GB2217732A

GB2217732A - Apparatus for the electrochemical treatment of radioactive product solutions

Info

Publication number: GB2217732A
Application number: GB8908693A
Authority: GB
Inventors: Detlef Stritzke; Karlheinz Haberland
Original assignee: Deutsche Gesellschaft fuer Wiederaufarbeitung von Kernbrennstoffen mbH
Current assignee: Deutsche Gesellschaft fuer Wiederaufarbeitung von Kernbrennstoffen mbH
Priority date: 1988-04-19
Filing date: 1989-04-18
Publication date: 1989-11-01
Also published as: GB8908693D0; DE3813017C2; FR2630134A1; DE3813017A1; FR2630134B1; JPH01312493A

Description

r 2217732 1 APPARATUS FOR THE ELECTROCHEMICAL TREATMENT OF RADIOACTIVE

PRODUCT SOLUTIONS The invention relates to an electrolytic through-flow cell for 5 the electrochemical treatment of radioactive product solutions.

Electrochpmical treatment in electrolytic through-flow cells is successfully employed in the treatment of product solutions in the production of nuclear fuels and in the reprocessing of irradiated nuclear fuels.

The PUREX process has been used successfully world-wide for more than 30 years in the reprocessing of irradiated nuclear fuels. In that process the first step of the extraction operation in the reprocessing procedure lies in the selective transfer of uranium and plutonium into an organic solvent, with the highly radioactive fission products remaining in an acid aqueous solution. Thereupon, uranium and plutonium must be separated from each other, which is achieved by reducing the valence of the plutonium. Thereafter the plutonium is no longer soluble in the organic phase.

The operation of reducing the plutonium to the trivalent state is preferably effected by using electrochemically produced uranium- IV as uranium is already a component of the PUREX process and therefore no further foreign substances are brought into the process when using that reducing agent.

The majority of U-IV electrolysis processes which are the subject of technical performance operate in a discontinuous mode. The discontinuous mode of operation suffers from serious disadvantages. Those disadvantages are considered to lie in the relatively long electrolysis times for producing high levels of U-IV concentration. In addition, large electrolysis apparatus and storage and preparation containers are required.

The path which, has therefore been followed for the continuous production of uranium-IV is that which involves the use of an electrolysis throughflow cell (E.G. Orebaugh, R.C. Probst: Electrolytic production of uranous nitrate, USAEC-Report, DP-1549, April 1980). The cell which is proposed therein is of a rectangular configuration, with the anode and the cathode being disposed in opposite relationship at a spacing of 1 mm. The catholyte is separated 2 from an anolyte by an ion exchange membrane. A cathode comprises a. platinized wire mesh which is coated with a thin iayer of mercury in order to reduce the hydrogen over voltage.

That continuous electrolysis cell suffers from the disadvantage that the small free cross-section means that the pressure drop between the inlet and the outlet for.the liquid is high. The through-put is therefore very restricted. The electrolysis gases formed must be removed from the electrode surfaces by the flow of liquid being caused to pulse. The use of mercury involves serious disadvantages.

Another continuous electrolysis cell is known from German laid-open Patent Application (DE-OS) No. 21 58 058. The balls which bear against each other involve poor transfer of current. The filling gives rise to a high pressure drop as it has a high level of flow resistance and the balls must be held in a wire mesh since they are not self supporting.

According to the invention there is provided an electrolytic through-flow cell for the electrochemical treatment of radioactive product solutions, comprising a tubular housing having two electrodes connected as an anode and a cathode with one of the electrodes in the -interior-of the housing in the form of a packing of static mixing elements extending for substantially the entire cross-section of the housing.

Such a cell, for the continuous treatment of nuclear solutions, can have a high through-put with a high level of yield.

Thus the electrode packing which comprises one or more static mixing elements can extend over the through-flow length of the electrolytic cell. The configuration of the packing, consisting of what are known as static mixing elements, can be selected as is known for example from German Patent-specification No. P 35 06 693. Static mixing elements of that kind, which are made up from p.LaLes, form open, mutually intersecting passages which extend inclinedly with respect to the main direction of flow along the axis of the cell. The fluid is divided up into flow portions being continuously separated, re-arranged and brought together again. The continuous cross-mixing action constantly renews the material-specific and therma-I flow boundary layers at the wall of the element, so that, over the cross-section of the cell, the arrangement provides for permanent equalization of %z I 1 k - 3 1 1 concentration and temperature differences, combined with short -residence times. The electrolysis cell of that configuration does not involve what are known as dead zones in which undesirable accumulations of reaction products can be formed.

In an electrolytic through-floW cell, a configuration according to the invention can provide a large electrode surface area, with a low level of flow resistance. Almost all the cross-section of the cell is available as a through-flow cross-section. The high ratio between the electrode surface area and the volume of electrolyte, which is achieved in that way, means that it is possible to attain high variations in concentration per unit of time.

Advantageously the mixing elements comprise a plurality of individual segments which are arranged in mutually superposed relationship and which are respectively arranged in mutually displaced relationship. Such a hybrid-element packing makes it possible to compensate for lack of homogeneity. Preferably, the successive segments are arranged displaced through 900 relative to each other.

