FR2579360A1 - METHOD AND DEVICE FOR THE CONDITIONING, BY HYDRAULIC BINDERS, OF LOW AND MEDIUM ACTIVITY RADIOACTIVE EFFLUENTS - Google Patents

Abstract

THE PRESENT INVENTION CONCERNS A NEW PROCESS FOR PACKAGING, BY HYDRAULIC BINDERS, BORATE RADIOACTIVE EFFLUENTS OF LOW AND MEDIUM ACTIVITY BY CHEMICAL TREATMENT AIMED AT THE PREPARATION OF CALCIUM BORATE TETRAHYDRATE CRYSTALS WITH REDUCTION OF THE VOLUME AND MIXING LESS A HYDRAULIC BINDER, CHARACTERIZED IN THAT THE VOLUME REDUCTION OPERATION IS CARRIED OUT BY EVAPORATION, UNDER REDUCED PRESSURE, OF SOLUTIONS AND OR SUSPENSIONS, THE SAID REDUCED PRESSURE BEING BETWEEN ABOUT 0.1 AND 0.3 BAR AND APPLIED AFTER THE PHASE FOR PREPARATION OF CALCIUM TETRAHYDRATE BORATE CRYSTALS.

Description

Method and device for packaging, by hydraulic binders,

  radioactive effluents of low and

average activity.

  The present invention relates to a method and a device for the packaging, by hydraulic binders, of low radioactive effluents and

average activity.

  Hydraulic binding of low and intermediate activity radioactive effluents has long been practiced in the nuclear industry because of the currently accepted simplicity of the processes;

  corresponding, and the low cost of the coating material.

  It is recalled that such packaging must make it possible to manufacture, from a given chemical effluent, a storable concrete in a given environment and regulatory context, said manufacture having to have

  take place in a nuclear context and at the least cost.

  The known methods for carrying out such a packaging generally comprise the following steps: a chemical pretreatment passing the effluent

  radioactive form to be conditioned in a

  tonnable (solid dispersed in an aqueous phase) - a reduction of volume of the treated product - and a mixing of the product obtained with a quantity

  Adequate binder to get the concrete.

  For the treatment of borated effluents, particularly from PWR plants; we have already advocated - a continuous system based on a reduction in volume

  by evaporation at atmospheric pressure and the use of mala-

  horizontal dimensions; such a system is particularly suited to large capacities and important activities; a discontinuous system in which the volume reduction is carried out by combining a liquid / solid separation and an evaporation of the liquid phase, and the use of industrial mixers with vertical axes; such a system is more particularly adapted to the treatment of diluted solutions in

fixed installations.

  The present invention relates to a particular mode

  implementation of packaging systems

  discontinuously, said embodiment being particularly adapted to the use of stations

mobile and transportable.

  The process according to the invention comprises a pretreatment, a volume reduction and a kneading is essentially characterized in that: the volume reduction operation is carried out by evaporation, under reduced pressure, solutions and / or suspensions - and in this that all operations are done in

  one or two or more mixer-mixer devices

evaporator (M.M.E.).

  The pretreatment implemented in the invention

  This principle is known in principle since it consists, after a possible neutralization of the effluent so as to bring the Na / B molar ratio to be close to 0.25, to add lime (in solid form or in solution). so as to target the subsequent precipitation of B203 CaO4H20. It is known that the precipitation of this calcium borate takes place in four stages: the first consists, after the addition of lime, in the formation, at a temperature of the order of C, of calcium salt seeds; during this step, the mixture substantially maintains a constant viscosity - the second which follows the first, and which takes place at the same temperature, consists in the formation of mixed salts, which results in a significant progressive increase in the viscosity of the mixture. mixing, the third, which follows the second, consists in forming, at a temperature of the order of -50 ° C., crystals of calcium borate at 4.20 hours; this formation of crystals results in a decrease in the viscosity of the mixture (P2) - the fourth, which succeeds the third, consists in the ripening of the crystals formed; this curing also carried out at a temperature of 40-50 ° C. is in fact constituted by a progressive magnification of the crystals (P3) It has been found and this constitutes another aspect of the present invention - on the one hand that the reduction operation volume of

  evaporation under reduced pressure can not

  under no circumstances be carried out during the second and third stages of calcium borate precipitation; said reduction of volume can therefore only take place at this stage of the packaging process during the first and / or fourth step of

the precipitation operation.

