More than 300 different nuclides have been observed as the primary products of fission. The term fission products usually refers to the primary fission products, i. e., the fission fragments and their daughters resulting from radioactive decay and neutron absorption. Only a few of the primary fission products are stable, the rest being beta-emitting radionuclides. As a fission — product radionuclide undergoes beta decay, its atomic number increases whereas its mass number remains constant. The direct yield of a fission-product nuclide is the fraction of the total fissions that yield this nuclide, essentially as a direct-fission fragment. The cumulative yield of a given nuclide is the fraction of fissions that directly yield that nuclide and its radioactive decay precursors in the constant-mass fission-product chain; i. e., it is the sum of the direct yields of that nuclide and its decay precursors. Many of the fission products have such short half-lives that no accurate measure of their direct yields as primary fission products is available. However, reasonably reliable data have been secured on the cumulative yields of many of the long-lived radionuclides and on the cumulative yields of all the nuclides in a fission-product chain of given mass number [B3, Wl], The cumulative yields by mass number in the fission of M3U, 235U, and 239Pu by slow neutrons and in the fission of 235U, 239Pu, 232 Th, and 238 U by fast neutrons are listed in Table 2.9 and are shown as the familiar double-hump mass-yield curves in Figs. 2.12 and 2.13.

This situation with regard to yield and radioactive decay at each mass number is illustrated for mass number 90 in Fig. 2.14. For accurate estimation of the amount of any nuclide produced at a given time, the differential equations appropriate to such a system of yields and decays must be set up and solved. This is illustrated in Secs. 6.3 through 6.5 for selected fission-product nuclides of mass 135 and masses 147, 149, 151, and 152, which are important neutron-absorbing poisons in thermal reactors.