This invention relates to a method of processing a high level radioactive waste liquor such as a radioactive waste liquor discharged from a nuclear fuel reprocessing step of a nuclear fuel used for atomic power generation.
As the nuclear fuel is used inside an atomic reactor, nuclear fission products having long half life such as caesium (Cs) and strontium (Sr) are built up and uranium 235 is consumed so that the fuel becomes gradually difficult to burn. Hence, the fuel must be taken out from the reactor and be replaced by a fresh fuel at a suitable timing. On the other hand, so-called nuclear fuel reprocessing has been carried out to extract plutonium that remains in the used fuel that is taken out from the reactor for replacement and to isolate the effective component such as uranium 235 from the fission products by applying chemical treatment to the used fuel. The chemical treatment to be conducted in this nuclear fuel reprocessing step in turn generates a high level radioactive waste liquor containing large quantities of fission products. Because the fission products have long half life, the high level radioactive waste liquor must be safely stored and disposed. The volume of the waste liquor must also be reduced since the waste liquor must be generally stored for an extended period of time.
When the high level radioactive waste liquor is evaporation-concentrated and is then stored in a tank in the form of liquid in accordance with one of the conventional methods of finally processing the waste liquor, corrosion of the tank becomes a serious problem because the waste liquor is turned into a dense acidic waste liquor by 2N or 3N nitric acid that is added for the chemical processing in the nuclear fuel reprocessing step. Since the waste liquor is a liquid, it is not easily tangible. As another method of finally processing the high level radioactive waste liquor, Japanese Patent Publication No. 480/1982 discloses a method of sealing the waste liquor into the strata such as rock salt, granite, basalt, clay and the like at a high temperature and a high pressure or a method of calcining or changing the waste liquor into ceramic for solidification and then processing it finally. In either methods, the nuclear fission products such as caesium and strontium must be prevented from being emitted to the environment by evaporation or permeation.
Whether the high level radioactive waste liquor is stored in the form of liquid for an extended period of time or is stored in the form of the solid, these methods of finally processing the waste liquor have both merits and demerits and have not yet been established sufficiently. Whichever method may be employed, the final processing method is preferably flexible sufficiently. From the aspect of safety, volume reduction of the waste and easy handling, the method is preferably flexible such that the waste can be stored intermediately and temporarily at least under the solidified state and if a decision is thereafter made to store the waste in the solid form, the method can change the waste to the final solid or if a decision is made to store the waste in the liquid form, the method can change again the waste from the solid to the liquid state.
BE-A-812 192 discloses a method for treatment of high level radioactive liquid waste by mixing it with a concentrated silicate solution, e.g. water glass, that reversibly changes from a liquid to a solid state and vice versa depending upon its pH value, and by heating the mixture in order to solidify it. During said heating step, however, the permeation or evaporation of fission products and contained in the mixture cannot be prevented.
It is therefore an object of the present invention to provide a method of processing a high level radioactive waste liquor for temporarily and intermediately storing the waste liquor, which method is sufficiently flexible to cope with any change of a final processing method of the high level radioactive waste liquor that might occur afterwards and prevents evaporation or permeation of nuclear fission products having long half life and contained in the waste liquor.
Said object is achieved according to the present invention by a method of processing a high level radioactive waste liquor, wherein the high level radioactive waste liquor is mixed with an organic or inorganic silicate, sodium aluminate or a boron compound as solidifying agent which reversibiy changes from a liquid to a solid and vice versa depending upon its pH value so as to solidify said waste liquor and wherein the high level radioactive waste liquor is also mixed with a fixing agent of the group consisting of organic and inorganic titanium compounds, zirconium compounds, metal ferrocyanides and zeolite which fixing agent reacts with fission products contained in said waste liquor and prevents the permeation or evaporation of said fission products, so as to solidify said waste liquor while said fission products are sealed therein.
A solid formed by solidifying said waste liquor for intermediate storage may be heated, whenever necessary, to form a solid for final storage.
To a solid formed by solidifying said waste liquor for intermediate storage may also be added an acid or an alkali, whenever necessary, so as to again return said solid for intermediate storage to the liquid.
The features of the present invention are based upon the following four findings which are then applied by the inventor of the present invention to a method of processing a high level radioactive waste liquor.
- (1) A sodium silicate solution (so-called water glass) solidifies upon the neutralizing reaction with an acid or an alkali. In other words, it changes from the liquid to the solid depending upon the pH value of the solution. In this instance, the water content is held inside the solid as the water of crystallization.
