Patent Number: 045086419
Section: description

The invention is described in detail purely by way of example in the following: The task is, for instance, to decontaminate in a continuously running process a nuclear reactor coolant circuit manufactured from a low alloy or stainless steel. The magnitude of the internal surface area as well as the volumetric capacity of the coolant circuit are known. According to the invention, as decontaminating solution an aqueous solution of formic acid and/or acetic acid and at least one reducing agent are used. Preferred reducing agents are those which are made up of C, H, O, as well as N and do not contain harmful foreign elements such as S. Such reducing agents are, for instance, hydrazine, oxalic acid, ascorbic acid, acetic anhydride, etc., while the decontaminating solution according to the invention preferably contains as reducing agent formaldehyde and/or acetaldehyde. At the contaminated surface radioactive materials are adsorbed in one layer in a mixture of iron oxides, and by a previous sampling the thickness and composition of the surface layer to be removed may be determined (CH-PS: Application No. 2184/80-7). On the basis of the available and determined data and the given possibilities, such as, in particular, the availability of time, of heating and cooling devices, etc., the expedient composition for the decontaminating solution, the required quantity and the fundamentals of the course of the process are determined. The oxides of the contaminated steel surfaces are dissolved directly and/or reductively by the decontaminating solution introduced into the coolant circuit and are converted into soluble iron-(II)-formate and/or iron-(II)-acetate which are stabilised by the reducing conditions established in the decontaminating solution principally by the reducing agent present therein, and in particular no oxidation to precipitating ferric compounds takes place. Thus, used decontaminating solution is coloured pale green but is clearly transparent, without turbidity, and contains at most the solid particles of the oxide layer that arise in the dissolution process, which do not represent a disturbing factor either in the decontamination itself or in the treatment of the used decontaminating solution for waste disposal. A decontamintating solution according to the invention that leads to generally satisfactory results is required to contain, e.g., only formic acid and formaldehyde, wherein for example 1 liter of decontaminating solution contains 7-22 ml formic acid and 12-36 ml formaldehyde. In the presence of O.sup.2- ions, such a decontaminating solution is characterised by the following formulae: (a) for the reducing agent formic acid EQU HCOOH+O.sup.2- .fwdarw.H.sub.2 O+2e.sup.- (1) and for the reducing agent formaldehyde EQU HCHO+O.sup.2- .fwdarw.HCOOH+2e.sup.- (2) the dissolution of the contaminated surface layer can be described as: ##STR1## One mole of iron reacts with two moles of formic acid and since the molecular weights of the materials used for the decontaminating solution are low (HCOOH: Mol. wt.=46.03, HCOH: Mol. wt.=30.03), and as has been shown experimentally, one liter of decontaminating solution can take up up to 30 g iron in the form of Fe.sup.2+, and so a relatively low chemicals consumption results for the decontamination while at the same time the cost of formic acid and formaldehyde is low, so that the process according to the invention with such decontaminating solution is particularly economical. This is also true when in place of or additional to the formic acid and formaldehyde acetic acid and acetaldehyde are used in the decontaminating solution, so that the decontaminating solution according to the invention excels by comparison with the known decontamination solutions in general by a low consumption of chemicals and low costs as well as high absorptive capacity for iron. The used decontaminating solution discharged from the coolant circuit is monitored during the dissolution process whereby the concentrations of Fe.sup.2+, acid and aldehyde are continuously controlled. Such a control is analytically simple and permits a reliable control of the whole decontamination process whereby an impermissible corrosion of the pure metallic surface is reliably excluded. The iron compounds contained in the decontaminating solution discharged from the coolant circuit are precipitated out and the used and thus purified decontaminating liquid is re-used, i.e., is regenerated for re-introduction into the coolant circuit. The precipitation of the iron compounds takes place preferably electrolytically, in that the used decontaminating solution is passed through an electrolysis stage which contains an iron cathode and a graphite anode. At the anode COOH.sup.- ions are oxidized to formic acid or to CO.sub.2 and water according to the formula: EQU COOH.sup.- +H.sup.+ .fwdarw.HCOOH (7) and at the cathode Fe.sup.2+ ions are reduced to metallic iron according to the formula: EQU Fe.sup.2+ +2e.sup.