EP1449221A1

EP1449221A1 - Method for treating radioactive waste and system for implementing the method

Info

Publication number: EP1449221A1
Application number: EP02767388A
Authority: EP
Inventors: Holger Weimer; Karl Heinz Kleinschroth; Johannes Abeln; Manfred Kluth; Helmut Schmieder; Hubert Goldacker; Jan Lamla; Eckhard Dinijus
Original assignee: Framatome ANP GmbH
Current assignee: Areva GmbH
Priority date: 2001-08-17
Filing date: 2002-08-15
Publication date: 2004-08-25
Also published as: WO2003017288A1; DE10140525A1; JP2005505756A

Abstract

The invention relates to a method for treating radioactive waste (A) existing, in particular, in the form of contaminated ion-exchange resin, with which a reliable intermediate storage or final storage of the waste (A) with a particularly small volume requirement is made possible. To this end, the invention provides that the waste (A) is mixed with transport water and the mixture is fed to an oxidation reactor (2) inside of which a wet oxidation of organic constituents of the waste (A) takes place under conditions that are supercritical for water (H2O).

Description

description

Radioactive waste treatment process and system for carrying out the process

The invention relates to a method for treating radioactive waste. It further relates to a system for processing organic waste containing waste from a nuclear facility suitable for carrying out the method.

Liquid waste containing organic components can be generated in a nuclear plant, in particular in a nuclear power plant. For example, ion exchange resins are usually used in nuclear plants to remove radioactive isotopes from a water or liquid stream. In this way, especially condensate, circulation or waste water flows are treated. The ion exchange resin used for cleaning can be in bed form (for example as a mixed bed filter) or in the form of

Precoat layers are used. Depending on the type of ion to be removed from the water or liquid stream, cationic and / or anionic ion exchange resins can be used. The respective ion exchange resin can be in powder form as a powder resin. Alternatively, ion exchange resin in the form of small spheres - also known as spherical resin - can also be used.

Ion exchange resin used in this way can no longer absorb any more ions after a certain operating period and must therefore be cleaned (regenerated) or discarded and disposed of. The ion exchange resin contains radioactive substances and thus forms radioactive waste containing organic components, so that appropriate safety standards corresponding disposal, conditioning or final storage must be ensured. However, such ion exchange resins conditioned by cementing, for example, require a comparatively large intermediate or final storage volume. For waste disposal to be suitable for final disposal, a volume reduction of waste of this type is usually initially desirable, but on the other hand, special safety standards with regard to reliable final storage capability must also be observed. When disposing of such waste, cementing and subsequent packaging in intermediate or final storage containers is usually provided. However, contrary to the volume reduction actually intended, such conditioning by cementing or bituminization even leads to an increase in the volume of the waste to be disposed of.

The invention is therefore based on the object of specifying a method for treating radioactive waste, which is present in particular in the form of contaminated ion exchange resin, by means of which particularly reliable intermediate or final storage of the waste is made possible with only a small volume requirement. Furthermore, a system for processing organic waste containing waste from a nuclear facility suitable for carrying out the method is to be specified.

With regard to the method, this object is achieved according to the invention in that the waste to be treated is fed to an oxidation reactor in which wet oxidation of organic components of the waste is carried out under conditions which are supercritical for water.

The invention is based on the consideration that a reliable intermediate or final storage for the waste is possible with a small footprint by concentrating the waste on its actually contaminated components or their salt load. For this purpose, the organic constituents formed, for example, by the ion exchange resins or their carrier material should largely be removed from the waste and a separate one adapted to them. disposal. For this purpose, a waste disposal facility, which is usually already present in nuclear facilities, should be usable. This is usually designed for the treatment of gaseous waste. Therefore, the organic constituents should be specifically converted into components that can be further processed in a simple manner and removed in this form from the waste.

As has surprisingly been found, a so-called wet oxidation of the organic constituents in supercritical water is particularly suitable for such a conversion. Supercritical water is water with a pressure above the critical pressure of 221 bar and a temperature above the critical temperature of 374 ° C. The hydrogen bonds are almost completely removed under parameters above these limit values, so that organic or other non-polar substances achieve almost complete miscibility with such supercritical water. This almost complete miscibility with supercritical water also applies to gases such as nitrogen (N ₂ ), oxygen (0 ₂ ) or carbon dioxide (C0 ₂ ) and to lower hydrocarbons (methane, ethane or the like). Such constituents of the waste, that is to say in particular its organic constituents, thus form a single phase with the supercritical water and an oxidizing agent, such as air or oxygen, for example, so that in this state the reaction of the organic constituents by reaction with the oxidizing agent does not is hindered by phase boundaries. In addition, the supercritical water in this state has a comparatively high diffusion coefficient with a relatively low viscosity, so that transport processes take place at a particularly high speed.

