Patent Number: 055457959
Section: description

DESCRIPTION OF PREFERRED EMBODIMENTS Laboratory tests which illustrate the decontamination method of this invention are described in detail below. A radioactively contaminated metallic object weighing approximately 200 kg, which in this laboratory test was a crane hook, was placed into an empty polypropylene tank with a capacity of approximately 300 l. The entire metal surface area of the crane hook was estimated to be approximately 2 m.sup.2. In a second step, 150 l of a 0.5% formic acid decontamination solution or agent was added to the bath. In a third step, the crane hook was left in the bath at an ambient temperature for 5 to 16 hours. Subsequently, the stoichiometrically depleted decontamination solution was pumped out. At this point the radioactivity of the used decontamination agent and the remaining radioactivity of the metallic object was measured, and the foregoing steps were repeated. These steps had to be repeated numerous times, depending on the extent of the radioactive contamination. After it was determined that the residual radioactivity of the crane hook was below the permissible threshold, the used decontamination agent was electrolytically treated in the same bath. The remaining sludge, comprising predominantly Fe, Fe (OH).sub.x, and other impurities, including the absorbed radioactivity, were solidified with cement after sedimentation and sanitized. In a final step, remaining water was then passed through an ion exchanger and subsequently delivered to a sewage treatment plant. In other laboratory tests, the time required for stripping a radioactive layer of metal from a sample of A43 steel was determined. The tests were performed on a sample weighing 200 g and having the dimensions of 50.times.100.times.5 mm. From these laboratory tests it was determined that with a decontamination solution having a very low formic acid concentration, such as 0.3 Mol/l, metallic stripping could be very precisely controlled by altering the bath temperature. Thus, it was determined, for example, that with a bath temperature of 19.degree. C. the stripping rate was 1.1 mg/cm.sup.2 .multidot.hr, while a bath temperature of 80.degree. C. produced a stripping rate of 35 mg/cm.sup.2 .multidot.hr. As in the laboratory test previously discussed, the used radioactively contaminated solution was subjected to anodic oxidation by means of electrolysis. The iron hydroxide sludge formed in this laboratory test absorbed the radioactivity. After sedimentation, the remaining water was used for further decontamination. A quantitative comparison between the method taught by U.S. Pat. No. 4,508,641 and a decontamination method according to this invention reveals that a decontamination method according to this invention produces 30 times less secondary waste than the method taught by the '641 patent. This comparison clearly shows the economic significance of the method of this invention. Although the examples cited herein utilize formic acid, the method of this invention can be performed absolutely identically using acetic acid instead of formic acid, as described, without changes regarding concentration or temperature. The two low-molecular carboxylic acids, formic acid and acetic acid, are the only carboxylic acids which are usable for this purpose. All higher-molecular carboxylic acids form complex byproducts which cause an increase in secondary waste. In the examples described hereinabove, contacting of the radioactive surfaces was performed by dipping the radioactively contaminated metallic object into a bath. Another form of contacting of the acid-containing aqueous solution with the radioactive surface comprises spreading the metallic objects to be decontaminated on a surface and drizzling or spraying the objects with the acid-containing aqueous solution. The acid-containing aqueous solution contacting the surface to be decontaminated is substantially stoichiometrically depleted of acid in the contact area. After a short reaction time, it is then possible to wash down the metallic surface with a stream of increased pressure. In the process, the substantially stoichiometrically depleted acid-containing aqueous solution is washed away, together with reaction products possibly formed on the metallic surface. Thereafter the metallic surface to be decontaminated can again be sprayed or drizzled. This treatment at intervals completely corresponds to a sequence of baths. Only a mechanical surface cleaning is performed by the spraying between two spraying or drizzling operations. This mechanical cleaning could also be achieved by brushes. The alternating drizzling and washing operations can be performed with the same acid-containing aqueous solution, which is always almost completely stoichiometrically depleted of acid in the contact area. This can be done until the entire amount of the acid-containing aqueous solution has been nearly totally stoichiometrically used up. It is preferred, in this method, that washing off the surface of the object with water is the last step. This method is usable for all mentioned metals or for alloys containing such metals. Tests of radioactive lead plates in particular have shown that this method is extremely simple and quick. The following qualitative conversion takes place during the process of this invention: EQU Pb+2 CH.sub.3 COOH+H.sub.2 O.sub.2 .fwdarw.Pb (CH.sub.3 COOH).sub.2 +H.sub.2 O+PB oxides A dark coating formed on the lead plates by this process is simply washed off by spraying. The stoichiometrically depleted solution is regenerated by separating off a sludge of Pb oxides by sedimentation, which solidifies and is processed as radioactive waste. The remaining solution is electrolytically treated in accordance with the following reactions: Reaction at the cathode: EQU Pb.sup.2+ +2e.sup.- .fwdarw.Pb.sup.o Lead precipitation Reaction at the anode: EQU COOH.sup.- +H.sup.+ .fwdarw.HCOOH Acid regeneration EQU Pb.sup.2+ +O.sub.2.sup.2- .fwdarw.PbO.sub.2 Lead oxide formation The lead precipitation products as well as the lead oxide are radioactive and are solidified with sludge and disposed of. The regenerated acid is radiation-free and suitable for reuse. It is only necessary to set the concentration again.