ITMI20120801A1 - METHOD FOR CONDITIONING SCORES ARISING FROM DISPOSAL OF NUCLEAR PLANTS - Google Patents

Description

Description

â € œMETHOD FOR THE CONDITIONING OF WASTE DERIVED FROM THE DISPOSAL OF NUCLEAR PLANTSâ €

The present invention relates to a method for the treatment of ferrous nuclear waste, typically waste produced in pickling operations of contaminated metal parts.

All radioactive by-products or residues, not reusable, of processes, or more generally of operations, in which radioactive substances have been generated or used are identified as "nuclear waste". Nuclear waste of any type and origin, due to their danger to man and his environment, must be treated and stored according to very special methods, which ensure the confinement to even very long times of radiation and nuclear elements or isotopes.

There are numerous types of processes in which nuclear elements or radiation are used, which produce waste at varying degrees of concentration and danger. A proposed classification, in use in Italy, divides these slags into:

- category 1, which includes all low level radioactive waste; It is the largest category, comprising, as mass, about 90% of the waste produced, but only 1% of the radioactivity (examples are the sanitary material used in nuclear medicine, the disposable clothing provided in a visit to a nuclear plant etc. ...);

- category 2, which includes all medium level radioactive waste; these require shielding, but constitute only 7% of the waste, with a total radioactivity of 4% (examples are the sheaths of the fuel elements of a reactor);

- category 3, which includes all waste with a high level of radioactivity, which constitutes only 3% of waste but represents 95% of radioactivity; they are the most dangerous due to the high dose of radiation that an accidental exposure would entail and to the decay in the order of millions of years of some of the radioactive isotopes they contain.

The different types of waste require different disposal procedures. The techniques studied and described in the last 60 years for this purpose are very numerous. The results are public and generally easily accessible; for the specific ones, concerning the long-term storage of types of waste containing long-lived and / or high-mobility isotopes, the conclusions are, however, still uncertain. The resources invested in these studies are, presumably, enormous; those to be invested for the conditioning and long-term storage of existing nuclear waste (including the remediation of the related sites) are partly known: for the US alone they have been valued in hundreds of billions of dollars.

The disposal of this slag generally requires a conditioning phase, which consists in the transformation of the slag into a form suitable for storage; and the storage of conditioned waste in suitable natural or industrially produced sites.

A particular type of nuclear waste, strategically very important, are those generated in the reclamation of nuclear reactors that are no longer active and nuclear sites that have become obsolete. In this case, nuclear waste is typically generated in the recovery and decontamination operations of large metal structures, which, when exposed to contact and / or radiation from radioactive isotopes, have in turn become radioactive (limited to the exposed surface), due to chemical contamination. or by nuclear mutation (under the effect of radiation). The whole of the operations linked to these reclamations is indicated in the sector with the English term â € œdecommissioningâ €, which will be adopted in the rest of the text. The dominant technique in the decontamination operations of metal surfaces is the so-called â € œ picklingâ €.

Many of the decommissioning waste thus generated belong to the above-mentioned category 3, and typically contain isotopes with a long, medium-life and high mobility, which in any case involve specific conditioning for highly dangerous waste. While industrially tested and economically useful processes have been identified for the complete management of nuclear waste belonging to categories 1 and 2, for those of category 3 important but still partial results have been obtained, especially for the anti-economic aspect of the required conditioning. , and to date no long-term storage facility is in operation.

As regards the conditioning of decommissioning waste, and typically those generated by pickling, the experts have come to the conclusion that it is necessary to use glass matrices with high chemical and thermo-mechanical stability for all long-lived radioactive isotopes and / or high mobility; see for example the article â € œGlass packages guaranteed far millions ofyears ", by à ‰. Y. Vernaz, Clefs CEA, n. 46 (2002), pp. 81-84. Numerous examples of vitrification of category 3 slag, even at an industrial level, which were however affected by process reliability problems and typically high costs.

