On-line Separation of Iodine Species in Reactor Water using Mixer-settlers

Abstract

A method for separation of iodine species from water has been developed. It is based on liquid-liquid extraction and separation is achieved in four extraction steps. A system based on this method for continuous separation of iodine species using mixersettlers has been developed. It consists of four mixer-settler batteries with 4,4,6 and 6 mixer-settler stages each. As organic phase an aliphatic kerosene is used and the separation is improved if the organic solvent has been pretreated with iodine carrier, stripped and washed. With an aqueous feed flowrate of 10 ml/min and mixer-settler battery phase flowratios of approximately 0.1 except for the elementary iodine strip battery with a phase flowratio of 1, the system separation performance is 92% for methyl iodide, 97% for iodate and elementary iodine and 99% for iodide. IntroductionIn all water cooled reactors radioactive iodine is released to the primary coolant directly from fuel defects or from outside fuel fission of tramp uranium in the core region. Iodine solution chemistry involves formation of a number of stable oxidation states and is, especially at low concentrations, complex. In BWR systems volatile iodine species such as elementary iodine, HIO and organic iodides are transported by the steam to out-side core systems (iodine carryover) and are one of the sources for radioactivity in the offgas system. This transport of volatile iodine must be considered in the design and operation of a BWR in order to minimize the release of radioactivity to the environment. It is also important in terms of system accessibility and in determination of the iodine source term to plantworkers during the yearly maintenance shut-down. Much effort has been done in recent research to investigate iodine accident chemistry while its chemistry under normal operation still is fairly unknown.Prediction of formation and destruction of iodine species in reactor systems demands accurate knowledge of the species concentrations under different types of reactor operating parameters. To obtain this information a characterization of iodine species must be performed. This involves a chemical separation where the different species are separated from each other and measured. The most widely used method to chemically separate iodine species in reactor water was originally described by Lin [1] and it is based upon liquid-liquid extraction. It has been used in batch type experiments and consequently it reveals information only about the particular batch sample at the time of sampling. Another drawback of the batch type technique is that it sets a practical limit to how much it is possible to concentrate the samples directly in the separation procedure.Concentration of the iodine species is important since some of the species exist in very low concentrations and require long measuring time to obtain statistically satisfying results. In order to obtain information over longer periods of time a continuous online separation technique using mixer-settlers has been developed. Experimental General sepa...