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).
Xenon ‘Poisoning’
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.
Xenon Removal
Neutron Absorption
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.
Beta Decay
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.
Xenon and Reactor Power Levels
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 haven’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 was outlined in an earlier 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.
Image Credit
Xenon image courtesy of Wikimedia Commons published under the CC license.
11 Comments
Somehow I have a feeling that this sentence is going to end up being quoted out of context.
You might be right, but anyone can quote just about any sentence out of context.
I think it’s crazy how people are so willing to jump on any imperfection or challenge with a process, idea, etc, and trump all the good based on so little “bad”. Brian, I am definitely not talking about you, I’m just philosophizing here. Sorry if it didn’t seem that way.
I try and use a term like “neutronic poisons” instead of just “poisons” to avoid that kind of potential quotemining
They’re welcome to take me out of context, it’s what they do and there’s really no avoiding it. Luckily reasonably intelligent as well as moderately stupid people see through it.
This was a very helpful post. Thank you.
One thing I was thinking about was the entire iodine issue. If you follow my posts on another list, you may have noticed me dealing with a man who believes people are getting sick in France, RIGHT NOW, from the iodine from Chernobyl. Yet iodine’s half-life is so short that by the time the “cloud” got to France, there couldn’t have been much left. I suppose I could have read this information in all sorts of publications, but seeing the short half-life of iodine as a problem for reactor operators was illuminating.
Meredith- I am also skeptical of radioactive Iodine currently being a problem in France from the Chernobyl disaster. However, there are several different isotopes of Iodine, and many of them can be produced from nuclear fission. I-129, for instance, has a half-life of 15.7 million years.
http://en.wikipedia.org/wiki/Isotopes_of_iodine
The idea of Iodine from Chernobyl causing a cancer problem in France is untrue. ‘They’ conjure up this garbage everywhere a nuclear plant is found because ‘they’ have nothing real to use against us.
For many thyroid conditions, Doctors prescribe massive doses of radioiodine as treatment, as much as 30 millicuries, or as Helen Caldicott would say “30 Million Picocuries!!!!”
This dose is MEANT to be INGESTED, not an accidental breath of a few thousand picocuries mind you, we’re talking millions of them ingested! This is what Doctors tell you to take. Nuclear plant employees receiving this treatment can’t go in to work because they would set off the radiation monitors from 300 feet away in the parking lot!
So I would seriously like to read the data they’re using for this ‘study’…I could use a good laugh.
One way to make Xe a non issue is to provide a sufficient Xe override capability with a bit of excess reactivity and control rods. It may not be the most efficient or technically elegant way to solve the problem, but it works fine and lasts a long time, 33 years in some particular systems that I know reasonably well.
This is, in fact, exactly how research reactors operate. The excess reactivity is limited by the NRC to about $8.00, but this is quite significant and will overcome any Xenon build up. I work at a reactor and when we start up from a clean (xenon free) core, we have to pull blades for about 16 hours until xenon equilibrates. If we shutdown and then start back up before the xenon decays away (~60 hours) we actually have to insert rods into the core because power creeps up. The rate of xenon buildup is very slow, so this is not a sudden thing, we are talking <1% power over 1 hour.
As Eugene Wigner once pointed out to Enrico Fermi, if you continuously remove Xenon-135, you can breed in a high power density thermal neutron reactor. How can you do that, Fermi asked.
Dissolve the uranium in a carrier fluid, run the fluid into a core with a large enough volume to achieve a critical mass, and then bubble the Xenon-135 out of the reactor, Wigner replied.
But U-235 will not give enough neutrons to breed in a thermal reactor, Fermi said.
Yes but U-233 will, Wigner answered
@Charles Barton:
I just love your attitude about the U-233! Someday it WILL happen.
@Rod Adams:
Yes, ca. 1985, one of the dual D2G machines (USS California) liked to go from “modlock” 15% or so power to 100%. In just a few minutes. And then back to 15%. These were “drills,” i.e. just so we could show we could do it– you know, practice. The core was 13 years old or so. (Don’t remember the EFPHs, but it didn’t wear out ’til ’92). Yeah, the reactor operator had to “shim” a little in and out for a few hours after, but hey, that was his job if the Captain wanted to water-ski behind a rooster tail for a few minutes.
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[...] on the 200 fission products had been incomplete, and it turned out that some fission products like Xe-135 had large cross-sections to absorb neutrons, so after a few hours enough had been produced to [...]