Patent Number: 055457959
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method for decontaminating radioactive metal surfaces with an aqueous solution containing acetic acid. 2. Description of Prior Art Several different methods are known for decontaminating radioactive metal surfaces. The use of fluoroboric acid to decontaminate radioactively contaminated surfaces is taught by U.S. Pat. No. 5,008,044. The method taught by the '044 patent is suited for decontamination of surfaces comprising metallic as well as mineral substances. The advantage of the method taught by the '044 patent is the high absorbency of the decontamination agent used, which provides a great stripping depth, making the method particularly suitable for cleaning medium and severely radioactively contaminated items of various materials. Appropriately, the method taught by the '044 patent is also used in decontamination efforts at Chernobyl, Russia. The high metallic content permits electrolytic regeneration of the metals. Decontamination of tanks is costly, however, and produces a large amount of waste because of the acid residue present. The toxicity of the decontamination agent poses an additional problem, particularly at higher temperatures, such as above 130.degree. C., when the decontamination agent pyrolizes into toxic borofluoride. Another decontamination method, taught by U.S. Pat. No. 4,508,641, uses formic acid and/or acetic acid as a decontamination agent and at least one reducing agent, such as formaldehyde and/or acetaldehyde. The '641 patent teaches a method for decontaminating reactor cooling coils, with which steel surfaces can be cleaned with relatively small quantities of chemicals and rinsing water, and wherein used decontamination solution is reprocessed. The addition of reducing agents causes the iron ions to remain stable in the solution, prohibiting the formation of compounds. In a system with closed loops, prohibiting the formation of compounds is crucial for preventing the formation of sediment from settling compounds. The iron compounds are only separated from the decontamination solution in a second step of the decontamination method taught by the '641 patent. Because the entire decontamination process takes place in a closed loop, either the decontamination agent must be continuously injected because it is stoichiometrically depleted, or high concentrations of the acids must be used. On the other hand, the decontamination of a tank does not present such problems. However, cleaning and decontaminating the entire cooling medium in a closed loop according to the decontamination method of the '641 patent is extremely problematic because of the formaldehyde that is present as a reducing agent. A complete decontamination below the permissible threshold of 0.37 Bq/cm, for example, is hardly possible. Nevertheless such complete decontamination of the entire cooling medium is not required inside the cooling loops of reactors. SUMMARY OF THE INVENTION It is an object of this invention to provide a decontamination method which uses a decontamination agent that is low in toxicity, a decontamination method which is economical, and a decontamination method which produces relatively little secondary waste. This and other objects are achieved by a decontamination method according to this invention in which a radioactively contaminated metallic object is contacted with a substantially non-radioactive, aqueous solution containing 0.05% to 5.0% volume acetic acid, preferably by immersion in said aqueous solution, until the acetic acid in the aqueous solution in contact with the metallic object is nearly completely stoichiometrically depleted, thereby forming an aqueous, stoichiometrically depleted solution. As used in this application, the term "non-radioactive" is intended to relate to an aqueous solution that is either completely free of radioactivity or has a very insignificant level of radioactivity. In one preferred decontamination method according to this invention, the radioactively contaminated object is again contacted with the acetic acid-containing aqueous solution until the acetic acid in the aqueous solution is nearly completely stoichiometrically depleted and this step is repeated until the radioactively contaminated metallic object has a residual radioactivity level below a permissible threshold level. Radioactively charged metallic oxides and metallic hydroxides are precipitated out from the aqueous stoichiometrically depleted solutions, forming a radioactive sediment. The radioactive sediment is solidified and separated from the aqueous, stoichiometrically depleted solution. The aqueous, stoichiometrically depleted solution may then be regenerated by adding acetic acid for additional decontamination of other radioactively contaminated metallic objects. A method of this type has the advantage that the solution need not be completely cleaned after each use. As a result, the level of secondary waste is relatively small. Only after the decontamination effort has been completed is the remaining aqueous solution completely cleaned with known agents. In a decontamination method according to this invention where the radioactively contaminated metallic objects comprise aluminum, lead, copper or nickel or alloys containing aluminum, lead, copper or nickel, an oxidizing agent, preferably hydrogen peroxide, is added to the aqueous solution containing acetic acid.