Stabilization of radionuclides into wastes

The specification discloses a process for stabilizing radionuclides extracted during the upgrading of minerals. The process comprises forming a composition of a radionuclide and a component and roasting the composition so that the component forms a crystalline phase having a structure that binds the radionuclides. Suitable components include a compound of a lanthanide and/or phosphorus and zirconia. Zirconia in its cubic form is useful in stabilizing uranium and thorium.

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: 
1. Ensuring the presence of a chemical composition and distribution in the 
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. 
2. Roasting of the stream in such a manner as to be effective in the 
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: 
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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 SiO.sub.4 
Perovskite (Gd, La).sub.0.5 Na.sub.0.5 TiO.sub.3 
Magnetoplumbite (Na, Cs).sub.0.5 La.sub.0.5 Al.sub.12 O.sub.19 
Hollandite (Ba.sub.x Cs.sub.y Na.sub.2) Al.sub.2 Ti.sub.6 O.sub.16 
1 
Non-fission product host phases 
Spinels (Mg, Ni, Fe) (Al, Fe, Cr).sub.2 O.sub.3 
Corundum Al.sub.2 O.sub.3 
Rutile TiO.sub.2 
Pseudobrookite Fe.sub.2 TiO.sub.5 
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While other ceramic phases exist in various waste forms the other phases 
are usually minor phases of less importance to waste stabilization. 
Methods for the formation of ceramic wastes typically involve sintering of 
ceramic precursors (possibly after preliminary drying and roasting) under 
high pressures (eg. 650 atmosphere) and at high temperatures (above 
1000.degree. C.) in order to produce ceramic monoliths of low surface area 
and therefore low reactivity. Nevertheless it has been demonstrated that 
such waste forms are accessible to environmental alteration, particularly 
in slightly acidic and in slightly basic aqueous solutions (as are 
frequently encountered in natural ground and surface water), and can 
deliver mobile radionuclides into the environment. The previously proposed 
methods are thus expensive and not fully effective. 
There has previously been very little work aimed at stabilizing 
radionuclides into low level radioactive wastes. There exists a need for a 
low cost process for the stabilization of uranium and thorium and 
radionuclides in the decay chains of uranium and thorium into wastes 
containing from tens of parts per million to percents of uranium and 
thorium. Such stabilization must be effected as to prevent dissolution of 
the contained radionuclides from the wastes at a rate greater than that 
which can be absorbed and removed by environmental processes without 
accumulation to unacceptable levels significant to biological function. 
Clearly there is considerable incentive to discover alternative methods for 
the stabilization of radionuclides into wastes which can be disposed of 
into the environment without significant risk of mobilisation, 
particularly for wastes derived in part from natural sources. 
Accordingly the present invention provides a process for stabilization of 
radionuclides derived from naturally occurring sources, the process 
comprising the steps of: 
(i) forming a composition of a radionuclide and sufficient of a stabilizing 
component to ensure that when the composition is roasted, a crystalline 
phase is formed having a structure that binds the radionuclide; 
(ii) roasting the composition under conditions sufficient to form a 
crystalline phase in which the radionuclide is bound to reduce its 
environmental mobility. 
The radionuclide bearing material may be in any form which is amenable to 
subsequent formation of the desired phases. It is particularly beneficial 
if the radionuclides are present in an aqueous solution to which the 
stabilizing component can be added in solution as an additive to provide 
excellent mixing. In such cases the aqueous solution may be evaporated 
prior to roasting if desired, and components in the solution may also be 
hydrolysed from salts to oxides, hydrated oxides and hydroxides prior to 
roasting. Alternatively solutions may be directly spray roasted, allowing 
evaporation, hydrolysis (pyrohydrolysis) and crystalline phase formation 
to occur simultaneously. 
The roasted products of the process which is herein disclosed are of high 
surface area (1-100 m.sup.2 per gram) and yet exhibit virtually no 
solubility of contained radionuclides. Expensive high pressure calcination 
may hence be avoided, demonstrating the superior performance of the waste 
form of the disclosed process by comparison with previously reported waste 
forms. Certainly it is not anticipated that it would be necessary to 
operate the process outside of normal chemical processing pressure ranges 
e.g. up to 20 atmospheres. 
