Iodine 135

Iodine 135

Xenon - 135. Neutron absorption and scattering. Comparison of cross-sections.
Xenon – 135. Neutron absorption and scattering. Comparison of cross-sections.
Source: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data Library

As was written, xenon 135 has a tremendous impact on the operation of a nuclear reactor. Xenon-135 is a product of U-235 fission and has a very large neutron capture cross-section (about 2.6 x 106 barns). It also decays radioactively with a half-life of 9.1 hours. Little of the Xe-135 results directly from fission, but most comes from the decay chain, Te-135 (β decay, 19 sec) to I-135 (β decay, 6.6 hr) to Xe-135. The instantaneous production rate of xenon 135 is dependent on the iodine 135 concentration and therefore on the local neutron flux history. On the other hand, the destruction rate of xenon 135 is dependent on the instantaneous local neutron flux. Its significance is primarily caused by two factors:

  • its enormous capture cross-section
  • its ultimate total yield, which is about 6.3%

In a typical nuclear reactor fueled with uranium 235, the presence of 135Xe as a fission product is of the highest importance in the power operation. At zero-power operation the presence of 135Xe may be neglected, since its concentration is very low. At power operation the situation is different.

Engineers and operators must perfectly know behaviour of a reactor with xenon 135. They are trained to properly anticipate and react to the transients induced by equilibrium or non-equilibrium xenon distribution. The presence of 135Xe is associated with the following operational issues:

  • Equilibrium Xenon – Xenon Poisoning
  • Equilibrium Xenon for Different Power Levels
  • Response to Reactor Power Changes
  • Iodine Pit – Response to Reactor Shutdown
  • Spatial Xenon Oscillations

To understand these issues, we have to perfectly understand the production and removal of xenon 135.

Production and Removal of Xenon 135

Table of total yields for various nuclei - thermal fission
Table - yields - xenon, iodine, promethium
Table of total yields for various nuclei – thermal fission.

Equilibrium Iodine

When the rate of production of iodine equals the rate of removal of iodine, equilibrium exists. This equilibrium also known as “xenon 135 reservoir”, since all of this iodine must undergo a decay to xenon. In equilibrium, the iodine concentration remains constant and is designated NI(eq). The following equation for the iodine equilibrium concentration can be determined from the preceding equation by setting the dNI/dt =0. Since the equilibrium iodine concentration is proportional to the fission reaction rate, it is also proportional to reactor power level.
iodine equilibrium - equation

See above:

Xenon 135