EP0497641B1 - Process and device for treating liquid radioactive effluents originating from a nuclear plant - Google Patents

Description

The invention relates to a method and a device for treating liquid radioactive effluents originating from a nuclear power station and in particular from a nuclear power station cooled by pressurized water.

The operation of nuclear facilities and in particular the operation of power generating plants comprising a nuclear reactor results in the production of radioactive waste and in particular of a large quantity of liquid effluents containing radioactive products which, according to their nature and their level of activity, can be either recycled in the installation itself, or, when their activity is important, put in a form allowing to ensure their storage in a safe place, or finally, when their activity is lower than the activity imposed by the regulations, be discharged directly into the environment.

In most cases, liquid effluents containing radioactive substances must be treated in such a way as to separate the dangerous radioactive substances from the part of the effluents that can be released into the natural environment of the installation or power plant.

In the case of an electro-nuclear power plant comprising a pressurized water type reactor, several circuits are used for the treatment of effluents, depending on their nature and their source.

A first circuit called primary effluent treatment circuit treats the effluents of the primary reactor circuit from the volumetric and chemical control circuit of the primary fluid, the pressurizer discharge tank, the leakage recovery circuit and the purge recovery circuit and vents.

These effluents are collected and stored in a tank supplying the treatment circuit, then filtered and purified by passage through a demineralizer of the mixed bed type and a specific demineralizer intended for the elimination of cesium. The effluents then pass through a degasser to reach intermediate storage tanks. The gases separated from the effluents are sent to a gaseous effluent treatment circuit. The effluents are then taken up and treated in an evaporator which separates the water from a solution concentrated in products contained in the primary fluid and in particular in boric acid. The separated water, if it meets the required standards, is recycled in the primary circuit. Otherwise, this water is sent to the spent effluent treatment circuit.

The concentrated boric acid solution can itself be recycled in the primary circuit or sent to the solid effluent treatment network or to the spent effluent treatment circuit.

The role of the used effluent treatment circuit is to collect, store, control and possibly recycle or reject the different liquid effluents coming from the various circuits of the power plant.

These effluents are divided into four categories, according to their chemical quality and their activity.

A first category constituting the residual drains are effluents related to the primary fluid of the reactor which contain boron-based products and which are aerated. These effluents can be constituted by the rinsing and transfer waters of the used resins from the demineralizers located on the volumetric and chemical control circuit or other primary fluid treatment or recovery circuits. These effluents can also come from flushing purges and water leaks from the primary circuit which cannot be recycled in this circuit.

A second category is made up of floor drains which are weakly active and charged effluents which come from unrecovered leaks or from the washing of the floors of reactor buildings, nuclear auxiliaries or fuel storage. These effluents can also be constituted by the rinsing waters of the ion exchange resins of the purge circuit of the steam generators.

A third category of effluents, called service effluents, are slightly active effluents which consist of laundromat effluents or coming from showers, sinks or changing rooms likely to cause the presence of radioactive substances, in the reactor maintenance block.

Finally, a fourth category of effluents, or chemical effluents, contains both radioactive substances and chemical substances. These effluents can be constituted by the products coming from the decontamination room of the reactor maintenance block, by the liquid for draining the purge circuit of the steam generators or by the emptying of the tanks of the nuclear auxiliaries building which are likely to contain reagents and chemicals.

The contaminated and chemically charged effluents from the spent effluent treatment circuit are sent to a filtration system and are then treated by an evaporator.

The evaporator consists mainly of a heater supplied by a steam flow, a evaporation column and a condenser supplied with a cooling water flow.

The concentrates are collected at the bottom of the evaporation column and are transferred using a pump to one of the storage tanks of the solid effluent treatment circuit.

In addition to the evaporation installation, a chain of demineralization units is also used to treat the active effluents which do not in principle contain inactive chemical compounds capable of saturating the resins of the demineralization units.

After their control, the very slightly contaminated effluents are filtered and directed, as well as the distillate resulting from evaporation, towards an evacuation system, before being discharged into the natural environment of the nuclear reactor such as a river. or the sea.

