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The problem at Chernobyl was the Xenon Transient.

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PamW Donating Member (566 posts) Send PM | Profile | Ignore Mon Apr-18-11 11:47 PM
Original message
The problem at Chernobyl was the Xenon Transient.
Incidentally, the accident at Chernobyl was as a result of an experiment where they were trying to use the main turbine as a power source post LOP-trip. While this experiment was not the root cause of the accident, at was what they were trying to accomplish when they allowed themselves to get all wrapped around the axel.

The operators were conducting an experiment with the Chernobyl RBMK Unit 4. However, just
as they started the experiment and had reduced reactor power, the load controller in Kiev
called and requested that they remain online since they really needed the power. The load
controller in Kiev released the plant to go offline about 12 hours later.

When you either shutdown or lower the power greatly on a reactor, it goes through what is
called a Xenon Transient. One of the most common fission fragments is Iodine-135 which decays
quickly to Xenon-135. Xenon-135 is both radioactive with a few hour half-life, and it is also
destroyed by neutron absorption. When you shutdown the reactor, you stop the production of I-135,
but there still is the equilibrium concentration in the core, so the rate of decay of I-135 and
hence production of Xe-135 doesn't change much. However, when you shutdown the reactor, you shutdown
the destruction of Xe-135 by neutron absorption. Since the destruction rate was in equilibrium
with the production rate, the production rate of Xe-135 now exceeds the destruction rate and
the amount of Xe-135 increases. It will build up for several hours and then drop back to zero when
all the I-135 and Xe-135 have finally decayed.

What's so important about this temporary build-up of Xe-135? Xe-135 is THE world champion
neutron absorber - it is a poison to the reactor.

This buildup of Xenon was attempting to shut the RBMK down. The operators where taking excessive
actions to keep it going - like bypassing interlocks that restricted how far they could withdraw
control rods. Because of that, they put the reactor in unstable territory, and a power excursion
was the result of the instability.

The Chernobyl design was an unstable design to begin with, as detailed in these course notes from
a class at MIT:


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Throckmorton Donating Member (1000+ posts) Send PM | Profile | Ignore Tue Apr-19-11 01:24 AM
Response to Original message
1. Thanks,
Edited on Tue Apr-19-11 01:24 AM by Throckmorton
I wasn't going to go into detail about why it happened, only that the experiment they were going to attempt was in support of an LOP scenario when the main turbine continued to supply bus voltage during coast-down.

Rule 1 from license school: We don't experiment with the reactor.
Rule 2 from license school: We don't troubleshoot with the reactor.
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Turbineguy Donating Member (1000+ posts) Send PM | Profile | Ignore Tue Apr-19-11 06:18 AM
Response to Original message
2. Thanks
There was a write up in the Bulletin of the Atomic Scientists at the time but I don't have that issue anymore.
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