Re: Radiation Hardness of materials

[GRAHNK@aol.com]

This info is from my previous life as a DOE consultant.

There was some work done in Idaho (INEL) a few years back.  They had some Be
reflector blocks from the Advanced Test Reactor that required disposal.  The
blocks were high in H-3 (approx 100,000 Ci each) but the H-3 was typically
not available for release unless the Be corroded.  The intended disposal
method was in concrete lined shafts within INEL LLW disposal facility.
 Since, the concrete liner wouldn't prevent water intrusion (and hence
corrosion) and the fact that the concrete itself also causes corrosion, it
was decided to provide an additional liner, between the waste form and the
concrete in the form of seamless, extruded High Density Polyethylene (HDPE)
pipe.  Since the HDPE had a wall thickness of almost 2", and the top and
bottoms caps were heat welded, it was thought that this additional liner
would be a very effective barrier to both the intrusion of water, and if any
release waste form occured, the release outside the shaft of H-3.
 Unfortunately,  HDPE (and other plastics like it, i.e. CPVC) becomes brittle
with gamma exposure.  
Co-60 is was found to be a minor constituent of the Be reflector blocks,
(don't recall the numbers, but the blocks were hot enough that they would
have to be transported in shielded casks, something like 20-50 R/hr).  In as
little as 79 days, the HDPE would have received enough exposure to cause some
embrittlement, including small microfractures, that would allow the intrusion
of water (or at least water vapor) and the release of tritiated water vapor
and/or tritium gas.  With increased exposure, the embrittlement became worse.
 Anyway, the decision was made to not allow disposal of the blocks.  As far
as I know, they're still being stored in the canals at ATR.  There was some
talk of attempting to melt them down for H-3 recovery for the weapons
stockpile, but I don't know what became of that.  A good contact at INEL is
Rajiv Bhatt 208-526-2773.  His group had the study done on embrittlement of
plastics in radiation environments.  I worked on evaluation of the proposed
disposal and the HDPE embrittlement was one of the factors considered in the
analysis.


Kelly Grahn
Illinois Department of Nuclear Safety
West Chicago Field Offfice

GRAHNK@aol.com
Voice 630-293-6348
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