FR2545638A1 - Process for removal of an aqueous solution of radioactive nuclides - Google Patents

Abstract

Process for removal and storage of radioactive nuclides in aqueous solution. This solution is diluted with water originating from a porous and submarine geological formation 7 and, after dilution, the solution obtained is injected into the same submarine geological formation. Field of application: removal and storage of radioactive waste.

Description

 The invention relates to a method for removing radioactive waste. The invention relates more particularly to a process for the evacuation of high activity liquid waste, by dilution and confinement.

 Spent fuel from nuclear reactors can be reprocessed for the recovery of diurium (and plutonium if desired). The reprocessing operation gives a residue consisting of radioactive waste. The latter, formed during reprocessing, can be classified into three types: (1) high activity liquid waste (HLLw), which is aqueous waste containing almost all (more than 99%) fission products and actinides , with a small amount of uranium and plutonium not recovered during reprocessing; (2) liquid intermediate activity waste (ILLW), which is aqueous waste containing fission products in concentrations much lower than that of high activity liquid waste, and does not contain appreciable amounts of uranium or plutonium; and (3) low activity liquid waste (LLLw), which is waste requiring minimal treatment before being discharged to the outside.

The reprocessing of fuel from a nuclear power plant is described in the book Nuclear Technology, volume 43, pages 244-258 (mid-April 1979)
The disposal of high-level liquid waste poses a difficult problem for the nuclear energy industry to resolve, as some of the radioactive species present in this waste have a very long lifespan. nuclear waste discharges given in the following works are indicated for reference:
Science, Volume 197, pages 519-527 (August 5, 1979); Oceanus,
Volume 20, NO 1 (Winter 1977); Nuclear Technology, Volume 44, pages 284-296 (July 1979); ERDA-76-43, Volume 1-5 (May 1976). As indicated in the aforementioned works, cited by way of example, the current methods for disposing of high-activity liquid waste relate mainly to the concentration and containment of this waste.

The main example of concentration and containment techniques is the proposed system in which radioactive waste is coated with borosilicate glass or in a similar solid mass unlikely to be exhausted by water leaching. The solid mass is further placed in a container and placed in a location (such as a geological formation) in which it can be expected that the high activity liquid waste will not be disturbed and will not be in contact with the biosphere for However, this system results in the formation of a concentrated source of radioactive nuclides and the release of their decay heat, with the resulting high temperatures, has a deleterious effect on the surrounding geological formation and on the longevity of the container and the properties of resistance to drawdown by leaching of the solid mass of waste. The failure of the container and the solid mass then leads to a release of radioactive nuclides at a very high local concentration, greatly increasing the concentration gradient back to the biosphere through the geological formations. The invention relates to a method of disposal of radioactive waste, such as high activity liquid waste, which eliminates the disadvantages of concentration and containment techniques.

 The invention relates to a method for disposing of radioactive waste such as high activity liquid waste, which eliminates the drawbacks specific to the techniques, conventionally proposed, of concentration and containment of high activity liquid waste.

The invention instead uses an aqueous dilution of the radioactive waste, with water from a geological formation, and the confinement of the resulting dilute aqueous solution of the waste in the geological formation, preferably a deep formation under -marine, located offshore. The invention provides particular benefits and advantages for the disposal of high activity liquid waste, as described in more detail below.

 According to the invention, there is provided a process for discharging an aqueous solution of radioactive nuclides, which consists in: (a) producing water from a geological formation containing water, porous, extending almost horizontally, permeable to water and located below adsorbent sedimentary geological layers; (b) forming a dilute solution of the radioactive nuclides by mixing the water produced with the aqueous solution; and (c) injecting the dilute solution into the geological formation.

The geological formation chosen to produce both dilution water and to constitute an injection site intended to retain the diluted solution is preferably located in an underwater zone. Storage at sea, in a formation. underwater geology, allows evacuation with practically the assurance that the evacuated nuclides will not enter an aquifer from which. water is taken to be used on the surface or from which a natural flow goes towards surface places located on land. The geological evacuation at sea also ensures an additional high and immediate dilution, by sea water, of all nuclides leaving by migration from the formation used for evacuation
The dilute nuclide solution is preferably injected into a central injection site located in the formation chosen for evacuation, while water from the formation is produced simultaneously from several production sites arranged peripherally around the injection site. This ensures a dispersion of the diluted solution injected into the formation over a large horizontal section of this formation and the exposure of the nuclides to a large bed of adsorbent solids, in order to allow immobilization of the nuclides by ion exchange and sorption chemical of these nuclides.

