IMMOBILIZATION OF THE MOLTEN SALT WASTES INTO NZP MATERIALS BY USING AN ION EXCHANGE, A PRECIPITATION OR A GRSS (GEL-ROUTE STABILIZATION/SOLIDIFICATION) METHOD

摘要

The NZP(sodium zirconium phosphate)-structure material has been considered as a prospective host material for the immobilization of various radionuclides. Molten LiCl salt wastes generated from an electro-metallurgical process for the conditioning of spent oxide fuels contains high contents of alkali elements, which would limit the selection of a suitable waste form and require a much more complex process for the stabilization/solidification. For this reason, this study presents more simple and manageable conditioning method by using the NZP precursor as a selective ion exchanger. For the separation of the radionuclides, amorphous Zr(HPO{sub}4){sub}2·2H{sub}2O could exchange metal cations based on the ion selectivity, Cs>Sr>Na>Li. The metal-exchanged form, Zr(MPO{sub}4)x(HPO{sub}4){sub}(2-x), was transformed into MZrP (LiZr{sub}2P{sub}3O{sub}12, NaZr2P{sub}3O{sub}12, CsZr{sub}2P{sub}3O{sub}12 or SrZr{sub}2P{sub}6O{sub}24) and ZrP (ZrP{sub}2O{sub}7) after a heat treatment at 700, 800 or 900°. For the direct waste-form fabrication without any separation of the nuclides, a synthesis of the NZP-structure material was carried out using three different methods, that is an ion exchange, a precipitation or a GRSS (Gel-Route Stabilization/Solidification) method, and a single crystalline MZrP without ZrP could be formed at below 800°. From the TG/DTA result, there was little possibility for the vaporization of Cs in all the samples. The leach rate of Cs after the 7-day PCT (Product Consistency Test) leaching test ranged from 10{sup}(-3)g/m{sup}2·day to 10{sup}(-5)g/m{sup}2·day. It is noted that the chemical durability of the products fabricated by the GRSS method was higher than those of the other methods. From these results, it could be concluded that the ion-exchange method is useful for the separation of the radionuclides to leave a relatively clean LiCl even though it requires many processing steps, while the GRSS method can directly fabricate a durable waste form by a simpler process, with no generation of the secondary radioactive wastes.