Procedure for ceramizing radioactive wastes

The present invention concerns a procedure for transforming radioactive wastes into ceramics. From the radioactive waste solution, the waste is bound to an inorganic ion exchanger either with batch equilibration or in columns. After the waste has been bound to the ion exchanger, it is transformed into ceramics by admixing it to tile clay or to another ceramizing material and by firing the mixture to a tile. This yields an extremely low soluble and mechanically durable ultimate disposal product of the nuclear waste. The ceramizing procedure is applicable with any inorganic ion exchanger.

BACKGROUND OF THE INVENTION 
The present invention concerns a procedure for transforming radioactive 
wastes into ceramics. 
Treatment of the radioactive waste solutions accruing in nuclear energy 
production aims at transforming the wastes into a safe form for ultimate 
disposal. In this context, safety implies low solubility of the final 
waste product, and good mechanical as well as thermal and radiation 
stability. 
In solidifying low and medium-active power plant wastes, the commonest 
procedures are embedding in concrete and in bitumen. The greatest drawback 
of the inexpensive and simple embedding in concrete is the high leach rate 
of radio nuclides from the solidified product. Bituminized products have a 
lower degree of solubility, but the process is a lot more difficult and 
risky, e.g., because of risk of ignition. The only procedure widely used 
in solidifying the high-active waste from reprocessing of spent fuel is 
vitrifying the waste, particularly in borosilicate glasses. However, 
experiments carried out with ceramic final waste products such as 
titanate, zirconate and niobate-based ceramic transformation products have 
proved that these are superior to glass products in stability, and are 
gaining ground in research. 
Titanates, in particular sodium titanate, are the most important base 
materials for ceramic products for ultimate disposal. The radioactive 
wastes are bound to them in the material synthesis, by ion exchange or by 
mechanical mixing in calcinate form. Thereafter, the product may be 
transformed into ceramics under high pressure and at high temperature. The 
most promising ceramic final waste product is SYNROC (A. E. Ringwood et. 
al., Immobilization of High Level Nuclear Reactor Wastes in Synroc: A 
Current Appraisal, Research School of Earth Sciences, Australia National 
University, Publication No. 1975, 1981). It is composed of three minerals, 
the main components of which are TiO.sub.2 (60%) and ZrO.sub.2 (10%). 
These minerals are analogous to minerals occurring in nature, and they 
have been found to have exceedingly low solubility and to tolerate 
radiation extremely well. 
The drawback encumbering the ceramizing procedures studied so far is their 
complexity and high cost. Expensive initial materials awkward to pre-treat 
and expensive compressing apparatus are used in them. 
SUMMARY OF THE INVENTION 
The present invention aims at improvement of the procedures known in the 
art. A more specific aim of the invention is to provide a procedure which 
is simple in its process technology, and economical, and wherein 
inexpensive and readily available initial materials are used, for instance 
conventional raw materials of the ceramic industry. The invention is 
applicable in connection with both low- and high-active wastes. The other 
objects of the invention and the advantages gainable with its aid will 
become apparent in the disclosure of the invention. 
The aims of the invention are achieved by means of a procedure which is 
mainly characterized in that the procedure comprises the following steps: 
(a) from the radioactive waste solution, the waste is bound to an inorganic 
ion exchanger, 
(b) the inorganic ion exchangers loaded with wastes are admixed to a 
ceramizing substance, and 
(c) the waste mixed with ceramizing substance is baked to become the final 
waste product. 