 The invention will be described in more detail with reference to the attached drawing by way of nonlimiting example and in which the single figure is a diagram of a preferred embodiment of an installation implementing the method of the invention.

 The drawing represents a marine platform 1 which rests on the bottom 3 of the sea and which rises above the surface 5 of the sea. The platform is arranged above a horizontal geological formation 7. The formation 7 is porous and is horizontally permeable to water. It constitutes a reservoir for the formation of water. Adsorbent sedimentary geological layers 9 extend between an upper limit 11 of formation 7 and the bottom 3 of the sea. Water is withdrawn from formation 7 by means of several peripheral production wells, two of which are represented by 13 and 15. The production wells draw water into production regions such as regions 17 and 19 in the case of wells 13 and 15, respectively. The water produced by these wells 13 and 15 is directed to an installation 21 pumping and treatment. One or more injection wells such as well 23 extend from platform 1 to one or more injection sites such as injection region 25, which is centrally located in formation 7 relative to to regions 17 and 19 of production in which the peripheral wells of production take water. The injection well 23 comprises a central main injection conduit 27 used for injecting a dilute solution of radioactive nuclides, and it has an annular space formed by a casing 29 and the lower end of which is closed by a packer 31. Most of the water produced from it; formation, suitably treated if necessary, is transmitted from the treatment installation 21, via a pipe 33, to the central injection pipe 27. A relatively concentrated aqueous solution of radioactive nuclides (HLLW) is directed towards the platform 1 from a fuel reprocessing installation, using means of transport such as a specially constructed ocean-going barge 35, or by any other conventional and suitable means of transport. high activity liquid waste is pumped, through a line 37, from the barge 35 into the injection line 27 and is diluted in the line 27 with the treated water coming from the formation. The resulting diluted formation is forced back down, through the conduit 27, to a perforated section 39 of the casing 29 and it is injected into the formation 7. A sufficient quantity of treated water coming from the formation is transmitted, from the treatment installation 21 via a conduit 41, in the annular space located inside the casing 29, in order to maintain in this annular space a pressure greater than that prevailing in the conduit 27. This avoids any discharge of radioactive nuclides into the aiuiular space following a leak or a communication that can be established between the conduit 27 and the annular space. By producing formation water, coming from the peripheral regions 17 and 19 of production, at the same time as a dilute solution of radioactive nuclides is injected into the central region of injection, there is a risk of horizontal dispersion. radioactive nuclides injected into formation 7, towards the peripheral wells.

 The invention provides a means for the disposal of aqueous solutions of high-activity liquid waste and similar solutions containing radioactive nuclides, such as fission products and actinide ions. High activity liquid waste is generally in the form of an acidic aqueous solution, for example a nitric acid solution from the reprocessing of spent nuclear fuel. Other aqueous solutions of radioactive nuclides, for example solutions containing other acids or other solubilizing agents, can be removed by the process of the invention.

 According to the invention, high activity liquid waste, or a similar, relatively highly concentrated solution of radioactive nuclides, the concentration relating to the specific radioactivity (expressed in Ci / m), are substantially diluted. (a dilution of about 100 times is generally suitable for high-level liquid waste) with formation water taken from a selected geological formation, preferably underwater. The expression “formation water” designates any aqueous liquid present in the geological formation chosen. The water present in the formation may be of natural origin or it may be introduced artificially. For example, mobile fossil water is suitable, and water introduced for secondary oil recovery, for example, is also suitable. If the chosen formation contains petroleum, it is within the scope of the invention to produce a mixture of petroleum and formation water and to recover the petroleum until the first appearance of radionuclides at the production well. This can allow additional recovery of oil after full flooding during normal secondary recovery. Means are provided for treating the formation water produced, for example by filtration, elimination of bacteria, adjustment of pH, ion exchange or addition of other agents, before mixing with high activity liquid waste. The treatment can be suitably carried out according to conventional techniques used to treat the injection water used in the secondary recovery of petroleum, with the additional requirement of preventing any precipitation of the radioactive aws when mixing the liquid waste d activity and formation water. An important advantage of diluting high activity liquid waste with large amounts of water is that the concentration (Ci / m3) of radioactive nuclides is significantly reduced, which has the effect of reducing the content (kW / m3) in heat of disintegration of high activity liquid waste before its storage in the geological formation. An exact importance of the dilution of the high activity liquid waste with the formation water is not critical for the process of the invention. Preferably, the dilution is sufficient to lower the concentration of iodine 129, in the resulting diluted solution at its maximum admissible concentration (MAC), in microcuries per milliliter (Ci / ml), in soluble form, above the natural background of the water in non-limited areas, i.e. 'equivalent to potable water. The CMA for for I-129 is specified by the "U.S. Nuclear Regulatory Commission" (Title 10, 10 CFR 20, Appendix B).

