The synthetic isotopes 237U and 233U occur in the 4л + 1 radioactive series, of which 237Np is the longest-lived member. Figure 5.3 shows the nuclear reactions that occur successively as these nuclides decay into their nearly stable end product 209Bi. Table 5.4 gives the half-lives of these radioactive species and their principal decay radiations. Radiations from 229 Th and its short-lived daughters will be the most important contributors to the remaining toxicity of high-level wastes from irradiated reactor fuels containing 237 Np, after such wastes have been in storage for several hundred thousand years, when these daughters will be in secular equilibrium with 162,000-year 233 U.