Stabilization of radionuclides into wastes

Details Stabilization of radionuclides into wastes Stabilization of radionuclides into wastes

1995-04-10

1997-03-18

Mai, Ngoclan

Hazardous or toxic waste destruction or containment

Destruction or containment of radioactive waste

Chemical conversion to a stable solid for disposal

588 15, G21F 900

Patent

active

056132433

DESCRIPTION:

BRIEF SUMMARY
This invention relates to the stabilisation of radionuclides derived from naturally occurring materials into forms which are not accessible to the environment and are therefore suitable for disposal.
In a particular embodiment the present invention provides a process whereby a stable solid waste is formed by hydrolysis and roasting of aqueous solutions or suspensions containing radionuclides, particularly radionuclides in the decay chains of naturally occurring radioisotopes of uranium and thorium. In a general aspect the process of the invention comprises two basic steps for stabilising radionuclides present in a process stream, namely: stream, which upon roasting of the stream will be effective in stabilisation of radionuclides into crystalline phases such as to prevent significant immediate redistribution of radionuclides upon disposal into the environment. formation of such phases.
Additional steps may be employed as will be described below.
Various processes for the treatment of ores, concentrates and processed materials have the effect of taking contained radionuclides into aqueous solution or rendering radionuclides sufficiently soluble to allow extraction by water in the environment. For example, the processing of uranium ores to yellowcake, the extraction of rare earths from monazite and processes for the production of upgraded products from mineral sands concentrates (for example ilmenite and zircon) result in the production of such materials.
In addition, various steps in the nuclear fuel cycle will have the effect of rendering both naturally occurring and synthetic radioisotopes accessible to environmental mobilisation. As a result, wastes from such processing must generally be stored in supervised and monitored repositories, despite the fact that the wastes are frequently of extremely low radioactivity.
A common problem in the conversion of radionuclide bearing wastes to stable forms is the multiplicity of radionuclides which are normally present. For example, the most common form of uranium, uranium 238 has 7 other elements in its decay chain which will all be present whenever uranium 238 is present. Similarly thorium 232 has 7 other elements in its decay chain. In order to prevent environmental mobility all of the multiplicity of radionuclides which are present in a waste stream must be simultaneously stabilised into environmentally inaccessible forms. In particular, uranium, thorium and radium must at least be stabilised. Few cost effective schemes to achieve such, outcomes exist. Those schemes which do exist commonly are suited to synthetic high level waste derived from nuclear reactors for which high cost waste disposal schemes can be contemplated. Further for these schemes there has been little effort or reported success with stabilisation of shorter lived decay progeny of uranium or thorium.
The only method for radium stabilisation which has previously been reported is coprecipitation, with sulphuric acid and barium chloride additions to form a radium bearing barium sulphate. This method requires large additions of expensive barium chemicals and is not fully effective. The solid wastes thus produced cannot be released safely into the environment as exposure to ground and surface water can result in solubilisation of contained radium.
The literature of radioactive waste forms (Harker, A. B., "Tailored Ceramics" in Radioactive Wasteforms for the Future, Lutze W. and Ewing R. C. eds., North Holland, 1988) lists the following crystalline ceramic phases as host phases for waste stabilisation:


______________________________________ Actinide and rare earth hosts Flourite structure solid solutions UO.sub.2 --ThO.sub.2 --ZrO.sub.2 Zirconolite CaZrTi.sub.2 O.sub.7 Pyrochlores (Gd, La).sub.2 Ti.sub.2 O.sub.9 Perovskites CaTiO.sub.3 Monazite (Gd, La) PO.sub.4 Zircon ZrSiO.sub.4 Strontium and alkaline earth hosts Magnetoplumbates (Ca, Sr) (Al, Fe).sub.12 O.sub.19 Perovskites (Ca, Sr)TiO.sub.3 Hollandite Ba Al.sub.2 Ti.sub.6 O.sub.16 Alkali Hosts Nepheline (Na, Cs) Al Si

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