 Iodine 129 has the longest period (16 million years) of all the fission products and actinides usually present in highly radioactive wastes such as high activity liquid wastes. Iodine 129 is considered to be the most mobile of all radioactive nuclides, with regard to movement through the geological layers, since it is present only in anionic form and is therefore less subject to chemical sorption and to an ion exchange in such layers. If the high activity liquid waste to be removed is diluted with CMA of iodine 129 before storage, and even in the case where iodine 129 ends up leaving by migration of the porous formation in which it has been confined, to pass into other geological layers where it risks encountering other water flows (or else to pass directly into sea water), it would be emitted in these flows at a concentration equal to or less than its CMA.

 The magnitude of the dilution chosen to reduce the diode 129 concentration may advantageously be sufficient to bring some of the other long-lived radioactive nuclides to lower blue concentrations than their respective CMAs while other nuclides may have concentrations exceeding their CMAs.

However, it is very likely that all of these other radioactive nuclides will be immobilized by ion exchange and chemical sorption in geological media. It is expected that all. the radioactive nuclides injected into the chosen confinement formation remain in this formation, but if other nuclides than iodine 129 were to. However, they would be immobilized by sorption in a large thickness of sedimentary geological layers covering the formation in which the nuclides are injected.

 The dilute nuclide solution resulting from the mixture of high activity liquid waste and formation water is injected into a suitable geological formation, preferably underdmarine. A training must have several properties to be chosen as an evacuation or storage site suitable for the process of the invention. The chosen formation must contain the necessary quantity of mobile water. It must extend horizontally over distances of the order of several kilometers. The combination of the horizontal extent, the vertical extent and the porositd must be such that the formation has a sufficient volume to allow the injection of a practical quantity of diluted radioactive solution and to give a practical quantity of formation water intended for the dilution of the liquid wastes of high activity, initially concentrated. Typical values suitable for the vertical extent are 5 to 10 meters or more and for the porosity, from 10 to 20% or more. In addition1 the formation must have a substantial horizontal permeability to water to allow easy removal of formation water for dilution, from a limited number of peripheral production wells, and for easily accepting the diluted solution of radioactive waste, under a pressure and at a reasonable flow rate, by one or a few centrally arranged injection wells.

 The upper limit of a geological formation suitable for the disposal of radioactive nuclides must be between several hundred and several thousand meters below the surface which is preferably the sea floor extending above of this training. The intermediate cover includes various geological layers. In general, some layers are substantially waterproof and other layers are highly adsorbent, as is the case with sedimentary deposits. A preferred geological formation for the disposal or storage of nuclides may be, for example, a spent or partially depleted petroleum reservoir containing mainly water.

 The formation water used to dilute the radioactive nuclide solution is produced from at least one production site or production region located in the formation of? evacuation or storage. The production sites are generally spaced horizontally from the dlinJecJc.ion site of radioactive nuclides. Preferably, the formation water is produced from several production sites spaced horizontally and peripherally around the injection site. At least 4 to 10 peripheral production wells are preferably used with each injection well. By producing the formation water peripherally, a uniform radial dispersion of the dilute solution of radioactive nuclides is promoted.

 The preferred embodiment described below constitutes a preferred embodiment of the invention. It goes without saying that many modifications can be made to this preferred embodiment of the invention, given without implied limitation.

