38
FREILING, CROCKER, AND ADAMS
Effect of Fission-product Concentration
and Volatile Soil Constituents
The concentration of a given fission product in the fireball is extremely small (of the order of 10~' mole/liter). This very low con-
centration level affects its behavior in the vapor phase, its interaction
with vaporized material, its species in the condensed phase, and, consequently, its distribution coefficient.
The effect of low concentrations on the vapor species can be viewed
from both kinetic and thermodynamic viewpoints. Consider a fission
product M that is formed initially as MO but whose normal species is
M,O. The formation of an M,O molecule requires the collision of two
MO molecules. At low MO concentrations such collisions will be comparatively rare with respect to collisions with macroscopic concentrations of vaporized soil or device constituents. Furthermore, kinetic
activation energies and transition probabilities for reactions involving
MO may be considerably different from those involving M,O. Thermodynamically, we would say that the low concentrations would have the
effect of impeding association to form polynuclear or polymeric compounds,
The fission-product species that wants to form a polynuclear molecule but is unable to find an identical species to pair up with may
settle for a reasonable approximation. Thus it is very unlikely that two
*°RbOH molecules would pair up in a silicate~surface burst to form the
dimer (°RbOH)>. It is much morelikely that each would pair up with a
NaOH molecule (formed from vaporized water and Na,O soil constit-
uents) to form the well-known °"RbOH-NaOH. One can visualize many
Similar situations which are likely but which have never been investigated. The net reaction between such monomers and the refractory
portion of the soil (indicated by R) can be written as Na,O°R(c) +
RbOH(v) = NaRbO-R(c) + NaOH(v). Here (c) and (v) indicate condensed
and volatile phases, respectively. The equilibrium constant for this
reaction is expressed in terms of activities as
K= a\aon ANaRbO-R
0
ARbDOH 4Na,OrR
Replacing vapor-phase activities by partial pressures and assuming
equal activity coefficients for the condensed species,
K= (PNaoH/Nnao-r)
(Prion/Nyarso-r)
According to this expression, K is essentially a selectivity coefficient,
the quotient of two distribution coefficients. From a knowledge of K,