Development of pyrolytic monolithic carbon composites for the conditioning of spent ion exchange resins
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Development of pyrolytic monolithic carbon composites for the conditioning of spent ion exchange resins Pamela B. Ramos1, Néstor O. Fuentes2,3 and Vittorio Luca2 1
U. A. Materiales – GIDAT – GAEN - CNEA Programa Nacional de Gestión de Residuos Radiactivos, GASNyA - CNEA 3 Instituto Sabato – UNSAM / CNEA Av. Gral. Paz 1499, B1650KNA – San Martín, Buenos Aires, Argentina. 2
ABSTRACT The pyrolysis of ion exchange resin beads that are used for the purification of water in reactor primary- and secondary-cooling circuits can result in stable and leach resistant carbonaceous products. However, free flowing beads are less desirable waste forms for disposal in sub-surface or surface repositories than monolithic masses of low porosity. We have investigated the pyrolysis of polymeric resin – cation exchange resin composites to give mechanically robust and chemically durable monolithic carbonaceous waste forms that are suitable for repository disposition. Also investigated was the dependence of product properties on various processing parameters (temperature ramp and final temperature). As a first approach, epoxy resins were used for the preparation of monoliths since such resins cure at room temperature and result in a relatively high carbon yield. Carbonaceous monolithic products were prepared at 400, 500, 600, 700 and 800 oC using a temperature ramp of 2oC/min. The products were maintained at the chosen temperatures for a period of one hour. Mass losses, volume reduction, hardness and compressive strength were measured and mathematical functions are proposed to describe the measured values of these properties. The carbon monoliths were observed to be mechanically robust. INTRODUCTION Polymeric ion exchange resins are employed worldwide in nuclear power plants (NPPs) for purifying water in primary and secondary cooling circuits. When the resins become saturated in radionuclides including 60Co and 137Cs they are replaced. Replacement usually occurs as soon as breakthrough of one or more contaminant elements is detected. Thus, at NPPs a large volume of spent resin is generated annually for which safe disposal is required. Among the contaminant metallic elements incorporated into cation exchange resins, particularly 137Cs is problematic as (i) it has a relatively long half-life, (ii) is highly radiotoxic, and (iii) it is extremely soluble and hence mobile in the biosphere. Although various options exist for disposing of such spent resins, there seems to be no universally accepted method. It is therefore important to develop adequate treatment and conditioning strategies for this waste. It has been recently reported [1 - 4] on the immobilization of contaminant elements in pyrolyzed cation exchange resin beads. These previous studies addressed the immobilization of + 2+ Cs , Sr , Co2+ and Ni2+ cations by pyrolysis of the beads in argon at temperatures in the range of 300 – 600 oC. Pyrolysis generates mechanically robust carbonaceous beads with reduced dimensions, and containing less oils and gases. All the contaminant metals are
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