ITMI20120817A1 - METHOD AND PLANT FOR THE CONDITIONING OF SCORES DERIVED FROM DISPOSAL OF NUCLEAR PLANTS - Google Patents

Description

Description

METHOD AND PLANT FOR THE CONDITIONING OF WASTE DERIVED FROM THE DISPOSAL OF NUCLEAR PLANTS

The present invention relates to a method for treating ferrous nuclear waste, typically waste produced in pickling operations of surface contaminated metal parts, as well as a plant for carrying out the method.

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 dangerousness for man and his environment, must be treated and stored according to very particular methodologies, 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 Î "1% of the radioactivity (examples are sanitary material used in nuclear medicine, disposable clothing provided during 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 USA 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 "decommissioning", which will be adopted in the rest of the text. The dominant technique in the decontamination operations of metal surfaces is the so-called "pickling".

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 € œG / ass packages guaranteed formillions 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 80% and even more preferably at least 90%, in the oxidation state 3, that is 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 Italian patent applications MI2010A002105, MI2012A000801 and MI2012A000803, the content of which is incorporated in the present description, teach various alternative methods for producing phosphatic glasses starting from metallic (iron-based) nuclear waste. The object of all three of these questions is a process that is divided into 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;

- raising the pH of the solution thus obtained to a value greater than 1.5 and less than 10, and preferably between 1.7 and 2.5, so as to cause the precipitation of the phosphate salts of iron and other 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 three questions differ from each other in the way in which the phase of raising the pH of the solution derived from the oxidation phase is carried out.

In particular, in application MI2010A002105 the pH is brought to a value higher than 1.5 by adding a solid base, for example Ca (OH) 2, or by electrochemical way. In the case of the electrochemical pathway, in detail this consists in providing an electrochemical cell separated into two half-cells by means of an anionic membrane; introducing the solution derived from the oxidation step into a first half-cell, which, during the operation of the method, will be brought to cathodic potential; introducing a first electrode (cathode) into this solution; introduce in the second half-cell, which during the operation of the method will be brought to anode potential, a solution of a composition similar to that of the solution to be treated, but which does not contain the metal ions to be precipitated; introducing a second electrode (anode) into this solution; bring the electrodes to their working potential, causing electrolysis of the water, which involves the hydrogen ion reduction reaction in the first half cell (cathode):

2H <+> + 2e <"> â †’ H2â €

and in the second (anodic) half-cell the reaction takes place:

H20 â † ’1/2 02â € 2H <+> + 2e <">;

consequently, H <+> ions are consumed in the first half cell, causing the desired increase in the pH value.

In application MI2012A000801 the raising of the pH is obtained with a method which consists in providing an electrochemical cell containing a first electrode and a second electrode; introduce in the cell the solution resulting from the oxidation operation of Fe <2+> to Fe <3+>; bring the first electrode to cathode potential and the second electrode to anode potential causing, in the vicinity of the cathode and the anode, respectively, the same two electrochemical reactions seen above for the cathodic half-cell and for the anode one; the potential difference applied to the electrodes must be 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 near the cathode to values higher than that necessary for produce the precipitation of insoluble metal phosphates. Alternatively, the same application teaches the implementation of the method in an electrochemical cell separated into two compartments by a cationic membrane. The same reactions seen above, which in the case without a membrane occur around the electrodes, in this case occur in the two half-cells that are formed, with the only difference that in the two half-cells the concentration of ions H <+> It is almost constant.

Finally, in application MI2012A000803 the pH increase is made to take place in a compartment not containing electrodes of an electrodialysis cell. More in detail, the method consists in providing a separate electrodialysis cell in at least three compartments by means of at least one cationic membrane and at least one anionic membrane; inserting a first electrode in the compartment defined between a cationic membrane and the cell walls; inserting a second electrode in the compartment defined between an anionic membrane and the cell walls; introducing the solution deriving from the oxidation operation in the compartment defined between said cationic membrane and said anionic membrane, and a dilute solution of phosphoric acid in the compartments in which said first and second electrodes are present; bringing the first electrode to cathode potential and the second electrode to anode potential, causing, respectively, in the cathode and anode compartment, the same reactions of the cathode half cell and the anode half cell seen above with reference to application MI2010A002105; in this way the elimination of phosphoric acid from the compartment defined between the membranes is achieved, with consequent increase in the pH value and precipitation of insoluble metal phosphates.

The methods taught by these three patent applications minimize the volumes of liquids to be used in the decommissioning processes and the energy required to evaporate the solvents and separate them from the solid fractions, and therefore represent a concrete improvement over those previously known.

However, these methods have the problem of being conducted in discontinuous mode, that is, in a single closed cycle (at most repeatable in a sequence of equal cycles), a mode not suitable for industrial use; furthermore, these methods are not optimal from the point of view of energy efficiency.

