WO2013168118A2

WO2013168118A2 - Method for the conditioning of waste arising from the decommissioning of nuclear plants

Info

Publication number: WO2013168118A2
Application number: PCT/IB2013/053753
Authority: WO
Inventors: Maurizio Masi; Luca Magagnin; Pier Paolo Costa; Lorenzo Costa
Original assignee: Ecir - Eco Iniziativa E Realizzazioni - S.R.L.
Priority date: 2012-05-10
Filing date: 2013-05-09
Publication date: 2013-11-14
Also published as: ITMI20120801A1; WO2013168118A3

Abstract

A method is described for the treatment of ferrous nuclear waste, produced in pickling operations of metal parts having surface contamination, which comprises the treatment, in an electrochemical cell (10), of the solution obtained from said pickling, following and oxidation operation. The method allows the volumes of liquid phases to be treated to be reduced to a minimum, and the recycling in the process of the by-products thereof.

Description

METHOD FOR THE CONDITIONING OF WASTE ARISING FROM THE DECOMMISSIONING OF NUCLEAR PLANTS

Field of the invention

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

Background of the invention

All by-products or non-reusable radioactive residues of processes, or more in general of operations, in which radioactive substances have been generated or used, are identified as "nuclear waste". Due to the danger it poses to man and his environment, nuclear waste of any type and origin must be treated and stored according to very particular methods, which ensure the containment of the radiations and of the nuclear elements or isotopes, for even very long periods of time.

There are many types of processes wherein nuclear elements or radiations are employed, which produce waste at varying degrees of concentration and danger. One proposed classification, in use in Italy, divides this waste into:

- category 1 , which comprises all waste having a low level of radioactivity; this is the largest category, comprising, as a 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 supplied during a visit to a nuclear plant, etc. ...);

- category 2, which comprises all waste having a medium level of radioactivity; this waste requires screening, but constitutes just 7% of the waste, with a total radioactivity of 4% (examples are the sheaths of the fuel elements of a reactor);

- category 3, which comprises all waste having a high level of radioactivity, which constitutes just 3% of the waste but represents 95% of the radioactivity; this is the most dangerous type of waste due to the high dose of radiation that an accidental exposure would entail and due to radioactive decay in the order of millions of years for some of the radioactive isotopes contained therein.

The different types of waste require different disposal procedures. The techniques studied and described in the last 60 years for this purpose are extremely numerous. The results are public and generally easily accessible; for specific results regarding the long-term storage of the types of waste containing long-life and/or high mobility isotopes, the conclusions are, however, still uncertain. The resources invested in these studies are, presumably, huge; the resources to be invested for the conditioning and long-term storage of existing nuclear waste (including the reclamation of the relevant sites), are in part known: in the USA alone these have been evaluated at hundreds of billions of dollars.

In general, the disposal of this waste requires a conditioning step which consists in the transformation of the waste into a form suitable for storage; and the storage of the conditioned waste in suitable, natural or industrially produced sites.

A particular type of nuclear waste that is strategically very important, is the type generated in the remediation operations of the nuclear reactors that are no longer operational and of the nuclear sites that have become obsolete. In this case, the nuclear waste is typically generated during recovery and decontamination operations of the large metal structures, which, exposed to contact and/or the radiations of radioactive isotopes, have in turn become radioactive (limited to the exposed surface), due to chemical contamination or to nuclear mutation (under the effect of the radiations). The complex of operations associated with these remediations is indicated in the sector with the 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".

Much of the decommissioning waste thus generated belongs to the above- mentioned category 3, and typically contain isotopes with long mean life and high mobility, which in any case entail specific conditioning for highly dangerous waste. While industrially proven and economically useful processes for the comprehensive management of category 1 and 2 nuclear waste have been identified, there have been important but as yet incomplete results for category 3 waste, especially in terms of the anti-economic aspect of the required conditioning, and to date, there is no operational deposit for long-term storage.