The objective of the treatment of effluents by the known technique, that is to say by evaporation and demineralization, is to minimize the activity of the effluents so as to discharge into the environment a maximum volume of effluents become inactive and store in drums a minimum volume of substances with high radioactivity. In order to achieve storage of radioactive substances, under very good safety conditions, the coating of these substances is carried out in an inert material before their storage.

The known processes for treating effluents by evaporation and by demineralization have certain drawbacks.

In particular, the decontamination factor for effluents by evaporation is too low and it is most often necessary to reprocess the distillates obtained. The use of a single evaporator for the whole seems to be effluent from the power plant results in an insufficient availability of this evaporator.

The use of ion exchange resins to carry out demineralization also has specific drawbacks. Demineralizers with ion exchange resins have a low chemical permissivity which results in acceptable ion concentrations in the treated effluents which are limited to relatively low values. Thus the maximum ion contents must be lower than the following limits: 15 ppm with regard to Na⁺ ions, 10 ppm with regard to Ca⁺⁺ ions and 20 ppm with regard to C1 ions ^{- .} In addition, the cost of coating the resins used for the treatment is high, which increases the cost of treating the effluents.

DE-A-3,634,180 discloses a method for treating radioactive effluents from a nuclear power station, so as to recover a reusable boric acid solution in the nuclear power station. The effluents are circulated so as to pass them through a reverse osmosis device. The permeate obtained is brought into contact with ion exchange resins and the concentrate containing a high proportion of radioactive substances is recovered as well as the substances separated from the permeate by ion exchange resins.

This process which aims at a thorough purification of the permeate before its recovery requires a second passage of the permeate as a whole in a reverse osmosis device. It is therefore necessary to circulate large quantities of liquid to obtain sufficient purification of the boric acid solution. This process is therefore not suitable for the treatment and elimination of all of the radioactive effluents from a nuclear power plant.

The object of the invention is therefore to propose a method for treating liquid radioactive effluents originating from a nuclear power station, in which the effluents to be treated are put into circulation so as to cause them to pass in contact with a surface of a fine filtration membrane, part of the effluents or permeate is collected on the side of the membrane opposite the face brought into contact with the effluents to be treated, the permeate is brought into contact with ion exchange resins so as to demineralize it , the demineralized permeate is removed, the part of the effluents remaining on the side of the contact surface of the membrane is collected and containing a high proportion of radioactive substances contained in the effluents, called concentrate, the radioactive substances contained in the concentrate are collected, the substances separated from the permeate are recovered by the ion exchange resins, and the radioac substances are stored in containers tives of the concentrate and the substances separated from the permeate by demineralization, a process which makes it possible to lower the activity of these effluents to a very low level before their discharge, to produce a minimum volume of radioactive substances separated from the effluents and to treat large volumes of effluents, the load of saline mineral matter and the clogging power can be high, the chemical composition of these effluents can also be variable in a wide range.

For this purpose, the treatment of the effluents is carried out during three successive passages in contact with the membrane, a first passage during which the effluents to be treated are put into circulation in contact with the membrane, the permeate collected is demineralized before be discharged into the environment and the concentrate is stored in a first tank, so as to be reincorporated into the effluents to be treated,
a second pass during which the concentrate obtained during the first pass is put into circulation in contact with the membrane, the permeate obtained is mixed with effluents to be treated and the concentrate sent to a second storage tank,
and a third passage during which the concentrate obtained during the second passage is put into circulation in contact with the membrane, the permeate is introduced into the first storage tank and the concentrate introduced into storage barrels, so as to achieve its elimination .

Preferably, a filtration membrane is used which makes it possible to separate the substances contained in the effluents, by reverse osmosis.

In order to clearly understand the invention, we will now describe, by way of nonlimiting examples, with reference to the appended figures, several embodiments of the method according to the invention and the corresponding devices in the case of treatment. liquid effluents in the spent effluent treatment circuit of a pressurized water nuclear reactor, by reverse osmosis.

Figure 1 is a schematic view of a device for implementing the method according to the invention and using reverse osmosis modules.

FIG. 2 is a functional diagram showing the use of a reverse osmosis treatment device in a circuit for treating spent effluents from a pressurized water nuclear reactor.

FIG. 3 is a diagram showing the general integration of a device for treating by reverse osmosis in the circuit for treating spent effluents from a pressurized water nuclear reactor.

