Xenon poisoning is a phenomenon that occurs during the operation of nuclear reactors and affects the reactivity of the reactor and, consequently, its safety and efficiency.
This concept is closely related to the internal nuclear processes of the reactor, in particular to the formation and disintegration of certain isotopes generated during nuclear fission.
What is xenon poisoning?
Xenon poisoning refers to the accumulation of xenon-135 (¹³⁵Xe) in the reactor core.
This isotope is a by-product of nuclear fission and has an extremely high capacity to absorb neutrons, which reduces the reactor's reactivity. Reactivity, in this context, measures the reactor's ability to sustain a chain reaction.
When too much xenon-135 accumulates, it can "poison" the core by absorbing neutrons that would otherwise drive the nuclear reaction.
Xenon-135 has a cross section (a measure of its propensity to capture neutrons) of about 2 million barns, making it one of the most powerful neutron absorbers known.
This property means that even small amounts of xenon-135 can have a significant impact on reactor operation.
How xenon-135 is generated
Xenon-135 is formed via two main pathways:
- Decay of iodine-135 : During nuclear fission, isotopes such as iodine-135 (¹³⁵I) are produced, which has a half-life of approximately 6.57 hours. This isotope decays by beta emission to form xenon-135.
- Direct production : A small fraction of xenon-135 is produced directly during the fission of nuclear fuel.
Once generated, xenon-135 can follow two paths:
- Absorb a neutron and become xenon-136, a stable isotope.
- Decays by beta emission with a half-life of 9.2 hours to form caesium-135 (¹³⁵Cs).
The concentration of xenon-135 in the reactor depends on a complex balance between its production and disposal.
Effects of xenon poisoning on reactor operation
Xenon poisoning has several important effects on reactor operation:
- Reduced reactivity : As the concentration of xenon-135 increases, neutron absorption increases, which decreases reactivity. This can make reactor control more difficult, especially during power changes.
- Post-shutdown effect : After a reactor is shut down, the concentration of xenon-135 increases due to the decay of residual iodine-135. This increase can prevent the reactor from restarting immediately, a phenomenon known as "transient poisoning."
- Power Oscillations : In large reactors, such as boiling water reactors (BWR), xenon poisoning can cause power oscillations in different regions of the core, posing problems in maintaining stable operation.
Dynamics of xenon poisoning
The behavior of xenon-135 in a reactor is influenced by several factors:
- Reactor power : At high powers, xenon-135 production is accelerated due to the increased fission rate. However, it is also eliminated more quickly by absorbing neutrons.
- Reactor shutdown : When the reactor is shut down, production of xenon-135 ceases, but its concentration continues to increase temporarily due to the decay of iodine-135.
- Reactor restart : During restart, the high concentration of xenon-135 can absorb so many neutrons that it becomes difficult to achieve a sustained chain reaction.
Strategies for managing xenon poisoning
Managing xenon poisoning is a critical task in the operation of nuclear reactors.
Some strategies include:
- Rod Control : Control rods can be adjusted to compensate for changes in reactivity caused by xenon-135.
- Operational Planning : Operators plan shutdowns and power changes taking into account the behavior of xenon-135 to minimize its impact.
- Advanced reactor designs : Some modern reactors use designs that reduce the effects of xenon poisoning, such as the use of moderating materials that affect neutron capture.
Importance of xenon poisoning in nuclear safety
From a safety perspective, xenon poisoning is a phenomenon that operators must constantly monitor.
Unexpected accumulation of xenon-135 can lead to dangerous operating conditions, such as the inability to control reactivity or restart the reactor after a shutdown.
In addition, power fluctuations caused by xenon can generate uneven heat distributions in the core, increasing the risk of fuel damage.
Historical examples
Xenon poisoning has played a role in historic nuclear incidents.
For example, in the Chernobyl disaster in 1986, inadequate handling of xenon poisoning contributed to the instability that led to the reactor explosion. Although this was not the only factor, it illustrates the importance of properly understanding and managing this phenomenon.