Recently, among the most promising glassy materials for the purpose of retaining radioactive isotopes, especially if in the presence of sulphates, chromates, phosphates and halides, the glassy phosphatic systems containing iron have been credited. Systems of this type are described in patents US 5,750,824 and US 5,840,638 and in patent application GB 2,371,542 A.

Among these, particularly interesting is US 5,750,824, which teaches the production of phosphate glasses containing from 30 to 70% by weight of phosphorus oxide (as P2O5) and from 22 to 50% of iron oxide, the rest being made up of from oxides of other metals, including those derived from nuclear waste; moreover, this document teaches that the best results are obtained with glasses in which iron is present for at least 50%, preferably at least Î "80% and even more preferably at least 90%, in the oxidation state 3, i.e. as Fe <3+> ion. According to this document, phosphatic glasses with a high Fe <3 +> / Fe <2+> ratio are characterized by the best properties of chemical resistance (for example, to leaching, i.e. washing with water), density and resistance thermomechanics.

The methods taught in these documents provide for the preparation of a mixture of powders of oxides or salts of phosphorus and iron in the desired weight ratios; the fusion of this mixture; the addition, before or during said melting, of the slag to be disposed of; and the solidification of the melt in special molds.

An open problem with these methods is the management of the considerable volumes of liquid of the solutions in which the decommissioning waste is initially dissolved. In fact, in some cases the solutions are added directly to the melt of oxides or salts of phosphorus and iron, generating however large volumes of vapors which must then be recondensed, decontaminated and disposed of; in other cases, the solutions are first brought to dryness, and the slag is added in the form of powder to the melt, but also in this case the obtaining of slag powders involves the evaporation of large quantities of liquid.

The purpose of the present invention is to provide an improved method for the conditioning of decommissioning slags, specifically those from pickling.

According to the invention, this purpose is achieved with a process which includes the following steps:

- dissolution of contaminated metal parts of nuclear plants with the use of phosphoric acid, forming a solution with a pH lower than 1.5;

- oxidation of iron ions in solution from Fe <2+> to Fe <3+> to obtain a Fe <3 +> / Fe <2+> ratio equal to or greater than 9;

- electrochemical raising, in a cell without membrane or with a cationic membrane, of the pH of the solution thus obtained to a value greater than 1, 5 and less than 10, causing the precipitation of the phosphate salts of iron and metal ions present in the solution;

- separation of the precipitated salts from the liquid phase; And

- heat treatment of vitrification of the mixture of precipitated solids.

The invention will be illustrated with reference to the following Figures, in which:

- Fig. 1 shows (schematically) in its upper part an electrochemical cell for the realization of the method of the invention, and in its lower part a diagram of the trend of the concentration of the H <+> ion at inside the cell, along the coordinate that goes from the cathode to the anode of the cell;

- Fig. 2 schematically shows an alternative electrochemical cell for carrying out the method of the invention.

The method of the invention has various advantages with respect to the known ones. In particular, it does not require the pretreatment of large volumes of solution to recover the salts of the radioactive metals to be then added to the precursors of the iron-phosphatic glass in suitable proportions, because with the present process the mixture of phosphorus and metals, in principle in the necessary proportions, is produced in situ in the solution, and is then obtained from this to be sent to heat treatments, while the remaining liquid phase can be recycled, after topping up with new concentrated phosphoric acid, in a subsequent dissolution cycle (pickling), precipitation and separation, without having to be disposed of separately. In this way, all treatments that can generate secondary contamination are avoided.

