Neutron absorber
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Neutron absorbers are isotopes of certain elements that absorb free neutrons creating heavier isotopes of the same element. The most prolific neutron absorbers are elements that become stable by absorbing a neutron such as xenon-133 (Xe-133), which absorbs a neutron to become Xe-134. Xe-133 has a half-life of 5.2 days while Xe-134 is stable. Xe-133 is formed in nuclear reactors through the splitting of actinide metals indirectly as a decay product of iodine-133 (I-133), which also has a short half-life. Other isotopes that are major neutron absorbers include helium-3 (He-3), which becomes He-4 and boron-11 (B-11) which becomes B-12, which has a half-life of only 0.2 seconds and decays into carbon-12 (C-12), the most common isotope in nature. Samarium-149 formed during the fission process is also a highly effective neutron absorber, with its very long half life it last effectively forever in the fuel until it absorbs a neutron and transmutes into Sm-150, which is stable. Other neutron absorbers used in nuclear power plants include cadmium and gadolinium, both of which consist of mixed isotopes some of which are voracious neutron absorbers.
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