Method for the treatment of radioactive and other concrete wastes

Description

[0001] This invention relates to a method for treatment of concrete wastes, especially, but not exclusively, radioactive concrete wastes.

[0002] Nuclear reactors employed in the production of electricity generally comprise, surrounding the nuclear reactor, a steel reactor vessel for containing the nuclear reactor and then, outside this, a thick concrete structure to serve as a biological shield. In some of the more recent gas cooled reactor systems an integrated circuit has been used and the reactor has been housed in a prestressed concrete structure in which the various cavities are lined with steel. With the passage of time the steel vessels and considerable quantities of the concrete used for shielding or other purposes become significantly radioactive. This poses problems during the dismantling of nuclear installations, not only because of the mass of active material to be disposed of, but also because of its form. Demolition of nuclear installations is indeed likely to result in the production of large quantities of active dust which varies in size from small particles of say 0.25 mm diameter to dust having a particle size of the order of a few microns, some of the finer dust being invisible under normal conditions. Coarser and less mobile dust may have a size up to about 2 mm. This latter dust as well as some of the fine dust will cover the concrete rubble which itself may vary in particle size from that of gravel i.e. about 2 mm diameter to the order of a cubic metre. It is necessary to be able to recover all of this material and generally immobilise it in suitable containers so that it can be disposed of in a designated repository.

[0003] It will be appreciated from the foregoing that there are two basic problems in packing the products of demolishing concrete components of nuclear installations. Firstly it is necessary to confine airborne dust and prevent it from entering the atmosphere. Moreover it is necessary to ensure that the dust which exists on the surface of concrete rubble is made to adhere well to it and is not distributed during the packing of the rubble.

[0004] A number of proposals for achieving dust control have been made. These may be divided into two classes, namely firstly prevention of breakdown of solid surfaces to form dust and secondly the prevention from becoming mobile of any dust which has already been formed. Thus it has hitherto been proposed to dustproof concrete to prevent a solid concrete surface from breaking down by coating it with a silicate material which is known to react with lime in the concrete and strengthen the surface. Moreover United States Patent No. 2,854,347 describes the use of natural and synthetic rubber lattices for example styrene-butadiene or acrylic rubber for stockpile dust control, in particular the prevention of wind losses of coal dust from outdoor coal and coal dust piles.

[0005] In neither case is there a need for the very rapid treatment of large quantities of mobile material. Indeed a single demolished nuclear reactor station may produce as much as 10,000 tons of active rubble of which 10% by weight may be dust which needs to be controlled.

[0006] It is an object of this invention to provide a method for the treatment of such mobile material as radio-active concrete waste which acts rapidly to confine airborne dust and prevent it from entering the atmosphere and which ensures that dust on the surface of concrete rubble adheres well to it.

[0007] According to the present invention, there is provided a method for the fixing of concrete dust produced on demolition of a concrete installation which comprises collecting such dust which is airborne and taking it up in a liquid medium which has the ability to wet and bind the dust particles together and set to a cohesive mass under ambient temperature conditions and conditions such that the setting is solely the result of reaction involving the chemical contents of the dust particles and said solution.

[0008] At the same time a liquid of the same or different general character will generally be employed to fix on to the surface of the rubble that dust which is not airborne.

[0009] A variety of materials may in principle, be used to carry out the method of this invention. However, many have inherent characteristics which detract from their ease and convenience of use. Among these drawbacks are the presence of inflammable or noxious components, the necessity of including in them additives in order to bring about a reaction by which the mixture of dust and liquid will set to a firm and readily handleable solid, high cost or limited availability, or lack of resistance to atomic radiation. The materials which have been found most suitable for use in carrying out this invention are aqueous solutions of the alkali metal silicates and more particularly those aqueous solutions which are based on sodium silicate.

[0010] Sodium silicate is manufactured in a range of silica/alkali ratios and the properties of the solution are largely defined by this ratio. It might be thought that a solution of high silica/alkali ratio will be more effective in carrying out the method of the invention than a solution of lower silica/alkali ratio and will enable the volume of material used to be kept down. In fact this is not the case. It is preferable to use relatively low silica/alkali ratios whereby a stronger product is obtained. Thus there is no advantage to be obtained in using silica/alkali ratios in the range of 3.4-4:1 and higher and in fact better results are usually obtained at much lower ratios e.g. 2-2.5:1.

[0011] Insofar as the solids content of the silicate solution employed is concerned it is preferred to use a solution strength of from 5-50% by weight solids. In fact for optimum performance the silica/alkali ratio and solids content are interrelated, and for silica/alkali ratios in the range 2-2.5:1 it is preferred to use a sodium silicate solution having a solids content of 25-40% by weight, the higher solids contents being more suitable for the coarser concrete dusts.

[0012] The setting of sodium silicate solution to form substantially solid material is a well known phenomenon in itself. A variety of processes are known for achieving this setting, for example loss of relatively small quantities of water and reaction with acid in solution or absorption of carbon dioxide from the atmosphere. This phenomenon has been employed for many years in the binding of sand in the production of foundry moulds, where the use of carbon dioxide and heat is required. The speed of reaction and strength of the gel formed depends inter alia upon the silica/alkali ratio and upon the solids content of the sodium silicate solution. It is a general characteristic of these known procedures that either heat or a chemical treatment is required to promote the setting reaction.

