Re: Question on Properties of Aluminum at Research Reactor

[John Andrews <andrewsjp@chartertn.net>]

Paul E. Benneche wrote:



>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)

>

Paul, it looks to me like you have plate-out from radon and thoron in 

the air where this device is located.  These nuclides are in the decay 

chain of thorium and uranium (or radium).  The precursors are gasses, 

and the radon (Em-222) has a halflife of 3.8+- days.  This is known to 

accumulate in areas where radium is present even in small quantities (or 

in low concentrations) where there is limited ventillation.  Your one 

month time frame makes this look like about the time for equilibrium in 

a still air storage location.  The presence of  Pb-212 must be because 

of some thorium in the surroundings.  Thorium's gaseous daughter, thoron 

(Em-220), has a much shorter half life, but the Pb-212  from the rapid 

decay of Po-216 to an atom of Pb-212 in free air makes it very likely 

that it will attach to whatever it first contacts.  The lead then 

accumulates because it has a longer half life of 10.6 hours.  I have no 

clue where these gasses are coming from, but they are in the air.  I 

suggest you take an air sample of that air and count it immediately and 

look for these same nuclides.  I am sure you will find them.  I suspect 

that you will not find the Co-60 or Eu-152 in the air sample.



Look at the ventillation of the area.  If you can, increase the air 

flow.  You might also introduce some clean aluminum in the same place 

and use it as a background model for this survey.  It will have equal 

surface contamintion from the radon and thoron in the air.



Look for stored items containing radium and thorium in the same area. 

 Many ceramics have these components.  So do some welding rods.  Radium 

is normally in concrete in low concentrations.  Both thorium and uranium 

may be present in bare soils nearby with gas conduction paths to the 

storage area with the aluminum.



An electrostatic air filter is a very effective radon and thoron 

daughter capture device.  Put a GM detector near it (as close to the 

collection plate as you can fit it) and let it run.  Watch the activity 

go up and down as the radon and thoron concentrations vary with the time 

of day.  This can be very instructive.



The Co-60 and Eu-152 may be old contamination that remains in the 

aluminum.  I can't really help here.



John Andrews

Knoxville, Tennessee



>  

>



************************************************************************

You are currently subscribed to the Radsafe mailing list. To unsubscribe,

send an e-mail to Majordomo@list.vanderbilt.edu  Put the text "unsubscribe

radsafe" (no quote marks) in the body of the e-mail, with no subject line.

You can view the Radsafe archives at http://www.vanderbilt.edu/radsafe/