Classifications

• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21F—PROTECTION AGAINST X-RADIATION, GAMMA RADIATION, CORPUSCULAR RADIATION OR PARTICLE BOMBARDMENT; TREATING RADIOACTIVELY CONTAMINATED MATERIAL; DECONTAMINATION ARRANGEMENTS THEREFOR
  • G21F9/00—Treating radioactively contaminated material; Decontamination arrangements therefor
  • G21F9/04—Treating liquids
  • G21F9/06—Processing
  • G21F9/16—Processing by fixation in stable solid media
  • G21F9/162—Processing by fixation in stable solid media in an inorganic matrix, e.g. clays, zeolites
• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21F—PROTECTION AGAINST X-RADIATION, GAMMA RADIATION, CORPUSCULAR RADIATION OR PARTICLE BOMBARDMENT; TREATING RADIOACTIVELY CONTAMINATED MATERIAL; DECONTAMINATION ARRANGEMENTS THEREFOR
  • G21F9/00—Treating radioactively contaminated material; Decontamination arrangements therefor
  • G21F9/28—Treating solids
  • G21F9/34—Disposal of solid waste
  • G21F9/36—Disposal of solid waste by packaging; by baling

Definitions

• the present invention concerns a process for the treatment of the decladding wastes, some medium level Wastes and the high level wastes produced by plants reprocessing irradiated nuclear fuels.
• the invention concerns a process for the insolubilisasion and self-solidification of the eflluents considered, after evaporation of the excess water. This process is simpler than those previously envisaged for the treatment of these particular efiluents.
• the object of the invention is to provide a process for the treatment, in the same operation, of the different types of solutions which constitute hese categories of effiuentsa process which has not been achieved before now.
• the invention enables a solid compact mass to be obtained which lends itself particularly well to:
• This covering or containment acts as a barrier against leaching of the radioactive components contained in the solid mass.
• the covering of the solid considered under (a) makes it possible to avoid dispersing the radioactive solids in an inert mass which serves at the same time as 'binder and fixing agent preventing leaching.
• the process according to the invention also achieves a large volume reduction of the solids to be stored compared With that achieved by the known dispersion methods, proposed notably for slurries with specific activity inferior to that considered here, since in these known techniques the volume of active material incorporated in the inert mass represents only 40-50% of the total volume to be stored.
• the process according to the invention does not require any operation at high temperature; this considerably reduces the risks of accelerated corrosion due to the chemical nature of the solutions treated, as well as the risks of volatilising certain radioactive species. It is possible, therefore, to make use of the present-day construction materials and to avoid complex and delicate installations for the purification of gases and vapours produced by the operation. Because of this, the process according to the invention represents a notable simplification compared with other techniques which have been envisaged, such as calcination of the efiiuents or their incorporation in vitrified masses.
• nuclear fuels are essentially made up of a can of a weakly neutron-absorbing material, generally aluminium, magnesium, stainless steel or zirconium and its alloys such as Zircaloy, and of an uranium core in the form of metal or oxide. After irradiation in a reactor, the core contains some plutonium and fission products in addition.
• Chemical decladding uses selective agents for dissolving the canning; these are caustic soda for aluminium, sulphuric acid for magnesium or stainless steel, and ammonium fluoride for zirconium or Zircaloy.
• These decladding solutions constitute a very large part of the volume of medium level wastes produced by a reprocessing plant, as they contribute from 0.5 to 10 m. per ton of uranium treated.
• the activity contained in these solutions arises partly from the fission products which passed into the solution in the course of decladding, and partly from the activation of certain elements contained in the canning material.
• the activity level measured in these solutions can reach 10 Ci/m. depending on the irradiation conditions and the cooling time of the fuel processed.
• the other medium level wastes considered in the process according to the invention are roduced essentially in the course of the third stage of reprocessing. They are made up of the concentrates resulting from the evaporation of different wastes of lower specific activity. Their composition is rather variable around the following values:
• Main component NaNO lO400 g./l.
• Quantity 5 m. /tonne U processed.
