Pilot Plant for treating of spent exchange resins

Spent exchange resins that have been accumulating during the last operational 30 years in Atucha I nuclear power plant (NPP) are a 'problematic' waste. These spent resins conform an intermediate level waste due to the total content of alpha, beta and gamma emitters (some samples of spent resins were analyzed in 2003). For this reason its treatment is more expensive since it is necessary to add more safety barriers for its final disposition and also for the radioprotection actions that are involved. Using sulfuric acid solutions it is possible to elute from the spent resins the ions that are retained. In the same operation are eluted Cobalt, Cesium and alpha emitters since that all these elements react as cations in aqueous solution. Decontamination by electrochemical methods was analyzed as an interesting method to apply after elution operation to these spent resins since that with the decontamination process it is possible to obtain a solid without activity and concentrate the activity in cells that are small in volume and its manipulation doesn't present any extra complication. Experiments made with active samples taken from the deposit were successful. Because of these results it was built a small plant to treat a batch of 100 dm3 of wet spent exchange resins. Some problems with the material that was in the deposit together with spent resins caused that we had to plan a more complex strategy to obtain a complete decontamination of the spent resins (in this stage we used the cobalt retention cell that was described in other paper to retain Cobalt and alpha emitters and a sample of zeolites from Argentina ores to retain Cesium). Due to alpha emitters act electrochemically like cations it was possible to retain altogether with ionic Cobalt on the copper amalgam electrode. Working in the non-active lab with alcoholic solutions it was possible to retain ionic Cesium on a copper electrode (copper is covered by mercury fine film which forms a solid amalgam) with a similar efficiency to other methods like adsorption on zeolites or on silico titanates. Some preliminary results show that the decontamination was effective and it is useful for evaluating all the parameters involved in the final design of an industrial facility. The amount of removed actives in the first batches was 80 - 94% for Cesium-137 and 36 - 75% for Cobalt-60. If we considered that alpha content in original spent resins varies from 244 Bq/g up to 774 Bq/g we have removed in these experiments about 46 - 89%. (author)