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.