• the high level wastes are also produced during the third stage of the reprocessing. They contain essentially the bulk of the fission products as well as a certain number of other salts originating in the reagents added during processing and from corrosion of the equipment. At the end of the process, they are usually concentrated by evaporation. These high level wastes have variable compositions, which can be summarized as follows:
• Fission and corrosion products $100 g./l.
• Compounds originating in the reagents 120 g./l.
• HNO 11 M 5+) activity Ci/m.
• Quantity 1 m. /t.
• Processes (a) and (b) present very serious technological and economic drawbacks due to the quantity of material to be treated; the salt content on the extremely corrosive nature of the solutions, the volatilsation of certain fission products, the operating temperatures and the risks of explosion involved.
• Method (c) is applicable only in regions with a particular geological structure, and only a limited number of these exist.
• the techniques classed under (e) involve fire and explosion risks, as well as risks of radiation damage to the 4 sheathing material, and above all the volume reduction is insufficient.
• the invention concerns a process for the solidification of radioactive wastes containing either water-soluble fluoride or a water-soluble mixture of fluoride and sulphate, possibly in the presence of a maximum of about 1 molar equivalent of nitrate for 3 molar equivalents of fluoride and/or sulphate.
• the process is characterized:
• the quantity of lime used being at least equal to that necessary to neutralize the free sulphuric acid and/ or to precipitate the free fluoride
• the first stage of the process according to the invention consists of precipitating the fluorides and sulphates from the decladding solutions with lime according to well known techniques.
• the normal treatment to apply would be the separation of the fluoride and sulphate precipitate, by filtration for example. This would lead to a powdery product diflicult to handle, from which the radioactive components could be easily leached. In addition, a large quantity of the radioactive substances would be left in the large volume of mother liquor, which would require a subsequent treatment.
• the second and third stages of the process according to the invenution therefore consist of distillation or evaporationof the water from the mixture until a paste containing 10 to 20% by weight of water is obtained, which is then allowed to harden to a dry cake.
• the leachability of the cake obtained by this process is then reduced to a very small amount (less than 2.10- g./cm. day) by covering the hard cake with an impermeable and insoluble material, which acts as a barrier between the cake containing the radioactive salts and the environment.
• This covering is the fourth step of the process according to the invention.
• An effective coating can be obtained by:
• Covering materials other than bitumen for example hardenable resins, cement, etc. can be considered.
• the block can be placed in a leak-tight metal cask of corrosion resistant alloy. In this case, the risk of leaching the block by external agents is non-existant.
• the principal advantage of the process according to the invention lies in the fact that the phenomenon of solidification of the concentrated slurry takes place over a large range of waste compositions to be treated. This property is quite unforeseen, particularly for solutions with a low sulphate content.
• the mechanism of solidification of a slurry obtained by the process described in the present invention can very well be explained for solutions containing only sulphates, because it is similar to the solidification of plaster of paris, although the preparation of this is completely different.
• the innovations of the present invention lie in the fact that it is not necessary to pass through a stage of forced drying as in the preparation of plaster of paris, and that it is possible, at the same time, to solify fluoride solutions from the dissolution of zirconium and Zircaloy, although it has been established that calcium fluoride crystallizes with no molecules of water of crystallization.
• the solidification process is not disturbed by the presence of soluble salts, such as MgSO NaNO etc. This allows great latitude in the choice of volumes of solutions for solidification, as is shown, for example, by the following table:
• the incorporation and the solidification of the high level wastes is carried out either after neutralization of the free nitric acid by Ca(OH) CaO, NaOH, etc. or after partial destruction of this acid by a reducing agent, such as sugar or formaldehyde, followed by neutralization of the remaining acid.
• a reducing agent such as sugar or formaldehyde