Figure 4 is a schematic view similar to the view of Figure 1 showing an alternative embodiment a reverse osmosis treatment device for implementing the method according to the invention (four-pass method).

In Figure 1, there is shown, in general, a reverse osmosis treatment device for implementing the effluent treatment method according to the invention.

For the purpose of simplification, there is shown in FIG. 1, a single module 1 of reverse osmosis treatment although the device according to the invention generally comprises a plurality of identical modules placed in parallel and supplied with liquid effluents, from 'a storage tank 2, via a supply line 3 on which is placed a pump 4 for circulating liquid effluents.

The module 1 has been represented symbolically in the form of a capacity separated into two compartments 1a and 1b by a fine filtration membrane 5 making it possible to implement a process for the separation of substances contained in the effluents, by the phenomenon of reverse osmosis.

The effluent to be treated is brought under pressure, thanks to the pump 4, inside the compartment 1a, so as to circulate in contact with one of the faces of the membrane 5 whose constitution allows the separation by reverse osmosis of substances contained in the effluent.

The compartment 1b is at a pressure lower than the pressure of the compartment 1a, this pressure of the compartment 1b can be close to atmospheric pressure.

The pressure difference on either side of the membrane 5 is greater than the osmotic pressure, so that pure or substantially pure water which represents a significant part of the effluents is capable of crossing the membrane 5 to penetrate into compartment 1b, during the circulation of the effluents in compartment 1a.

The part of the effluents crossing the membrane 5 and entering the compartment 1b constitutes the permeate, while the remaining part of the effluents which is discharged at the outlet of the compartment 1a by the pipe 7 constitutes the concentrate.

In the case of effluents containing radioactive substances, most of these radioactive substances are found in the concentrate, at the outlet of module 1.

A recirculation pump 10 is placed on a pipe 8 bypassed with respect to the module, so as to send the concentrate collected by the pipe 7 to the inlet of the module.

Line 7 is also connected, with the interposition of a valve 12, to a concentrate discharge assembly comprising a first line 13 connected to two intermediate storage tanks for concentrate 14 and 15 and a second line 16 connected to a unit 17 filling storage drums and evacuating the concentrate.

The supply line of the intermediate storage tanks 14 and 15 is connected by a line 18 to the supply line of the module 1.

A permeate evacuation line 20 is connected to compartment 1b, so as to evacuate the permeate, either in a first line 21 connected to a demineralization installation by ion exchange resins, or to a second line 22 connected to the reservoir intermediate storage 14, or finally to a third line 23 connected to the storage tank 2 of the device.

The device as shown diagrammatically in FIG. 1 makes it possible to carry out the treatment of liquid effluents with a determined number of passages of the effluent stream in the reverse osmosis module (s), this stream of effluents possibly incorporating a part of the concentrate or permeate obtained at each passage.

In order to reduce the necessary surface area of the membranes, depending on the flow rates to be treated and the concentration to be obtained for the materials separated from the effluents, it is preferable to use a parallel mounting of the filtration modules by reverse osmosis with overall recirculation of the parts of the effluent separated by the module.

In the case of the treatment of effluents from a pressurized water nuclear reactor, in the circuit for treating spent effluents, according to the embodiment which will be described below, a treatment in three passages in the filtration modules (described in Figure 1) has proven to be very good.

In such a three-pass treatment, the first concentrate is treated during a second pass and the concentrate obtained after the second pass is treated again.

The intermediate concentrates from the reverse osmosis treatment are temporarily stored in one of the tanks 14 and 15, before being reprocessed in the reverse osmosis unit.

The permeate from the first treatment is directed by line 21 to the demineralization unit at the outlet from which the demineralized permeate is discharged into the environment, after verification of its activity, outside the plant.

In the case of effluents from a pressurized water nuclear power plant treated in the circuit spent effluents, the permeate from the second pass through the reverse osmosis module is mixed with the effluents to be treated, this permeate being returned by line 23 in the direction of supply to the module.

The permeate from the third pass in the reverse osmosis module is evacuated through line 22 and mixed with the concentrate from the first pass, inside the storage tank 14.

