Table 2.2 lists the fission products present in greatest abundance after 10% burnup of typical fast reactor fuel (30% plutonium with typical concentrations of the higher plutonium isotopes). The behaviour of a chemical system with so many components is obviously extremely complex and is certainly not understood in detail. The broad outlines are given in this section but the complexities are such that the actual behaviour in a particular fuel element with a slightly different com­position, cladding, temperature or irradiation history may differ quite widely. It is convenient to divide the most abundant fission products into groups as follows. Elements in the same group behave roughly similarly.

Inert Gases (Kr, Xe). These are mainly released from the fuel, but some are retained in solution or in small bubbles within the grains in the cooler parts of the fuel (see section 2.3.4).

Alkali Metals (Rb, Cs). These are very volatile in elemental form and migrate to the cool periphery of the fuel. This migration is illus­trated in Figure 2.16 which shows the distribution of 137Cs as determ­ined by y — spectroscopy. In some cases the isotopes that are daughters of inert gases, such as 133Cs which is produced from 133Xe which decays with a half-life of 5.3 d, and 87Rb produced from 87Kr decaying with a half-life of 78 min, can appear in the gas plenum to which the precurs­ors have migrated.

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Figure 2.16 The distribution of fission products in irradiated fuel.

Metals forming refractory oxides (Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Pm, Sm). By and large, having formed oxides these elements do not migrate and are found uniformly distributed through the fuel, as illustrated by the data for 144Ce in Figure 2.16. But again the isotopes that are daughters of volatile or gaseous precursors, such as 138Ba and 140Ba (from 138Cs and 140Cs respectively), are less uniform. 89Sr (a daughter of 89Rb that has a half-life of 15.4 min) migrates farther than 90Sr (from 90Rb, half-life 2.7 min).

Metals that do not form oxides (Tc, Ru, Rh, Pd, Ag, Te). These are found as metallic inclusions, sometimes dispersed through the fuel and sometimes, especially if the central temperature is high, having migrated to the central void. There they form droplets of metal that are molten while the reactor is operating and are found as small ingots when the fuel is examined subsequently. Such ingots usually contain uranium and plutonium as well. Figure 2.16 shows the loss of ruthenium from the hottest part of the fuel.

Metal that may form an oxide (Mo). The fate of the molybdenum depends on the oxygen potential of the fuel as explained in section 2.3.3. If it is low molybdenum is found in the metallic inclusions; if it is high it is found as MoO2.