Xenon-135 forms directly from fission and from decay of iodine-135, and has a neutron absorption cross section of roughly 2.6 million barns. At steady power its production and burnout balance; after a shutdown or power reduction, iodine keeps decaying into xenon while burnout stops, so xenon peaks about 8 to 11 hours later and can make restart impossible until it decays, the so-called xenon dead time. In large cores, xenon can also drive slow spatial power oscillations.
We tripped six hours ago, we're heading into the xenon peak, no restart until tomorrow.
A quick restart before the xenon peak is sometimes possible after a trip; wait a few hours too long and the core is locked out until xenon decays.
Also heard as
- xenon
Related terms
Poison
Any material that absorbs neutrons strongly, whether added deliberately for control or produced by fission.
Reactivity
A measure of how far a reactor is from criticality, positive when power will rise and negative when it will fall.
Shutdown margin
How far subcritical a reactor would be with all rods inserted except the single most valuable one stuck out.
Half-life
The time it takes for half the atoms of a radioactive isotope to decay, a fixed value for each nuclide.