- (2) When the solid is heated, the water of crystallization is emitted and a glass solid not containing the water of crystallization is formed.
- (3) When an acid or an alkali is added, this solid returns to the solution. In other words, it reversibly changes from the solid to liquid depending upon the pH value.
- (4) When a mixture of a high level radioactive waste liquor and a sodium silicate solution is further mixed with a fixing agent of the group consisting of organic and inorganic titanium compounds, zirconium compounds, metal ferrocyanides and zeolite prior to solidifying said mixture, said fixing agent reacts with fission products contained in said waste liquor and prevents the permeation or evaporation of said fission products during solidification.
- Figure 1 is a flow sheet of typical nuclear fuel reprocessing steps;
- Figure 2 is a flow sheet of the processing steps of the high level radioactive waste liquor in accordance with one embodiment of the present invention;
- Figure 3 is a diagram showing the free water content of the water of crystallization in sodium silicate with the change in pH values;
- Figure 4 is a schematic view showing the structural formula in solidification of sodium silicate;
- Figure 5 is a diagram showing the change of evaporation and permeation ratios of strontium with the change in the addition amount of titanium tetrachloride;
- Figure 6 is a diagram showing the change of evaporation and permeation ratios of caesium with the change in the addition amount of copper ferrocyanide;
- Figure 7 shows the change of the weight of water glass with respect to the change in the heating temperature; and
- Figure 8 is a schematic view useful for explaining an apparatus to be employed in one embodiment of the present invention.
Hereinafter, one embodiment of the present invention will be described with reference to the accompanying drawings. This embodiment primarily deals with a waste liquor which is extracted from the nuclear fuel reprocessing step and contains principally nitric acid, as the high level radioactive waste liquor.
Figure 1 shows a typical nuclear fuel reprocessing flow diagram. The used fuel generated in an atomic power plant is stored in a storage basin for several months. During storage, the radioactivity of the fuel decays. After the decay, the cladding of the used fuel is removed mechanically or the fuel rods are cut into pieces as such and are then dissolved by nitric acid inside a dissolving tank. Concentrated nitric acid of 7N to 13N is used. After dissolution, the solid content is removed as a solid waste while gaseous products of nuclear fission such as krypton, xenon and the like are discharged in the off gas. On the other hand, the remaining fuel solution is transferred to the co-decontamination step after the nitric concentration is adjusted to about 3N. In this co-decontamination step, the high level radioactive waste liquor containing caesium, strontium and the like is separated and extracted from the organic phase containing uranium and plutonium. The organic phase containing uranium and plutonium which is thus separated from the fission products as described above is distributed to uranium and plutonium in the distribution step and uranium and plutonium are then purified, enriched and then stored for reuse.
The solution consisting principally of nitric acid that contains the nuclear fission products and is formed in the co-decontamination step in the fuel reprocessing is the typical example of the high level radioactive waste liquor.
Figure 2 shows the flow of the processing of the high level radioactive waste liquor. In the flow diagram, the high level radioactive waste liquor containing the fission products such as caesium and strontium is mixed with a solidifying agent for solidifying the high level radioactive waste liquor and with a fission product-fixing agent for stably fixing the fission products contained in the high level radioactive waste liquor in the waste liquor or in the solidified matter. In the high level radioactive waste liquor containing 2 mol/I- concentrated nitric acid and having a radioactivity concentration of 806 Ci/I, the fission products account for 800 Cill. A sodium silicate solution or so-called "water glass" is used as the solidifying agent. As expressed by Na20. nSiOs - xH20, water glass contains the water of crystallization of xH20. Copper ferrocyanide or titanium tetrachloride is used as the fixing agent for the fission products. They are expressed by the formulas Cu2[Fe(CN)6] and TiC14, respectively.
The mixing ratio of the high level radioactive waste liquor and the water glass must be taken into consideration when mixing them together. In consideration of the properties of the resulting solidified matter and the decay heat, the proportion of oxides such as the fission products in the solidified matter and the glass component in the water glass is preferably about 1/10. In terms of the mixing ratio of both solutions, this proportion is from about 1/2 to about 1/5, though varying to some extents depending upon the waste liquor and the water glass component.