For these reasons, under supercritical conditions, an almost complete conversion of the organic components of the waste into carbon dioxide (C0 ₂ ) and water (H ₂ 0) is possible in a comparatively short reaction time of typically only a few minutes. Organically bound atoms (for example Cl, P, S) form mineral acids, and nitrogen compounds are converted into molecular nitrogen (N ₂ ). As a result, such treatment of the radioactive waste enables an almost complete degradation of its organic components, in particular the ion exchange resins as such, with a comparatively fast reaction time and high degradation rate, with the formation of dioxins or NOχ being reliably avoided.

A comparatively good manageability can also be achieved while maintaining high safety requirements by advantageously mixing the waste first with transport water and then feeding this mixture to the oxidation reactor. On the one hand, the supply of water to the oxidation reactor to carry out the wet oxidation is necessary anyway. On the other hand, this water, which is required anyway, can also be used as transport water for the waste, in which it is diluted and kept in a pumpable suspension. This makes the waste particularly easy to treat. The reaction rate during wet oxidation can also be influenced by a suitable choice of the mixing ratios or the metering, as a result of which, in particular, overloading of the oxidation reactor and / or soot formation can be reliably avoided.

In principle, such a conversion of organic constituents can be carried out by wet oxidation in the manner of a self-sustaining reaction if the temperature in the oxidation reactor is set to a value of more than about 480 ° C. However, an auxiliary fuel is advantageously fed to the oxidation reactor for particularly high flexibility in carrying out the reaction and in particular for influencing reaction rates or degradation rates as required. By burning the auxiliary fuel in the oxidation reactor, the reaction can be influenced in a targeted manner and maintained with particularly high reliability. Especially when treating radioactive waste from a nuclear facility, for example ion exchange resins, ethanol is expediently used as the auxiliary fuel.

It has proven to be particularly advantageous for the treatment of contaminated waste from nuclear plants that advantageously in the oxidation reactor a system temperature of about 700 ° C and / or a system pressure of about 250 to 300 bar is set.

In order to ensure reliable degradation of the organic constituents of the waste and their conversion into easily processable products such as carbon dioxide (C0 ₂ ) and water (H ₂ 0), air and / or oxygen (0 ₂ ) is advantageously supplied to the oxidation reactor as an oxidizing agent. In order to facilitate an adaptation to the operating conditions to be set there already when the oxidizing agent is fed into the oxidation reactor, the oxidizing agent is preheated in a further advantageous embodiment before it is fed into the oxidation reactor.

With regard to the system for the treatment of waste containing organic constituents from a nuclear plant, the stated object is achieved in that the waste can be fed to an oxidation reactor, the pressure housing of which has means for setting a pressure of more than the critical pressure of water and means for setting a temperature of are assigned more than the critical temperature.

Accordingly, the system for processing the waste is designed such that its oxidation reactor is suitable for providing comparatively high pressures and comparatively high temperatures in its interior. To ensure this, the oxidation reactor expediently has a pressure housing which is designed to be subjected to an internal pressure of more than 221 bar without appreciably affecting its mechanical stability. Likewise, the pressure housing is expediently designed, in particular with regard to its wall or its suspension, to be subjected to a temperature of more than 374 ° C.

The setting of the temperature inside the oxidation reactor can be influenced by a suitable choice of a feed rate of the waste to be treated and of the oxidizing agent. Furthermore, the system temperature in the Interior of the oxidation reactor can also be influenced by the choice of a suitable feed rate of an auxiliary fuel. However, in order to allow a particularly high degree of flexibility in setting the operating parameters, the oxidation reactor can advantageously be heated by a number of heating elements arranged on the outer wall of its pressure housing. With such an arrangement, the temperature can thus be set from the outside and independently of the feed of substances into the interior of the oxidation reactor. The ignition of the auxiliary fuel can also be suitably supported by the heating elements, so that in particular the start-up process in the treatment of the waste can be suitably influenced.

To adjust the pressure in the interior of the pressure housing, the treatment system advantageously includes a high-pressure pump connected to a supply line for the waste connected to the oxidation reactor. A feed pressure that is adapted to the actual reaction conditions can already be set in the feed line for the waste.