  and on the other hand, that, in order to save time, the formation of calcium borate tetrahydrate could be obtained by passing through the calcium borate hexahydrate; to do this the third step of the precipitation is carried out at a temperature between 10 and 30 C and it is only then (fourth step) that the temperature of the mixture is raised to -50 C; in this case there will be simultaneously, in this fourth step, formation of calcium borate tetrahydrate from calcium borate hexahydratg,

  magnification of crystals of calcium borate tetra-

  hydrated and, preferably, reducing the volume of

  mixture by evaporation under reduced pressure.

  By the foregoing explanations, it is understood that the mixture obtained at the end of crystallization may comprise calcium borates having several stages of hydration. Therefore, when in the present application, it is indicated that calcium borate tetrahydrate is prepared, it should be understood that we will aim at the preparation of this product which is revealed

  the most stable during packaging and under normal conditions.

  storage of the conditioned product, but that other calcium borates (eg with 2.6 or 9 water molecules) may be present. However, it is generally possible to prepare a mixture of calcium borates in which

  the borate tetrahydrate represents at least 90% of the borates.

  It has been seen that the volume reduction operation by vacuum evaporation could take place, (and was preferably carried out to achieve a maximum of

  save time) during the fourth stage of the

  tation. It was also indicated that this same operation could take place, if necessary, during the first stage of the precipitation and this is done when

the effluents are diluted.

  But the volume reduction operation can also

  take place before the crystal operation begins

  and must continue after the fourth step of the crystallization process to obtain the best volume reduction. It should be remembered that the concreting (addition of cement) is subjected to an aqueous mixture containing between

and TO% dry extract.

  In view of this objective and the various stages at which the volume reduction operation can be applied by evaporation under reduced pressure, the following rules and possibilities can be stated: 1. if the starting effluent has a corresponding boric acid concentration at about 200 to 250 g / l; it is possible not to reduce the volume before the fourth

  step of the crystallization operation.

  The reduced pressurization of the mixture for the purpose of volume reduction will therefore be carried out only during said fourth step and may be extended beyond said step if necessary. 2. if the starting effluent has a boric acid concentration of less than about 200g / l,

  experience has shown that it is then desirable

  table to also perform a volume reduction during the first phase of the crystallization operation and / or before said first phase. This reduction in volume can indeed be undertaken on the solution (or suspension) of effluent received, before any lime addition to said effluent, the advantage of operating this reduction in volume at this time of the procedure is that we can reduce the pressure on a liquid heated to a temperature

greater than 60 C.

  In any case, and whatever the treatment stage where the volume reduction is carried out, it is understood that this reduction is carried out by placing the solution or the suspension at a pressure of between 0.1 and 0.3. bar at the temperature of said solution or

  suspension at said phase. It is easy to understand that

reading the invention it is possible

  - to aim at the preparation of a suspen-

  concreting at about 30% by weight of dry matter

  - to aim at the preparation of a suspen-

  concretely containing up to about 70%

  by weight of dry matter, which will give rise to

  to a lower concrete volume.

  An essential advantage of the invention is that all operations can be carried out in a single apparatus and possibly for questions of convenience and volumes treated in two appliances of the same type provided that a blender-mixer-evaporator is provided which is provided with ; a device (double jacket for example) for maintaining the temperature of the product to be treated at a value determined in advance between

and about 100 C.

  an effective stirrer having preferably

  several agitation regimes and

  can be used with fluids (or sus-

  pensions) with very different viscosities

  rents (these different viscosities being

  in the crystallization period and in the concreting period) - and a device making it possible to produce in the apparatus a reduced pressure

  from 0, there 1 bar. It is clear that, if the operation

  volume reduction is carried out by causing the pressure to be between about 0.1 and about 0.3 bar, it is possible and sometimes desirable, for reasons of confinement, to carry out all or part of the other operations while operating under a slight depression, for example at 0.7 or

0.9 bar.