The first operation of the process consists in washing and dissolving in phosphoric acid of the metal parts contaminated on the surface (pickling), deriving from the dismantling of the nuclear plant. These are conveniently reduced into pieces of suitable size and weight, for example in the order of kilograms or tens of kilograms. The metal parts derived from nuclear plants are typically made of steel, and therefore consist mainly of iron, with smaller quantities of elements typical of steel metallurgy (chromium, nickel, manganese, ...) and of radioactive elements that are deposited in surface, or that are produced, by contact with radioactive isotopes or radiation. It has been observed that the appropriate concentration by weight of metals in the acid solution is between about 5 and 12%, preferably about 10%, of the weight of the overall solution. This solution is subsequently standardized to the desired concentration and pH (for example 9% by weight and pH 1). Operating under these conditions in phosphoric acid, a concentrated solution is obtained, ideal for the subsequent oxidation operations of iron from Fe <2+> to Fe <3+> and sufficiently close to the saturation point for the precipitation of iron-phosphate salts. The resulting solution has a weight ratio between phosphorus and iron suitable for the production of ferrophosphatic glasses endowed with the characteristics necessary for the conditioning of the radioactive metals initially present in the solution. After the separation of the solid precipitate from the liquid phase of the solution, a final check of the elemental composition of the material obtained takes place and, if necessary, a retouching of the composition with the addition of the defective component if necessary. For the purposes of the invention, the solution must contain iron and phosphorus in a molar ratio between 33/66 and 45/55, and preferably about 40/60, and have a pH lower than 1.5.

The solution obtained can optionally be analyzed to determine its chemical composition, and in particular the molar ratio of Fe / P and the ratio of Fe <3 +> / Fe <2+>. For the purposes of the invention, it is preferable that the molar ratio Fe / P is about 40/60; this ratio can be adjusted to around the optimal value by adding, to the solution directly obtained from pickling, a soluble salt of iron or phosphoric acid, depending on which of the two components is the result of the analysis at fault with respect to said optimal ratio. Furthermore, it is preferable that the Fe <3 +> / Fe <2+> ratio has a high value, greater than 9 and typically greater than 24.

Since in the initial solution obtained by pickling the iron is present almost exclusively in the form of Fe <2+> ions, the method of the invention provides a second operation, which consists in the oxidation of at least part of said ions to bring said ratio in the preferred interval; the oxidation operation can be carried out for example by adding hydrogen peroxide, ion permanganate, by bubbling oxygen in the solution, or by any other known method.

The third operation of the process of the invention consists in causing the precipitation of the metal salts present in the solution, raising the pH of the latter without increasing its volume. In this operation the pH of the solution is brought to a value between 1, 5 and 10, and preferably between 1, 7 and 2.5.

According to the present invention, the raising of the pH is obtained by electrochemical way, by applying a potential difference between two electrodes immersed in the solution obtained from the previous operation.

In a first embodiment of the method, the electrochemical treatment is carried out in an electrochemical cell with a single compartment, ie where there are no separation elements between anode and cathode, of the type shown in Fig. 1.

The solution to be treated, 11, deriving from the oxidation operation, is introduced into the cell 10, consisting of a single tank. Two electrodes, 12 and 13 are also inserted in the cell. An adequate potential difference is applied to the electrodes (in the figure, the electrode 12 is brought to cathodic potential and the electrode 13 to anode potential), so as to generate an electric current of suitable value consequent to the establishment of the electrolysis reaction of the water.

Since the solution introduced into the electrochemical cell is highly acidic, the hydrogen ion reduction reaction takes place at the cathode:

2H <+> + 2e <"> â †’ H2â €

which consequently consumes them causing an increase in the pH value; symmetrically, the oxidation reaction takes place at the anode:

H20 â † ’1/2 02â € 2H <+> + 2e <~>

which introduces into the solution a quantity of H <+> ions equal to that consumed at the cathode.

Overall, therefore, there is no alteration of the pH of the solution as the circulation of electric current causes both a consumption of H <+> ions at the cathode and the formation of the same ions in a quantity equivalent to the anode.

However, to ensure the transport of ions, in particular of H <+>, from the anode to the cathode, a concentration gradient is established, which depends on the applied current density; the gradient is such that the concentration of H <+> is maximum at the anode (where H <+> is formed) and minimum at the cathode (where H <+> is consumed), as shown in the lower diagram in Figure 1 (the horizontal coordinate in the diagram represents the distance between the cathode, position â € œCâ € on the abscissa axis, and the anode, position â € œA "on the same axis). pH rises to the point of triggering the precipitation of FeP04.

