Accumulating fission product poisons[edit]
There are numerous other fission products that, as a result of their concentration and thermal neutron absorption cross section, have a poisoning effect on reactor operation. Individually, they are of little consequence, but taken together they have a significant effect. These are often characterized as lumped fission product poisons and accumulate at an average rate of 50 barns per fission event in the reactor. The buildup of fission product poisons in the fuel eventually leads to loss of efficiency, and in some cases to instability. In practice, buildup of reactor poisons in nuclear fuel is what determines the lifetime of nuclear fuel in a reactor: long before all possible fissions have taken place, buildup of long-lived neutron-absorbing fission products damps out the chain reaction. This is the reason that nuclear reprocessing is a useful activity: solid spent nuclear fuel contains about 97% of the original fissionable material present in newly manufactured nuclear fuel. Chemical separation of the fission products restores the fuel so that it can be used again.
Other potential approaches to fission product removal include solid but porous fuel which allows escape of fission products[6] and liquid or gaseous fuel (molten salt reactor, aqueous homogeneous reactor). These ease the problem of fission product accumulation in the fuel, but pose the additional problem of safely removing and storing the fission products. Some fission products are themselves stable or quickly decay to stable nuclides. Of the (roughly half a dozen each) medium lived and long-lived fission products, some, like 99
Tc, are proposed for nuclear transmutation precisely because of their non-negligible capture cross section.
Other fission products with relatively high absorption cross sections include 83Kr, 95Mo, 143Nd, 147Pm.[7] Above this mass, even many even-mass number isotopes have large absorption cross sections, allowing one nucleus to serially absorb multiple neutrons.
Fission of heavier actinides produces more of the heavier fission products in the lanthanide range, so the total neutron absorption cross section of fission products is higher.[8]
In a fast reactor the fission product poison situation may differ significantly because neutron absorption cross sections can differ for thermal neutrons and fast neutrons. In the RBEC-M Lead-Bismuth Cooled Fast Reactor, the fission products with neutron capture more than 5% of total fission products capture are, in order, 133Cs, 101Ru, 103Rh, 99Tc, 105Pd and 107Pd in the core, with 149Sm replacing 107Pd for 6th place in the breeding blanket.[9]
Decay poisons[edit]
In addition to fission product poisons, other materials in the reactor decay to materials that act as neutron poisons. An example of this is the decay of Tritium to Helium-3. Since Tritium has a half-life of 12.3 years, normally this decay does not significantly affect reactor operations because the rate of decay of Tritium is so slow. However, if Tritium is produced in a reactor and then allowed to remain in the reactor during a prolonged shutdown of several months, a sufficient amount of tritium may decay to helium-3 to add a significant amount of negative reactivity. Any Helium-3 produced in the reactor during a shutdown period will be removed during subsequent operation by a neutron-proton reaction. Pressurized Heavy Water Reactors will produce small but notable amounts of Tritium through neutron capture in the heavy water moderator, which will likewise decay to Helium-3. Given the high market value of both Tritium and Helium-3, Tritium is periodically removed from the moderator/coolant of some CANDU reactors and sold at a profit.[10] Water boration (the addition of boric acid to the moderator/coolant) which is commonly employed in pressurized light water reactors also produces non-negligible amounts of Tritium via the successive reactions 10
5B(n, α)7
3Li and 7
3Li(n,α n)3
1T or (in the presence of fast neutrons) 7
3Li(n,2n)6
3Li and subsequently 6
3Li(n,α)3
1T. Fast neutrons also produce Tritium directly from boron via 10
5B(n,2α)3
1T.[11] All nuclear fission reactors produce a certain quantity of Tritium via ternary fission.[12]