FR2615315A1 - METHOD FOR IMMOBILIZING RESINS EXCHANGING RADIOACTIVE IONS BY A HYDRAULIC BINDER - Google Patents

Abstract

THE INVENTION RELATES TO A PROCESS FOR IMMOBILIZING RADIOACTIVE ION EXCHANGER RESINS BY A HYDRAULIC BINDER, IN PARTICULAR BORATE RESINS. DEPENDING ON THE PROCEDURE, THE RESINS ARE DECANTED THEN CONTACT FOR 3 HOURS AT THE MOST WITH A ELUANT SOLUTION OF 100 TO 300 GL IN CA (NO) DUE TO 1 TO 2 LKG OF DECANTED RESINS AND THAT, IN THE MIDDLE OF PH 9 IS ADD A HYDRAULIC BINDER WITH LOW HEAT OF HYDRATION SO THAT THE WATER RATIO OF THE ELUANTELIANT SOLUTION (BY WEIGHT) IS BETWEEN 0.3 AND 0.5 AND THE RATE OF INCORPORATION OF DECANTED RESINS ENROBED (BY WEIGHT) IS BETWEEN 3 AND 10.

Description

Method for immobilizing radioactive ion exchange resins

by a hydraulic binder.

  The invention relates to a method for immobilizing radioactive ion exchange resins (REI) with a binder.

hydraulic.

  The radioactive REIs to be processed come from

  essentially nuclear reactors in which they are used to purify the water of various reactor circuits and sometimes the water of the storage pools for irradiated fuel elements. In particular, in PWR pressurized water reactors, anionic REIs are placed on the primary circuit whose water is added with boric acid playing the role

moderator.

  The anionic REIs can then serve as a "boron lung" to maintain the desired boron concentration in the circuit. Plant operators estimate that used REIs to be treated may contain borates up to the equivalent of 1000 g

  boric acid per kg of dry REI.

  In addition to the borates, these REIs (cationic, anionic, mixed bed) can contain the lithium, ammonium, iron, cobalt, chromium, nickel, cesium cations and the hydroxide, sulphate,

  phosphate, silicate, fluoride, chloride, bicarbonate.

  - REIs are also used in installations for reprocessing irradiated fuel elements for the purification of water in storage pools and for treatment

liquids.

  REIs are placed in columns or cartouches.

  Currently, they are regenerated in place before immobilization. They then essentially contain H, OH and

  non-eluted active metal cations.

  To avoid the dissemination in the environment of radioactive substances, we seek to immobilize the waste containing them in a matrix resistant to mechanical, chemical agents ... likely to damage it during the storage of this waste. One way to do this is to mix said waste with a hydraulic binder which, by setting followed by hardening, gives a certain mechanical resistance to the mixture.

  and some resistance to chemical attack.

  The values of these mechanical and chemical resistances that immobilized waste (also called final or coated product) must reach in order for its storage to ensure sufficient safety for man and the environment, are set by safety standards

nuclear.

  These are established by the national authorities and

  may therefore vary from country to country.

  French safety standards for waste immobilized in a hydraulic binder are among the most restrictive: few countries have succeeded in reaching such standards with waste containing REI. In France, since 1982, the treatment of REI by concreting has also been interrupted, the processes used do not lead to acceptable mixes

according to safety criteria.

  The treatment of REI with a hydraulic binder poses two essential problems that one does not encounter with

  other types of nuclear waste.

  The first problem is that of ion exchange between

  the REI and the medium containing the hydraulic binder.

  The ions in the medium which exhibit a greater affinity for the REIs than that which the ions contained in these REIs have for them, attach themselves to the REIs in place of the ions which they contained, these ions being released into the medium. There is ion fixation and simultaneously salting out of other ions. So that the medium is depleted in ions of the hydraulic binder (Ca ++ and SQ4 essentially) and on the other hand is enriched in ions originating from S4 of nuclear installations (active metal cations,

  phosphates, sulfates .. H +, OH and borates).

  The depletion of binder ions, in particular of Ca and SO4, causes a change in the setting (delay, uncontrollability, incomplete setting). Furthermore, the ions brought in by the REIs and released into the medium can interfere with setting, hardening or influence the resistance of immobilized waste over time. - Zn ++ has a retarding or even inhibiting action; - Mg can be exchanged with Ca of calcium hydrates, after taking and therefore modifies the behavior over time of the product; - H the hydration reactions of the binder take place in basic medium, a lowering of the pH to acid values, delays the setting see the inhibits;

  - phosphates also have a delaying effect on setting.