The reactions illustrated above are only one of the possible redox pairs that can be used to cause the pH increase in the volume of solution adjacent to the cathode; other reactions can be obtained from tables reported in electrochemistry manuals.

In a second embodiment, the electrochemical treatment according to the invention is carried out in an electrochemical cell, schematized in the upper part of Fig. 2, separated into two compartments by a cationic membrane, 14. The solution 11 is introduced into the cathode compartment. deriving from the previous oxidation operation, while a dilute solution of phosphoric acid 15 is introduced into the anodic compartment, necessary to guarantee the electrical continuity of the system and to have ions compatible with those of solution 11.

The membrane 14 allows the passage between the two half-cells only of positive ions, and due to the great difference in mobility in solution and in the permeability of the H <+> ions with respect to the other cations, the permeation of the former is about two orders of magnitude higher than that of the latter, so that in fact the ionic transport in solution is almost completely attributable to the H <+> ions alone. Due to the potential difference existing between anode and cathode, the migration of the H <+> ions occurs from the anodic to the cathode compartment. In this case, as shown schematically in the diagram shown in the lower part of Fig. 2, the concentration of the H <+> ion is almost constant inside each half-cell, showing a sensitive gradient only around the electrodes. Since in this case the `` leap '' of ion concentration H <+> occurs largely in correspondence with the membrane (due to the mobility of these ions greater in solution than inside the membrane itself, which causes these are rapidly neutralized at the cathode as soon as they enter the cathode compartment), the decrease in the concentration of H <+> ion is more homogeneous in the cathode compartment than in the case without a membrane; this decrease in concentration determines a localized increase in pH, which causes the precipitation of phosphates in correspondence in this compartment.

The embodiment described above for the case of the cell separated into compartments can be easily generalized to a system with several electrodes, in which a series of electrodes alternatively placed at cathode and anodic potential are present; a system with several electrodes of this type allows to work in parallel on larger quantities of solution, thus increasing the productivity of the method.

The electrochemical precipitation method is ideal, because it avoids having to add other solutions to the one derived from pickling, which would lead to an increase in the overall volume of liquids to be treated and disposed of, after appropriate conditioning treatments, at the end of the process. By operating in this way, the component useful for vitrification (the phosphates, mainly consisting of ferric phosphate plus the traces of the other phosphates of the metals originally in solution, or simply of adsorbed metals) is precipitated in the optimal proportion, without compromising the quality of the glass and preserving the starting phosphoric acid solution for a further pickling cycle, ie reusing all the components of the original slag and the co-products of the process; this route is therefore the ideal method to avoid any secondary contamination.

Once the phosphates have precipitated from the solution, they can be recovered by eliminating the liquid phase above, for example by simple transfer. If necessary, the precipitate can then be centrifuged, to obtain a better separation from the liquid phase. The mixture of wet phosphates is then preferably mechanically mixed, to homogenize it. In fact, during the precipitation phase, it is possible that the different phosphates precipitate at different times, giving rise to a precipitate in which the different phosphates are stratified according to the order of precipitation. An inhomogeneous solid phase thus obtained could give rise to a glass that is not perfectly homogeneous: despite the precipitate being subjected to melting, the viscosity of the melt could be such as not to allow complete homogenization during melting, with the risk of obtaining a glass final composition not perfectly homogeneous and therefore parts of the same (especially any parts poor in iron) that do not have the characteristics required by the application.

The precipitate is then vitrified, implementing all the precautions required by the presence of radioactivity. Typically the FeP04 precipitate melts and vitrifies at a temperature not exceeding 1100 ° C. The presence of other cations co-precipitated with the iron phosphate can generate a relatively wide thermal band within which vitrification takes place, typically between 800 ° C and 1300 ° C.

The liquid phase, still containing considerable quantities of phosphoric acid, and potentially various metal ions, even traces of radioactive isotopes, is recovered with operations typical of the known art, for example by decantation, and / or centrifugation and can be reused in a subsequent cycle of dissolution of metal parts and precipitation of phosphates, after topping up the phosphoric acid to reintegrate that consumed in the precipitation.

