When Enrico Fermi fired up the first nuclear reactor at Hanford in 1944, he was in for quite a surprise. Shortly after the reactor went critical, power stalled and the reactor shut down. A few hours later, the reactor unexpectedly started up again all by itself. This was the result of poisoning brought on by Xenon 135 (Xe).
In nuclear terms, poison is used to describe a substance that has a strong ability to absorb neutrons. Think of neutrons as the oxygen needed for a fire to burn. Put water on the fire, and you rob the fuel of the oxygen needed for the fire, and the flames cease. Likewise, when you remove the free neutrons from a fission reaction, the reactor goes sub-critical and shuts down.
Xenon 135 is one of the most common and troublesome poisons found in nuclear reactors. It is a radioactive isotope of Xenon which is predominately formed by the beta decay of Iodine 135. The iodine is a common byproduct of the fission of Uranium 235. When the Uranium splits, it releases several neutrons and also two smaller daughter elements, among them Iodine 135.
Once it appears in the reactor, Xenon is removed in one of two ways. The first is by neutron absorption. When Xe-135 captures a neutron, it becomes Xe-136, a stable isotope which does not absorb neutrons. This process is referred to as the burn up or burn out of Xenon, where the poisonous Xenon is removed.
The second way Xenon is removed is by beta decay. Xe-135 has a half life of about 9 hours. I-135 has a half life of about 6.5 hours. This time differential is one of the factors that makes Xenon such a problem for nuclear reactors. Since Xenon takes longer to decay than the Iodine takes to build in the Xenon, then there is a natural tendency for Xenon levels to increase in a reactor when not at equilibrium.
When a reactor is at equilibrium Xenon, the rate that Iodine decays into Xenon-135 (build in) is equal to the rate Xe-135 decays plus the rate of Xenon burn out.
The real problem with Xenon comes into play when power levels in the reactor change. When power rapidly decreases in the reactor, the rate of Xenon burn out drops. However, the existing I-135 continues to decay and produce more Xe-135. This causes Xenon levels to increase, bringing the available neutrons down and lowering power. A few hours later, as I-135 production slows, the Xe-135 levels off and power rises again. So you havn’t touched anything, but power is now higher than you left it.
The converse is also true. When power is rapidly increased, the rate of Xenon-135 burn out rises sharply but the I-135 decay remains unchanged. This causes a lowering of Xe-135 concentration and an increase in power. Eventually, the rate of I-135 production and decay along with the rate of X-135 production and burnout reach equilibrium. Now power is lower than you left it because Xenon has built back in.
The end result of this is Xenon 135 is a major nuisance to Nuclear Reactor Operators and Core Engineers. The solution is placing limits on the rate at which a plant raises and lowers power. This enables operators to keep a close eye on Xenon and make sure the reactor is running in a safe and controlled manner. The operators at Chernobyl didn’t. As you will see in a later post, Xenon-135 was one of the many factors that combined to cause the catastrophe at the Russian reactor that would change the nuclear industry forever.
~Man Overboard
Image used in this Post
Xenon image courtesy of Wikimedia Commons published under the CC license.
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So I suppose “we’re all created equal” doesn’t apply?
haa ~R
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[...] low power, with fewer neutrons in the core, Xenon-135 built in at an accelerated rate, and the operators noticed that power was dropping further. They [...]