  By far the most troublesome ions are the borates. Their effect is known on hydraulic binders, and according to their concentration in the medium they delay or inhibit setting, whether they are in free form or associated with certain ions such as

lithium to form Li2B407.

  The ionic exchanges can continue after setting, in particular during leaching tests, between the leaching medium and the REIs made accessible in the mix for various reasons (permeable matrix, poor homogeneity, high porosity, etc.). Released ions can generate reactions

  harmful to the mix, for example sulphates.

  The second type of difficulty encountered in the treatment of REIs specifically results from the migration of water from REIs to the medium containing the hydraulic binder. The REI sell a fraction of their water according to the principle of the balance between the water of the REI and the water of the environment. As the hydration reactions of the binder are exothermic, the outflow of water continues during setting. At the end of the setting, the partially dehydrated REI grains can, if the final product is brought into contact with water, take up water. This is the well-known phenomenon of swelling and cracking of submerged coated materials after setting and even hardening: the swelling due to the uptake of water causes cracking of the

  material and can result in a total disintegration of the material.

  To these two categories of problem, industrial solutions must be provided in which: 1) the volume of asphalt mixes to be stored in the long term must be as small as possible to minimize the costs of storage facilities; 2) the implementation of the immobilization process must be technologically feasible by means as simple as possible, reliable and rapid; In the patent FR-A-75 33 518 are added additives having the function of preventing the penetration of water into the grains of REI. Such additive substances form a protective layer around the grain of REI. These are organic compounds (organic ester, polyvinyl propionate) or minerals (alkali silicate). However, it is not certain that the borate ions possibly contained in the REIs cannot pass into the aqueous medium. In addition, the implementation and the cost make this process

not very interesting.

  In order to limit the transfer of water between the REI and the binder during setting, another patent FR-A-80 21 524 recommends the use of blast furnace cement under precise conditions and water saturation REIs. Conditions: Mixing water / cement (by weight) = 0.20 to 0.40 and incorporation rate = Dry resin / Asphalt (by weight) <15% for a powdered REI and 25% for a granular REI . Note that the mixing water is water

  added to -REI saturated with water to ensure the setting of the cement.

  In such a process, ion exchanges likely to occur between the cement and the REI are absolutely not taken into account. such a process cannot be applied to borated REIs: the released borated ions inhibit the setting of cement in the

above conditions.

  Furthermore, solidification by hydraulic binder of borated effluents is known from patent FR-A-85 04 222 which describes a process in which before the addition of cement, the borated effluents are treated with lime to precipitate borates calcium structure determined under specific conditions. A solution was then necessary to treat the borated REIs consisting of eluting them to extract the borated ions from the REIs and putting them back into solution, then to separate the REIs from the eluting solution, to rinse in order to remove traces of borates as well as possible, and finally concreting the REI on one side and the effluents

  borated on the other according to the methods described above.

  The elution of radioactive REIs has already been used before solidification for asphalting or by polymerization of a thermosetting resin. In patent FR-A-76 24 624, the eluting solution is a solution of sodium hydroxide, ammonia, lime aluminum chloride, citrate, oxalate or sodium acetate, or else an amine. The REI obtained are decanted or wrung out and then mixed with the resin

  thermosetting which caused polymerization.

  Such pretreatment makes it possible to eliminate the H ions from cationic REIs, these ions acting on the crosslinking accelerator added to the thermosetting resin: the H + ions are

  extracts from the REIs, dissolved and then separated from the REIs.

  In patent EP-157 683 the elution is carried out with a solution of Ca, Be or Sr salts (anicns nitrates, formiates or acetates), the REI are separated from the eluting solution,

  rinsed, suspended in water and then asphalted.

  The pretreatment has the function of replacing the H +, Na +, OH, Cl ions of the REIs by the ions of the larger eluting solution which modify the three-dimensional network of the REIs so as to prevent the penetration of water into the bituminous mixes immersed in the leach medium. So the risk of

  swelling is extremely reduced.

  According to these REI treatment methods employing an elution to remove the annoying REI ions - these ions being annoying either by their action on the solidifying medium or by their ability to allow water to be taken up by the REIs.

  are separated from the eluting solution before being immobilized.

  The present application proposes an industrial scale process for treating REIs possibly loaded with borates by a hydraulic binder in one go, on the same place, at the same time and this in order to obtain coated materials meeting the

  safety standards in force in the country.

  Said method comprises a pretreatment step by elution followed by a solidification step by taking a hydraulic binder, the conditions of the elution making it possible to surprisingly obtain a solidifiable medium, as obtained, with a hyrdaulic binder although being charged with various ions,

especially in borates.