Additives (used in small proportions) for use as the stabilising component 
which have in particular been found to be beneficial in the process herein 
disclosed are lanthanide compounds and phosphorus compounds. Even a small 
addition of a lanthanide compound in the presence of phosphorus can result 
in highly effective stabilization of uranium and thorium. Stabilization of 
radium can be effect in the process. Any additives having the desired 
effect of stabilization of radionuclides into wastes and not interfering 
with the disclosed effects may be used. In some circumstances it will not 
be necessary to make additions to the stream to be treated by the process 
in order for the process to be effective. 
The process as disclosed is not otherwise constrained. It may be conducted 
in any equipment and on any solution or other waste material which is 
capable of forming the desired phase combination. For most waste streams 
only small additions of additives will be required. 
It is the combination of at least two elements (for example phosphorus and 
a lanthanide), under the conditions described which results in the 
complete effectiveness of the presently disclosed scheme in stabilizing 
the full range of important radionuclides. No other ceramic waste form 
which specifically stabilizes by chemical means uranium, thorium and all 
decay progeny simultaneously has previously been disclosed. A lanthanide 
that has been found to be particularly useful is cerium.

The following examples further illustrate the invention. 
EXAMPLES 
Chloride solutions having the compositions indicated in the attached Table 
1 were first evaporated to dryness at 80.degree. C. to produce solid 
residues. These residues were then held under a flow of steam at 
200.degree. C. for one hour and then under a flow of steam and air at 
800.degree. C. for two hours, ensuring both the completion of all possible 
hydrolysis and the development of crystalline properties. The granular 
solid residues were then allowed to cool in air. 
The solid wastes were then leached at room temperature (62.5 gpL) in 
synthetic groundwater (5 gpL sodium chloride, 500 mgpL sulphuric acid) 
maintained at pH below 5 by periodic additions of acetic acid. The leach 
was continued for 24 hours, after which the residue was filtered, washed 
with fresh synthetic groundwater and dried. 
Roasted and leached wastes were subjected to chemical analysis and gamma 
spectroscopy analysis for major elements and radionuclides. Radionuclide 
extraction from the solid wastes in leaching is also indicated for each 
case in the attached Table 1. 
Clearly those samples having lanthanide (eg. Ce) and P additions under 
circumstances which produced a waste needing little or no acid addition to 
maintain pH below 5 provided wastes which did not subsequently allow 
leaching of radionuclides. The absence of these elements or conditions 
resulted in a far less stable waste. However, it is expected that other 
elements may substitute for these main constituents, allowing for a range 
of effective compositions, provided that the effective circumstances as 
disclosed are maintained. 
Further, the addition of barium salts (made to liquor A1-9 of the attached 
table in a separate test) was found to have a strongly negative impact on 
the stability of uranium and radium in the wastes produced by otherwise 
identical treatment. Hence wastes containing barium, lanthanide and 
phosphorus (as have previously been produced in waste forms, due to the 
composition of wastes from nuclear fuel processing which contain zirconium 
and phosphorus) are herein disclosed as ineffective for the purposes for 
which the present invention is practised. In general where the 
effectiveness of the process depends on the presence of phosphorus and 
lanthanides the presence of elements which form more stable phosphates 
than lanthanides may require the addition of incremental compensating 
phosphorus for all other identical conditions. 
Solutions derived from the production of synthetic rutile by acid leaching 
of thermally treated ilmenite to which additives were made to result in 
solutions having the composition indicated in the attached Table 2 were 
also treated according to the method described above. 
Roasted and leached wastes were subjected to chemical analysis and gamma 
spectroscopy analysis for major elements and radionuclides. Radionuclide 
extraction from the solid wastes in leaching is also indicated for each 
case in the attached Table 2. 
TABLE 1 
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Liquor Compositions and Waste Stability, Illustrating the Process 
Disclosed. 