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;

- raising, by chemical or electrochemical way, of the pH of the solution thus obtained to a value higher than 1.5 and lower 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; characterized in that said operations are carried out continuously. The invention will be described in detail below with reference to the Figures, in which:

- Fig. 1 schematizes a plant for carrying out the method according to the operating mode in which the precipitation of phosphates is achieved by adding a solid base to the pickling solution;

- Fig. 2 schematizes a plant for carrying out the method according to the operating mode that uses an electrochemical cell separated into two half-cells by means of an anionic membrane; And

- Figs. 3 and 4 schematize energy recovery systems that use hydrogen and oxygen formed, respectively, in the plant of Fig. 2, and in a plant based on electrodialysis.

For the description of the chemical conditions with which to carry out the operations of the process, please refer to the contents of the questions MI2010A002105, MI2012A000801 and M12012A000803 mentioned.

According to the present invention, the process is carried out continuously.

Fig. 1 schematically represents a plant in which the operation of raising the pH, with consequent precipitation of phosphates, takes place by adding a solid base to the solution obtained from pickling. In a reactor, 100, the metal parts to be treated are fed (the feed is generically indicated with the arrow 101) and a solution of concentrated phosphoric acid through a line 102. The dissolution of the metal part takes place in the reactor (pickling) . In the dissolution reaction, gaseous hydrogen is released, which is extracted from the system through the degassing line 103, and preferably sent to a fuel cell (of known type) for energy recovery purposes. The solution obtained from the pickling is sent through the line 104 to a second reactor, 110, in which the oxidation of the Fe <2+> ions formed during the pickling to Fe <3+> takes place; the oxidation can take place by continuously adding a solution of hydrogen peroxide or a permanganate through line 111, or by bubbling gaseous oxygen through line 111â € ™. The solution enriched in Fe <3+> is sent, through the line 112, to the reactor 120, where the ferric phosphate precipitates. A solid base 130, for example calcium hydroxide, in the form of a powder, is continuously fed into the reactor 120 from a container 131 (e.g., a hopper); inside the reactor 120 the pH reaches a value such as to cause the precipitation of the ferric phosphate, FeP04, and of the phosphates of the other metals initially present in the metal part subjected to treatment; the precipitate is extracted from the lower part of the reactor 120 through a line 121, while the solution no longer containing the metals is removed from the system through the line 122.

Fig. 2 schematically represents a plant in which the operation of raising the pH, with consequent precipitation of phosphates, takes place in an electrochemical cell separated into two half-cells by means of an anionic membrane. In a pickling reactor, 200, the metal slag to be treated (arrow 201) and the concentrated phosphoric acid solution are introduced through line 202. Gaseous hydrogen is produced which is removed through line 203 and preferably sent to energy recovery. The solution thus produced is sent, through line 204, to the oxidation reactor, 210, where the oxidation of the Fe <2+> ions to Fe <3+> takes place as described above; in the case of use of solutions of oxidizing compounds, these are fed through the line 211, while in the case of use of gaseous oxygen, this is fed through the line 211â € ™. The solution leaving the oxidation reactor is sent, through the line 212, to the precipitation reactor 220. The solution present in this reactor is continuously recirculated, through a line 221 and a pump 222, to the cathode half-cell of the electrochemical reactor 230 . The reactor 230 is separated into two half-cells through the anionic membrane 231, one of which contains an electrode placed at cathodic potential (indicated in the figure by the sign â € œ- ") and one an electrode placed at anode potential (indicated in the figure by the sign â € œ + "). As previously described, H <+> ions are consumed in the cathode half cell, causing the pH increase which causes the precipitation of phosphates. The solution contained in this half-cell, containing suspended phosphate powders, is sent back, through the line 232, to the precipitation reactor 220; in this reactor the solution decants, the phosphates collect on the bottom, from which they are then extracted through line 223, while the hydrogen gas is extracted through line 224. The system includes an accumulation tank 240 containing more diluted phosphoric acid of the one fed through the line 202, which is recirculated in the anode half-cell of the reactor 230; the sending of the acid from tank 240 to reactor 230 takes place through line 241 and pump 242, while the return of the solution containing gaseous oxygen to tank 240 takes place through line 233; the tank is also fed continuously with water, necessary for the anodic reaction, through line 243, and has an oxygen degassing line, 244; the oxygen withdrawn from tank 240 is preferably sent to an energy recovery apparatus together with the hydrogen withdrawn through lines 203 and 224. The feeding solution to the cathodic half-cell is continuously circulated from the half-cell to the precipitation reactor 220 and from this to the half cell. Similarly, this happens for the circulation of the phosphoric acid solution between the anodic half-cell and the storage tank. By doing this, a system is obtained which rapidly reaches stationarity conditions and which, with a good approximation, can be considered perfectly mixed and with an almost uniform composition. In order for this to occur without any variation in the volumes involved (which inevitably change due to the effect of the electrochemical reactions and the action of the membrane), it is necessary to continuously remove a part of these solutions in such quantities as to compensate for the supply of fresh pickling solution (to the cathode circuit) and diluted phosphoric acid (to the anode circuit). The two solutions collected in this way, having the average composition of the two tanks, are both recycled to the dissolver. These conditions are obtained, as regards the cathode circuit, by sending the solution taken from the precipitation reactor through line 221, partly to the cathode half cell through line 221â € ™, and partly to pickling reactor 200 through line 221 " ; for the anode circuit, part of the solution present in the storage tank 240 is in turn sent to the pickling reactor 200 through line 245. The invariance of the volumes involved therefore occurs not only in relation to the electrochemical cell but to everything The overall result is that the metal and fresh phosphoric acid are fed to the process and metal phosphates are extracted from the process.

