The present invention concerns the treatment of radioactive effluent.
In particular, the invention concerns the treatment of radioactive materials arising from the reprocessing of irradiated nuclear fuel in order to isolate that which is radioactive and confine it in a form which is capable of storage in a safe environment.
It is generally accepted that there are three levels of radioactive arisings from nuclear fuel reprocessing operations: low level, which is capable of being discharged to sea under carefully controlled conditions; and intermediate level and high level, both of which need treatment to concentrate the radioactive material and to confine it in a form capable of being stored in a safe environment for the periods necessary to ensure that its activity does not present a health hazard.
In particular, in the intermediate level arisings, treatment may be give which produces a floc which retains the radioactivity. This treatment includes the consecutive formation of precipitates of a ferrocyanide, hydroxides, barium sulphate, and a sulphide. Such intermediate level arisings are, for example, as a result of the reprocessing of irradiated Magnox fuel from the first generation of British nuclear power stations-uranium metal fuel encapsulated in magnesium alloy (Magnox) cans. Such arisings further include sludges and ion-exchange resins which result from storage and in-pond corrosion of the irradiated fuel in the storage ponds at reactor sites and at the reprocessing site, and also include pieces of the magnesium alloy cans which have been stripped from the irradiated uranium metal fuel. Furthermore, they can include various ferric/aluminium flocs resulting from the treatment of reprocessing liquid streams to remove therefrom, or reduce to acceptably low level, radioactivity prior to discharge to sea. The arisings are generally given a conditioning treatment to ensure that materials which could adversely affect subsequent encapsulation or storage are removed. Such materials may include salts, such as sodium sulphate and ammonium nitrate. The flocs, conditioned where necessary, include insoluble precipitates to which are tightly attached radioactive species, and considerable quantities of free water containing dissolved salts, which are non-radioactive. However, the amount of floc which can be incorporated into a solid fixation material matrix (by subsequent treatment with bitumen, cement matrices or the like) is limited by the weight of said non-active salts, as well as by the radioactive precipitates content, therefore requiring a greater quantity of fixation material than would be justified by the weight of radioactive precipitates alone. This leads to greater quantities of fixation material being used than is strictly necessary.