Re: Chart of the Nuclides -Reply

[Brian_Gaulke@hc-sc.gc.ca]

Brian Gaulke
01/28/98 11:07 AM

I don't have any experience in actually measuring Cs-135 to Cs-137 ratios,
but simple calculations indicate that the atom ratio produced in a fission
reactor will be less than one.  The fission yields for the mass 135 and
mass 137 isobars are nearly equal, only about 5.7% higher for mass 135.  On
the other hand, in a high thermal neutron flux, such as found in a reactor
core, a large fraction of the Xe-135 produced will absorb a neutron and
move to the mass 136 isobar before decaying to Cs-135.  My calculations are
as follows:

lambda(Xe-135, radioactive decay)  = ln(2) / (9.1 hrs x 3600 sec/hr)
                                   = 0.000021 sec^-1.

lambda-eff(Xe-135,neutron absorption)    = sigma x neutron flux
                                   = 2.6E-18 cm^-2 x 1E13 cm^-2sec^-1
                                   = 0.000026 sec^-1.

Thus around half the Xe-135 will absorb a neutron before radioactive decay
in a typical in-core thermal neutron flux of 1E13 cm^-2sec^-1.   This
swings the atom ratio clearly in favor of Cs-137.

Note that the situation is very different for weapons produced cesium,
where the neutron flux will not be present during the time the xenon is
decaying.

(If there's something wrong with my thinking or calculations, I'm always
happy to learn something new.)

Brian R. Gaulke, CHP
Head, Dosimetry Section
Radiation Protection Bureau
Health Canada
brian_gaulke@hc-sc.gc.ca





CAW @ nrc.gov on 98/01/28 09:17:38

Please respond to radsafe@romulus.ehs.uiuc.edu

To:   radsafe @ romulus.ehs.uiuc.edu
cc:    (bcc: Brian Gaulke)
Subject:  Chart of the Nuclides -Reply




Robert,
All the information I have about thermal neutron fission of U-235 indicates
that
such fission produces more Cs-135 atoms than Ca-137 atoms.  The great
difference
in half-lives, 2,300,000 years and 30.07 years respectively, accounts for
the
difference in the activities of the two.

Charlie Willis
caw@nrc.gov