• This process according to the present invention can be applied to the final treatment and storage of radioactive waste solutions, and particularly to the treatment of decladding waste solutions, since the solid cakes obtained, suitably covered with an impermeable, insoluble material and placed in an appropriate containment, such as inexpensive casks, either can be stored in the open with no protection than an enclosure preventing entrance to the radiation zone, or can be thrown into the sea. In this case, the long-term storage costs are reduced to a minimum. For high level wastes, storage should probably be carried out for a certain period under shelter with forced ventilation to ensure the dissipation of the heat released by fission product decay.
• EXAMPLE 1 265 g. of lime (technical grade containing about 90% Ca(OH) are added in -20 minutes to 1.500 cm. of Zircaloy decladding waste solution with a specific activity of 1 Ci/l. and mixed by a radial flow turbine type agitator. After distillation of 1.150 cm. of water, the slurry obtained is cooled in a detachable cylindrical mould. A lifting hook is placed in the viscous mass, which is allowed to cool in air. After one hour, the paste has changed into a hard cake. After the cylindrical mould has been dismantled, the block is plunged, by means of the hook, into two successive baths of hot bitumen, the first at 180 C., the second at 130 C.
• the coated block is then placed in a bath of distilled water kept between 21 C. and 30 C. for a period of days, the :water being renewed every 12 hours.
• the 80 litres of water used in this test are concentrated to 1 litre, and no activity is detected in the water.
• the apparent density of the block, after it has been kept for 2 hours at 110 C. is 1.5 g./cm. and the volume reduction obtained (volume of initial solution/ volume of block) is 3.3.
• EXAMPLE 2 133 g. of technical grade lime are mixed in 10 to 20 minutes with 500 cm. stainless steel decladding solution containing 3 M free H 50 and 5 Ci radioactivity/l, in the way described in Example 1. 350 cm. water are distilled and the resulting slurry treated as described in Example 1. The paste hardens in 45 minutes. No activity is detected in the water used for the leach tests. The apparent density of the block is 1.5 g./cm. and the volume reduction obtained is about 2.
• EXAMPLE 3 500 cm. magnesium decladding solution with a specific activity of 300 mCi/l. are treated with 46 g. of technical grade lime as in the previous examples. After evaporation of 425 cm. of water, the paste hardens in 50 minutes. No activity is detected in the water used for leach tests on the block after it has been coated with bitumen. The apparent density of the block is 1.4 g./cm. and the corresponding reduction is 4.
• EXAMPLE 5 500 cm. magnesium decladding solution identical to that of Example 4, are treated with 26 g. technical grade lime. Then 468 cm. aluminum decladding solution iden tical to that used in Example 3 are added. After evaporation of 700 cm. water, the paste hardens in 75 minutes. No activity is detected in the water used for leach tests. The apparent density of the block is 1.2 g./cm. and the corresponding volume reduction is about 4.5.
• EXAMPLE 6 250 cm. stainless steel decladding solution (see Example 2) are mixed with 250 cm. magnesium decladding solution (see Example 4), the neutralized with g. technical grade lime. After evaporation of 350 cm. water, the paste hardens in 1 hour. No activity is recorded in the leach test water. The apparent density of the block is 2.2 g./cm. and the volume reduction is 4.3.
• EXAMPLE 7 250 cm. stainless steel decladding solution (see Example 2) mixed with 250 cm. decladding solution (see Example 4) are neutralized with 80 g. technical grade lime. Then 234 cm. aluminum decladding solution (see Example 3) are added. After evaporation of 550 cm. water, the paste hardens in 80 minutes. No activity is detected in the leach test water. The apparent density of the block is 2, and the corresponding volume reduction is 3.3.
• EXAMPLE 8 350 cm. stainless steel decladding solution (see Example 2), after neutralization With 80 g. technical grade lime, are mixed with 1.500 cm. Zircaloy decladding solution (see Example 1). After evaporation of 1.570 cm. water, the paste hardens in 80 minutes. The activity detected in the water from the leach tests corresponds to 10 ppm. of the total activity involved in the test. The apparent density of the block is 1.6 g./cm. and the volume reduction is 6.5.
• EXAMPLE 9 550 cm. stainless steel decladding solution (see Example 2), after neutralization by 138 g. technical grade lime, are mixed with 300 cm. aluminum decladding solution (see Example 3) and 1.500 cm. Zircaloy decladding solution. After distillation of 2.000 cm. water, the paste hardens in 90 minutes. The activity recorded in the water from the leach tests corresponds to 25 p.p.m. of the total activity involved in the test. The apparent density of the block is 2 g./cm. and the volume reduction is 7.4.