A1-6 
A1-9 
A2-1 
A2-2 
A2-3 
A2-4 
A3-1 
A3-2 
A3-4 
A3-5 
A1-2 
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Liquor, g/L 
Fe 0.25 
23.2 
0.27 
0.27 
0.27 
0.27 
0.27 
0.27 
58.1 
0.27 
36.8 
Zr 2.01 
2.01 
1.98 
1.98 
1.98 
1.98 
1.98 
1.98 
-- 1.98 
0.73 
Si 0.058 
0.058 
0.29 
0.29 
0.29 
0.29 
0.29 
0.29 
-- 0.29 
-- 
Ti 0.064 
0.064 
0.064 
0.064 
0.064 
0.064 
0.064 
0.064 
-- 0.064 
-- 
Y 0.172 
0.172 
-- 0.172 
-- 0.172 
0.172 
0.172 
0.172 
0.172 
-- 
Mg 0.169 
0.169 
-- -- 0.169 
0.169 
0.169 
0.169 
-- 0.169 
-- 
Al 0.43 
0.43 
0.43 
-- -- 0.43 
0.43 
0.43 
-- 0.43 
-- 
P &lt;0.020 
&lt;0.020 
-- -- -- -- 0.09 
0.09 
0.09 
0.135 
-- 
Ca 6.50 
6.50 
6.50 
6.50 
6.50 
6.50 
6.50 
6.50 
-- 6.50 
4.69 
Ce 0.01 
0.01 
-- -- -- -- -- 0.011 
-- 0.011 
-- 
Hf 0.062 
0.062 
0.062 
0.062 
0.062 
0.062 
0.062 
0.062 
-- 0.062 
-- 
Cl 71.0 
114.7 
71.0 
71.0 
71.0 
71.0 
71.0 
71.0 
110.7 
71.0 
78.5 
Na -- -- 0.42 
0.42 
0.42 
0.42 
0.42 
0.42 
-- 0.42 
-- 
U -238 0.028 
0.028 
0.029 
0.029 
0.029 
0.029 
0.029 
0.029 
0.029 
0.029 
0.5 
Th -232 0.070 
0.070 
0.070 
0.070 
0.070 
0.070 
0.070 
0.070 
0.070 
0.070 
0.5 
Ra -226 400 400 400 
400 
400 
400 
400 
400 
400 
400 
6000 
H.sub.2 SO.sub.4 
14.2 
14.2 
15.7 
15.7 
15.7 
15.7 
15.7 
15.7 
15.7 
15.7 
10.9 
Addition (g/l) 
Waste Leach Results 
Acetic 0 0 47.6 
33.2 
41.4 
46 3.9 
48.4 
0 0 0 
Acid Addition 
0.5.M mL/L 
U Extraction 
0 6.2 8.0 
3.6 
8.6 
6.5 
13.0 
0 13.3 
0 69 
Th Extraction 
0 3.6 6.1 
0 3.8 
0 16.0 
0 14.6 
0 0 
% 
Ra Extraction 
0-10 
21 18 34 71 44 0 28 11 15 44 
% 
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TABLE 2 
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Liquor Compositions and Waste Stability 
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Liquor, g/L 
A4-1 A4-2 A4-3 
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Fe 84.4 86.9 83.8 
Zr 0.009 5.15 5.12 
Si 0.023 0.028 0.028 
Ti 0.177 0.171 0.150 
Y 0.011 0.012 0.012 
Mg 2.29 2.41 2.10 
Al 0.146 0.175 2.70 
P 0.097 1.38 2.65 
Ca 0.110 0.115 0.116 
Ce 0.048 0.158 0.168 
Hf -- -- -- 
Cl n.d. n.d. n.d. 
Na 0.515 0.555 0.546 
U -238 0.180 0.182 0.158 
Th -232 0.102 0.106 0.090 
Ra -226* 
H.sub.2 SO.sub.4 Addition (g/l) 
0 0 0 
Waste Leach Results 
Acetic Acid Addition 
0.5 .M mL/L 0 5.2 5.0 
U Extraction % 19.8 0.13 0.08 
Th Extraction % 0.11 0 0 
Ra Extraction % 3 7 4 
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n.d. = not determined 
*in radiochemical equilibrium with uranium