The system illustrated above also has the advantage of allowing precipitation and degassing away from the electrodes and membranes, thus avoiding their fouling and reducing current losses.

A continuous system based on an electrochemical reactor not divided into half-cells by a membrane, or divided into two half-cells with a cationic membrane, as described in application MI2012A000801, can be realized according to the same principles illustrated above in the case of the electrochemical reactor with anionic membrane, as will be evident to the technicians of the sector.

Similarly, a continuously operating system based on an electrodialysis reactor, as described in application MI2012A000803, can be made without difficulty by an expert in the field; this system will differ from the one described above with anionic membrane in that the dilute solutions of phosphoric acid are in this case fed both into the cathode and anode compartment of the electrodialysis reactor (instead of only into the anode compartment as occurs in the case illustrated above ), and the solution containing suspended phosphate powders, to be sent to the precipitation reactor, is extracted from the compartment (or compartments) between the cationic and anionic membranes of the reactor.

In a preferred embodiment, the gaseous hydrogen and oxygen produced in systems with electrochemical reactors are recovered and reused to produce electric current, for example by means of a fuel cell in order to reduce the energy needs of the process. This embodiment is illustrated in Fig. 3, with reference to the case of the use, to precipitate the phosphates, of an electrochemical cell with anionic membrane (the application of the principle to the other possibilities, i.e. cell without membrane, cell with cationic membrane, or electrodialysis cell, will be evident to those skilled in the art). The electrochemical cell, 30, is divided into a cathodic and an anodic half-cell by means of the anionic membrane 31; in the cathode half cell, the solution deriving from pickling and oxidation is fed through line 32, and the spent solution is extracted through line 33; in the anodic half-cell, a solution of diluted phosphoric acid is fed through line 34, and concentrated phosphoric acid is extracted through line 35. From the cathode half cell, moreover, gaseous hydrogen is extracted, which is sent to a fuel cell, 36; gaseous oxygen is extracted from the anode half cell and sent to the same cell 36; in the cell, from the controlled reaction between hydrogen and oxygen, an electric current is generated which is reintroduced into circulation, feeding the electrodes of the reactor 30.

The energy recovery system in the case of a plant that includes an electrodialysis cell is schematically represented in Fig. 4, which indicates the meaning of the inlet and outlet lines from the cell and from the fuel cell, and the symbols MA and MC indicate, respectively, the anionic and the cationic membrane.

Taking into account that on average the current efficiency of an electrochemical process is approximately 50% and that of a fuel cell is 60%, the possible energy recovery is of the order of 30% and therefore completely significant in the economics of the process.

Claims (4)

CLAIMS 1. A method for conditioning 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; - raising, by chemical or electrochemical way, of the pH of the solution thus obtained to a value higher than 1.5 and lower 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; characterized in that said operations are carried out continuously. 2. Method according to claim 1, in which the solution resulting from the oxidation operation is continuously sent to a precipitation reactor (120), into which a solid base (130) is continuously introduced from a container (131) in the form of powder, bringing the pH inside said precipitation reactor to a value such as to cause the precipitation of phosphates, which are continuously extracted from the lower part of said precipitation reactor through a line (121), and removing from the system the solution no longer containing the metals through a drain line (122). 3. Method according to claim 1, wherein: the solution resulting from the oxidation operation is continuously sent to a precipitation reactor (220); the solution present in this reactor is continuously recirculated, through a line (221) and a pump (222), to the cathode half-cell of an electrochemical reactor (230) separated into two half-cells through an anionic membrane (231); in said cathode half cell, the consumption of H <+> ions determines the increase in pH which causes the precipitation of phosphates, which are transported in the form of suspended powders to said precipitation reactor, where they are decanted and extracted from the bottom of said reactor through a discharge line (224). 4. Method according to claim 3, wherein the gaseous hydrogen and oxygen produced respectively in said cathode half cell and in an anode half cell of the electrochemical reactor (30) are sent to a fuel cell (36), in which from the controlled reaction between hydrogen and oxygen an electric current is generated which is reintroduced into circulation, feeding the electrodes of said electrochemical reactor.