Indeed, the concentration of permeates in ⁶⁰ Co which is a critical element among the radioactive substances contained in the effluents is of the same order of magnitude in the permeate from the second passage as in the effluents to be treated (1.48 10⁻⁵ ppm ). On the other hand, the concentration of ⁶⁰Co in the permeate from the third pass is equivalent to the concentration of ⁶⁰Co in the concentrate from the first pass (6.84 10⁻⁵ ppm).

The concentrate from the first passage which is returned to the storage tank 14 and the permeate from the third passage as well as the concentrate from the second passage which is returned to the storage tank 15 are reintroduced into the supply line of module 1 , by line 18.

The device comprises valves on the different branched pipes which allow the different liquid flows to be directed, so as to carry out the operations mentioned above.

The reverse osmosis module which has been described in a simplified manner with reference to FIG. 1 is in reality constituted by a capacity inside which is arranged a very long tube made of a porous substance whose surface is covered by a deposited layer constituting the reverse osmosis membrane.

The fluids to be treated are sent under pressure by the pump 4 inside the tube whose wall constitutes the support for the reverse osmosis membrane.

Preferably, the membrane is produced in a dynamic manner, that is to say that this membrane is produced in situ on the porous support constituted by the wall of the tube. This wall can be constituted for example by a sintered metal and the membrane by a deposit of a polymeric material of an appropriate composition.

The membranes produced in situ on the filtration walls can also be eliminated and regenerated in situ on the treatment installation, after its commissioning, which implies a single initial investment.

FIGS. 2 and 3 show reverse osmosis treatment devices similar to the device shown in FIG. 1 and operating on an identical principle, incorporated in a circuit for treating spent effluents from a pressurized water nuclear reactor .

The effluents to be treated in the spent effluent circuit of the power station are at a temperature close to 35 ° and have a pH of around 7, a boron concentration of around 1000 ppm and a cobalt concentration of which l order of magnitude is 10⁻⁵ ppm.

For the purpose of simplification, the fluid treated in the spent effluent circuit is considered to be a fluid comprising two solutes, namely boron and cobalt.

Cobalt is taken as a representative element of the effluent activity since its contribution represents the major part of this activity.

The experience acquired in pressurized water nuclear power plants shows that the total volume to be treated annually is around 2300 m³, which volume of effluents being treated by evaporation and demineralization.

This annual volume of 2300 m³ constitutes part of the residual drains (1700 m³ per year) of which 300 m³ are treated by evaporation, 1300 m³ treated by demineralization and 100 m³ discharged, floor drains (1800 m³ per year) of which 600 m³ are treated by evaporation and chemical effluents (900 m³ per year) of which 100 m³ are treated by evaporation.

It has been shown that these 2300 m³ of effluents can be treated in an installation implementing the method according to the invention which will be described below, in particular with reference to FIGS. 2 and 3.

In FIGS. 2 and 3, a device for treatment by reverse osmosis is substantially analogous to the device represented in FIG. 1 generally designated by the reference 25 and incorporated in the circuit for treating spent effluents from a nuclear water reactor. under pressure.

As can be seen in FIG. 2, the reverse osmosis treatment device 25 comprises a plurality of reverse osmosis modules such as 26 and 26 ′ which are arranged in parallel, each on a line such as 27 and 27 ′.

In the case of the application of the device to the circuit for treating spent effluents from a pressurized water nuclear reactor, seventeen reverse osmosis modules were used, arranged in parallel on seventeen corresponding connected lines, for their group supply, single line 28.

Line 28 is itself supplied with effluents produced in the nuclear power station, via reservoirs such as 29 intended to collect effluents such as waste drains.

The effluents are sent under high pressure to the interior of the modules 26, 26 ′, by means of a pump 30 arranged on the supply line 28.

A line 31 on which are disposed a recirculation pump 32, a heat exchanger 33 and a valve 34 is placed in bypass with respect to all of the modules 26 and of the lines 27.

Each of the modules 26 comprises a separation membrane by reverse osmosis separating the interior volume of the module into a first part in which the effluent to be treated and the concentrate circulates and a second part in which the permeate is collected.

