Neutron shields - lessons not learned

["tom_dixie" <tom_dixie@email.msn.com>]

We recently completed a characterization of a facility that used californium
neutron sources for radiography.  During the characterization of the storage
shield we noted a simple detail that will cost the customer a significant
amount for disposal, neutron activation of the shield.  If you are in need
of a neutron shield in the future, you may want to pay attention.

The radionuclides of concern are Co-60, Eu-152, Eu-154, and Mn-54.  (Thanks
to those who responded tmy earlier question regarding the expected
radionuclide composition).

The item that I would like to pass on and maybe save someone a lot of $$ and
trouble is shield construction.  The following items may help you:

1.  Don't make the shield any bigger than it has to be - you are just making
it more difficult to remove the activity when you remove the shield from
service - or you may have to throw the whole thing away as radwaste.

2.  Consider something other than concrete.  It may be less expensive to
construct with concrete - but radwaste costs down the road may hurt.

3.  Consider a poly shield inside your concrete shield.  This will reduce
the neutron fluence rate at the concrete and reduce the activation of your
shield.  If you do this, allow an access for removal of the poly shield
(without serious concrete cutting) so that the poly can be removed easily
and the inside surface of your concrete is available for survey without
coring the concrete.  This will also save $$ and time.

4.  If you do have to sample to determine the degree of activation of your
shield, segregate the water collected from the coring operations.  (Do this
during the cutting operations during the D&D also).  You will generate some
water with respectable concentrations of activation products.  If you don't
segregate, the amount of potentially radioactive water collected may be
substantial.

5.  When you sample, the number of samples needed may be reduced by using
direct readings and evaluating an area based on a ratio of response rather
than sample analysis.

Tom O'Dou, CHP, RRPT
MKM Engineers, Inc.
Las Vegas, NV
tom_dixie@msn.com




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