The hardening reaction develops when the high level radioactive waste liquor, the water glass as the solidifying agent and the fixing agent of the fission products are mixed together, forming the solidified matter. The mechanism why the solidified matter is formed will be explained with reference to Figure 3, which illustrates the change in the behaviour of the water glass depending upon the pH value (hydrogen ion concentration). The abscissa represents the pH value of the water glass and the ordinate does the free water content. The diagram illustrates the proportion of the water of crystallization to the free water. In other words, if the free water content is 0%, the water glass is a solid and if the free water content is 100%, the water glass is a liquid. It can be seen from Figure 3 that the water glass is solid at a pH 6 to 8 and the free water content becomes greater on both acidic and alkaline sides so that the water glass gradually changes into the liquid. The present invention makes use of this property of water glass. As described already, the high level radioactive waste liquor generated from the fuel reprocessing step contains principally the nitric acid solution and is hence acidic. The invention makes use of the neutralizing reaction between the water glass which is alkaline and the waste liquor which is acidic. Both solutions are mixed and are then left standing for two to five days. In the interium, the hardening reaction expressed by the following formula occurs and a solidified matter having a sufficient strength is formed:
It is believed that the structure of the solidified matterthus formed constitutes three-dimensional net-like macromolecules of the formula (Si02)n as shown in Figure 4.
Next, the fixing agent for the fission products will be described. As described already, the fission products. contained in the high level radioactive waste liquor such as caesium and strontium are likely to evaporate from the waste liquor together with the vapor or to permeate into the water during storage. Any fixing agents indicated in claim 1 may be employed so long as they incorporate the fission products into the large molecular structure or converting them into substances having low solubility, thereby changing the fission products to stable products. Examples of the fixing agents include copper ferrocyanide and titanium tetrachloride. The mechanism of fixing the fission products by these fixing agents will be explained with reference to Figures 5 and 6.
In Figure 5, the solid line represents the change in the evaporation quantity of strontium with the change in the addition amount of titanium tetrachloride contained in the solidified matter when the solidified matter formed in the manner described above is heated at 1,200°C for 6 hours and dotted line represents the change in the permeation quantity of strontium when the solidified matter is left standing in water for 30 days. On the other hand, the solid line in Figure 6 represents the change in the evaporation quantity of caesium with the change in the addition amount of copper ferrocyanide in the solidified matter when the solidified matter is heated at 1,200°C for 6 hours while dotted line represents the change in the caesium permeation quantity when the solidified matter is left standing in water for 30 days. It can be seen from these diagrams that in conjunction with the change in the evaporation quantity, the effect of the fixing agent is relatively small for strontium because strontium is originally not easy to evaporate, but a large evaporation inhibiting effect can be seen for caesium because the element is highly easy to evaporate. In conjunction with the change in the permeation quantity, the fixing agent shows a large permeation inhibiting effect for both strontium and caesium. Copper ferrocyanide and titanium tetrachloride can check the evaporation and permeation of the fission products because they have such characteristics as to selectively take the alkali or alkaline earth metal such as caesium and strontium into their crystal lattice. In other words, caesium and strontium are caught into the net-like structure of the macromolecules of (Si02)n described already. When copper ferrocyanide is used as the fixing agent, it reacts with caesium as expressed by the following reaction formula:
Accordingly, being incorporated in the large molecular structure, caesium can not easily escape physically from the net-like structure of the water glass and can not easily permeate chemically in water because its solubility with water drops. The same also holds true of the relation between titanium tetrachloride and . strontium. Since the fixing agent is mixed with the water glass and the high level radioactive waste liquor, the fixing agent reacts with caesium, strontium and the like and forms a non-volatile non-permeable compound. Thus, the solidified matter fixing therein caesium, strontium and the like is formed.
The solidified matter thus obtained is stored as the solid for intermediate storage until the final processing method is decided.
When the final processing method is decided in Figure 2, the optimal treatment of the solid is carried out to change it into a storage body suitable for the final storage. If the final processing method is one that stores the waste in the form of liquid, for example, an acid or an alkali is added to the solid for intermediate storage. As explained already with reference to Figure 3, the form of the water glass changes with its pH value. The solid for intermediate storage remains solid in the pH range of 6 to 8. If the pH value of this solid is changed to at least 10 or below 4, the solid dissolves and again returns to the liquid. Nitric acid which is primarily contained in the radioactive waste liquor is used as the acid while sodium hydroxide or the like is used as the alkali. In either case, the pH value of the solid for intermediate storage may be selected from a range in which the solid is soluble. The solid for storage that is again returned to the liquid is charged into rocks or strata or into tanks in accordance with the final processing method selected.
