236 U and 237 U are produced by successive neutron captures in fuel containing 238 U. Both isotopes are detrimental contaminants. Long-lived 236 U is a neutron absorber that reduces the fuel’s reactivity. It has an atomic mass between 23SU and 238 U, which makes subsequent isotopic reenrichment more difficult, as described in Sec. 15 of Chap. 12. The 6.7-day half-life of 237 U necessitates storage of irradiated uranium for around 150 days if its radioactivity is to be no higher than that of natural uranium, as explained in Chap. 8. И7и decays to 2.14- million-year 237 Np, the longest-lived member of the 4л + 1 radioactive decay series.

1.3 239U

239U is produced by neutron capture in fuel containing 238 U. It decays to 239 Pu through two successive beta emissions, as described in Fig. 3.1. Because of its short, 23.5-min half-life, it is not present after irradiated fuel has been stored. It is, however, a significant contributor to decay heat production immediately after reactor shutdown.