Xe Poisoning Effect

As mentioned in the previous post, poison is a type of material used to control the rate of fission reaction. In this post, we are going to discuss on one of the most important poisons in the nuclear reactor which is Xe.

As known, a major contribution to the sequence of events leading to the Chernobyl nuclear disaster was the failure to anticipate the effect of "xenon poisoning" on the rate of the nuclear fission reaction in the Chernobyl nuclear reactor.

So, we are going to examine its effect on reactivity of fission reactions through the example of Chernobyl accident.

One of the extraordinary sequences in the operation of a fission reaction is that of the production of I, iodine-135 as a fission product and its subsequent decay into Xe, xenon-135. 6% of the fission products are Iodine-135. It has a rather small probability for absorbing a neutron, so it is not in itself a significant factor in the reaction rate control. But it has a half-life of about 6.7 hours and decays into xenon-135 (half-life 9.2 hours). And Xe-135 likes neutrons very much since it has a very high probability for neutron absorption.

Xenon is produced either by decay chain from Iodine- 135 or direct yield from fission reaction. It is lost by either absorption (burn-up) of a neutron to become Xe -136 which is a weak neutron-absorber. In the normal operation of a nuclear reactor, Iodine-135 is produced, decays into xenon-135 which absorbs neutrons. It establishes the balance of the operating conditions. There is an equilibrium concentration of both iodine-135 and xenon-135.

When a new reactor is started up, it is xenon-free. When the reactor goes critical at low power (low neutron flux), a negligible Xenon is presented. When the reactor is brought up to higher power, Xenon builds up, the operators have to withdraw control rods in order or the reactor to maintain the critical state.

When a reactor is shutdown, the neutron flux is reduced. Xe is no longer produced by fission and is no longer removed by burn up (since no neutrons to be absorbed). So the only operation is I-135 decay to Xe 135 and Xe -135 decays itself.

In the Chernobyl case, there were some failures in understanding in Xenon effect. When the persons conducting the tests on the Chernobyl reactor tried to increase the power at some point in their tests, it would not respond. They apparently did not have the understanding that the failure to increase was due to the absorption of neutrons by the xenon, so they completely removed the control rods to force the increase. The increased power then burned away the xenon and also caused voids in the cooling water, both of which rapidly increased the reaction rate, driving it out of control.

So, it is essentially important for nuclear engineers to have a proper understanding and detailed knowledge as well as careful control in dealing with nuclear reactor.