  More specifically, the subject of the invention is a process for immobilizing radioactive ion-exchange resins (REIs) which can contain borates in a quantity which can range up to the equivalent of 1000 g H3 using a hydraulic binder. B03 / kg of dry REI, characterized in that the REI are decanted and then placed in contact for at most 3 h, with an eluting solution of at 300 g / l of Ca (NO3) 2 at a rate of 1 to 2 l / kg Decanted REI and that, in the middle of pH> 9, is added a hydraulic binder at low heat of hydration so that the water ratio of the eluting solution / binder (by weight) is between 0.3 and 0.5 and that the incorporation rate F = REI settled / coated (by weight)

is between 3 and 10%.

  Ion exchange resins from nuclear installations (cationic, anionic or mixed bed) are collected, stored and then sent to the processing unit. It is therefore not known, in a precise manner, in general before treating it, their composition, the nature and the quantity of the ions which they contain. In any case, it is not easy to indicate a precise limit of the quantity of borates which can be fixed since condensed molecules can form and fix. We estimated at 1000 g eq H3 BO3 a significant amount, an average amount would be

of 500 g eq H3 B03.

  The stored REIs are in the form of a suspension. According to the process which is the subject of the invention, the REIs to be treated are initially left to settle, the supernatant is removed (pumping,). The water-saturated REIs obtained (called 100% decanted) are then weighed. The weight of 100% decanted REI introduced for treatment will be the reference from which will be calculated

  the quantities of material added thereafter.

  The purpose of bringing the REIs into contact with the eluting solution is: - to fix the ions of the eluting solution on the REIs, ions favorable for immobilization by the hydraulic binder; - to put in solution the REI ions, among which are troublesome ions for immobilization by the hydraulic binder; - to precipitate in the solution said troublesome ions in the form of insoluble solids under the conditions of immobilization

by the hydraulic binder.

  The precipitation combines with the elution so that the efficiency of the elution is clearly improved: the eluted ions precipitating, their concentration in the solution decreasing, the balance between the borates in the REI and the borates in

solution is moved.

  The eluting solution chosen is an aqueous solution of

  calcium nitrate causing calcium borates to precipitate.

  The favorable effect on the elution of the precipitation allows a rapid contact time: less than 3 h, and preferably 1 h. This contact time was determined, as well as the amount of equivalents -g cation or anion and the amount of water provided by the eluting solution per kg of REI from

  numerous tests carried out by the applicant.

  It was indeed not possible to choose process values - knowing neither the composition nor the capacity

  theoretical exchange of starting REIs, nor their borate content.

  Furthermore, the volume of eluting solution introduced for this elution has a direct influence on the next step of treatment with the hydraulic binder, because the entire volume is

kept for this treatment.

  In fact, the water ratio of the eluting solution (by weight) / binder (by weight) must be included within strict limits. So that the weight of the added binder depends on the volume of the eluting solution, therefore the weight of the mix (binder + solution

  elected + REI) also depends on it.

  However, it is not possible to inconsiderately increase the weight of the mix, handling and storage problems arise.

pose.

  It was therefore necessary to choose the concentration and the volume of eluting solution so that: - the elution was effective (effective precipitation and fixation); - there is no need to add water to respect the E / C and F ratios; - the weight of the final mix is not excessive; - The volume of solution is not too large for elution and treatment with the binder can take place in the same device. Simultaneously, it was necessary to determine the contact time compatible with the process constraints: - sufficient elution and precipitation for the delaying effect

  on taking borates is no longer sensitive.

  - industrial constraints: shortest contact time

  possible to be able to produce as much asphalt as possible per day.

  The experiments carried out by the applicant show that the optimum is reached with an aqueous solution of Ca (NO3) 2 containing 100 to 300 g / l of calcium nitrate at a rate of 1 to 2 l /

  Kg of REI 100% decanted and a contact time of 3 hours at most.

  It is quite obvious that the contact time and the quantity of ions brought depend on the content of borates which is

generally unknown.

  The preferred values correspond to coated materials in accordance with French safety standards: concentration of the eluting solution = approximately 200 g / l; approximately 1 l / kg REI 100% decanted; 1 hour contact time. The operator can choose other values in the given ranges according to the standards he will have to respect: for less restrictive standards, he can advantageously reduce the contact time. It is certain that the longer the contact time, the greater the elution, therefore the annoying ions are

  blocked in solution by precipitation.

  To increase the efficiency of elution, it is advantageously added at the same time as the eluting solution of lime (preferably in solid form so as not to add water)

  at a rate of less than 200 g / kg of REI 100% decanted.

  The elution therefore takes place according to a discontinuous process in a single step: to the 100% decanted REI is added, with stirring, the eluting solution. Advantageously, decantation, elution and treatment with the hydraulic binder take place in

  the same device (a mixer).