The supply lines 27 of the modules 26 are connected to the first part of the module in which the effluent and the concentrate circulate. This first part of the module is connected, at the outlet, to a pipe 36, the various pipes 36 of the modules 26, 26 ′ arranged in parallel being connected to a common pipe 38. The recirculation pipe 31 is connected, to one of its ends, to the line 38 for outlet of the concentrate and, at its other end, to the supply line 28.

Two isolation valves 35a and 35b are arranged, on either side of each of the modules such as 26, on the supply line 27 and on the concentrate recovery line 36, respectively, so as to isolate or, conversely, to commission module 26.

A valve 39 is arranged on the line 38 for recovering the concentrate, downstream of the branch of the recirculation line 31.

Line 38 opens, at its downstream end, into a station 40 for storing concentrates.

Each of the parts of the modules such as 26 and 26 ′ intended to receive the permeate is connected by a line 42 leading into a common line 43, to a demineralization unit with ion exchange resins 41.

The parts of each of the modules such as 26, 26 ′ intended to receive the permeate are also connected by a line 45 to a unit 46 comprising two intermediate storage tanks 47a and 47b.

The concentrate recovery line 36 of each of the modules such as 26 is also connected, by means of a bypass line 48, to the storage unit 46.

The outlet of the intermediate storage tanks 47a and 47b is also connected to each of the supply lines such as 27 and 27 ′ of the modules such as 26 and 26 ′, via a return pipe 50 on which is placed a pump 51 making it possible to reintroduce under pressure into the lines such as 27 and 27 ′, the liquids contained in the tanks 47a and 47b, in order to mix, as indicated in the invention, the different permeates and concentrates which have similar concentrations.

It is therefore apparent that the treatment device 25 shown in FIG. 2 and integrated into the circuit for treating spent effluents from a pressurized water nuclear reactor is substantially identical to the device shown schematically in FIG. 1.

As explained above, it is possible to advantageously use a treatment method involving three successive passages of the effluent in each of the reverse osmosis modules (as indicated in FIG. 2).

In FIG. 3, there is shown schematically, the device 25 for treatment by reverse osmosis which has been described with reference to FIG. 2 and the entire reactor effluent treatment circuit.

This spent effluent treatment circuit comprises four groups of storage tanks 55, 56, 57 and 58 intended to receive the chemical effluents, the service effluents, the floor drains and the residual drains respectively.

The groups of reservoirs 55, 56 and 57 are each constituted by two reservoirs with a capacity of 20 m³ and the group 58 by two reservoirs with a capacity of 35 m³.

The groups of reservoirs 55, 56, 57 and 58 are supplied by means of collection lines for the effluents from the nuclear power station 60, 61, 62 and 63 respectively.

At the outlet, the groups of reservoirs are connected on the one hand to lines 64, 65, 66, 67 opening into a pipe 68 supplying the treatment device 25 and on the other hand to lines 69, 70, 71 and 72 intended to supply a low-activity effluent treatment circuit generally designated by the reference 75.

Circulation pumps respectively 69 ′, 70 ′, 71 ′, 72 ′ are arranged on the lines 69, 70; 71 and 72 to ensure the evacuation of the effluents in the direction of the treatment circuit 75.

Valves are arranged on branches of the outlet pipes of the tanks, so as to direct the effluents either to the reverse osmosis treatment device 25, or to the circuit 75 for treating effluents having a low activity.

The effluent reverse osmosis treatment device which has been shown schematically in FIG. 3 is identical to the processing device 25 represented in FIG. 2.

A single reverse osmosis treatment module 26 has been shown. The part of this module in which the effluents and the concentrate circulates is connected, at one of its ends, to the supply line 27 on which the high pressure pump 30 is disposed and at its other end to the line of outlet of the concentrate 38 which is connected on the one hand to the storage station for the concentrates 40 and on the other hand, via the line 48, to the intermediate storage tanks 47a and 47b.

The part of the module 26 receiving the permeate is connected on the one hand to the demineralizer 41 by a pipe 43 and on the other hand to the intermediate storage tanks 47a and 47b by a pipe 45. In addition, a line 76 provides the junction between the permeate outlet line and 55 chemical effluent storage tanks.

The weakly active effluent treatment circuit 75 includes an inlet line 77 on which a filter 78 is placed, connected to the module of the demineralization unit 41. A filter 79 is placed at the outlet of the demineralization unit.