As regards the conditioning of decommissioning waste, and typically pickling- generated waste, experts have come to the conclusion that, for all long-lived and/or high-mobility radioactive isotopes, it is necessary to use glass matrices with both chemical and thermo-mechanical high-stability; see for example the article "Glass packages guaranteed for millions of years", by E. Y. Vernaz, Clefs CEA, no. 46 (2002), pp. 81-84. Numerous examples of the vitrification of category 3 waste have been proposed, even at industrial level, which have however proved to be conditioned by process reliability problems and by typically high costs.

Phosphate glass systems containing iron have recently been accredited among the more promising vitreous materials for the retention of radioactive isotopes, especially in the presence of sulphates, chromates, phosphates and halides. 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.

Of these, particularly interesting is US 5,750,824, which teaches the production of phosphate glass containing from 30 to 70% by weight of phosphorous oxide (as P₂O₅) and from 22 to 50% of iron oxide, the rest being made up of the oxides of other metals, including those arising from nuclear waste; in addition, this document teaches that the best results are obtained with glass in which the iron is present in at least 50%, preferably at least 80% and even more preferably at least 90%, in the oxidation state 3, i.e. as Fe³⁺ ion. According to this document, phosphate glass with a high Fe³⁺/Fe²⁺ is characterized by the best chemical resistance (for example, to leaching, namely washing out with water), density and thermo-mechanical resistance properties.

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

An issue as yet unresolved with these methods is the management of the significant volumes of liquid of the solutions in which decommissioning waste is initially dissolved. Indeed, in some cases the solutions are added directly to the melt of oxides or salts of phosphorus and iron, however generating large volumes of vapours, which must then be recondensed, decontaminated and disposed of; in other cases, the solutions are first brought to dryness, and the waste is added in powder form to the molten material, but in this case too the obtainment of the powders of waste entails the evaporation of high amounts of liquid.

Summary of the invention

The aim of the present invention is to provide an improved method for the conditioning of decommissioning waste, specifically pickling waste.

According to the invention, this aim is achieved with a process that comprises the following steps:

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

- oxidation of the iron ions in solution from Fe²⁺ to Fe³⁺ so as to obtain an Fe³⁺/Fe²⁺ ratio equal to or greater than 9;

- raising by electrochemical means, in a cell without membrane or with cationic membrane, of the pH of the solution thus obtained to a value greater than 1.5 and lower than 0, causing precipitation of the phosphate salts of iron and of the metal ions present in the solution;

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

- thermal vitrification treatment of the mixture of precipitated solids.

Brief description of the drawings

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

- Fig. 1 shows (schematically), in the upper part thereof, an electrochemical cell for embodying the method of the invention, and in the lower part thereof, a diagram of the evolution of the H⁺ ion concentration within the cell, along the coordinate that runs from the cathode to the anode of the cell;

- Fig. 2 schematically shows an alternative electrochemical cell for embodying the method of the invention, and in the lower part thereof, a diagram of the evolution of the H⁺ ion concentration within the cell, along the coordinate that runs from the cathode to the anode of the cell.

Detailed description of the invention

The method of the invention has several advantages compared to the known ones. In particular, it does not require the pre-treatment of large volumes of solution to recover the salts of the radioactive metals to then be added to the precursors of the iron-phosphate glass in suitable proportions, because with the present process the mixture of phosphorus and metals, essentially in the necessary proportions, is produced in situ in the solution, and is then obtained therefrom to be sent to the thermal treatments, while the remaining liquid phase can be recycled, following top-up with new, concentrated phosphoric acid, in a subsequent dissolution (pickling), precipitation and separation cycle, without having to be disposed of separately. This way, all treatments that can generate secondary contamination are avoided.