  This type of apparatus is marketed for example by the company GUEDU (France), the principle

  mixers used in the mixer-dryers corresponding to

  having been applied to the pharmaceutical industry

  and fine chemicals (kneading, chemical treatments,

  vacuum poration) as in the field of civil engineering since

many years.

  The paste obtained at the end of the operation of reducing the volume by evaporation under reduced pressure,

  is mixed, after cooling, with a hydraulic binder

  lic. Said dough may have previously been fluidized, either by acidification at pH <10, or by addition of a

  fluidifying, either by adding an untreated effluent.

  The composition of the concrete obtained (solid / water / binder) is set by the following specifications, for a given hydraulic binder: consistency (function of

  the amount of liquid in the dough) and mechanical strength

  nique (function of the binder / water ratio). The dry extract of the overconcentrated paste (solid) (solid + water) is thus fixed by these two criteria, and will give the volume

condensate to obtain.

  The type of binder to be used will depend on the specifications of the final product and will commonly be: - Portland cement - Blast furnace slag cement - Semi-binding additives cement (pozzolans, fly ash ...) Typical performances proven both at the laboratory scale and at the pilot scale, inactive or active, are as follows: Volume reduction Up to 420 g H3B03 per liter of concrete, ie a reduction in volume (effluent / concrete ) of 3.5 for an initial effluent

at 21,000 ppm boron.

  quality of the mix (based on 420 g H3B03 / 1 concrete) compressive strength> 200 bar after 28 days wet air cure time 30. water resistance: after 3 months immersion, no modification of the mechanical strength, volume variation 1% leaching: Co 60: <10-6 cm / day <104 cm / day The following nonlimiting examples illustrate the invention.

EXAMPLE 1

  Sodium neutralized boric acid is treated in a molar ratio Na / B = 0.26 and 240 g / l in boric acid. It is assumed that the production of concentrates for a PWR 900 MWe is 2.5 m 3 / year. A mobile packing station is

  equipped with a GUEDU type 2500 dry type feeder,

  cited useful 1600 about 1 and about 10 m2 of surface

total exchange.

  The evaporation capacity under the conditions of use of the process is of the order of 80 kg / h. 1.6 m3 of borated effluent is fed into the apparatus, then

  lime is introduced in a molar ratio Ca / B = 0.5.

  The precipitation reaction is carried out generally in 5 hours, the speed of rotation of the kneader being slaved to the measurement of a vibrating needle viscometer. This results in a suspension of insoluble calcium borates, mainly in CaO, B 2 O 3, 6H 2 O form, the reaction yield being 85%. The dry extract at 60 C of the

  obtained solution is about 32%.

  The suspension obtained above is then overconcentrated at an absolute pressure of 0.25 bar at approximately 60 ° C. to a solids content of 45%. Mainly obtained tetrahydrate CaO, B203, 4H20 which is stable. The operation lasts about 7 hours. After cooling, CLK cement is added, such that the free water / cement ratio is 1.45. After 11 min of mixing, 1160 l of density concrete 1, 625 are obtained. The total duration of the operating cycle is approximately 13 h.

  Volume reduction: 1.38 (effluent volume / concrete volume).

  Processing the annual production of a slice:

hours (in equivalent).

EXAMPLE 2

  Sodium-neutralized boric acid is treated in a Na / B molar ratio of 0.25 and 120 g / l.

in boric acid.

  In a GUEDU 2500, 1600 l of initial solution is fed, and evaporated under

  vacuum up to 240 g / l at a pressure of 0.1 bar absolute

  read, gradually feeding the initial solution

  in order to keep the volume constant.

  The evaporation capacity under these conditions is 120 kg / h. After treatment of 3200 l of initial solution, 1600 l of 240 g / l solution of acid is obtained.

  boric, the operation lasting about 14 hours.

