopic composition changes. The effect is illustrated in Figure 1.17 which shows what would hap­pen, in theory, to 1 kg of 238U if it could remain in a reactor until it was entirely consumed by fission (something that is impossible in practice because the buildup of fission products would completely disrupt the fuel material). The quantity of 239Pu would increase, followed by 240Pu, then 241Pu and lastly by 242Pu, all in successively smaller amounts. On the linear scale of Figure 1.17 243Pu and all the americium and curium isotopes do not appear because the quantities produced are very small. This reflects the fact that, as far as any effect on the oper­ation of the reactor is concerned, they are quite unimportant. Their effect on the waste stream, which is far from unimportant, is discussed later.

The extent of irradiation is indicated in Figure 1.17 in terms of the burnup, the fraction of the “heavy atoms” (i. e. nuclei of uranium or plutonium) that have been fissioned. Since all fissions, of whatever isotope, liberate roughly the same amount of energy (about 200 MeV), burnup can also be measured in terms of the energy released. 100% burnup — i. e. complete destruction by fission — would yield about 80 TJ/kg. Another convenient way of measuring burnup is in terms

of megawatt-days per tonne (MWd/t). Complete burnup is equivalent to about 106 MWd/t.

Figure 1.18 shows how the isotopic composition of the plutonium in Figure 1.17 changes with burnup. The most notable feature is the steady increase of the 240Pu fraction. The concentrations of 241Pu and 242Pu are always small. The composition tends asymptotically towards 239:240:241:242 — 56:36:5:2. (The precise ratios depend on the spec­trum of the neutron flux.)

Figure 1.19 shows how the quantities of the various isotopes would vary with burnup in an idealised uranium-cycle reactor with a continu­ous feed of fertile 238U. The reactor is assumed to generate 2500 MW (thermal) from a core containing a total of 7.4 tonnes of fissile and fertile material (i. e. heavy atoms). The initial enrichment in 239Pu is

Figure 1.19 The evolution of fuel composition in a uranium-cycle reactor (2500 MW thermal, 7.4 tonnes of heavy atoms).

23% but this declines as the concentration of fissile 241Pu builds up. The buildup of the other plutonium isotopes can also be seen.

Figure 1.20 shows the same information for a thorium-cycle reactor that has an initial loading of 239Pu to make it critical. The uranium iso­topes build up very slowly and for a prolonged period 239Pu has to be added to maintain criticality. The fact that only very small quantities of 237U and 238U, the sources of higher actinides (see Figures 1.14 and 1.15), are produced indicates that thorium-cycle reactors pro­duce less of the hazardous higher-actinide waste than uranium-cycle reactors.

However, as stated earlier, Figures 1.19 and 1.20 are theoretical because in practice the fertile feed is not continuous but comes in batches when the core is reloaded after irradiated fuel is removed for reprocessing. The maximum burnup achievable in practice is around 20%, at which point in a uranium-cycle reactor the plutonium contains around 77% 239Pu and 21% 240Pu. If plutonium with a higher 239Pu

concentration is required the fuel has to be removed and reprocessed much sooner, after at most a few percent burnup. Plutonium rich in 239Pu is sometimes called “high-grade plutonium”.

Figure 1.21 shows how the isotopic abundances vary typically in a reactor. While it operates the quantities of both 238U and 239Pu decrease (usually they are said to “burn down”), the reactivity being maintained by withdrawing control absorbers from the core. At the end of a period of operation (usually called a “run”) some of the irradiated fuel is removed and taken for reprocessing. The plutonium is separated, some fresh plutonium is added to it to replace that con­sumed, and it is returned to the reactor. In the example of Figure 1.21 it is assumed that the plutonium in the core at startup and added at each reload is pure 239Pu. The quantities of the higher plutonium isotopes increase steadily from run to run. 242Pu is present but its abundance is indistinguishable on the scale of Figure 1.21.