The first operation of the process consists in the washing and dissolution in phosphoric acid of the surface-contaminated metal parts (pickling) resulting from decommissioning 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 coming from nuclear power plants are typically made of steel, and thus primarily consist of iron, with minor amounts of elements typical of steel metallurgy (chrome, nickel, manganese, ...) and of radioactive elements that have deposited on the surface, or that have been produced through contact with radioactive isotopes or radiations. It has been observed that the appropriate concentration by weight of metals in the acidic solution is between about 5 and 12%, preferably around 10% by weight of the overall solution. This solution is subsequently standardised at the desired concentration and pH (for example 9% by weight and pH 1). Operating under these conditions in phosphoric acid produces a concentrated solution that is ideal for the subsequent oxidation operations of iron from Fe²⁺ to Fe³⁺ and sufficiently close to the saturation point for the precipitation of iron-phosphate salts. The resulting solution has a phosphorus to iron weight ratio suitable for the production of iron-phosphate glass endowed with the necessary characteristics for the purposes of the conditioning of the radioactive metals initially present in the solution. After 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, an adjustment of the composition with the possible addition of the component in defect. For the purposes of the invention, the solution must contain iron and phosphorus in a molar ratio of between 33/66 and 45/55, and preferably of about 40/60, and have a pH lower than 1.5 . The solution obtained can be optionally analysed to determine the chemical composition thereof, and, in particular, the Fe/P molar ratio and the Fe³⁺/Fe²⁺ ratio. For the purposes of the invention, it is preferable that the Fe/P molar ratio be approximately 40/60; this ratio can be adjusted around the optimal value by adding, to the solution directly obtained from pickling, a soluble iron salt or phosphoric acid, depending on which of the two components resulted from the analysis to be in defect with respect to said optimal ratio. Furthermore, it is preferable that the Fe³⁺/Fe²⁺ ratio have a high value, greater than 9 and typically greater than 24.

Since in the initial solution obtained by pickling iron is present almost exclusively in the form of Fe²⁺ ions, the method of the invention provides for a second operation, which consists in the oxidation of at least part of said ions to bring said ratio into the preferred range; the oxidation operation may for example be achieved by adding hydrogen peroxide, permanganate ion, by means of bubbling oxygen into the solution, or with any other known method.

The third operation of the process of the invention, consists in causing precipitation of the metal salts present in the solution, by raising the pH of the latter without increasing its volume. In this operation, the pH of the solution is brought to a value of 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 means, by applying a potential difference between two electrodes immersed in the solution obtained by the previous operation.

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

The solution to be treated, 11 , arising from the oxidation operation, is introduced into the cell, 10, formed by a single tank. Two electrodes 12 and 13 are also introduced into the cell. A suitable potential difference is applied to the electrodes (in the figure, electrode 12 is brought to cathode potential and electrode 13 to anodic potential), so as to generate a suitable electric current following the onset of the electrolysis reaction of water. Since the solution introduced into the electrochemical cell is highly acidic, the reduction reaction of the hydrogen ions takes place at the cathode:

2H⁺ + 2e^" → H₂†

which accordingly consumes them causing a raising of the pH value; symmetrically, the oxidation reaction takes place at the anode:

H₂O → 1/2 0₂† + 2H⁺ + 2e^"

which introduces into the solution an amount of H⁺ ion equal to the amount consumed at the cathode.

Thus there is generally no alteration of the pH of the solution in that the circulation of electrical current causes both a consumption of H⁺ ion at the cathode and the formation of the same ions in an equivalent amount at the anode.

To ensure, however, the transport of ions, in particular of H ⁺, from the anode to the cathode, a concentration gradient, which depends on the density of current applied, is established; the gradient is such that the concentration of H⁺ is maximal at the anode (where H⁺ forms) and minimal at the cathode (where H⁺ is consumed), as illustrated in the lower diagram in Figure 1 (the horizontal coordinate in the diagram represents the distance between the cathode, position "C" on the axis of abscissa, and the anode, position "A" on the same axis). Therefore, the pH rises up around the cathode to the point of triggering the precipitation of FeP0₄.