  The concentrated solution obtained is then treated as in Example 1. In this second example, the total duration of an operating cycle is of the order. of

27 hours.

EXAMPLE 3

  Boric acid is treated at 120 g / l.

  An installation is used with two GUEDU tumble dryers: a type 2500, capacity 1600 1 and exchange surface 10 m, and type 820,

  500 1 capacity, 5 m2 of exchange area.

  1.6 m3 of boric acid solution is fed into the 2500 type dough mixer and neutralized with sodium hydroxide in a molar ratio Na / B = 0.26,

  lime is added in a molar ratio Ca / B = 0.5.

  Phase P1 of pretreatment and evaporation are then simultaneously carried out under an absolute pressure of 0.25 bar. The evaporation capacity under these conditions is about 90 kg / h. This phase lasts approximately 2 h 40 min. Then we stop evaporation

  during the P2 phase of the pretreatment, ie 2.5 h approx.

  Then, the concentration of the pretreated solution is taken up again under a pressure of 0.1 bar until about 500 l of concentrated suspension containing the equivalent of 384 g / l of boric acid is obtained. dry extract of about 51%. This last operation lasts approximately 12 hours, ie an overall time of this first cycle of 17 hours.

  The concentrated solution is then trans-

  in the GUEDU 820, whose evaporation capacity will be of the order of 40 l / h. The overconcentration is finished to a solids content of 56%. The operation lasts 30 minutes

about.

  The overconcentrated dough is then acidified

up to pH 8.

  Mix for 10 min with CLK 45 cement in a free water / cement ratio of 0.8, with addition of 3% of water reducing plasticizer (by weight

compared to cement).

  500 1 of concrete, density 1.78- is obtained.

  Total duration of a cycle assuming continuous work: 17 h.

Volume reduction: 3.2.

Claims (6)

  1. Process of. conditioning, by hydraulic binders   low and medium activity borated radioactive effluents by chemical treatment for the preparation of crystals of calcium borate tetrahydrate, volume reduction and mixing of the product obtained with at least one hydraulic binder, characterized in that   the volume-reduction operation is carried out by evapo-   under reduced pressure, solutions and / or   said reduced pressure being between about 0.1 and 0.3 bar and applied after the   preparation of crystals of calcium borate tetrahydrate.   2. Process for the conditioning, by means of hydraulic binders, of low and intermediate activity borated radioactive effluents by chemical treatment aimed at   to the preparation of crystals of calcium borate tetra-   hydration, volume reduction and mixing of the product obtained with at least one hydraulic binder, in which process the preparation phase of blister crystals   Calcium tetrahydrate has a first stage of ad-   junction of lime and germination, a second stage of formation of mixed salts, a third stage of   formation of said borate and a fourth step of   crystals of said borate, characterized in that a volume reduction, under reduced pressure, is also performed during at least one of the first and fourth stages, and optionally after the   for the preparation of tetra calcium calcium borate crystals hydrate.   3. A method of packaging according to one of the claims   cations 1 and 2, characterized in that the effluents   are also subjected to evaporation under pressure -   reduced before the formation phase of borate crystals of calcium tetrahydrate.   4. Method according to one of claims 1 to 3,   characterized in that the whole of the crystal phase   is carried out under reduced pressure between about 0.7 and 0.9 bar.   5. Method according to one of claims 1 to   4, characterized in that the preparation of the crystals of calcium borate tetrahydrate is carried out by formation during the third step of calcium borate hexahydrate, which is subsequently, during said   fourth stage, transformed into a titrated calcium borate   hydrate. 6. Device for implementing the method   according to one of claims 1 to 5, characterized   this process is carried out batchwise,   in one (or two) apparatus which is a mixer-mixer   evaporator and which comprises a device for maintaining the temperature, a stirrer with several stirring regimes and effective with fluids of variable viscosities and a device making it possible to carry out   a pressure of about 0.1 to 1 bar.