Accumulating Fission Product Poisons
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 impact. 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 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.
Other fission products with relatively high absorption cross sections include 83Kr, 95Mo, 143Nd, 147Pm. 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.
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.
Read more about this topic: Neutron Poison
Famous quotes containing the words accumulating, fission, product and/or poisons:
“A way of certifying experience, taking photographs is also a way of refusing it—by limiting experience to a search for the photogenic, by converting experience into an image, a souvenir. Travel becomes a strategy for accumulating photographs.”
—Susan Sontag (b. 1933)
“The pace of science forces the pace of technique. Theoretical physics forces atomic energy on us; the successful production of the fission bomb forces upon us the manufacture of the hydrogen bomb. We do not choose our problems, we do not choose our products; we are pushed, we are forced—by what? By a system which has no purpose and goal transcending it, and which makes man its appendix.”
—Erich Fromm (1900–1980)
“Labor is work that leaves no trace behind it when it is finished, or if it does, as in the case of the tilled field, this product of human activity requires still more labor, incessant, tireless labor, to maintain its identity as a “work” of man.”
—Mary McCarthy (1912–1989)
“The internal effects of a mutable policy ... poisons the blessings of liberty itself.”
—James Madison (1751–1836)