(407e) Modeling of Self-Heating in Anion Exchange Columns for Plutonium Recovery

A finite element code has been developed to model radiolytic self-heating
in anion exchange columns.  Anion exchange columns are used to extract plutonium
from nitric acid solutions as part of the Purex process.  When columns are used
to extract Pu-238, significant rates of radiolytic heating pose a risk that
column temperatures could rise to the point where a runaway resin degradation
reaction occurs.  A runaway degradation reaction can lead to a potentially violent
thermal excursion, where gas generation from the degradation reaction causes
the column to eruct.  The finite element code focuses on predicting the maximum
temperature due to radiolytic heating in order to ensure that this maximum
temperature remains below the temperature required to initiate a reaction
excursion.  The code also can be used to calculate the cumulative radiological
dose to the resin.

The finite element code calculates concentration profiles and
temperatures for normal operation of a column, including loading, washing, and
elution of the resin, and for natural circulation during flow stoppages.  Resin
loading is modeled by combining a chemical equilibrium absorption relation with
models for axial dispersion and solid phase diffusion.  Desorption is modeled
by combining the axial dispersion model with a shrinking core diffusion model. 
Natural convection calculations are based on the Ergun relation for flow in a
packed bed, using pressure gradients obtained from temperature and composition
differences in the column solution. 

The model was applied to calculate maximum temperatures in anion exchange
columns used to extract Pu-238 for the Cassini mission at the Savannah River
Site and for a Pu-238 laboratory-scale column at Los Alamos National
Laboratory.  More recently, the model was used to predict doses to the resin
for feeds containing plutonium, americium, and curium.  For this calculation,
it was assumed that the resin absorbs plutonium much more strongly than either
americium or curium.  Results are presented for all three studies.