Patent Number: 045086419
Section: summary

The invention concerns a process for the decontamination of steel surfaces, particularly in nuclear reactor coolant circuits, by the removal of the contaminated surface layer with an acid-containing aqueous decontaminating solution and for the preparation of the decontaminating solution containing the dissolved radioactive materials for waste disposal. To decontaminate nuclear reactor coolant circuits aqueous solutions of mineral acids are frequently used. Mineral acids are aggressive (corrosive) materials and it is therefore extremely difficult to control the course of the decontamination process by the sole means of adjusting the acid concentration, i.e., such that the contaminated surface layer is effectively removed within an acceptable time while the pure metal of the coolant circuit is not corroded. Corroded spots in the coolant system can lead to leaks which, because of the serious consequences, cannot be permitted. Consequently, complicated decontamination processes have been developed, one of the best known being the so-called "AP-CITROX" process ("Kernenergie" Volume 11, 1968, p. 285-290). In the first stage of this two-stage process the contaminated metallic surface is prepared in a treatment lasting several hours with an oxidising alkaline permanganate solution. In the second stage dissolution takes place with a reducing aqueous solution of a dibasic ammonium citrate, which also requires several hours. Each stage is followed by flushing with water. A similar two-stage decontamination process is described in U.S. Pat. No. 3,873,362. In the first process stage, aqueous solutions of alkali metal permanganates, nitric acid, sodium persulphate, sodium bromate an preferably hydrogen peroxide are used for oxidising the contaminated steel surface layer. For the reducing second process stage, aqueous solutions of mixtures of mineral acids, such as sulphuric acid and/or nitric acid and complex-forming materials, such as oxalic acid, citric acid or formic acid are provided, to which corrosion inhibitors, e.g., iron-(III)-sulphate, iron-(III)-nitrate, nitric acid, phenylthiourea or others may be added. The utilization of hydrogen peroxide in the first process stage has, by virtue of its ready decomposition into water and oxygen, the special advantage that the subsequent flushing with water can be dispensed with. Thereafter, the dissolved metallic components, together with the radioactive materials, are precipitated from the used decontaminating solution of the second process stage. For precipitation the sulphuric and oxalic acid contained in the decontaminating solution can be neutralized with calcium hydroxide so that calcium sulphate and calcium oxalate are formed which contain a great part of the radioactive materials present and which are then separated from the liquid by filtering. Alternatively, potassium permanganate may first by added to the used decontaminating solution in order to decompose the oxalic acid and to obtain manganese dioxide and manganese sulphate, which then can be precipitated by adjustment of the pH value to about 10 with, e.g., calcium hydroxide. Although here also the greater part of the radioactive material is removed with the precipitate, in both cases the filtrate is still contaminated and must be passed to nuclear waste disposal. Such two-stage decontamination processes may be performed as continuous processes or as batch processes. However, in addition to the long duration, the high consumption of chemicals and water are also unsatisfactory, and above all, in addition to the relatively high amount of solid radioactive waste, liquid radioactive waste is also obtained whereby the waste disposal of the used decontaminating solutions is a difficult problem. With the known processes the decontamination of nuclear reactor coolant circuits is laborious and relatively expensive, especially when corrosion of the pure metallic surfaces is excluded from consideration due to the safety requirements. Accordingly, the task of the present invention is to provide a decontamination process for nuclear reactor coolant circuits which requires lesser amounts of chemicals and flushing water for the decontamination of steel surfaces of the same area as the known two-stage processes, which permits a preparation of the used decontamination solution in which only minimum amounts of solid radioactive waste materials are present and wherein the liquid waste contains at most a low radioactivity, most likely lying below the permitted threshold value, which enables an easy control of the decontamination process and practically excludes the possibility of corrosion of the pure steel surfaces. The solution of the task according to the invention consists in the process defined in claim 1. SUMMARY OF THE INVENTION In the process according to the invention the decontamination solution contains formic acid and/or acetic acid and a reducing agent, preferably formaldehyde and/or acetaldehyde. These chemicals are not only very cheap but also relatively non-toxic, so that in the handling of this decontaminating solution no special safety measures are required. On contact with the steel surfaces to be decontaminated, Fe.sup.2+ ions go into solution. Accordingly, the decontamination process according to the invention is a single-stage process, which in contrast to a two-stage process assures a gain of time and cost. By means of the reducing agent contained in the decontaminating solution the Fe.sup.2+ ions are held stably in the solution. The liquid is of pale green colour, but is clear and transparent, without cloudiness, and its composition may be relatively easily monitored during the treatment of the steel surface. It has been shown that by such a decontaminating solution ion oxide is removed 10-15 times faster than the pure basic material and this permits the decontamination process to be conducted without great difficulties and in such a manner that an attack on the pure steel surface, which would lead to damaging corrosion by the decontaminating liquid, is practically impossible. For waste disposal iron compounds are precipitated from the decontaminating liquid. Since the used decontaminating solution contains only Fe.sup.2+ ions, no problems arise in precipitation. The deposits that form have the property of adsorbing the radioactive materials in the solution so that by separation of the deposit very high precipitation decontamination factors are achievable. The separated solid deposit contains then practically all the radioactive materials from the decontaminating solution while the liquid contains at most an unimportant residual activity which lies or may lie beneath the tolerance limit, and thus the liquid may be regenerated for re-use or may be subject to a simple chemical waste disposal by decomposition of the dissolved materials into gaseous products and water, NaOH, and possibly Na.sub.2 CO.sub.3 . The chemical composition of the decontaminating solution provided according to the invention permits the Fe.sup.2+ ions to be precipitated in the form of iron compounds, the density of which roughly corresponds to the density of iron oxide or which can be readily converted into such iron compounds. The radioactive waste obtained by a performed decontamination process is then approximately equal to the material removed from the contaminated surface and thus represents a minimum.