Re: Potassium Iodide -Reply -Reply -Reply -Reply

[Charles Willis <CAW@nrc.gov>]

Emil,

Xenon poisoning is the phenomenon of  6.57 hour I-135 decaying to 9.1 hour
Xe-135.  Xe-136 has a large (2.6 million barn) neutron cross section so as long as
the reactor is operating, the Xe-133 is kept at a low concentration by the neutron
absorption process.  After shutdown, the Xe-135 builds up and effectively reduces
the core reactivity so that extra reactivity must be built into the reactor if it is to
restart after being shut down.  The effect is strongest 10 or so hours after
shutdown, but can be significant after a much shorter period of time.  This effect
caused considerable consternation in the first production reactor at Hanford, but
now is simply a part of reactor design.

Xenon poisoning does not eliminate the radioiodine dose potential, which is
dominated by 8.02 day I-131, with  20.6 hour I-133 being next most important.  

In the TMI accident, the reactor was operating at the start of the accident but was
automatically shut down quickly.  The core damage did not start until the reactor
coolant pumps were shut down about an hour and a half  into the accident.  Initially,
radioactive material was released was released into water and then through a
charcoal adsorber which seems to have effectively "scrubbed" the iodine, so that
the early release to the atmosphere was predominately noble gas.  It is argued that
little iodine was released because the of the formation of CsI, which is much less
volatile than is elemental iodine.   Because of the limited sampling capability, we
really do not know how much of the radioiodine was released to containment, but
the evidence is abundant that little iodine (<32 curies) was released to the
atmosphere; the noble gas release was about 13 million curies.

Charlie Willis
caw@nrc.gov