Xenon - An Introduction to Neutron Poisons


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 behavior of xenon-135 is shown below.

xenon11.gif (6826 bytes)

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 simple terms, the equilibium concentration of the xenon is determined by the equation:

Xeeq = lII0 + gISfF   


 lXe + sXeF   


Symbol Represents
Xeeq Equilibrium xenon concentration


Iodine-135 decay constant, (0.693 / t1/2) and t1/2 is half life for iodine 135, or 6.58 hrs.
lXe + sXe Xenon-135 decay constant,  (0.693 / t1/2) and t1/2 is half life for xenon 135, or 9.14 hrs.
Ieq         Equilibrium iodine 135 concentration
gI Fission yield of iodine 135
S Macroscopic fission cross section
sXe Microscopic cross section for xenon-135
F Neutron flux

  • In reality, since reactors contain the various isotopes of uranium and plutonium in the fuel in varying concentrtaions throughout the core life, the actual equation must account for the production of xenon and iodine from these various fissile nuclides.
  • Following a shutdown the xenon concentration increases as shown in the figure below:

    xenon13.gif (12720 bytes)

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

    Further investigation shows that the size of the increase and the time to peak depends on the power level the reactor has been operating at, as shown below:

    xenon12.gif (16335 bytes)

    If the reactor is to be started up when the xenon is at the peak, there must be sufficient fuel and control rod worth to overcome the effects of the xenon.

    Other neutron poisons buildup in the reactor and their effects must be similarly considered in the design and operation. These include:

    Other neutron poisons are purposely used in the design and operation of reactors. These include:

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