Neutron poison

However, neutron-absorbing materials, also called poisons, are intentionally inserted into some types of reactors in order to lower the high reactivity of their initial fresh fuel load.

Because these two fission product poisons remove neutrons from the reactor, they will affect the thermal utilization factor and thus the reactivity.

The inability of a reactor to be restarted due to the buildup of xenon-135 (reaches a maximum after about 10 hours) is sometimes referred to as xenon precluded start-up.

During periods of steady state operation, at a constant neutron flux level, the xenon-135 concentration builds up to its equilibrium value for that reactor power in about 40 to 50 hours.

Thus, the dynamics of xenon poisoning are important for the stability of the flux pattern and geometrical power distribution, especially in physically large reactors.

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.

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.

Of the (roughly half a dozen each) medium lived and long-lived fission products, some, like 99Tc, are proposed for nuclear transmutation precisely because of their non-negligible capture cross section.

[7] Above this mass, even many even-mass number isotopes have large absorption cross sections, allowing one nucleus to serially absorb multiple neutrons.

Fixed burnable poisons are generally used in the form of compounds of boron[13] or gadolinium that are shaped into separate lattice pins or plates, or introduced as additives to the fuel.

[15] Soluble poisons, also called chemical shim, produce a spatially uniform neutron absorption when dissolved in the water coolant.

The changing of boron concentration in a PWR is a slow process and is used primarily to compensate for fuel burnout or poison buildup.

[14] All commercial PWR types operating in the US (Westinghouse, Combustion Engineering, and Babcock & Wilcox) employ soluble boron to control excess reactivity.

[citation needed] One known issue of boric acid is that it increases corrosion risks, as illustrated in a 2002 incident at Davis-Besse Nuclear Power Station.

During SCRAM the operators can inject solutions containing neutron poisons directly into the reactor coolant.