The outlet part of the circuit 75 communicates with reprocessing effluent discharge conduits 82 allowing their evacuation into the environment of the nuclear power plant.

The installation as shown in FIG. 3 makes it possible to treat part of the chemical effluents, part of the residual drains and part of the floor drains, containing radioactive substances inside the reverse osmosis treatment device 25, the manner which has been described with reference to FIG. 1.

The permeate recovered during the first pass, in the case of an operation in three passes as described above, is sent to the demineralization unit 41 through the pipe 43 before being discharged into the environment, after verification and decay radioactive if necessary.

The radioactive substances contained in the concentrate are stored, processed and placed in drums at the processing and storage station 40.

In addition, the radioactive residues retained by the resins of the demineralization unit 41 are also stored and placed in barrels.

The three-pass operation of the reverse osmosis treatment device is identical to the operation of the device shown in FIG. 1.

The permeate from the second effluent passage is sent to the chemical effluent tanks 55 via the line 76 and the permeate obtained during the third passage is sent to the reservoir 47a by the line 45, to be mixed with the concentrate collected via line 48, during the first pass.

The concentrate obtained during the second pass is sent by line 48 to the tank 47b.

The concentrate collected during the third pass is evacuated to the processing and storage station 40.

The liquids contained in the reservoirs 47a and 47b are reintroduced into the supply line 68 of the treatment device 25.

In the case of a circuit for treating spent effluents from a pressurized water nuclear reactor ensuring the annual treatment of 2300 m³ of effluents, as described above, a device for treating by reverse osmosis was used. comprising seventeen parallel modules totaling a 119 m² membrane surface.

The treatment capacity of the reverse osmosis unit is 3.5 m³ / h. The recirculation rate using the seventeen modules in parallel and an overall recirculation of the concentrate is 8.23.

The length of the tube constituting the support wall of the reverse osmosis filtration membrane of a module is 147 m and the inside diameter of the tube is 15 mm.

The pressure of the effluent at the inlet of the module is 70 bars and the supply speed is close to 3 m / s.

The permeability of the membrane at 35 ° C and under 70 bars of pressure is 30 l / h / m².

The boron rejection rate is 35%, the NaCl rejection rate is 88% and the ⁶⁰Co rejection rate is 96%.

The permeate flow through the wall of the modules is 3.23 m³ / h. The concentrate flow is 80 l / h.

The effluent volume reduction factor is 1750 and the volume of radioactive substances to be stored in barrels is 1.3 m³. The effluent activity concentration factor is 96.

The éatCo concentration of the permeate is 3 × 10 et ppm and the boron concentration of the concentrate is 1607 ppm.

We will now give below comparative results between the process according to the prior art for treating effluents by demineralization and evaporation and the treatment process according to the invention by reverse osmosis and by demineralization of the permeate.

In the process according to the prior art, the volume of effluents from the waste drains which are treated by demineralization represents an annual volume of 1300 m³. This demineralization of 1300 m³ of effluents is accompanied by the production of one cubic meter of chemical and radioactive substances which must be eliminated by storage in barrels.

All the effluents which are treated by evaporation to carry out the separation and elimination of the radioactive substances represents an annual volume of 1000 m³, these effluents having an activity of 5.7 10⁻³ Ci / m³. The evaporative treatment produces a distillate which is released into the environment and a volume of 7 m³ of radioactive substances which must be eliminated by storage in barrels. The activity of these stored radioactive substances is 800 10⁻³ Ci / m3.

In the case of the process according to the invention, an effluent volume of 2300 m³ is treated by reverse osmosis, the final concentrate obtained being stored in barrels and the permeate being discharged into the environment after demineralization.

The effluents to be treated have a cobalt content of the order of 10 et ppm and a boron content of the order of 1000 ppm. The activity of these effluents is 5.7 10⁻3 Ci / m³.

The volume of concentrate corresponding to the 2300 m³ of effluent which is stored in drums represents a volume of 1.3 m³. The activity of the substances stored in the barrels is 547 10⁻³ Ci / m³. The cobalt content of these substances is 96 10⁻⁵ ppm and the boron content of 1607 ppm.

