[ RadSafe ] Changing in alpha risk as spent fuel ages

[ROMANOWICH Larry(L) - BRUCE POWER]

Are there any technical reports which quantify changing lung absorption
parameters (Fr, Sr, Ss) for a fuel matrix particle as it is allowed to
decay outside of the core (5 micron assumption)?  I have some empirical
evidence suggesting that exposures to freshly discharged fuel will be
close to the default of type S, but if you let the same source term sit
for a while, away from the fission process, it will be come more
soluble, i.e. tends to type M.  My explanation would be that the alpha
recoil energy damages the fuel matrix, causing microcracks, thus more
surface area, thus increased solubility for the main dose drivers of Pu,
Am and Cm buried within the particle.  This is key because the fuel
particle is designed to be insoluble, though by themselves, Am and Cm
are generally considered type M.  Microcracks would allow more than just
the surface layer of Pu, Am and Cm to be exposed to the lung fluids. 
We use natural uranium; the primary dose constituents of Pu, Am, and Cm
are numerically much less than the U238 component in the spent fuel
particle.

Larry Romanowich
AHP Dosimetry 
B13
Bruce Power
519 389 4046 ext 15556