The above-exemplified reactions are just one of the possible redox pairs that can be used to cause the raising of the pH in the volume of solution adjacent to the cathode; other reactions can be obtained from tables set out in electrochemistry manuals.

In a second embodiment thereof, the electrochemical treatment according to the invention is carried out in an electrochemical cell, schematically shown in the upper part of Fig. 2, separated into two compartments by a cationic membrane, 14. Solution 1 1 coming from the preceding oxidation operation is introduced into the cathode compartment, while a diluted phosphoric acid solution, 15, necessary to ensure the electric continuity of the system and to have ions that are compatible with those of the solution 11 , is introduced into the anode compartment.

Membrane 4 allows the passage of only positive ions between the two half- cells, and due to the great difference of mobility in solution and of permeability of the H⁺ ion compared to the other cations, the permeation of the former is about two orders of magnitude greater than that of the latter, so that the ionic transport in solution is in fact almost completely attributable to the H⁺ ions alone. Due to the difference in potential that exists between anode and cathode, migration of the H⁺ ion takes place from the anode compartment to the cathode compartment. In this case, as schematically shown in the diagram provided in the lower part of Fig. 2, the concentration of H⁺ ion is almost constant within each half-cell, only showing an appreciable gradient around the electrodes. Since in this case the "gap" in concentration of the H⁺ ion largely takes place at the membrane (due to the greater mobility of these ions in solution than within the membrane itself, which ensures that they are rapidly neutralised at the cathode as soon as they enter the cathode compartment), the decrease in concentration of the H⁺ ion is more homogeneous in the cathode compartment with respect to the case without membrane; this decrease in concentration determines a localised rise in the pH, which causes precipitation of the phosphates at this compartment.

The above-described embodiment for the case of cell subdivided into compartments is easily generalisable to a system having multiple electrodes, wherein there are a series of electrodes placed alternately at cathodic and anodic potential; a multiple-electrodes system of this type allows working in parallel on greater amounts of solution, thus increasing the productivity of the method.

The electrochemical precipitation method is ideal, since it avoids the need to add other solutions to the one arising from pickling, which would lead to the increase of the overall volume of liquids to be treated and disposed of, following suitable conditioning treatments, at the end of the process. Operating in this way, there is precipitation of the useful component for vitrification (the phosphates, mainly consisting of ferric phosphate plus traces of the other phosphates of the metals originally in solution, or simply of adsorbed metals), in optimal proportion, without compromising the quality of the glass and while preserving the starting phosphoric acid solution for a further pickling cycle, i.e. by reusing all of the components of the original waste and the process co-products; this way thus constitutes the ideal method for preventing any secondary contamination. Once the phosphates have precipitated from the solution, they can be recovered by eliminating the overlying liquid phase, by simple pouring for example. If necessary, the precipitate can then be centrifuged to obtain a better separation from the liquid phase. The mixture of moist phosphates is then preferably mechanically mixed, for the homogenisation thereof. Indeed, during the precipitation step, it is possible for the various phosphates to precipitate at different times, resulting in a precipitate wherein the different phosphates are stratified according to the order of precipitation. A non-homogeneous solid phase thus obtained could give rise to a glass that is not perfectly homogeneous: despite the precipitate being subjected to fusion, the viscosity of the melt could be such as to not allow complete homogenisation during the fusion, with the risk of obtaining a final glass of not perfectly homogeneous composition and thus parts thereof (especially any parts with a low iron content) that do not exhibit the characteristics required by the application.

The precipitate is then vitrified, implementing all the precautions that the presence of radioactivity imposes. Typically the FePO₄ precipitate melts and vitrifies at a temperature not greater than 1100 °C. The presence of other cations co-precipitating with the iron phosphate can generate a relatively broad thermal range within which the vitrification occurs, typically between 800 °C and 1300 °C.

