W. J. Crooks, III, E. A. Kyser, III, and S. R. Walter
Westinghouse Savannah River Company
Aiken, South Carolina, 29808

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Despite reports^1,2,3,4 of the relative inertness of Reillex^TM HPQ anion exchange resin to chemical and radiological degradation, a low temperature exothermic reaction was identified for this resin in nitric acid using the Reactive System Screening Tool⁵ (RSST). The purpose of this work is to characterize the low temperature exothermic reaction, investigate its origin, and evaluate the risks it introduces to plutonium processing at the Savannah River Site.

The RSST calorimeter allows for efficient screening of runaway reactions as a function of temperature. The reaction of Reillex^TM HPQ, Ionac^TM A-641 and irradiated Reillex^TM HPQ^TM anion exchange resins (all in nitrate form) with nitric acid were compared in the RSST. The thermal effects were evaluated as a function of nitric acid concentration, cerium (IV) loading (as a simulant for plutonium (IV)), and damage caused by previous irradiation of the resin. Much of this work focused on the low temperature exothermic reaction observed for Reillex^TM HPQ-nitric acid mixtures. Off-gases associated with this low temperature exotherm were identified by GC-MS, the final nitric acid concentration was determined by titration, and the solid resin residue was characterized by FTIR.

Incidents involving undesirable self-accelerated exothermic reactions between organic resins and nitric acid solutions have been well documented. As expected, all resins studied displayed runaway reaction behavior at temperatures in excess 100^oC.

Generally, for all resin-nitric acid mixtures studied, higher nitric acid concentrations decreased the time to the maximum rate of reaction but did not increase the intensity of the exotherm, and cerium (IV) had little to no effect on the reactivity of the resin-nitric acid systems.

Surprisingly, Reillex^TM HPQ displayed an additional low temperature exotherm that initiated at about 69^oC and reached a maximum rate about 80^oC (see Table). While this low temperature exotherm is small in magnitude (2^oC/min ± 1), gaseous products were identified in the off-gas, leading to concerns of pressurization in the ion exchange column. FTIR analysis of the solid resin residue identified a carbonyl stretching frequency at 1700 cm^-1 that was not present in the unreacted resin. A study of the reaction of various model compounds with nitric acid in the RSST suggests the reactive sites are the ethylbenzene pendant groups, which were introduced during resin production as an impurity in the divinylstyrene reactant.

Interestingly, the Reillex^TM HPQ that had been irradiated prior to the test showed no low temperature exotherm, evidence that irradiation caused the exothermic reaction to occur. These results indicate that the low temperature exothermic reaction may be initiated by radiological or chemical attack. Therefore, a chemical pretreatment was developed that eliminates the exotherm.

1. C.T. Busher, R.J. Donohoe, S.L Mecklenburg, J.M. Berg, C.D. Tait, K.M. Huchton, and D.E. Morris, Applied Spectroscopy, 1999, 53 (8), 943-953.
2. S.F. Marsh, "The Effects of In Situ Alpha-Particle Irradiation on Six Strong-Base Ion Exchange Resin", Los Alamos National Laboratory Report LA-12055 (April 1991).
3. S.F. Marsh, "The Effects of Ionizing Radiation on Reillex^TM HPQ, a New Macroporous Polyvinylpyridine Resin, and on Four Conventional Polystyrene Anion Exchange Resins, Los Alamos National Laboratory Report LA-11912 (November 1990).
4. S.F. Marsh, "Reillex^TM HPQ: A New, Macroporous Polyvinylpyridine Resin for Separating Plutonium Using Nitrate Anion Exchange", Solvent Extraction and Ion Exchange, 1989, 7(5), 889-908.
5. Manufactured by Fauske and Associates, Inc., 16W070 West 83^rd Street, Burr Ridge, IL, 60521