The demineralization of the permeate before its release into the environment uses a volume of resins on which the substances separated from the 0.6 m³ permeate, this volume of resins to be removed by storage in barrels.

It therefore appears that the process according to the invention makes it possible to carry out the treatment under better conditions by producing a volume of substances to be eliminated, whether in the form of concentrates or in the form of resins, which is less than the volume of substances to be eliminated in the case of treatment according to the prior art. Treatment is therefore less expensive.

In addition, the demineralization of the already purified permeate makes it possible to eliminate any trace of cobalt in the discharges which are discharged into the environment.

In addition, the method of treatment by reverse osmosis according to the invention which is carried out at a temperature close to ambient temperature makes it possible to significantly reduce the degradation of the installations by thermal effect.

The reverse osmosis treatment device also has high chemical inertness vis-à-vis the treated effluents, satisfactory behavior in the presence of corrosive fluids, good mechanical and physico-chemical resistance as well as very good clogging resistance, insofar as effluents and concentrates are constantly kept in circulation in contact with the surface of the filtration membrane.

It is also possible to perform an in situ regeneration of the membranes used and the retention rate of the substances to be eliminated is very high.

A substantial energy saving is also obtained, the energy consumption of the reverse osmosis modules being limited to the energy required to pressurize the effluents by means of the supply pump; this energy is less than the evaporation energy of the effluents, for the same quantities treated.

Indeed, in the case of an evaporation treatment according to the prior art, the energy consumption of the evaporator is 1.4 kW / h per liter of treated effluents.

The amount of energy required to carry out the treatment of 1000 m³ of effluents is therefore 1,400,000 kWh.

In the case of the reverse osmosis treatment according to the invention, the energy consumption corresponds to the consumption of the pumps, that is to say of the recirculation pump and of the feed pump.

The power of the recirculation pump is 12 kW and the corresponding consumption in the case of treatment of an effluent volume of 2300 m³ is 7886 kWh.

The power of the feed pump is 9.6 kW and the consumption of this feed pump for the treatment of 2300 m³ of effluents is 6298 kWh.

It follows that the total annual energy consumption for the treatment of effluents from a pressurized water plant by the process according to the invention is 7886 + 6298 = 14,200 kWh.

The treatment method according to the invention using a reverse osmosis filtration device therefore consumes approximately 100 times less energy than the method according to the prior art by evaporation.

In Figure 4, there is shown an alternative embodiment of a reverse osmosis treatment device in which the treatment is carried out in four successive passes.

The device comprises an effluent storage tank 80 allowing the supply, by via a high pressure pump 90, fifteen reverse osmosis filtration modules 81 arranged in parallel and associated with a circuit 83 for recirculating the concentrate.

The device also includes three intermediate storage tanks for concentrates 84, 85 and 86.

The concentrate resulting from a first pass is stored in the tank 84 to be reintroduced into the supply line of the modules 81, through a pipe 87. The concentrates resulting from a second pass are themselves stored in the tank 85 and the concentrates resulting from a third passage in the tank 86.

The concentrates resulting from the fourth passage through the modules 81 are evacuated by a line 88, to a barrel storage station.

The permeate resulting from the first passage is evacuated via a pipe 89 to the demineralization unit, before being discharged into the environment.

The permeate resulting from the second passage is evacuated by a line 91, in the supply line of the modules.

The permeate resulting from the third passage is evacuated via a pipe 92, into the storage tank 84.

The permeate resulting from the fourth pass is discharged into the concentrate storage tank 85.

However, it appeared that such a four-pass treatment device which has certain advantages is less economical to operate than the three-pass treatment device which has been described above.

The invention is not limited to the embodiments which have been described.

Thus the fine filtration membrane in contact with which the effluents are circulated can be constituted by a reverse osmosis membrane of any type, such a microporous membrane being able to be constituted for example by a sintered metal.

The structure and design of the processing circuits integrated into the existing circuits of the installation or of the nuclear power plant can be arbitrary.

It is possible to use a number of reverse osmosis or ultrafiltration modules different from the numbers indicated above in the nonlimiting examples.

Any number of effluents and concentrates can be passed through these modules and any recirculation rate suitable for the type of treatment to be used can be used.

