XENON PRODUCTION & RESPONSE

XENON

Xenon-135 Response to Reactor Shutdown

Because the decay rate of iodine-135 is faster than the decay rate of xenon-135, the xenon concentration builds to a peak. The peak is reached when the product of the terms λINI is equal to λXeNXe (in about 10 to 11 hours). Subsequently, the production from iodine decay is less than the removal of xenon by decay, and the concentration of xenon-135 decreases. The greater the flux level prior to shutdown, the greater the concentration of iodine-135 at shutdown; therefore, the greater the peak in xenon-135 concentration after shutdown. This phenomenon can be seen in Figure 5, which illustrates the negative reactivity value of xenon-135 following shutdown from various neutron flux levels.


During the early operation of the Stagg Field and Hanford reactors, the scientists found that some nuclides were produced that caused the reactor to shutdown. These nuclides came to be known as - neutron poisons - because they stopped the neutrons from being absorbed in the fuel and causing fission. Xenon-135 was one of the first poisons identified. For a reactor just starting up, the behaviour of xenon-135 is shown below.

Xenon-135 (Xe135)
Xenon-135 is one of the many products produced from the fissioning of uranium, thorium, or plutonium nuclides. Xe135 is produced directly from fission and from the BETA decay of Tellurium-135, as shown below. The Xe135 subsequently beta decays to Cesium-135 then to Barium-135. The half-lives are shown in RED below the line.

Te135 ==============> I135 ==============> Xe135 ==============> Cs135 ==============> Ba135

<0.5 min 6.7 hr 9.2 hr 2 x 106 yr
In reality, since reactors contain the various isotopes of uranium and plutonium in the fuel in varying concentrations throughout the core life, the actual equation must account for the production of xenon and iodine from these various fissile nuclides.

Note that the xenon peaks then falls off to almost xenon-free after 72 hours.