Preferably the mixing element packing comprises plates in the form of expanded sheet metal. That construction can ensure good c urrent penetration from the anode to the cathooe. The mesh width can be suited to the respective procedure involved in or,der to provide for the optimum ratio between electron through-flow and electrode surface area.

Advantageously the entire packing of the mixing elements is _divided into individual zones which can be supplied with different levels of current strength. That form of different current supply can be used with advantage as, with increasing electrolysis time, the level of concentration of the substance produced rises. According to the operating procedure employed, individual electrode elements or electrode zones can be operated with different current densities.

Preferably the housing and the mixing element package are of conical form whereby variation in current density can be produced by way of the conical variation of the static mixing element packing, which is necessarily linked to the shape of the housing. With only one current supply unit, the arrangement provides for. a reduction or an increase in the level of surface current density in the product discharge direction because the cross-section of the mixing element 4 packing varies.

A further advantageous construction for producing different levels of current density is-provided if the tubular housing is of a configuration which increases or decreases in diameter in a stepped form, in a product discharge direction. The step-wise variation in the diameter of the electrolysis cell means that it is possible to attain a variation in the current density over the cross-section, while only one current supply unit needs to be employed. That is to be considered as a means for carrying out the process in a controlled fashion.

Advantageous perforated plate portions which are connected as counter-electrodes are arranged between the segments of the mixing element packing. Rings which are connected as counter-electrodes may also be arranged between the hybrid elements.

Preferably the housing of the through-flow eel.11 is constructed as an insulator.

as the anode.

Advantageously the housing of the through-flow cell is connected Such electrolyte through-flow cells are suitable for continuous production of the uranium-IV which is required as a reducing agent in the PUREX process. The use of those constructions reduces the danger of re-oxidation of the U-IV produced, in the through-flow cell.

Providing one electrode in the form of a static mixing element gives a particularly large act-ive surface area for the electrochemical reaction. That gives an advantageous relationship between the electrode surface area and the volume of eleeLroiyte.

The invention is particularly suitable for producing, according to demand, the uranium-IV which is required as a reducing agent in the U/Pu separation operation. That makes it unnecessary for the uranium1V to be stored or produced beforehand. By using an eiectrolyte through-flow cell according to the invention, the respective requirement of uranium-IV can be met by direct production thereof, in parallel relationship with the Pu/U separation procedure. That is possible for the reason that an adequate concentration of U-IV can be attained immediately by virtue of the configuration of electrolytic through-flow cell according to the invention.

There is no need for a pulsation effect in such a through-flow cell as the gas bubbles produced are effectively carried away. The 1; t through-flow cell can be totally filled so that there is also no need to provide for a supply of scavenging air.

The invention is diagr ammatically illustratea by way of example in the accompanying drawings, in which:- Figure 1 shows one embodiment of an electrolytic thprough-flow cell according to the invention with central anode configuration and a diaphragm screening; Figure 2 shows a view on an enlarged scale of a hybrid-element packing to be connected as a cathode in an electrolytic through-flow cell according to the invention; Figure 3 shows another embodiment of an electrolytic through-flow cell according to the invention with a housing connected as the anode and the cathode being divided up into individual zones; Figure 4 shows an electrolytic through-flow cell according to the invention with its diameter increasing in a stepped configuration in the product discharge direction; and Figure 5 shows an electrolytic through-flow cell according to the invention of conical shape, with the diameter thereof increasing in the product discharge direction.

The arrangement shown in Figure 1 comprises a circular -section tubular housing 11 which is closed at its ends by c-osure plates 13 and 15. The lower closure plate 13 has a feed connection 19 for treated product; olution. Individual static mixing elements (segments) 21 are disposed in the housing 11 in superposed relationship. That packing of static mixing elements 21 is fixed with respect to the housing 11 by way of an electrically insulated suspension arrangement 23. The housing 11 is also an insulator.

Each static mixing element 21 comprises snort, inclinedly folded or corrugated plates 25 (see Figure 2). They are arranged in layers with each other in such a way as to form open and muLually intersecting passages which extend inclinedly with respect to the main flow direction (axis of the cell). The mixing elem-ents 21 are successively arranged in a condition of being displaced through 900 relative to each other. The static mixing elements 21 are provided at their centre with an opening 25 through which is passed a concentrically disposed bar anode 27. The bar anode 27 is surrounded by a diapnragm 28 which is in the form of a tube and which is intended to prevent oxidation of the POOR QUALITY 6 hydrazine which is added in the production of uranium-IV.

The static mixing elements 21 are connected as a cathode by way of an electrical connection 31. The static mixing elements 21 are externally surrounded by a jacket sleeve 33. The jacket sleeve 33 establishes the through-flow cross-section of the eiectrolytic throughflow cell.

Arranged concentrically with respect to the through-flow cell in the lower closure plate 13 is a connection 35 for the feed of an anolyte which acts as liquid coolant.

