The term “higher actinides” is used for the man-made nuclides of elements with atomic number of 93 or greater. They originate from neutron capture in uranium and are sometimes called “trans-uranium elements” or “trans-uranics”. In practice the term is also often used for the man-made nuclides formed by neutron capture in thorium.

The most important of the higher actinides, both for reactor oper­ation and commercially, are 239Pu and 233U produced by neutron cap­ture in 238U and 232Th respectively, as explained in the Introduction. They are both beginnings of long and complicated chains of reactions, mainly neutron captures and p decays, that produce many isotopes of several elements. Figures 1.14 and 1.15 are simplified diagrams of these chains. The horizontal arrows represent neutron captures and the vertical ones p — (upward) or p+ (downward) decays.

Figures 1.14 and 1.15 include the reactions that are of greatest prac­tical importance, but many details have been omitted in the interest of clarity. For example 243Am is p -active but this is not significant because its half-life is 7370 years. Almost all the nuclides in both diagrams that are not p-active are subject to а-decay with half-lives of greater than 1000 years so they are of little importance as far as reactor operation is concerned. (They are much more important in reprocessing and waste storage because the radiation is hazardous.) 241Am however has

a half-life of 433 years to produce 237Np, which in turn is а-active with a half-life of 2.1 x 106 years. This makes it one of the longest lived hazardous nuclear waste products.

Many of the nuclides decay by spontaneous fission but in most cases it is unimportant because the half-lives are so long. The excep­tions are 242Cm and 244Cm which have spontaneous fission half-lives of 7.2 x 106 and 13.2 x 106 years respectively. The neutrons generated are insignificant in normal operation of the reactor but not when it is shut down. As the quantity of 241Pu changes with burnup, so do the quantities of the curium isotopes and hence the neutron source strength. As a result the relationship between shutdown reactivity and subcritical power level varies both with burnup and also with time while the reactor is shut down due to 242Am and 244Am.

Almost all of the (n, y) reactions indicated in both diagrams are mirrored by (n,2n) reactions going in the opposite direction. Most of the latter can be ignored because the (n,2n) cross-sections are very small. However, there is one that is of some significance because it leads to the production of a particularly unpleasant waste product. Figure 1.16 show how the (n,2n) reaction in 233Pa leads eventually, after a series of a — and p-decays, to the production of 208Tl. This is intensely radioactive, emitting 2.6 MeV ys that pose a serious problem in a reprocessing plant.