[0013] In contrast thereto, the method of the present invention can be carried out under ambient conditions without the need for either application of heat or addition of any outside chemical agents. It is thought that the presence of calcium ions from lime in the concrete is sufficient to bring about the setting reaction. In fact the age of the concrete may affect the speed of the setting reaction. Thus old concrete in which the lime has largely been converted to carbonate may react with the silicate solution more slowly.

[0014] The value of such simplicity of operation can readily be appreciated when consideration is given to the demolition of nuclear installations where large quantities of radioactive concrete dust are produced in a short period of time and must not be allowed to enter the atmosphere.

[0015] It is not within the scope of this invention to define precisely the method by which the active dust in a large scale demolition process may be collected and treated with the silicate solution. However, in principle, the containment vessel in which the concrete rubble and dust are present after demolition can be held at below atmospheric pressure to prevent escape of the dust into the atmosphere and the finer dust can be extracted by suction, separated from the air stream by one of a variety of available methods using cyclones, filters or electrostatic precipitators and thereafter mixed with the silicate solution to form a slurry which is allowed to set, or otherwise mixed with the silicate solution to form pellets or lumps of a convenient size. The dust which is left adhering to the lumps of rubble may be fixed thereto by spraying with the same silicate solution after dilution thereof.

[0016] In addition to using silicate solution, it is possible in carrying out this invention to employ the silicate in powdered form, to mix it with the concrete dust, and then to add water to enable the setting reaction to take place.

[0017] The following Examples illustrate this invention:

EXAMPLE 1

[0018] 1.5 parts by weight of concrete ground to a dust of particle size less than 300 microns were mixed with 1 part by weight of a sodium silicate solution having a silica/alkali ratio of 2.85:1 and a solids content of 33.4% by weight. The suspension formed was allowed to stand for 18 hours under ambient conditions and at ambient temperature. At the end of this time, the suspension had set to form a solid body which was easily handled without disintegration or dusting. After standing 76 hours under similar conditions the solid body was sufficiently hard for a 5 cm cube cut from it to have a compression strength of 6373.25 Pa.

EXAMPLE 2

[0019] 3.5 parts by weight of a 40 days old concrete ground to a dust of particle size passing a 75 micron sieve were mixed with 1 part by weight of sodium silicate solution having a silica/alkali ratio of 2.85:1 and a solids content of 30% by weight. The resulting suspension was allowed to stand for 5 hours under ambient conditions and at ambient temperature. At the end of this time it had set to a hard cohesive solid body which was easily handled without undergoing disintegration or surface dusting.

EXAMPLE 3

[0020] 6.7 parts by weight of a 5 years old concrete ground to a particle size passing a 75 micron sieve were mixed with 1 part by weight of sodium silicate solution having a silica/alkali ratio of 2.85:1 and a solids content of 30% by weight. The resulting suspension was allowed to stand for 5 hours under ambient conditions and at ambient temperature. At the end of this time a hard cohesive solid which was easily handled without undergoing disintegration or dusting had formed.

EXAMPLE 4

[0021] 3.6 parts by weight of concrete ground to a particle size passing a 75 micron sieve were mixed with sodium silicate in powdered form having a silica/alkali ratio of 2:1 and a solids content of 84% and then 2.3 parts by weight of water were added and mixing was effected. The resulting suspension was allowed to stand for 18 hours under ambient conditions and at ambient temperature. At the end of this time it had set to a solid cohesive mass which was easily handled without undergoing disintegration or dusting.

[0022] Although for reasons of safety, non-radioactive concrete was used in the foregoing Examples, it will be appreciated that equivalent results would have been obtained if the concrete had been radioactive. It is for this reason that this invention is not limited to the treatment of radioactive concrete waste.

Claims

1. A method for the fixing of concrete dust produced on demolition of a concrete installation, characterised in that such dust which is airborne is collected and taken up in a liquid medium which has the ability to wet and bind the dust particles together and set to a cohesive mass under ambient temperature conditions and conditions such that the setting is solely the result of reaction involving the chemical contents of the dust particles and said solution.

2. A method as claimed in claim 1, wherein the dust results from the demolition of a radioactive concrete installation, finer dust particles being removed from a containment vessel in which concrete rubble and dust particles are present by suction and allowed to contact a said liquid medium and coarser dust particles being contacted by a said liquid medium while it is in situ in said vessel.

3. A method as claimed in claim 1 or 2, wherein the liquid medium is an aqueous alkali metal silicate solution, preferably sodium silicate solution.

4. A method as claimed in claims 2 and 3, wherein the finer dust particles are mixed with the silicate in powdered form and water is added to enable the setting to take place.

5. A method as claimed in claim 3 or 4, wherein both said finer and said coarser dust particles are taken up by aqueous alkali metal silicate solution, the solution which contacts the coarser dust particles being more dilute than that which contacts the finer dust particles.

6. A method as claimed in any one of claims 3 to 5, wherein the liquid medium is an aqueous sodium silicate solution in which the silica/alkali ratio is in the range of from 2-4:1.

7. A method as claimed in claim 6, wherein said ratio is from 2-2.5:1.

8. A method as claimed in any one of claims 3 to 7, wherein the silicate solution has a solids content of from 5 to 50% by weight.

9. A method as claimed in any one of claims 3 to 5, wherein the liquid medium is an aqueous sodium silicate solution having a solids content of 25 to 40% by weight and a silica/alkali ratio of from 2-2.5:1.