The mode of operation of the above-described cell will be described hereinafter by way of example with reference to the production of uranium-IV.

At the feed connection 17, uranium-VI nitrate solution from the reprocessing process or from a storage container is introduced into the through-flow cell. As it passes through the cell, the uranium-IV nitrate solution is subjected to electrochemical treatment by the application of voltage to the anode 27 and the cathode 31. The electrochemical reactions result in a rising concentration of uraniumIV nitrate solution which is taken off at the discharge connection 19 and which, after -separation of electrolyte gases, is passed into the procedure for the separation of U and Pu for the purposes of reducing the Pu to Pu-III.

The modified construction in Figure 2 has perforated plate portions 41 which are connected as anodes, between the individual segments of the static mixing packing, which is connected as the cathodes. In addition or alternatively, the arrangement may have a central bar anode 27.

The through-flow cell in Figure 3 comprises a housing 43 which is connected as the anode and which is provided at its-lower end with an intake connection 17 for the uranium-VI nitrate solution. Static mixing elements 21 are arranged in superposed relationship in the housing 43, with the static mixing elements being divided up into three zones 45, 47 and 49. Provided between the individual zones are empty or free spaces 51 and 53. Each mixing packii- ' g zone 45, 47, 49 is separately put under voltage, as a cathode. The static mixing element packings 21 are surrounded by a diaphragm 55. A vent opening 57 for the electrolyte gases is disposed at the upper end. In the upper 1 1 7 region, at the side thereof, the product solution produced is.discharged by way of the discharge connection 19. An anolyte is introduced in the annular space 59 between the anode and the cathode, by way of a connection 61. The anolyte can act as liquid coolant and 5 is discharged by way of a connection 62.

Figure 4 is a view in diagrammatic form of a housing which is of a stepped configuration and which.contains a mixing element packing 73. The packing 73 which increases in size in a stepped configuration is connected"as the cathode, with the housing 71 being connected as the anode. An insulator 86 is provided between the housing 71 and the mixing element packing 73. The cathodes are supplied with the same current and voltage by way of a direct current source 75. Different current densities are produced in the individual step portions of the housing by virtue of the different diameters.

A further construction for producing different levels of current density is shown in Figure 5. Therein a housing 81 is of a construction which increases in width in the product discharge direction. A mixing element packing 83 is divided up into individual zones which can be supplied from a voltage source 85. An insulator 86 is disposed between the housing 81 and the packing 83.

i 8

Claims

1. An -electrolytic through-flow cell for the electrochemical treatment of radioactive product solutions, comprising a tubular housing having two electrodes connected as an anode and a cathode with one of the electrodes in the interior of the housing in the form of a packing of static mixing elements extending for substantially the entire cross-section of the housing.

2. An electrolytic through-flow cell according to claim 1, in which the packing of static mixing elements comprises a plurality of individual segments which are arranged in mutually superposed relationship and which are respectively arranged in mutually displaced relationship.

3. An electrolytic through-flow cell according to claim 1 or claim 2, in which the mixing element packing comprises plates of expanded sheet metal.

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4. An electrolytic through-flow cell according to claim 1 or claim 2, in which, considered in the through-flow direction, the mixing element packing is divided into a plurality of zones and each zone has a separate current feed and can be supplied with a different currentstrength.

5. An electrolytic thrbugh-flow cell according tc, any one of claims 1 to 3, in which the housing and the mixing element packing are of conical form.

6., An electrolytic through-flow cell according to any one of claims 1 to 3, in which the tubular housing is of a configuration which increases or decreases in diameter in a stepped form, in a product discharge direction.

7. An electrolytic through-flow cell according to any one of claims 2 to 6, in which disposed between the segments of the mixing element packing are perforated plate portions which are connected as counter- Z 4 9 electrodes.

8. An electrolytic through-flow cell according to any one of claims 2 to 6, in which disposed between the segments of the mixing element packing are rings which are connected as counter-electrodes.

9. which: (a) An electrolytic through-flow cell according to claim 1, in the housing of the through-flow cell is constructed as an insulator, (b) a bar anode is concentrically arranged centrally over the length of the housing, the bar anode is surrounded by a tubular diaphragm, and segments which are disposed in superposed relationship, of a static mixing element packing are connected as the cathode.

10.

which:- (a) the housing of the through-flow cell is connected as an anode, (b) segments of the static mixing element packing, which are disposed in superposed relationship in the housing, are connected as the cathode, and the segments are divided into individual zones which each have a respective separate current supply.

(c) An electrolytic through-flow cell according to claim 1, in

11. An electrolytic through-flow cell for the electrochemical treatment of radioactive product solutions substanialiy as hereinbefore described and illustrated with reference to the accompanying drawings.

PubUshed 1989 at The Patent Ofnee. C HOUSe, 86,71 High Holborn, W.J-.. Vid.R 4TP. Further copies nlaybe obtIned from The PatentOfBU. Wes Branch, St Mary.. OrpIn9WIL, Xent BM 3BD. Printed tY Multiplex techWques ltd, St, Rent, Son. l/e7