The liquid phase, still containing significant amounts of phosphoric acid, and potentially of various metal ions, even traces of radioactive isotopes, is recovered with operations typical of the prior art, for example by decanting, and/or centrifugation and can be reused in a subsequent dissolution cycle of metal parts and precipitation of phosphates, following top-up of the phosphoric acid to replenish the acid consumed in the precipitation.

The process of the invention thus achieves the result of disposing of the metal parts arising from decommissioning while avoiding the need to treat high volumes of liquid phases, typical of the processes of the prior art. Indeed, with the methods of the prior art, the volumes of liquid phases generated are proportional to the weight of the metal parts themselves (since for each treatment cycle of a unit of weight of metal parts it is necessary to employ a given volume of solution), while in the case of the present invention, the volume of the liquid phase is essentially the volume necessary for a single dissolution operation of a portion of metal parts.

Claims

1. Method for the conditioning of waste arising from the disposal of nuclear plants, comprising the following operations:

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

- raising by electrochemical means, in a cell without membrane or with cationic membrane, of the pH of the thus solution obtained to a value greater than 1.5 and lower than 10, causing the precipitation of the phosphate salts of iron and of the metal ions present in the solution;

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

- thermal vitrification treatment of the mixture of precipitated solids.

2. Method according to claim 1 , further comprising the recovery of said liquid phase and recycling in a subsequent cycle of operations of the method.

3. Method according to any one of the preceding claims wherein, in the dissolution operation of the contaminated metal parts, said parts are added in an amount 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 elemental analysis of the chemical composition of the solution is performed, and, if it is determined that the Fe/P molar ratio is outside the range of between 33/66 and 45/55, the component in defect is added to the solution in order to bring said ratio into said range.

5. Method according to any one of the preceding claims, wherein, in said oxidation operation, the Fe³⁺/Fe²⁺ ratio is brought to a value equal to or greater than 24.

6. Method according to any one of the preceding claims, wherein said oxidation operation is carried out through the addition of hydrogen peroxide, of permanganate ion, or through bubbling of oxygen into the solution.

7. Method according to any one of the preceding claims, wherein the operation of raising the pH by electrochemical means comprises the following steps: - providing an electrochemical cell (10) containing a first electrode (12) and a second electrode (13);

- introducing into the cell the solution (11 ) arising from the oxidation operation;

- bringing the first electrode to cathodic potential and the second electrode to anodic potential, causing the following reactions to take place, around the cathode and the anode respectively:

2H⁺ + 2e^" → H₂†

and

H₂0 → 1/20₂T + 2H⁺ + 2e^_

and applying a potential difference between the electrodes such as to induce a circulating electric current density that gives rise to an H⁺ ion concentration gradient between anode and cathode such as to bring the pH in proximity of the cathode to values greater than the value necessary to produce precipitation of the insoluble metal phosphates.

8. Method according to any one of claims 1 to 6, wherein the operation of raising the pH by electrochemical means comprises the following steps:

- providing an electrochemical cell (10) containing a first electrode (12) and a second electrode (13), and subdivided into two compartments by means of a cationic membrane (14);

- introducing into the compartment containing the first electrode the solution (11 ) arising from the oxidation operation;

- introducing into the compartment containing the second electrode, a diluted solution of phosphoric acid (15);

- bringing the first electrode to cathodic potential and the second electrode to anodic potential, causing the following reactions to take place, in the cathodic and in the anodic compartment respectively:

2H⁺ + 2e^"→ H₂†

and

H₂0 → 1/20₂† + 2H⁺ + 2e^"

so as to determine in the cathodic compartment the raising of the pH value and the inducing of precipitation of the insoluble metal phosphates.

9. Method according to claim 8, wherein a series of electrodes alternatively brought to cathodic and anodic potential is employed.

10. Method according to any one of the preceding claims, wherein, in said operation of raising the pH, said pH is brought to a value of between 1.7 and 2.5.

1 1. Method according to any one of the preceding claims, wherein said thermal treatment is carried out at a temperature of between 800 °C and 1300 °C.