It is possible to use a single intermediate storage tank for the concentrates or any number of tanks depending on the number of successive passages in contact with the membrane.

Finally, the method and the device according to the invention can be applied to treat liquid effluents from different installations of a pressurized water nuclear reactor.

Claims (10)

1. Process for treating liquid radioactive effluents originating from a nuclear power station, in which the effluents to be treated are circulated so as to bring them into contact with one surface of a fine filtration membrane (5), a part of the effluents or permeate is collected from the side of the membrane (5) opposite that of the surface put in contact with the effluents to be treated, the permeate is put in contact with ion exchange resins so as to demineralize them, the demineralized permeate is discharged, the part of the effluents remaining on the same side as the contact surface of the membrane and containing a high proportion of radioactive substances contained in the effluents, referred to as the concentrate, is collected, the radioactive substances contained in the concentrate are collected, the substances separated from the permeate by the ion exchange resins are recovered, and the radioactive substances of the concentrate and the substances separated from the permeate by demineralization are stored in containers, characterised by the fact that the treatment of the effluents is effected during three successive passages in contact with the membrane (5), a first passage during which the effluents to be treated are circulated in contact with the membrane (5), the permeate collected is demineralized before being discharged to the environment and the concentrate is stored in a first reservoir (14), so as to be reincorporated into the effluents to be treated, a second passage during which the concentrate obtained during the first passage is circulated in contact with the membrane (5), the permeate obtained is mixed with effluents to be treated and the concentrate sent into a second storage reservoir (15), and a third passage during which the concentrate obtained during the second passage is circulated in contact with the membrane (5), the permeate is introduced into the first storage reservoir (14) and the concentrate introduced into storage drums, so as to effect its disposal.
2. Process according to Claim 1, characterised by the fact that the membrane (5) is a reverse osmosis separation membrane and that a pressure difference is maintained between the two sides of the membrane (5).
3. Process according to Claim 2, characterised by the fact that the pressure difference between the two sides of the membrane (5) is around 70 bars.
4. Process according to any one of Claims 1 to 3, characterised by the fact that at least part of the effluents to be treated is demineralized by being passed over ion exchange resins before being discharged to the environment, without being put in contact with a fine filtration membrane.
5. Process according to any one of Claims 1 to 4, characterised by the fact that the effluents to be treated consist of liquid effluents from a nuclear power station having a pressurized water reactor, recovered in the treatment circuit for waste effluents from the station.
6. Device for implementing the process for treating liquid radioactive effluents originating from a nuclear power station according to one of Claims 1 to 5, characterised by the fact that it includes at least one reverse osmosis filtration module (1, 26), at least one reservoir (2) for storing the effluents to be treated, a pump (4) for supplying effluents at high pressure to the module (1), at least one pipe (20, 43) for recovering the permeate, connected to the reverse osmosis module (1, 26), at least one pipe (7, 38) for recovering the concentrate, connected to the module (1, 26), at least one intermediate reservoir (14, 15) for storing the concentrate and a station (17, 40) for storing the concentrate in containers.
7. Treatment device according to Claim 6, characterised by the fact that it includes a plurality of reverse osmosis treatment modules (26, 26′) disposed in parallel with each other, between a line (3, 28) for supplying effluents to be treated and a line (7, 38) for discharging concentrate, and a recirculation pipe (8, 31) placed so as to by-pass the plurality of modules (26, 26′) connected, at one of its ends, to the pipe (3, 28) for supplying effluents to be treated and, at its other end, to the pipe (7, 38) for discharging the concentrate, on which a recirculation pump (10, 32) is placed.
8. Device according to either one of Claims 6 and 7, characterised by the fact that the pipe for recovering permeate (20, 43) is connected to a unit (41) for demineralizing the permeate.
9. Device according to any one of claims 6 to 8, characterised by the fact that it has at least two intermediate reservoirs for storing the concentrate (14, 15, 47a, 47b) connected to the pipe (7, 36) for discharging the concentrate.
10. Device according to Claim 9, characterised by the fact that the pipe (20, 42) for discharging the permeate is connected to the intermediate storage reservoirs (14, 15, 47a, 47b).

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