 A preferred embodiment of the invention may be more appropriately described with reference to the accompanying drawing to illustrate an installation designed to dispose of or store the high activity liquid waste from a nuclear fuel reprocessing installation having a reprocessing capacity of 10 tonnes of heavy metal (uranium, pluto nium) per day, for 300 days per year, over an injection period of 10 years (this is equivalent to the reprocessing of nuclear fuel required to operate approximately 66 nuclear power plants 1000 mol, or approximately the capacity of all nuclear power plants currently in service in the United States of Amdriaue). The expected volumetric flow rate of high activity liquid waste produced by reprocessing 10 tonnes / day of spent nuclear fuel is, in accordance with the preferred embodiment of the invention, of approximately 61.5 m3 / day and comprises approximately 60 m3 / day of non-concentrated waste, of high activity, and approximately 1.5 m3 / day of concentrated waste of low activity. This high activity liquid waste releases up to 1.31 kW / m3 of radioactive decay heat (1.4 years after leaving the reactor). According to the invention, the high activity liquid waste is diluted 100 times by mixing with 6150 m3 / day of formation water produced, treated if necessary to prevent the formation of precipitates of radioactive nucleides during mixing. On the basis of previous seismic and other exploratory determinations, including the sampling of the layers and fluids of the formation, the platform 1 is installed at sea. The central injection well 19 is drilled in carefully collecting the cores and fluids from the formation to identify the depth and vertical extent of injection formations that may be desirable.

 The preferred formation 7 is found at a depth of, for example, 1600 meters below the sea bottom; it has a vertical extent of approximately 8 meters, a porosity of approximately 25% and a horizontal permeability to water of at least 200 millidarcies.

The casing 29 is installed in the well 23 to a depth corresponding to a level located just below the bottom of the formation 7. The casing 29 is perforated on the vertical height of the formation 7 (perforated section 39). The natural hydrologic flow and direction of flow through the perforations are measured to verify that formation 7 is not part of an active aquifer. Ten peripheral production wells, two of which are indicated in 13 and 15, are produced by directional drilling from platform 1 to cross formation 7 in order to form production sites 17 and 19 located at a certain distance. The radial distance is chosen to give formation 7 a pore volume equivalent to the volume of high activity liquid waste, increased by the volume of formation water used for dilution, over time. 10 years (3000 days) from the injection period, this volume value being preferably increased by 20 g to take account of an incomplete effect of scanning and displacement. For example, if formation 7 has a vertical height of 8 meters and a porosity of 25%, the radial distance between the central injection site and the peripheral production sites should be equal to approximately 1890 meters. production peripherals are cased and suitable pumping means are installed so that each production well gives 615 m3 of formation water per day. The formation water is treated, if necessary, in a treatment device 21, then it is introduced into the injection pipe 27, passing through the pipe 33, except for the flow of water which it can enter. necessary to introduce through the conduit 41 into the annular space located inside the casing 29 to maintain a relatively high pressure. If it turns out to be impossible to maintain a complete recycling of the formation water at the desired flow rate advantageously, via the injection well 23, other injection wells, which are centrally arranged and which are identical to the well 23, can also be used. After an appropriate flow of formation water flow has been established in the injection pipe 27, the high activity liquid waste can be introduced into this stream via the pipe 37. The solution diluted passes through the perforations of the section 39, moves radially through the formation 7 in the direction of the sites 17 and 19, and disperses inside the formation 7. The estimated increase in temperature of the formation 7 near the injection site, towards the end of the 10-year injection period, should not exceed 90 C.

 The fluids discharged from the production wells are monitored using conventional radioactivity indicators. If a radioactive breakthrough is indicated from the injection site to any of the production wells, that well is internally cemented.

Claims (6)

 1. Method for discharging an aqueous solution of radioactive nuclides, characterized in that it consists in producing water from a porous geological formation, containing water, extending slightly near horizontally, permeable to water and located below adsorbent sedimentary geological layers (9), to form a dilute solution of said nuclides by mixing water produced with said aqueous solution, and to inject the dilute solution into the formation.  2. Method according to claim 1, characterized in that the formation is an underwater formation.  3. Method according to claim 2, characterized in that the upper limit of the formation is located at least 1600 meters below the bottom (3) of the sea.  4. Method according to claim 1, characterized in that the formation comprises a partially or entirely spent oil tank.  5. Method according to claim 4, characterized in that petroleum is produced from said formation, in mixture with the formation water from which it is separated.  6. Method according to claim 1, characterized in that the formation water is produced from several production sites (17, 19) located in said formation, these production sites being spaced horizontally and roughly peripherally around d an injection site (25), and the diluted solution being injected into this injection site.