Neutron Poison

2 October 22, 2024

Neutron poisons are materials that absorb neutrons, thereby reducing the rate of a nuclear reaction. They are used to control the reactivity in a nuclear reactor.

One of the well-known neutron poisons in nuclear reactors is xenon-135, not “zenon-125”. Xenon-135 is a fission product and has a high cross-section for neutron absorption. After a reactor shutdown or when operating at low power, xenon-135 can build up in the reactor core.

When the reactor is at high power, the xenon-135 produced is burned off as quickly as it is produced, because the high flux of neutrons converts it into stable xenon-136. However, at low power levels, the neutron flux is insufficient to burn off the xenon-135, leading to its accumulation. This accumulation can cause a reactor to become temporarily “xenon-precluded”, making it challenging to increase the reactor power until the xenon-135 decays (with a half-life of about 9.2 hours) or is burned off. At high power levels this burn-off keeps the xenon levels stable. A lumped system for the decay concentrations is
$$\begin{matrix}
\dot I=\lambda_II+\sigma_f\Phi Y_IU^5\
\dot X_e=\lambda_II-(\sigma_{Xe}\Phi+\lambda_{Xe})Xe+\sigma_f\Phi Y_{Xe}U^5\
\dot U^5=-\sigma_a^5\Phi U^5\
\dot P_m=-\lambda_{pm}Pm+\sigma_f\Phi Y_{Pm}U^5\
\dot S_m=\lambda_{pm}Pm-\sigma_{Sm}\Phi Sm
\end{matrix}$$
Where \(Xe\), \(Pm\), \(Sm\), \(U\), are the concentrations of the relative elements.[^3] \(Y\) is the yield factor, \(\lambda\) is the decay coefficient, and \(\sigma\) is the microscopic cross-section.[^3] The subscripts \(f\) is for fission and \(a\) is for absorption.[^3] In this model, the reactivity is defined as
$$|\rho_{poison}|=f\frac{\sum_{poison}}{\sum_f^{\text{fresh fuel}}}$$

[[Iodine 135]] – can contribute to xenon increases due to decay after reactor power is lowered.
[[Nuclear Reactor Instability]] – partially caused by neutron poisoning effect
[[High Enriched Uranium]] – can overcome xenon dead-time
Submarine Reactors – use burnable poisons
Reactor Dead Time – denoted by the time for neutron poisons to decay
[[Boron]] – also used as a neutron poison in control rods
[[Nuclear Fuel]] – naval reactors must have enough reactivity to override the xenon dead time
[[Sm-149]] – samarium 149
[[Pm-149]]

Sources

Backlinks

Chernobyl Disaster
Nuclear Reactor Reactivity
[[Nuclear-Powered Submarines]]
[[Xenon-135]]