• EXAMPLE 1 1 250 cm. stainless steel decladding solution (see Example 2) added to 50 cm. magnesium decladding solution (see Example 4) are neutralized by 72 g. technical grade lime. After addition of 856 cm. concentrate with a specific activity of 10 Ci/l. and 300 cm. Zircaloy decladding solution (see Example 1), 1.100 cm. water are distilled off. The paste hardens in 130 minutes. The activity detected in the water from the leach tests corresponds to 135 p.p.m. of the total activity incloved in the test. The apparent density of the block is 2.1, and the volume reduction is 4.7.
• EXAMPLE 12 250 cm. stainless steel decladding solution (see Example 2), added to 50 cm. magnesium decladding solution (see Example 4), are neutralized by 72 g. technical grade lime. After addition of 120 cm. high level waste (100 ci/l.) previously partially neutralized by 43 g. technical grade lime and 300 cm. Zircaloy decladding solution (see Example 1), 550 cm. water are distilled off. The paste hardens in 20 minutes. The activity detected in the water from the leach tests corresponds to 85 p.p.m. of the total activity. The apparent density of the block is 1.8 g./cm. and the corresponding volume reduction is 3.3.
• EXAMPLE 13 250 cm. stainless steel decladding solution (see Example 2) and 50 cm. magnesium decladding solution (see Example 4) are neutralized by 72 g. technical grade lime. After addition of 856 cm. of concentrate (see Example 11) and 300 cm. Zircaloy decladding solution (see Example 1), 120 cm. high level waste are added (see Example 12), the acidity of which, previously reduced to 0.5 M HNO by treatment with sugar, has been completely neutralized by 3 g. technical grade lime. After distillation of 1.250 cm. water, the resulting paste hardens in 65 minutes. The activity detected in the water from the leach tests corresponds to 115 p.p.m. of the total. The apparent density of the block is 1.9, and the volume reduction is 4.6.
• EXAMPLE 14 250 cm. high level waste solution (see Example 13) mixed with 125 cm. 12 M sulphuric acid are reduced by distillation to 250 cm. To this concentrate is added 250 cm. stainless steel decladding solution (see Example 2) and the whole is neutralized by 176 g. technical grade lime. After addition of 300 cm. Zircaloy decladding solution (see Example 1), 600 cm. water are distilled off. The paste hardens in 35 minutes. The activity detected in the leach test water corresponds to p.p.m. of the total. The apparent density of the block is 2 g./cm. and the volume reduction is 4.9.
• EXAMPLE 15 cm. 12 M sulphuric acid are added to 500 cm. high level waste solution (see Example 13). The mixture is evaporated down to a volume of 250 cm. Then 80 g. technical grade lime are added. 175 cm. water are distilled off. The paste hardens in 30 minutes. The ac tivity detected in the leach water corresponds to p.p.m. of the total. The apparent density of the block is 1.7 g./cm. and the volume reduction is about 4.
• a process for solidifying radioactive wastes containing at least one member selected from the group consisting of water-soluble fluoride, water-soluble sulphate and mixtures thereof comprising mixing with agitation at least one liquid radioactive waste with sufficient lime to neutralize the free sulphuric acid and to precipitate the free fluoride, evaporating the water present in the mixture until a hardena'ble paste is obtained, hardening said paste and enclosing the resulting block in a water-tight covering.
• sufiicient water is evaporated to obtain a paste containing from about 10 to about 20% by weight of Water.
• impermeable material is selected from the grou consisting of bitumens, asphalts, parafiins, and hardenable resins.
• waste contains nitrate in a concentration of about 3 molar equivalents per molar equivalent of fluoride and sulphate.
• medium level waste is added to the decladding waste, said medium level waste having an activity of from about 10 to about 4 times 10 Ci/m.

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• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• General Engineering & Computer Science (AREA)
• High Energy & Nuclear Physics (AREA)
• Environmental & Geological Engineering (AREA)
• Chemical & Material Sciences (AREA)
• Inorganic Chemistry (AREA)
• Processing Of Solid Wastes (AREA)

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Cited By (17)

Families Citing this family (3)

• 1966
  • 1966-04-07 BE BE679231D patent/BE679231A/xx unknown
  • 1966-11-18 GB GB51682/66A patent/GB1116319A/en not_active Expired
  • 1966-11-18 US US595325A patent/US3557013A/en not_active Expired - Lifetime
• 1967
  • 1967-01-03 NL NL6700070A patent/NL6700070A/xx unknown

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