Neutron poison

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 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 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. 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.

Decay poisons

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, as helium-3 has a high absorption cross section for thermal neutrons, producing tritium and hydrogen again. 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. 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. All nuclear fission reactors produce a certain quantity of tritium via ternary fission.

Control poisons

During operation of a reactor, the amount of fuel contained in the core decreases monotonically. If the reactor is to operate for a long period of time, fuel in excess of that needed for exact criticality must be added when the reactor is fueled. The positive reactivity due to the excess fuel must be balanced with negative reactivity from neutron-absorbing material. Movable control rods containing neutron-absorbing material is one method, but control rods alone to balance the excess reactivity may be impractical for a particular core design as there may be insufficient room for the rods or their mechanisms, namely in submarines, where space is particularly at a premium.

Burnable poisons

To control large amounts of excess fuel reactivity without control rods, burnable poisons are loaded into the core. Burnable poisons are materials that have a high neutron absorption cross section that are converted into materials of relatively low absorption cross section as the result of neutron absorption. Due to the burn-up of the poison material, the negative reactivity of the burnable poison decreases over core life. Ideally, these poisons should decrease their negative reactivity at the same rate that the fuel's excess positive reactivity is depleted.

Fixed burnable poisons are generally used in the form of compounds of boron or gadolinium that are shaped into separate lattice pins or plates, or introduced as additives to the fuel. Since they can usually be distributed more uniformly than control rods, these poisons are less disruptive to the core's power distribution. Fixed burnable poisons may also be discretely loaded in specific locations in the core in order to shape or control flux profiles to prevent excessive flux and power peaking near certain regions of the reactor. Current practice however is to use fixed non-burnable poisons in this service.

Non-burnable poison

A non-burnable poison is one that maintains a constant negative reactivity worth over the life of the core. While no neutron poison is strictly non-burnable, certain materials can be treated as non-burnable poisons under certain conditions. One example is hafnium. It has five stable isotopes, ¹⁷⁶
Hf through ¹⁸⁰
Hf, which can all absorb neutrons, so the first four are chemically unchanged by absorbing neutrons. (A final absorption produces ¹⁸¹
Hf, which beta-decays to ¹⁸¹
Ta.) This absorption chain results in a long-lived burnable poison which approximates non-burnable characteristics.

Soluble poisons

Soluble poisons, also called chemical shim, produce a spatially uniform neutron absorption when dissolved in the water coolant. The most common soluble poison in commercial pressurized water reactors (PWR) is boric acid, which is often referred to as soluble boron. The boric acid in the coolant decreases the thermal utilization factor, causing a decrease in reactivity. By varying the concentration of boric acid in the coolant, a process referred to as boration and dilution, the reactivity of the core can be easily varied. If the boron concentration is increased (boration), the coolant/moderator absorbs more neutrons, adding negative reactivity. If the boron concentration is reduced (dilution), positive reactivity is added. The changing of boron concentration in a PWR is a slow process and is used primarily to compensate for fuel burnout or poison buildup.

The variation in boron concentration allows control rod use to be minimized, which results in a flatter flux profile over the core than can be produced by rod insertion. The flatter flux profile occurs because there are no regions of depressed flux like those that would be produced in the vicinity of inserted control rods. This system is not in widespread use because the chemicals make the moderator temperature reactivity coefficient less negative. All commercial PWR types operating in the US (Westinghouse, Combustion Engineering, and Babcock & Wilcox) employ soluble boron to control excess reactivity. US Navy reactors and Boiling Water Reactors do not. One known issue of boric acid is that it increases corrosion risks, as illustrated in a 2002 incident at Davis-Besse Nuclear Power Station.

Soluble poisons are also used in emergency shutdown systems. During SCRAM the operators can inject solutions containing neutron poisons directly into the reactor coolant. Various aqueous solutions, including borax, sodium polyborate, and gadolinium nitrate, are used.