Thorium is important in nuclear technology as the naturally occurring fertile nuclide from which neutron capture produces fissile 233 U by the succession of reactions

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In thermal-neutron reactors 233 U has an important advantage over 235 U or 239 Pu in that the number of neutrons produced per thermal neutron absorbed, rj, is higher for 233 U than for the other fissile nuclides. Table 6.1 compares the 2200 m/s cross sections and neutron yields in fission of these three nuclides. Thorium has not heretofore been extensively used in nuclear reactors because of the ready availability of the 235 U in natural or slightly enriched uranium. As natural uranium becomes scarcer and the conservation of neutrons and fissile material becomes more important, it is anticipated that production of 233 U from thorium will become of greater significance.

Compared with 239Pu, the other synthetic fissile nuclide, 233U, has the advantage that it can be “denatured,” made less available for use as a nuclear explosive, by isotopic dilution with 238 U in a mixture containing less than 12 percent 233 U. Production of a nuclear explosive from such a mixture would require costly and difficult isotope separation (Chap. 14). No similar means exists for denaturing 239Pu, which can be more readily separated from 238U by chemical reprocessing (Chap. 10).

In addition to its potential use in nuclear power systems, thorium has had minor industrial use in Welsbach mantles for incandescent gas lamps, in magnesium alloys to increase strength and creep resistance at high temperatures, and in refractories.