Hydraulic Permeability of Resorcinol-Formaldehyde Resin

Abstract

An ion exchange process using spherical resorcinol-formaldehyde (RF) resin is the baseline process for removing cesium from the dissolved salt solution in the high-level waste tanks at the Hanford Site, using large scale columns as part of the Waste Treatment Plant (WTP). The RF resin is also being evaluated for use in the proposed small column ion exchange (SCIX) system, which is an alternative treatment option at Hanford and at the Savannah River Site (SRS). A recirculating test loop with a small ion exchange column was used to measure the effect of oxygen uptake and radiation exposure on the permeability of a packed bed of the RF resin. The lab-scale column was designed to be prototypic of the proposed Hanford columns at the WTP. Although the test equipment was designed to model the Hanford ion exchange columns, the data on changes in the hydraulic permeability of the resin will also be valuable for determining potential pressure drops through the proposed SCIX system. The superficial fluid velocity in the lab-scale test (3.4-5.7 cm/s) was much higher than is planned for the full-scale Hanford columns to generate the maximum pressure drop expected in those columns (9.7 psig). The frictional drag from this high velocity produced forces on the resin in the lab-scale tests that matched the design basis of the full-scale Hanford column. Any changes in the resin caused by the radiation exposure and oxygen uptake were monitored by measuring the pressure drop through the lab-scale column and the physical properties of the resin. Three hydraulic test runs were completed, the first using fresh RF resin at 25 C, the second using irradiated resin at 25 C, and the third using irradiated resin at 45 C. A Hanford AP-101 simulant solution was recirculated through a test column containing 500 mL of Na-form RF resin. Known amounts of oxygen were introduced into the primary recirculation loop by saturating measured volumes of the simulant solution with oxygen and reintroducing the oxygenated simulant into the feed tank. The dissolved oxygen (DO) concentration of the recirculating simulant was monitored, and the amount of oxygen that reacted with the resin was determined from the change in the DO concentration of the recirculating simulant solution. Prior to hydraulic testing the resin for runs 2 and 3 was covered with the simulant solution and irradiated in a spent fuel element at the Oak Ridge National Laboratory High Flux Isotope Reactor (HFIR). Both batches of resin were irradiated to a total gamma dose of 177 Mrad, but the resin for run 2 reached a maximum temperature during irradiation of 51 C, while the resin for run 3 reached a temperature of 38 C. The different temperatures were the result of the operating status of HFIR at the time of the irradiation and were not part of the test plan; however, the results clearly show the impact of the higher-temperature exposure during irradiation. The flow rate and pressure drop data from the test loop runs show that irradiating the RF resin reduces both the void fraction and the permeability of the resin bed. The mechanism for the reduction in permeability is not clear because irradiation increases the particle size of the resin beads and makes them deform less under pressure. Microscopic examination of the resin beads shows that they are all smooth regular spheres and that irradiation or oxygen uptake did not change the shape of the beads. The resin reacts rapidly with DO in the simulant solution, and the reaction with oxygen reduces the permeability of a bed of new resin by about 10% but has less impact on the permeability of irradiated resin. Irradiation increases the toughness of the resin beads, probably by initiating cross-linking reactions in them. Oxygen uptake reduces the crush strength of both new and irradiated resin; however, the pressures that caused the beads to crush are much higher than would be expected during the operation of an ion exchange column. There was no visible evidence of broken beads in any of the resin samples taken from the test loop. Reaction with oxygen reduces the cesium distribution coefficient of the resin beads, as does irradiation. Higher temperatures during irradiation or during contact with the simulant solution further reduce the cesium distribution coefficient.