New development in treatment and conditioning of spent ion exchange resins

A significant quantity of spent ion exchange resins has accumulated at the Atucha I Nuclear Power Plant (NPP) and this is at the present a 'problematic' waste. These spent resins conform to the definition of an intermediate level waste due to the total content of alpha emitters in addition to the levels of activation and fission products. This categorisation makes their management more difficult by conventional means due the significantly high number of cemented drums that would be produced and the associated expense. The mass ratio between the radioactive elements present in these wastes and their final form, i.e. cemented and conditioned resins, is less than 10-7. It was considered very useful to try to separate the radioactive material from the spent resins as efficiently as possible in order to minimize the volume of final waste and avoid the requirement of cementing the spent resins. This laboratory and pilot scale project is based on a process to remove radioactivity from the spent resins by the use of strong acid reagents which also dissolve the metallic oxides ('crud') that may still be present with the resins. Subsequent treatment steps on the active solutions for separation of Co-60, Cs-137, other cations and alpha emitters are applied. At the end of the total process both the activity of the spent resins and processed solutions are very low. Since the alpha emitters in aqueous solutions act as cations the study of an electrodeposition process was conducted. This method can allow the deposition of the elements in their metallic state within a small volume of product which could facilitate the disposal options. In the case of the Cs-cation this direct aqueous electrolysis process is not possible. Promising results for caesium were obtained by using an electrolyte comprising a mixture of alcohol and water and an Hg-Cu amalgam cathode. Natural Argentinian zeolites were also tested and showed an adequate capacity to retain Co and Cs. A range of experiments with inactive and active ion exchange resins were done at laboratory scale. On the basis of these tests a small pilot plant was designed to treat batch quantities of wet spent ion exchange resins. After treatment at batch pilot plant scale spent resins were typically reduced to 0.7 % of the original Cs-137 value and 2.0 % of the original Co-60 value. At the laboratory scale the spent resins could be totally decontaminated with most depleted resins reporting less than detection limits for Co-60, Cs-137 and total alpha. The main difference was assessed to be the efficiency of contact between the acid solution and the resin. This aspect is a prime consideration for the design to process the full inventory of spent resins. At Atucha I NPP the exchange resins contained lithium in the form of Li-7. The extraction and recovery of Li-7 during the treatment of spent resins has important economical implications. It was shown that lithium could be recovered during the alcohol bearing part of the process. (author)