The chemical composition of fission products in discharge fuel is controlled by the long-lived and stable species. The amounts of most of the fission-product chemical elements change but little for thousands of years after discharge. Those elements that do change significantly in amount over long decay periods include:

1. Cesium, which includes appreciable quantities of 30-year 137Cs

2. Hydrogen, which consists entirely of 12.3-year 3H

3. Niobium, which consists almost entirely of 35-day 9sNb

4. Promethium, which consists entirely of 2.62-year 147Pm

5. Strontium, which contains appreciable quantities of 27.7-year 90Sr

6. Technetium, which consists entirely of 2.12 X 105-year "Tc

The elemental composition of the fission products in fuel discharged from the uranium-fueled PWR (Fig. 3.31) is listed in Table 8.2 and is plotted in Fig. 8.2. The composition expressed in elemental atoms per fission-product pair is the effective long-term elemental yield per fission, so the sum over all elements is equal to 2.