In order to enable a particularly effective and effective conversion of the organic components, the oxidation reactor is advantageously preceded by a mixer for mixing the waste to be treated with an auxiliary fuel and / or an oxidizing agent. With such an arrangement, an approximate homogenization and particularly uniform distribution of the individual constituents of the mixture can thus be produced even before it enters the actual oxidation reactor.

For a particularly high operational reliability, the oxidation reactor is advantageously designed as a so-called sweat wall reactor, in which the actual reaction zone for carrying out the wet oxidation is spatially decoupled from the surrounding pressure housing via an inner tube. In a further advantageous embodiment, the inner tube is porous, expediently made of ceramic material. The advantages achieved by the invention consist in particular in that the wet oxidation of organic constituents of the waste to be treated under supercritical conditions achieves a considerable volume reduction, particularly in the case of ion exchange resins, from nuclear plants. Almost complete degradation rates of the organic constituents can be achieved in a comparatively short reaction time, these being converted into easily treatable components, namely in particular carbon dioxide (C0 ₂ ) and water (H ₂ 0). The remaining residues essentially correspond to the actual radioactive materials bound in the ion exchange resins, which can thus be sent to a targeted interim or final storage without any significant space requirement. The residues can also be brought into a particularly concentrated form by a subsequent evaporation step.

An embodiment of the invention is explained in more detail with reference to a drawing. The figure shows schematically a system for processing waste from a nuclear facility.

The system 1 is provided for the treatment of waste A containing organic constituents from a nuclear plant, not shown in detail. Waste A can in particular be used and thus contaminated ion exchange resins. System 1 is designed to treat waste A with the aim of reducing its volume. A transfer into a state capable of being stored in an interim or final storage facility can take place in particular in connection with a subsequent evaporation step.

For this purpose, the system 1 is designed for wet oxidation of the organic constituents of the waste A to be treated under conditions which are supercritical for water. For this purpose, the system 1 comprises an oxidation reactor 2, to which the waste A, which is held in a storage container 4 and mixed with transport water, can be fed via a feed line 6. The supply line 6, which can be shut off with a valve 8, into which a high-pressure pump 10 and a preheater 12 are connected are, on the output side, open into a mixer 14 directly upstream of the oxidation reactor 2.

On the input side, a supply container 18 for an auxiliary fuel B and a supply container 22 for an oxidizing agent O are connected to the mixer 14 via a fuel line 16. A high-pressure pump 24 and a preheater 26 are also connected to the fuel line 16. Another preheater 28 is connected in the gas line 20.

On the output side, the oxidation reactor 2 is connected to a phase separator 36 via a waste water line 32, into which a cooler 34 is connected. On the output side of the phase separator 36, a control valve 38 is connected, via which the pressure in the oxidation reactor 2 can be adjusted. The control valve 38 further reduces the pressure of the exhaust gas to approximately 1 bar, by means of which it can be supplied to an exhaust system, for example an exhaust system, via a downstream exhaust line 40. Furthermore, the phase separator 36 is connected on the output side to a sewage system, not shown, via a sewage line 44 provided with a control valve 42. The fill level in the phase separator 36 can be set via the control valve 42 and can in particular be kept constant.

A sample system 46 is connected to the oxidation reactor 2, to the waste water line 32, to the waste gas line 40 and to the waste water line 44, via which the medium carried in the respective component can be monitored continuously and in particular as a basis for the process control.

The oxidation reactor 2 is designed as a so-called sweat wall reactor and comprises a pressure housing 50 in the manner of an outer jacket. An inner tube 52 is arranged inside the tubular pressure housing 50. The inner tube 52 is designed to be porous or perforated, so that overflow on the media side from the intermediate space surrounding the inner tube 52 in a ring between the pressure housing 50 and the inner tube 52 in the inner region of the inner tube 52 is fundamentally possible. In the exemplary embodiment, the inner tube 52 is designed as a ceramic tube made of a ceramic material with a suitable porosity.