By means of the procedure of the invention, a number of remarkable 
advantages are achieved. The invention describes a ceramizing procedure 
for inorganic ionic exchangers based on inexpensive and readily available 
initial materials, on conventional raw-materials of the ceramic industry 
and on a simple process technology, appropriate for both low- and 
high-active wastes. The raw materials for bricks and tiles are cheap and 
readily and continuously available. The manufacturing technology of tiles 
is simple, and the firing temperature of tiles is relatively low, thus 
preventing evaporation of certain radioactive substances during the baking 
process. It is possible to add to the tiles synthetic or natural 
additives, such as vermiculite or apatite, which improve the stability of 
certain substances in the tiles. In tile firing, no complex pressing 
apparatus is required, and this greatly reduces the cost and simplifies 
the process. Clay tile containing titanate is glazed in the course of 
firing and becomes very low soluble. It can be coated with an inactive 
surface layer. Thereby no metal container is needed for tiles loaded with 
medium-active wastes. Compared with bituminized and concreted products, by 
means of the procedure of the invention a remarkable saving in volume is 
achieved, and the ultimate decrease in volume is of the same order of 
magnitude as with vitrified products.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the procedure of the invention, the radioactive wastes in solution form 
are bound to an inorganic ion exchanger, such as titanate, niobate, 
zirconate or zirconium dioxide. For better binding in the tiles of certain 
radionuclides such as Cs, one may add synthetic or natural additives, such 
as vermiculite, laumontite or apatite to the tiles. In case batch 
equilibrating is used, the ion exchanger need not be dried and ground, and 
the tile clay may be added to the waste ion exchanger mixture directly 
after equilibrating so that the water content of the mixture will be about 
23 to 27%. The mass ratio of ion exchanger to tile clay is 1/9 to 2/8. 
The materials used to serve as ceramizing substances include red clay, 
kaolin, montmorillonite, feldspar, illite and quartz. 
After mixing the tile clay, the mixture is stirred with care so as to make 
it bakable. Hereafter, it is shaped into tiles in a mould. The tiles may 
be pressed to make them less porous. The tiles are left to dry overnight. 
Thereafter, they are dried at about 150.degree. C. for at least four hours 
and allowed to cool over night. 
The firing of the tiles is accomplished as follows. The kiln is heated at a 
rate of approximately 100.degree. C. per hour up to 
1020.degree.-1060.degree. C. The tiles are kept at peak temperature for 
4-10 hours. After the firing, the tiles are allowed to cool in the kiln. 
The tile kiln may be lined with thin inactive tiles in order to bind 
volatile substances. These lining tiles are replaced from time to time and 
disposed of along with the waste tiles. The tile firing may also be made 
continuous, applying experience gained in the ceramic industry. 
The quality factor of the tiles most important in view of the ultimate 
disposal is solubility from them of the waste nuclides. The leach rates of 
Sr, Cs and Co from sodium titanate or ZrO.sub.2 /red clay tiles loaded 
with evaporator waste concentrate are 10.sup.-6 to 10.sup.-7 g per 
cm.sup.2 x d in the declining order mentioned above. The solubility of Sr 
from sodium titanate/red clay tiles loaded with high-active waste is 
higher by one order of magnitude. Addition of vermiculite (2%) to the 
tiles causes some decrease of solubility. Thus, the leach rates are of the 
order of those of the best borosilicate glasses. 
The solubility properties of the tiles may be improved either by glazing 
their surface or by baking an inactive layer upon the surface of the tile 
of the tile clay that is being used. Even adding titanate to the tile 
clays will cause glazing of the tiles, and titanate/red clay tiles are 
rather less porous than the plain red clay tiles. The tile would be ideal 
when its solubility properties would allow it to be ultimately disposed 
without any extra shells. This may be contemplated at least in the case of 
tiles loaded with medium-active wastes. 
The tiles present very high mechanical durability, a feature important with 
a view to handling and transporting. The tiles have flexural strengths on 
the order of 20-30 MN/m.sup.2 (meganewtons per square meter). 
When the amount of ion exchangers in the tiles is 15% at the most, the 
evaporation of metals therefrom is minimal: at the most, something like 2% 
when the firing temperature is 1020.degree. C. With increasing amount of 
ion exchanger, and with temperature higher than mentioned, higher 
evaporation is also incurred. The optimum values for minimum evaporation 
are: 15% ion exchanger loading in the tile, firing temperature 
1020.degree. C., and firing time 4 hours. 
The procedure of the invention can be used for transforming into ceramics 
at least the most important wastes, such as evaporation waste 
concentrates, waste nuclides eluted from spent reactor resins, and 
high-active reprocessing waste. 
Various details of the invention may vary within the scope of the inventive 
idea described above.