3.1 Neptunium Isotopes

Table 9.11 lists the isotopes of neptunium important in nuclear technology and some of their important nuclear properties.

J36Np. The isotope 236 Np is formed in reactors by (n, 2n) reactions in 231 Np. It undergoes beta decay, with a half-life of 22 h, to form 236 Pu.

237Np. The isotope 237Np is formed in considerable quantities in reactors, by the nuclide chains initiated by (n, 7) reactions in 235 U and by (n, 2n) reactions in 238 U. Neutron capture by 237 Np leads through 238 Np to 238 Pu, which is the principal alpha-emitting constituent of plutonium in power reactors. To produce 238 Pu for use as a heat source for thermoelectric devices, neptunium has been recovered from irradiated uranium to form target elements for further irradiation in reactors. Commercial processes designed for this recovery are discussed in Chap. 10.

In normal reprocessing of irradiated uranium fuel neptunium appears in the high-level wastes. Because of its long half-life of 2.14 X 10* years, 237Np persists in these wastes long after most of the fission products and other actinides have decayed. It undergoes alpha decay in the 2n + 1 decay chain to form 233 Pa, which subsequently decays to 233 U, to 229 Th, and thence to 225 Ra and its decay daughters. Because of its half-life and the radiotoxicity of its daughters, 237Np is the source of important long-term toxicity in high-level wastes. If the radionuclides in these wastes ever become dissolved in groundwater, the chemistry of neptunium is such that it may not be as effectively retarded by sorption in geologic media as are the other actinides in these wastes.

238Np. The isotope 238Np is the 2.1-day beta emitter formed by neutron capture in 237Np. With the availability of separated 237 Np from fuel reprocessing, 238 Np is easily made by irradiation of the 237Np target. It has displaced 239Np as a tracer for chemical studies. The high capture-to — fission ratio for 237 Np results in only a relatively small contamination by fission products, which are easily removed chemically [K2].

Source: C. Keller, The Chemistry of the Transuranium Elements, Verlag Chemie, Weinheim, 1971.

239Np. The isotope 239Np is formed by neutron capture in 238U or by decay of 243 Am. The latter method is the easiest for laboratory preparation, if separated americium is available. Reactor — produced americium will not produce pure 239 Np, however, because of the presence of241 Am, which decays to 237Np.