Arranged in the outer area of the pressure housing 50 is a heating device 54, shown only schematically in the figure, which in the exemplary embodiment can be operated electrically via a supply unit (not shown in detail). The heating device 54 comprises a number of heating elements arranged in contact with the pressure housing 50. As indicated by the arrows 56, condensation water SW can also be applied to the annular space formed by the pressure housing 50 and the inner tube 52. A directed flow into the interior of the oxidation reactor 2 can be maintained by supplying condensation water SW. This prevents media-side overflow from the inner region of the inner tube 52 into the space between the pressure housing 50 and the inner tube 52 surrounding it. In particular, the supply of medium from this interior to the inner wall of the pressure housing 50 can be prevented, so that the corrosion of the pressure housing 50 is particularly low. The condensation water SW can be supplied in the lower region of the oxidation reactor 2 in a cooled form as so-called quench water QW. This serves as a coolant in order to dissipate the heat of reaction occurring in the oxidation reactor 2, it being possible for waste water A 'emerging from the oxidation reactor 2 to be cooled to a temperature of, for example, approximately 250.degree.

System 1 is designed for wet oxidation of organic constituents of waste A. For this purpose, the waste A mixed with transport water is introduced into the mixer 14 via the feed line 6 and mixed there with oxidizing agent O, in the exemplary embodiment with air or oxygen 0 ₂ . If necessary, an auxiliary fuel B, in the exemplary embodiment ethanol, is also added. The mixture formed in this way is conveyed from the mixer 14 into the interior of the inner tube 52 of the oxidation reactor 2. There are over- critical conditions, i.e. an operating pressure of about 300 bar and thus of more than 221 bar and an operating temperature of between about 600 ° C and about 800 ° C and thus more than 374 ° C, set. The operating pressure is adjusted as required via the high-pressure pumps 10, 24 and / or via a compressor 58 assigned to the reservoir 22. The operating temperature, however, is set by independent contributions during the actual wet oxidation, by combustion of the auxiliary fuel B as required and / or by additional ones Heating via the heater 54.

As a result of the operating parameters set in this way within the oxidation reactor 2, the water therein is placed in a supercritical state. In this state, the water has a particularly good solubility for comparatively non-polar substances due to the then almost completely disappearing hydrogen bonds. Accordingly, the organic constituents of waste A are dissolved with particularly high effectiveness in the supercritical water thus formed, as are the oxidizing agent O and / or auxiliary fuel B. Since there are no longer any phase boundaries in the supercritical water, an oxidation reaction of the organic constituents of waste A particularly favored with the oxidizing agent O. As a result, the organic constituents of waste A are reacted at a high reaction rate and largely completely with the oxidizing agent O to form carbon dioxide C0 ₂ and water H ₂ 0. Only the radioactive constituents originally incorporated in the ion exchange resins to be treated as waste A remain in the residual water, nitrogen-containing compounds being essentially converted into molecular nitrogen N ₂ . The remaining waste A 'is then transferred to the phase separator 36 via the waste water line 32 in the waste water. There, the gaseous constituents, in particular the gaseous oxidation products formed during the wet oxidation of the organic constituents of waste A, are separated off and fed to the exhaust air system via the exhaust line 40. The remaining waste stream is fed via the waste water line 44 to a further waste water treatment, for example an evaporation. Due to the operating conditions provided in the oxidation reactor 2, its components are exposed to increased corrosion. In order to protect the pressure housing 50, which is under considerable mechanical stress due to the system pressure to be set of more than 221 bar, from damage or impairment due to corrosion, the oxidation reactor 2 is constructed in the manner mentioned. The inner tube 52 serves to coat the actual reaction zone without the inner tube 52 being exposed to the considerable mechanical loads of the actual pressure housing 50 even as a result of the pressure conditions. The corrosion protection via the inner tube 52 is thus decoupled from the actually mechanically loaded component, namely the pressure vessel 50. The application of condensation water SW to the annular space between the pressure housing 50 and the inner tube 52 takes place with the proviso that a flow through the inner tube 52 from the outside inwards and thus reliably prevent corrosive material entry into this space. By flowing through the inner tube 52, material deposits on its inner wall and thus blockages are also reliably avoided.

The condensation water SW is supplied in the lower region of the oxidation reactor 2 in a cooled form as quench water QW. Characterized the reaction mixture is cooled there to a temperature of about 200 ° C to 300 ° C, so that u. a. precipitating inorganic constituents are at least partially dissolved again and thus blockages in the sewage pipe are reliably suppressed.

