RE: Molten Uranium

["Joe Alvarez" <jalvarez@auxier.com>]

I am not aware of measurements made on the surface of molten uranium.  As 
other responses to your question have noted, progeny is concentrated on the 
surface of solidified uranium after it is melted.  Molten metals are good 
solvents for many impurities that will precipitate as the metal cools. 
 This is the basis for zone refining, whereby, a melted zone is passed 
along the length of a bar of metal (by induction heating) so the impurities 
concentrate in the melt.  After multiple passes the bar is usually of high 
purity except for the end where it was last melted.  The point of this 
discussion is that likely, molten uranium will have the progeny evenly 
distrubuted throughout the melt, not on the surface.  Once the uranium 
solidifies it is a different matter.  There are two reasons why the progeny 
is found concentrated on the surface.  The first reason is that if a slag 
were involved, then the progeny will follow the slag.  The molten slag is a 
better solvent for the impurities than the metal, the reason for adding the 
slag in the first place.  Slag that sticks to the  outside of the metal on 
the oxide layer will have a higher concentration of progeny than  the bulk 
of the metal.  The other reason is the precipitation of the progeny as the 
metal solidifies.  The progeny will precipitate as inclusions between the 
metal grains, then diffuse to the surface along grain boundaries.  This 
diffision process is quite fast in the hot solid.  Some impurities are very 
surface-active, they are found on the surface in much higher amounts than 
in the bulk.  Therefore, molten uranium would not be expected to have as 
high a dose rate as solid uranium before the surface is cleaned.  Note: 
surface active diffusion of impurities has been observed at room 
temperature for particularly active elements.

Joe Alvarez
jalvarez@auxier.com
-----Original Message-----
From:	Mike McNaughton [SMTP:mcnaught@lanl.gov]
Sent:	Thursday, January 22, 1998 2:41 PM
To:	Multiple recipients of list
Subject:	Molten Uranium

Does anyone know of actual measurements of the dose rate at the surface of
molten uranium. It is said that the Th and Pa daughters concentrate on the
surface, thus increasing the dose rate. However, the thermal energy of an
atom is so much greater than the gravitational potential energy that the
thermal motion (Brownian motion) will counteract this, to some extent. Are
there any actual measurements of how large this effect is?

Thanks, mike

"Shlala gashle" (Zulu greeting, meaning "Stay safe")
mike (mcnaught@LANL.GOV)