page_title: Iodine pit
section_title: Iodine pit behavior
hierarchical_section_title: Iodine pit / Fission products decay and burnup / Iodine pit behavior
caption_attribution_description: Русский: Иодная ямаГрафик зависимости концентрации ксенона и реактивности ядерного реактора с момента его выключения English: Iodine PitGraph showing the concentration of Xenon and the reactivity of the nuclear reaction from the moment the reactor is shutdown.
context_page_description: The iodine pit, also called the iodine hole or xenon pit, is a temporary disabling of a nuclear reactor due to buildup of short-lived nuclear poisons in the reactor core. The main isotope responsible is ¹³⁵Xe, mainly produced by natural decay of ¹³⁵I. ¹³⁵I is a weak neutron absorber, while ¹³⁵Xe is the strongest known neutron absorber. When ¹³⁵Xe builds up in the fuel rods of a reactor, it significantly lowers their reactivity, by absorbing a significant amount of the neutrons that provide the nuclear reaction.
  The presence of ¹³⁵I and ¹³⁵Xe in the reactor is one of the main reasons for its power fluctuations in reaction to change of control rod positions.
  The buildup of short-lived fission products acting as nuclear poisons is called reactor poisoning, or xenon poisoning. Buildup of stable or long-lived neutron poisons is called reactor slagging.
context_section_description: The reactivity of the reactor after the shutdown first decreases, then increases again, having a shape of a pit; this gave the "iodine pit" its name. The degree of poisoning, and the depth of the pit and the corresponding duration of the outage, depends on the neutron flux before the shutdown. Iodine pit behavior is not observed in reactors with neutron flux density below 5×10¹⁶ neutrons m⁻²s⁻¹, as the ¹³⁵Xe is primarily removed by decay instead of neutron capture. As the core reactivity reserve is usually limited to 10% of Dk/k, thermal power reactors tend to use neutron flux at most about 5×10¹³ neutrons m⁻²s⁻¹ to avoid restart problems after shutdown.
  The concentration changes of ¹³⁵Xe in the reactor core after its shutdown is determined by the short-term power history of the reactor (which determines the initial concentrations of ¹³⁵I and ¹³⁵Xe), and then by the half-life differences of the isotopes governing the rates of its production and removal; if the activity of ¹³⁵I is higher than activity of ¹³⁵Xe, the concentration of ¹³⁵Xe will rise, and vice versa.
  During reactor operation at a given power level, a secular equilibrium is established within 40–50 hours, when the production rate of iodine-135, its decay to xenon-135, and its burning to xenon-136 and decay to caesium-135 are keeping the xenon-135 amount in the reactor constant at a given power level.
  The equilibrium concentration of ¹³⁵I is proportional to the neutron flux φ. The equilibrium concentration of ¹³⁵Xe, however, depends very little on neutron flux for φ > 10¹⁷ neutrons m⁻²s⁻¹.
  Increase of the reactor power, and the increase of neutron flux, causes a rise in production of ¹³⁵I and consumption of ¹³⁵Xe. At first, the concentration of xenon decreases, then slowly increases again to a new equilibrium level as now excess ¹³⁵I decays. During typical power increases from 50 to 100%, the ¹³⁵Xe concentration falls for about 3 hours.
  Decrease of the reactor power lowers production of new ¹³⁵I, but also lowers the burn rate of ¹³⁵Xe. For a while ¹³⁵Xe builds up, governed by the amount of available ¹³⁵I, then its concentration decreases again to an equilibrium for the given reactor power level. The peak concentration of ¹³⁵Xe occurs after about 11.1 hours after power decrease, and the equilibrium is reached after about 50 hours. A total shutdown of the reactor is an extreme case of power decrease.
caption: Development of (1) concentration of 135Xe and (2) reactor reactivity after reactor shutdown. (Until shutdown the neutron flux was φ=1018 neutrons m−2s−1.)
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