The preheaters 12, 26, 28 serve to keep the temperatures when waste A, oxidizing agent O and auxiliary fuel B enter the oxidation reactor 2 comparatively high in order to minimize the heating-up time in the actual oxidation reactor 2. As a result, a comparatively longer residence time is available for oxidation in the oxidation reactor 2. In addition, soot formation is largely avoided. The preheaters 12, 26, 28 are designed such that the medium flowing through them at each outlet has a temperature of has about 400 ° C. The increase in the operating temperature of these media to the operating temperature provided in the oxidation reactor 2 of approximately 600 ° C. to approximately 800 ° C. can thus be achieved with only limited expenditure.

Downstream of the oxidation reactor 2, the cooler 34 ensures that the waste water A 'is present with operating parameters that can be used for the phase separator 36, in particular with sufficiently cooled operating temperature. Further wastewater treatment of the wastewater A 'flowing out of the oxidation reactor 2 is thus also made possible.

The wet oxidation of organic constituents of the waste A in the oxidation reactor 2 of the system 1 enables a considerable volume reduction of the waste A to be disposed of. The organic constituents of waste A, which can make up by far the largest proportion of its volume, are reliably removed and converted into comparatively easy-to-handle substances, namely in particular carbon dioxide (C0 ₂ ) and water (H ₂ 0). For these substances with the exhaust air or. Waste water systems a problem-free further treatment possible. The remaining waste, on the other hand, can be fed into a reliable intermediate or final storage, especially after a further treatment by evaporation, with only a very small space requirement. In addition, in the case of such waste treatment, the oxidation in the liquid phase means that any cesium which might otherwise pass into the gas phase as a volatile element remains dissolved and thus does not contribute to gaseous waste requiring treatment. Compared to thermal combustion of organic radioactive waste, a comparatively complex emission control system can thus be dispensed with in the present case. LIST OF REFERENCE NUMBERS

1 system H ₂ 0 water

2 Oxidation reactor N ₂ nitrogen

4 storage tanks o ₂ oxygen

6 Supply line O oxidizing agent

10 high pressure pump QW quench water

12 preheater SW condensation water

14 mixers

16 fuel line

18 storage containers

20 gas line

22 storage containers

24 high pressure pump

26, 28 preheater

32 sewage pipe

34 cooler

36 phase separator

38 control valve

40 exhaust pipe

42 control valve

44 sewage pipe

46 sample system

50 pressure housing

52 inner tube

54 heating device

56 arrows

58 compressor

A waste

A 'remaining waste / wastewater

B auxiliary fuel

C0 ₂ carbon dioxide

Claims

Expectations

1. A process for the treatment of radioactive waste (A), in which the waste (A) is fed to an oxidation reactor (2), in which a wet oxidation of organic components of the waste (A) is carried out under supercritical conditions for water (H ₂ 0) ,

2. The method according to claim 1, wherein the waste (A) is first mixed with transport water and the mixture is fed to the oxidation reactor (2).

3. The method according to claim 1 or 2, wherein the auxiliary reactor (B) is fed to the oxidation reactor (2).

4. The method according to claim 3, in which ethanol is used as auxiliary fuel (B).

5. The method according to any one of claims 1 to 4, in which a system temperature of about 700 ° C is set in the oxidation reactor (2).

6. The method according to any one of claims 1 to 5, in which a system pressure of about 300 bar is set in the oxidation reactor (2).

7. The method according to any one of claims 1 to 6, in which the oxidation reactor (2) as the oxidizing agent (O) air and / or oxygen (0 ₂ ) is supplied.

8. The method according to claim 7, wherein the oxidizing agent (O) is preheated before it is fed into the oxidation reactor (2).

9. System (1) for the treatment of organic waste (A) from a nuclear plant, in which the waste (A) can be fed to an oxidation reactor (2), the pressure housing (50) means for adjusting a pressure of more than that critical pressure of water (H ₂ 0) and means for setting a temperature higher than the critical temperature of water (H ₂ 0) are assigned.

10. System (1) according to claim 9, whose oxidation reactor (2) via a on the outer wall of its pressure housing (50) arranged heating device (54) can be heated.

11. System (1) according to claim 9 or 10, the means for adjusting the pressure comprise a high-pressure pump (24) connected to a supply line (6) for the waste (A) connected to the oxidation reactor (2).

12. System (1) according to any one of claims 9 to 11, the oxidation reactor (2), a mixer (14) for mixing the waste to be treated (A) with a fuel and / or an oxidizing agent (O) is connected upstream.

13. System (1) according to one of claims 9 to 12, whose oxidation reactor (2) is designed as a sweat wall reactor.

14. System (1) according to claim 13, whose oxidation reactor (2) has a porous inner tube (52).