The process of the invention thus obtains the result of disposing of the metal parts derived from decommissioning avoiding the need to treat high volumes of liquid phases, typical of the processes of the known art. In fact, with the processes of the known art, the volumes of liquid phases generated are proportional to the weight of the metal parts themselves (because for each treatment cycle of a unit of weight of metal parts it is necessary to use a given volume of solution ), while in the case of the present invention the liquid phase volume is essentially that necessary for a single dissolution operation of an aliquot of metal parts.

Claims (10)

CLAIMS 1. Method for the conditioning of waste resulting from the disposal of nuclear plants which includes the following operations: - dissolution of the contaminated metal parts of nuclear plants with the use of phosphoric acid, forming a solution with a pH lower than 1.5; - oxidation of iron ions in solution from Fe <2+> to Fe <3+> to obtain a Fe <3 +> / Fe <2+> ratio equal to or greater than 9; - electrochemical raising, in a cell without membrane or with a cationic membrane, of the pH of the solution thus obtained to a value greater than 1.5 and less than 10, causing the precipitation of the phosphate salts of the iron and of the metal ions present in the solution; - separation of the precipitated salts from the liquid phase; And - heat treatment of vitrification of the mixture of precipitated solids. The method according to claim 1, further comprising the recovery of said liquid phase and recycling in a subsequent cycle of method operations. 3. Method according to one of the preceding claims, in which in the dissolution operation of the contaminated metal parts, said parts are added in an amount comprised between 5 and 12% by weight of the total weight of metal and phosphoric acid. 4. Method according to any one of the preceding claims, in which after said dissolution operation, an elementary analysis of the chemical composition of the solution is carried out and, if it is determined that the molar ratio Fe / P is outside the range between 33/66 and 45/55, the component in defect is added to the solution to bring this ratio back within said range. Method according to any one of the preceding claims, in which in said oxidation operation the ratio Fe <3 +> / Fe <2+> is brought to a value equal to or greater than 24. Method according to any one of the preceding claims, wherein said oxidation operation is carried out by adding hydrogen peroxide, permanganate ion, or by bubbling oxygen into the solution. 7. Method according to any one of the preceding claims, in which the electrochemical raising operation of the pH comprises the following steps: - providing an electrochemical cell (10) containing a first electrode (12) and a second electrode (13); - introducing the solution (11) deriving from the oxidation operation into the cell; - bring the first electrode to cathode potential and the second electrode to anode potential, causing the following reactions to occur, respectively around the cathode and the anode: 2H <+> + 2e <~> â † ’H2â € And H20 â † ’1 / 202â € 2H <+> + 2e <~> and applying a potential difference between the electrodes such as to induce a density of circulating electric current that determines a concentration gradient of H <+> ions between anode and cathode such as to bring the pH in the vicinity of the cathode to values higher than that necessary for produce the precipitation of insoluble metal phosphates. 8. Method according to any one of claims 1 to 6, wherein the electrochemical raising operation of the pH comprises the following steps: - providing an electrochemical cell (10) containing a first electrode (12) and a second electrode (13), and separated into two compartments by a cationic membrane (14); - introducing the solution (11) deriving from the oxidation operation into the compartment containing the first electrode; - introducing a dilute solution of phosphoric acid (15) into the compartment containing the second electrode; - bring the first electrode to cathode potential and the second electrode to anode potential, causing the following reactions, respectively in the cathode compartment and in the anode compartment: 2H <+> + 2e <"> â †’ H2â € And H20 â † ’1 / 202â € 2H <+> + 2e ~ so as to determine in the cathode compartment the increase of the pH value and the induction of the precipitation of insoluble metal phosphates. 9. Method according to claim 8, in which a series of electrodes alternatively placed at cathodic and anodic potential is used. Method according to any one of the preceding claims, in which in said pH raising operation, said pH is brought to a value comprised between 1, 7 and 2.5. 1 1. Method according to any one of the preceding claims, in which said heat treatment is carried out at a temperature between 800 ° C and 1300 ° C.