[ RadSafe ] Thorium in Welding Rods - Release of Thoron

[Leo M. Lowe]

In the many recent comments on thorium in welding rods, a 
few  related to the release of thoron (Rn-220) from large quantities 
of the rods.  It was suggested that the short half-life of thoron (56 
s) would result in insignificant emission rates.  However, that may 
not be the case.

For thorium (actually Ra-224) - bearing sources, the surface flux of 
thoron is inversely proportional to the square root of the half-life 
of thoron.   This also applies to radon (Rn-222) from uranium ( 
Ra-226)-bearing sources, with the result that the flux of thoron from 
soil containing equal concentrations of Ra-224 and Ra-226 is about 80 
times that of the radon flux.  This is discussed in many 
publications, including those of UNSCEAR.

Thus, the thoron flux from welding rods could be substantial (other 
factors are also important).  Of course, the resultant air 
concentrations of thoron would be greatly mediated by its short-half 
life and any local ventilation and air dispersion, as it is in the 
general environment.

L. Lowe