On the other hand, if the selected final processing method is one that stores the waste in the solid form, the solid for intermediate storage is subjected to treatment which changes it into the solid for final storage. This final treatment will be explained with reference to Figure 7. The diagram of Figure 7 shows the change of the weight of the water glass with respect to the change in the heating temperature. When the water glass is 100% by weight, it contains the water of crystallization and other water contents. When heated up to 500°C, the weight drops down to about 80% and the water of crystallization starts evaporating. When heating is further continued, the water glass loses the water of crystallization, its weight becomes about 50% by weight and the glass water changes into a vitreous solid. Though this embodiment forms the solid for final storage by heating the water glass to about 1,200°C, the solid for final storage may be a vitreous solid which is obtained by heating to about 600°C.
In the manner described above, the solid for intermediate storage of the high level radioactive waste liquor can be formed by utilizing the property of the solidifying agent whose form changes reversibly between the liquid and the solid by the addition thereto of the acid or alkali. If the solid for intermediate storage is formed, the solid sufficiently flexible to cope with any method of final processing, whether the selected method finally stores the waste in the liquid form or in the solid form.
Next, Figure 8 depicts an apparatus for forming the solid for intermediate storage in accordance with the present invention. Tanks 1, 2 and 3 store therein the high level radioactive waste liquor, the water glass as the solidifying agent and copper ferrocyanide as the caesium-fixing agent and titanium tetrachloride as the strontium-fixing agent, respectively. Flow regulating valves 8, 9 and 10 are disposed at the intermediate portions of pipes 5, 6 and 7 for connecting the tanks 1, 2 and 3 to a mixing tank 4, respectively. The mixture is sufficiently mixed inside the mixing tank 4 by a mixer 11. A pH meter 12 detects the pH value of the mixed solution inside the mixing tank 4 and the openings of the valves 8, 9 and 10 are adjusted so that the pH value of the mixed solution falls between 6 and 8. After the three kinds of solutions are thus mixed, the mixed solution is transferred to an intermediate storage tank 13, where the mixed solution is left standing for two to five days. After these procedures are completed, a solid 14 for intermediate storage which incorporates therein the fission products as the compound can be formed.
Though this embodiment uses sodium silicate as the solidifying agent, the same result can be obtained by use of alkali silicates such as potassium silicate, calcium silicate and so forth. An organic liquid silica compound such as ethyl silicate may also be used. Though an aluminum compound such as sodium aluminate can also form the solid for intermediate storage, it is preferred in this case to add glass components such as silica in forming the solid for final storage by heating the solid for intermediate storage. A boron compound such as expressed by B203 may also be used either alone or as a mixture with sodium silicate. Since the composition becomes analogous to that of so-called borosilicate glass in this case, a glass solidified matter having excellent weatherability and radiation resistance can be formed.
Though the foregoing embodiment uses copper ferrocyanide and titanium tetrachloride as the fixing agent of the fission products, it has been confirmed that the following compounds can likewise be used.
Organic and inorganic titanium compounds which are liquid at normal temperature, such as titanium tetraisopropoxide [Ti(OC3H7)41, and zirconium compounds can be used as the strontium-fixing agent. Titanium-containing oxides which are obtained by hydrolyzing these titanium compounds such as a compound of the formula Ti(OH)4 can also be used but since the compound is solid, it must be mixed in the fine powder form to ensure sufficient homogeneity.
Besides copper ferrocyanide, other metal ferrocyanides such as nickel ferrocyanide can provide the same effect as the caesium-fixing agent but it has been confirmed experimentally that the copper compound reduces the addition amount by 10 to 20% as compared with other metal compounds. Zeolite can further be used either alone or as a mixture with the metal ferrocyanides.
Since the foregoing embodiment is directed to the acidic high level radioactive waste liquor containing primarily nitric acid or boric acid, the waste liquor is neutralized with the alkaline fixing agent. If the high level radioactive waste liquor is alkaline, however, an organic phosphoric acid is used for the neutralizing reaction. This acid has the function of hardening further the solidifying agent and hence, serves as the hardening agent.
In accordance with the present invention, the high lever radioactive waste liquor is mixed with the solidifying agent having the property such that it reversibly changes between the liquid and the solid depending upon the pH value, thereby forming the solid for intermediate storage. Hence, the present invention provides the method of processing the high level radioactive waste liquor, which method is flexible such that it can form the liquid or solid for final storage in accordance with the selected final processing method. Since the fixing agent that seals the fission products such as caesium and strontium in the solid for storage under the stable state is added, it is possible to prevent the permeation and evaporation of the fission products.