- .fwdarw.Fe.degree. (8) The metallic iron adsorbs at least a significant proportion of the radioactive materials contained in the decontamination solution. The decontaminating solution discharged from the electrolysis stage is recycled into the cooling circuit optionally after topping up its formic acid and/or formaldehyde content. In place of electrolytic precipitation, a chemical precipitation of Fe.sup.2+ may also be provided whereby care must be taken that through the precipitation process no harmful materials, above all no S ions are introduced. In general, therefore, an electrolytic precipitation is preferred. A further advantage of the decontamination process according to the invention is that on the dissolution of the contaminated surface layer the reactions take place irreversibly and accordingly an entrainment of radioactive materials on surface areas which are not contaminated or are no longer contaminated is not expected to occur. After the removal of the anticipated thickness of the layer, the decontaminating solution is discharged from the coolant circuit. After the discharge certain residues will always remain in the coolant circuit. In the decontamination process according to the invention, as a consequence of the composition of the decontaminating solution, only such residues are present which may, by means of a simple heat treatment of 175.degree.-300.degree. C., be decomposed thermally into iron oxide and into gaseous decomposition products, particularly CO, CO.sub.2 and H.sub.2 O, i.e., into decomposition products which belong to the coolant circuit and thus have have no harmful influence on the operation. The thermal decomposition of the residue can be undertaken by the introduction of heated air or heated water, but in general is dispensed with because on restarting operation the coolant circuit heats up to the required temperature in a short time. A coolant circuit having residual radioactivity after the decontamination may be rendered "reactor pure" by flushing in the usual manner by means of ion-exchange. Such a flushing should, however, only be required in exceptional cases because the residual activity is easily prevented by corresponding removal of layer thickness. The discharged used decontaminating solution is further processed for waste disposal. In the decontaminating solution according to the invention the carrier for the discharged radioactive material is the iron that went into the solution and not any other additional material, so that, by precipitation of the iron from the decontaminating solution, practically all the radioactivity is caught in the precipitate and the separated liquid contains at most a permissible amount of radioactivity. In precipitating for waste disposal the aim is to adsorb all the radioactive materials in the used decontaminating solution in the smallest amount of precipitate, that the precipitate should be readily disposable and that the separated liquid should give rise to the minimum amount of "load" on the environment. In contrast to the precipitation arising in the regeneration of the used decontaminating solution, in precipitation for waste disposal any desired materials such as also sulphur compounds may also be used, provided that with these economically satisfactory precipitation results may be achieved. The precipitation process that may be considered here is very well described in the literature (e.g. L. Hardinger "Taschenbuch der Abwasserbehandlung", Parts I and II, Karl Hanser-Verlag 1977), so that it is not necessary to go into details. By way of summary the following essentials are here mentioned: (a) precipitation of Fe.sup.2+ as FeS with (NH.sub.4).sub.2 S according to EQU Fe(CO.sub.2 H).sub.2 +(NH.sub.4).sub.2 S.fwdarw.FeS+NH.sub.4 (CO.sub.2 H), (9) which can be decomposed by heat and/or catalytically to CO, CO.sub.2, H.sub.2 O and NH.sub.3 and water-insoluble iron (II)-sulphide of density 4.6, is precipitated, which has a relatively low molecular weight of 87.9, is well filterable and, for instance in comparison with iron hydroxide, has the advantage of low water content in the filter cake, but which is more difficult in terms of disposal because it for instance is difficult to incorporate into concrete. Additionally, because of the sulphur, this precipitation had better be used only when the separated liquid is to be disposed chemically and is not to be processed for re-use as decontamination solution. (b) Precipitation of Fe.sup.3+ and Fe.sup.2+ as hydroxide according to EQU Fe.sup.2+ +2OH.sup.- .fwdarw.Fe(OH).sub.2 (10) EQU Fe.sup.3+ +3OH.sup.