Question on Properties of Aluminum at Research Reactor

["Paul E. Benneche" <peb@VIRGINIA.EDU>]

We are completing the decommissioning work at the University of Virginia

Nuclear Reactor Facility and are having a few problems with some of the

enbedded aluminum structure in the reactor pool area.



Over the lifetime of our two megawatt research reactor (1960-1998) the

aluminum reactor pool gate frame was in constant contact with reactor pool

water, except for a nine month period in 1968.  The rest of the 70,000

gallon pool was epoxy paint coated concrete.  The pool was permanently

drained about six months ago.



While full, the pool water was always kept very pure, with a conductivity

between 1 and 3 micromhos/cm.  The water was recirculated through a carbon

filter and a mixed bed demineralizer.  It appears that the uncoated

aluminum may have either absorbed dissolved radionuclides out of the pool

water or it was manufactured with trace amounts of uranium or thorium

related radionuclides within the material.  These constituents seem to be

near the surface and can be removed with aggressive cleaning with either

high pressure water or lye.  However, some time after cleaning the

radionuclides reappear on the surface of the aluminum (detectable at the

surface with a thin window, large area, gas flow proportional counter, but

not readily removable).  We hypothesize that if there is uranium, thorium

or some other radionuclide within the aluminum that the cleaning that we

are doing may be selectively removing shortlived radioactive daughter

products from the surface that are then returning by building in over time

from their parent isotope.  Note, this gate frame was far enough distant

from the reactor that there was no possibility of neutron activation of

the material.



When examined by a long count on a germanium detector the following

radionuclides were observed above background levels in the gate frame

material:



Pb-212, Pb-214, Bi-214, Co-60 and Eu-152



The lead and bismuth may be from naturally occurring uranium / thorium

that may be in the aluminum and the cobalt and europium are activation

products that may have been "absorbed" out of the water.



Does anyone have any insight as to what we may be observing, as to why we

can clean the surface of the aluminum but the contamination returns in

about a month.  Any help would be appreciated.  Email or call me.



Paul E. Benneche

University of Va. Reactor Supervisor

peb@virginia.edu (e-mail)

434-982-5440 (work phone)





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