Patent Number: 047042354
Section: description

The invention will now be described in conjunction with some non-limiting examples. EXAMPLES In a number of decontamination tests carried out on samples of Inconel 600 taken from a number of different positions of steam generator in a PWR after an operation time of about 8 years, which samples were the most difficultly decontaminable ones to be obtained, the samples were treated at room temperature (about 20.degree. C.) for 48 hours in an oxidizing solution in accordance with the invention. This solution consisted of an aqueous solution, made acidic to pH 1.4 with nitric acid, of 1.5 g/l of cerium(3)nitrate, 0.1 g/l of chromic acid and 12 g/l of boric acid to which ozone was continuously supplied. The decontamination factors obtained at these experiments were 20-300. In comparative tests carried out on similar samples, i.e. tubes of Inconel 600, with the most effective of the soft processes tested at the Agesta and Surry-II projects, both requiring an operational temperature of 80.degree.-90.degree. C., decontamination factors with an average of merely 6.2 were obtained. In addition to the decontamination tests reported above, corrosion tests have been carried out on blank, non-preoxidized samples of Inconel 600. The test pieces were 3 pieces of steam generator tubes with lengths of 5 cm. To simulate the condition in the rolled zone in the tubes of the tube plate these samples had been rolled internally to half the lengths thereof. The cold deformation obtained was about 5%. The three samples were exposed in parallel in a 100 ml glass container in an aqueous solution containing 12 g/l of boric acid, 1.5 g/l of Ce(3)nitrate and 0.1 g/l of CrO.sub.3 in nitric acid at a pH of about 1.4. Oxygen gas with about 2.5 percent by volume of ozone was bubbled into the same container at a rate of about 0.1 l/min. The temperature was 20.degree. C. and the exposure time was 48 hours. The weight losses during this exposure are accounted for in the Table below. A further exposure cycle, identical with the first one was carried out. The weight losses at this exposure are also presented in the Table. The material loss after each exposure cycle is on an average well below 1 .mu.m, which must be regarded as extremely satisfactory. No signs of local corrosion have been observed. TABLE ______________________________________ Corrosion tests carried out on 3 identical rolled tube samples of Inconel 600 exposed in parallel to each other. Exposure times Cycle Time hours ______________________________________ 1a 48 1b 48 ______________________________________ Tube sample Material loss (.mu.m) No. Cycle 1 Cycle 2 ______________________________________ 0 0.86 0.68 1 0.91 0.58 2 0.98 1.11 ______________________________________