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From: (B. Alan Guthrie)
Newsgroups: talk.environment,sci.environment,,sci.chem,,sci.physics
Subject: Re: Nuclear wins biomass versus nuclear debate
Date: 3 Oct 1997 13:01:17 GMT

In article <6117hp$>,
Michael Kagalenko <> wrote:
>Greig Ebeling ( wrote in article <>
>]On 1 Oct 1997 02:18:36 -0400,
>](Scott Nudds) wrote:
>]>(Michael Kagalenko) wrote:
>]>:  Before
>]>:  experiment on RMBK reactor at Chernobyl plant commenced, the reactor
>]>:  was operated for several days at low power. That was explicitely
>]>:  prohibited,
>]>:  but operators were forced to do it because of political pressure to
>]>:  continue
>]>:  to supply energy. That resulted in iodine poisoning of the core and
>]>:  made possible the accident.
>]>  This simply does not make sense.  Reactors are not run at low power to
>]>produce energy, they are run at high power.
>]As I understand it, the low power output was due to the xenon
> You understand wrongly. Please stop spreading nonsense.

     The neutron absorption cross section for iodine-135 is so
     small that I have no idea what it is.  When performing
     reactivity calculations (which I do almost daily), I do
     not account for the neutron absorption in I-135.  Xenon-135,
     OTOH, has a neutron absorption cross section of several
     million barns (depending on the neutron spectrum).  Dr.
     Ebeling is indeed correct - xenon poisoning was a major
     factor in the Chernobyl accident.

     By comparison, the neutron absorption cross section in
     U-235 is something around 500 or 600 barns (all cross
     sections herein quoted are for thermal neutrons in a
     PWR spectrum).

B. Alan Guthrie, III          |   Quis Custodiet Ipsos Custodes?
                              | |   My opinions only

From: (B. Alan Guthrie)
Subject: Re: Iodine and Xenon Poisoning
Date: 10 Oct 1997 15:19:28 GMT

In article <>,
Michael C Baker  <> wrote:
>Axel Berger wrote:
>> I don't know the term either, but that is no surprise, as it is quite
>> common for Americans to treat international groups as local ones and
>> fill them with (for us foreigners) unintelligible Americanisms. What
>> one can say is, that common or not the term makes sense. After all the
>> Xenon itself is at equilibrium during periods of constant load and it
>> is the decay of the Iodine, which you can't do anything about, which
>> leads to the buildup after every decrease of the power level.
>The decreased rate of neutron capture by xenon is also a significant
>factor in the initial increase in core xenon concentration.  I believe
>that the initial rate of change is more dependent on this factor due to
>the rather long half life of iodine (6.7 hours) although the peak
>xenon concentration during the transient is dependendent on the iodine
>concentration prior to the transient.

   I am not disagreeing with what you wrote above.  I just want to
   take this opportunity to clarify, I hope, the dynamics of xenon.

   Our reactor is chugging along at full power, with the iodine and
   xenon in equilibrium (ie, the rates of change of the iodine and
   the xenon concentrations are xero.

   The peritent reactions are

        dI/dt  =  production from fission  -  decay to Xe-135

	dX/dt  =  production from fission + decay from I-135 -
	          decay of Xe-135 - absorption of neutron by Xe-135

   The fission yield of I-135 is something like 6% while the fission
   yield of Xe-135 is much less (but not negligible).  The half-life
   of I-135 is about 6.5 hrs while that of Xe-135 is 9.2 hours or so.
   At this time, the destruction of xenon by decay and neutron
   abosorption is the same as production xenon by I-135 decay and
   xenon production from fission.  The neutron absorption cross section
   for Xe-135 is several million barns in a PWR spectrum.

   The reactor power is reduced.  The production of Xe-135 from
   decay of I-135 does not change initially since the iodine concentration
   initially is the full-power equilibrium value.  But the
   destruction of xenon by neutron absorption is much less because
   the neutron flux has been decreased.  Thus, the xenon concentration
   will begin to increase.  Since reactivity does not really depend
   on the flux level (although it does depend on the flux distribution),
   the increase in xenon concentration will be a negative reactivity
   insertion, and reactor power will go down, if no other reactivity
   components are allowed to change.

   Now, of course, the iodine is decaying away, while it is not being
   produced at the full-power rate.  The production of xenon from
   I-135 decay slows, and eventually the Xe-135 concentration peaks
   as the production of Xe-135 becomes less than the destruction from
   Xe-135 decay and neutron absorption.

   Now, note that at some point the I-135 concentration can essentially
   be zero, while there is still substantial Xe-135 in the core.  Or
   look at another way, after my reactor has been shutdown for several
   days, the concentrations of I-135 and Xe-135 are both essentially
   zero.  My point here is that it is not the absense of I-135 with
   makes the reactivity transient;  rather, it is the presence of
   Xe-135 which causes the transient.

B. Alan Guthrie, III          |   Quis Custodiet Ipsos Custodes?
                              | |   My opinions only

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