Page 117 - Radiochemistry and nuclear chemistry

Radionuclides in Nature 103

occurrence. However, Np has been discovered in the spectrum of some stars. All known
Np-isotopes are presented in Figure 16.1.
Very small amounts of 237Np, as well as of 239pu, have been discovered on earth; the
half-lives of 239pu (in the 4n + 3 series) is 2.411 x 104 y. Both isotopes are too short-lived
to have survived the 4 cons since the solar system was formed. However, they are always
found in minerals containing uranium and thorium and it is believed that the neutrons
produceA in these minerals through (a,n) and (7,n) reactions with U and Th as well as by
spontaneous fission of 238U form the neptunium and plutonium through n-capture and
/3-decay processes. The n-production rate in the uranium,,/]~ne~;~l, pitchblende (containing
--50 % U) is about 50 n/kg s. The typical value for the ~~176176 ratio in minerals is 3
• 10 -12
The end product of the neptunium series is 209 Bi, which is the only stable isotol~ of
bismuth. Seven a- and four/3-decays are required in the sequence from the parent 237Np
to 209 Bi. An important nuclide in the neptunium decay series is the uranium isotoI~ 233 U,
which has a half-life of 1.59 • 105 y (the most stable intermediate) and, like 235U, is
fissionable by slow neutrons.
The long-lived plutonium isotope 244pu (belonging to the 4n series; see also Fig. 16.1),
which decatvs through a-emission and spontaneous fission (0.13 %) with a total half-life of
8.26 • 10"y, was discovered in rare earth minerals in 1971. If this is a survival of
15
l~rimeval 244 Pu, only 10- % of the original can remain. An alternate possibility is that this
244pu is a contaminant from cosmic dust (e.g. from a supernova explosion in more recent
times than the age of the solar system).

5.4. Thorium

5.4.1. Isotopes

Natural thorium consists 100% of the isotope 232Th which is the parent nuclide of the
thorium decay series. The specific radioactivity for thorium is lower than that of uranium,
and it is normally treated as a non-radioactive element. For radioactive tracer studies the
nuclide 234Th (tl,~ 24.1 d) is used after separation from natural uranium.

5.4.2. Occurrence and production

Thorium is somewhat more common in nature than uranium, with an average content in
the earth's crust of 10 ppm (by comparison the average abundance of lead is about 16 ppm
in the earth's crust). In minerals it occurs only as oxide. The content of thorium in sea
water is < 0.5 • 10 3 g/m 3, which is lower than that of uranium because of the lower
solubility of Th 4+ compounds (the most stable valency state of Th).
The most common thorium mineral is monazite, a golden brown rare earth phosphate
containing 1 - 15 % ThO 2 and usually 0.1 - 1% U30 8. It is also found in small amounts in
granite and gneiss. The largest deposits of monazite are found in India, Egypt, South
Africa, the USA, and Canada, with 200 - 400 kton ThO 2 in each country. The size of
natural resources are def'med in terms of ore reserves which can be economically processed.
Thus, the total reserves at commercial price in 1991 was estimated to > 2 Mt ThO 2.