  It is an important characteristic of the process to treat the entire mixture obtained at the end of the elution step. Indeed, as the prior art shows, when there is elution on REIs, there is then separation of the REIs from the solution. To the mixture obtained is therefore added with stirring the hydraulic binder at low heat of hydration, the medium being at a pH

at least equal to 9.

  Preferably, it is a slag cement, which, when hardened, has a low porosity and low permeability. The slag cements contain variable levels of clinker (the clinker being responsible for the exothermicity of the hydration reaction), let us quote as

example:

  In France cement: CLK> 80% dairy, <3% additives CHF 40 to 75% dairy, 3% CLC additives 20 to 45% dairy, <3% additives, 20-45% ash. In the USA: Portland Blast furnace 25-65% dairy In FRG: Eisenportland cement> 40% dairy In Japan: BLast furnace surround type C 60-70% dairy Among these cements, those with a high proportion of dairy (> 60%) are preferred . In France, the choice will focus on cement

CLK.

  With the hydraulic binder, other additives can be introduced such as fillers, plasticizers ... The final matrix in which the REI are immobilized, is based on hydraulic binder but may contain other elements in lesser

quantity.

  The amount of binder added is such that the weight ratio of water of the eluting solution / weight of binder is between 0.3 and 0.5 and preferably 0.4 for a rate of incorporation of REI F = weight of Decanted REI / mix weight between

3 and 10%.

  The weight of the mix will be equal to the sum of the weight of the eluting solution plus that of the 100% decanted REIs plus that of the binder.

  Examples will better illustrate the invention.

EXAMPLE 1

  Either a batch of borated REI in suspension to be treated - Neither the composition of this batch, nor the exact type of REI, nor their content in

  borates are only known for treatment.

  In fact these REI contain 350 g. eq H3 B03 / kg REI

  dry, these are anionic REI Amberlites IRN 78 LCL.

  The batch of REI is brought into a mixing mixer built by the company GUEDU, said device is associated with a weighing device. After rest during which the REI decant,

  the supernatant solution is pumped.

  The device is weighed, the weight of the 100% decanted REI is determined. To the 16.5 kg of 100% decanted REI obtained, 30 l of aqueous Ca (NO 3) solution 2 to 200 g / l are added as well as 0.8 kg of solid lime, the whole is stirred for 1 h.

  75 kg of CLK cement are then added with stirring.

  The whole is kneaded, poured into a container according to the usual conditions for the treatment of waste with a hydraulic binder. The weight of the mix obtained is approximately 120 kg, which represents an incorporation rate F 'of 4% in dry resins, a total water / cement ratio = 0.5 (total water = water in the saturating solution + REI constitution water + interstitial water present in 100% decanted REIs) and an increase factor

of volume f of 4.7.

  Core drilling is carried out in this asphalt and the

  samples taken are tested for mechanical strength.

  The mechanical resistance to compression is more than bars at 7 days, more than 200 bars at 14 days, and more than 300

bars at 2 x 28 days.

  Free 2 The same batch as above is decanted, weighed (16.5 kg) and eluted with 21.5 I of an aqueous solution containing 200 g / l of Ca (NO3) 2 -51 kg of

CLK cement are added.

  We then have F '= 8% (in dry resins) f = 3.5 In return for better incorporation there is less hardening since the compressive strength is

  less than 10 bars at 7 days and more than 200 bars at 28 days.

  This process, requiring to be implemented a single apparatus composed of a mixer, applicable to any REI used in nuclear environment, brings a very important simplification at the industrial level in the concreting of radioactive REI. The process which is the subject of the invention therefore has several essential advantages for industrial exploitation in a nuclear environment. The first is to avoid transfers of active materials: it suffices to bring the waste to be treated. No racking, apart from pouring the final product into a container, is

  necessary, nor parallel treatments of extracted effluents.

  This is because all of the processing can be done in

one device: the mixer.

  The second advantage is the simplicity of implementing the method. An REI arrives, we know nothing of its past. It can be immobilized so as to be acceptable according to safety criteria. It suffices to determine its weight in the decanted state and to place itself within the ranges of values indicated by the process which is the subject of the invention.

  Furthermore, elution is an easy operation.

  All the easy operations, all the simple apparatuses are sought in nuclear environment o the least problem raises difficulties as for the human invention (manual manipulation on the spot) and as for the decontamination or

  containment of radioactive material.

  A third advantage is to be able to process any REI coming from a nuclear installation, whether or not it contains borates and even in large quantities. There's the

  still a uniqueness in the treatment.