- .fwdarw.Fe(OH).sub.3, (11) whereby as precipitation reagent e.g. NaOH may be used. Precipitation as iron-(II)-hydroxide has the advantage that less NaOH is used but has the disadvantage that the precipitate is somewhat more difficult to filter than iron-(III)-hydroxide. When this is undesired the Fe(II) formate in the used decontaminating solution is first oxidized to Fe-(III)-formate, e.g., with hydrogen peroxide according to ##STR2## whereby the iron-(III)-formate is present as the formate of triiron-(III)-hexaformate base (Fe.sub.3 (HCO.sub.2).sub.6 (OH).sub.2 HCO.sub.2).4H.sub.2 O in the structure ##STR3## and a ratio of Fe:(HCO.sub.2)=3:7 is to be observed. The thus obtained iron-(III)-hydroxide is easier to separate from the liquid, e.g. by filtering as iron-(II)-hydroxide but for precipitation nevertheless requires more precipitating agent than does iron-(II)-hydroxide. With NaOH as precipitating agent the following reactions arise: EQU Fe(CO.sub.2 H).sub.2 +2NaOH.fwdarw.Fe(OH).sub.2 +2NaCOOH (14) and EQU Fe(CO.sub.2 H).sub.3 +3NaOH.fwdarw.Fe(OH.sub.3 +3NaCOOH. (15) In the precipitated iron hydroxide at least a very large portion of the radioactive material present in the decontaminating solution is adsorbed and the liquid separated from the precipitate, in the present case an aqueous solution of sodium formate with formaldehyde residues, is not really active or hardly active at all. The sodium formate can then be oxidatively decomposed to NaOH, Na.sub.2 CO.sub.3, CO.sub.2 and H.sub.2 O. An advantage of this precipitation process consists in that the weight of the separate precipitate corresponds to that of the material removed by decontamination, i.e., practically no weight increase occurs and also that the precipitate may without further processing readily be disposed by mixing with cement, whereby expediently a ferro-cement-like product is produced and a particularly low yield of contaminated material to be disposed of is assured. A further advantage of this iron hydroxide precipitation process is the decomposability of the resulting sodium formate. Instead of subjecting the whole mass of used decontaminating solution resulting from the decontamination of a coolant circuit all at once, expediently the decontamating solution is divided into several batches. After an optional treatment with hydrogen peroxide a small amount of precipitating agent, e.g. NaOH, is added to the first batch and after separation of the precipitate, the thus obtained sodium formate is decomposed as described above oxidatively, electrolytically or pyrolytically. The obtained liquid product is then used for precipitating the second batch of decontaminating solution, and so on. Thus, a significantly lower amount of precipitate results and the precipitate to be disposed of the used decontaminating solution can be formed as a recirculatory process or built into a continuous decontamination process as such. It is particularly favourable to proceed in such a way when the liquid separated after the precipitation still contains a certain amount of residual radioactivity because then a corresponding attenuation or dilution of the activity is achieved. The choice of the precipitation process to be used in a given case is determined from the apparatus actually available, from the possibilities of performing the process and particularly also from the volumetric capacity of the coolant circuit and the quantity of material to be decontaminated. The separation of the deposit precipitate and the liquid can be performed by simple filtering. For easy filtering flocculating agents such as polyacrylamide may be added to the used decontaminating solution whereby the precipitated particles agglomerate into larger particles. As a preferred flocculating agent, the precipitate of a preceding precipitation process is used. As mentioned, the separated liquid may either be processed for re-use as decontaminating solution, or may be "chemically" disposed of. For chemical disposal the formaldehyde is oxidized to formic acid; and thus obtained formic acid together with the present formic acid is decomposed to H.sub.2 O and CO.sub.2 by means of an oxidising agent according to the formulae: EQU HCOH+1/2O.sub.2 .fwdarw.HCOOH (16) and EQU HCOOH+Oxisising Agent.fwdarw.H.sub.2 O+CO.sub.2 (17) and salts of formic acid are disposed of in the same way. The thus obtained waste products are harmless to the environment and do not lead to any problems in their disposal. Any desired oxidising agent may be used and a choice thereof is influenced essentially only by the economy, i.e., to the low cost, and attention must be paid to ensuring that the advantageous chemical waste disposal is not affected deleteriously by the oxidising agent. In the foregoing, the invention was extensively described by reference to a simple decontaminating solution with formic acid and formaldehyde. However, it should be understood without further explanation that the above is also valid for all other desired composition of the decontaminating solution according to the invention. The decontamination process according to the invention may be carried out as a continuous process with the decontamination solution recirculated in a loop as well as a batch process, the advantages achieved being the same. It has in particular been shown that contaminated surfaces of low alloy steel as well as stainless steel have been effectively decontaminated by means of the decontamination process according to the invention. Thus, for instance, in a test with stainless steel, the surface of which containing mainly magnetite had an activity of 8 .mu.Ci/cm.sup.2 had its radioactivity lowered to 0.025 .mu.Ci/cm.sup.2 by the decontamination process according to the invention, which at a rate of material removal of about 10 mg/cm.sup.2 gives rise to a high decontamination factor of 330.