Patent Number: 055235131
Section: description

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS In the process illustrated in FIG. 1, boxes represent steps in the process, full lines with arrows represent flow of liquids and broken lines with arrows represent transfer of solids. A body to be decontaminated (not shown) is contacted in a contacting stage 1 with HBF.sub.4 solution from a supply 3. A spent HBF.sub.4 decontamination liquor containing material including contaminants removed from the body surface is produced thereby. The liquor is passed through a filtration stage 5 to remove solid matter, eg paint, algae and some undissolved radionuclides or metals etc removed from the body surface. When the liquor has been filtered it is passed to a treatment vessel in which precipitation 7 is carried out to provide HBF.sub.4 regeneration. Firstly, oxalic acid from a source 9 is added to the liquor to form oxalate precipitates in the manner described above. The liquor is then passed through a filtration stage 11 to remove the oxalate precipitates and returned to the treatment vessel for further precipitation treatment. Nest, KMnO.sub.4 in solid form is applied from a source 13 to the decontamination liquor. A trace volume of H.sub.2 O.sub.2 from a source 15 is added to the liquor to increase the reaction by the KMnO.sub.4. The liquor is then passed through a filtration stage 17 to remove the precipitate so formed. The filtered liquor comprising nearly pure regenerated HBF.sub.4 solution is thereafter passed through an ion exchange stage 19 to provide further purification of the solution. The clean HBF.sub.4 solution produced thereby may be re-applied at further discrete stages or continuously to the contacting stage 1 via a recirculation loop 20 to provide further decontamination of the object. At the end of the decontamination of the object, the decontamination liquor is returned to the vessel in which precipitation 7 is carried out. Calcium hydroxide is applied from a source 21 to neutralise the HBF.sub.4 acid. The precipitate so produced is filtered in a filtration stage 23 and the resultant neutralised, filtered liquor, which may be further purified by passage through the ion exchange stage 19, is subsequently discharged as a substantially clean, neutral liquid effluent stream 25. Solid matter comprising filtrate and filters containing them from the filtration stage 5, 11, 17 and 23 and spent ion exchange material (eg in the form of a cartridge) from the ion exchange stage 19 is transferred to a common solids waste 27 which is treated where appropriate by calcining for subsequent assay, storage and onward transport and disposal as ILW or LLW as appropriate, In a variation of this process the oxidation treatment by addition of potassium permangonate may be replaced by photolytic oxidative treatment of the decontamination liquor using UV radiation in a known way. This treatment may be followed by the addition of H.sub.2 O.sub.2 and the other following steps described above. EXAMPLE 1 In a first example of a method embodying the present invention decontamination of very large, highly contaminated components was carried out by a remotely controlled process. A number of very large lifting beams, used for the transfer of spent nuclear fuel in skips around fuel storage ponds, were required to be decontaminated. FIGS. 2 and 3 illustrate the size and shape of the lifting beams. Contamination levels were measured as giving rise to contact dose rates of 20 mSv/hr which required the method to be performed remotely. Initial laboratory trials were performed to determine the concentration and temperature of tetrafluoroboric acid to be used. It was apparent that an immersion method would require a very large volume of decontamination liquor owing to the size of the lifting beams. Spraying was therefore chosen as the contacting method. Further laboratory trials were conducted to optimise the spraying. A 1% solution of HBF.sub.4 at a temperature of 60.degree. C. was identified as the optimum for the decontamination solvent. Lifting beams of the types shown in FIGS. 2 and 3 (four in total) were decontaminated removing 0.7 GBq of Co.sup.60 and 0.64 GBq of Cs.sup.137, in total 1.34 GBq were removed. Treatment of the contaminated liquor was performed generally in the manner shown in FIG. 1. The first treatment involved the addition of oxalic acid. Further treatment in the manner described above was applied using potassium permanganate addition followed by H.sub.2 O.sub.2 addition. Subsequently, the resulting liquor was filtered and then passed through inorganic ion exchange adsorbers for extraction of residual caesium left in solution after filtration of the oxalates. At the end of the several decontamination cycles of the process the HBF.sub.4 solution was finally neutralised with calcium hydroxide. The calcium fluoride thereby produced was removed by filtration and the near neutral solution which was found to contain 8 Bq/ml Co.sup.60 and 3 Bq/ml Cs.sup.137 was discharged to a pond water treatment plant for further purification and de-activation. Results obtained in this Example are shown in FIG. 4 where gamma and beta activities of the liquor containing contaminants before acid regeneration and dissolved iron concentrations are plotted together on the vertical axis against sample numbers on the X axis. Samples of the liquor were taken and measured every 24 hours throughout a 17 day continuous decontamination programme. FIG. 4 demonstrates correlation between iron removed and contamination removed, validating the findings of the laboratory contamination profiling experiments which had previously been carried out. The efficiency of the filters is demonstrated by the graph between samples 1 and 2 where activity notably decreases when a fouling layer has been built up and again, after sample 8, when a filter has been replaced; activity increases and then falls when a fouling layer has been built up after this filter replacement. After ten days it was decided to increase acid concentration in the process to 5% by volume. This improved decontamination rates due to improved reaction kinetics, this improvement is demonstrated by the steep rise in the graph between samples 10 and 12. The final lifting beam, a lightly contaminated, heavily painted component, was introduced after sample 12. The graph shows a slight increase in radionuclide inventory at this stage arising to the light contamination and moderate increase in dissolved iron due to the item being largely protected by a paint layer. The rapid fall in beta and gamma activity and the almost complete removal of dissolved iron between samples 14 and 15 in FIG. 4 corresponds with the first regeneration/waste removal step using oxalic acid/potassium permanganate. A further decrease between samples 15 and 16 represents the addition of inorganic ion exchange adsorbers and the final decrease in beta and gamma activity between samples 16 and 17 corresponds with the addition of calcium hydroxide. The graph shows that the waste treatment/acid regeneration step applied after sample 14 provided a 100% reduction in dissolved iron, a 99.9% reduction in beta activity and 99.95% reduction in gamma activity. Complete removal into waste form was thereby obtained for dissolved iron and a 99.9% removal into solid waste form was obtained for the radioactive contaminants. As a result of the above treatment all of the beams were decontaminated to an activity level less than a 100 .mu.Sv target in less than a month. The beams were suitable to return to service thereafter. The volume of solid waste produced was only 1.1 m.sup.3 which was suitable for disposal via an existing LLW route. This waste contained 99.9% of the removed radionuclide contaminants. The volume of spent near neutral liquor which was discharged to a water treatment plant was only 2 m.sup.3. EXAMPLE 2 In a second example of a method embodying the present invention, a highly contaminated redundant plant components which had been employed in the production of mixed uranium oxide/plutonium oxide fuel were required to be decontaminated. In this example, the tetrafluoroboric acid solution was continuously regenerated during the decontamination process to ensure that levels of fissile material were maintained at sub-critical masses below the acceptance criteria for plutonium contaminated waste of 450 g/200 L of waste. This had been achieved in laboratory trials using a 5% solution of tetrafluoroboric acid at a temperature of 80.degree. C. to decontaminate mild steel, stainless steel, painted steel, aluminium, brass and plastics components having maximum alpha contamination levels in excess of 3000 cps. Using a 1 liter batch of HBF.sub.4 solution the decontamination of the contaminated plant components was carried out by concentrating dissolved iron to a level of 22 g/L and 2.2.times.10.sup.8 Bq of alpha activity/L before requiring regeneration. Regeneration of the acid decontaminant was accomplished by oxalic acid coprecipitation of iron and plutonium oxalate and americium adsorption using potassium permanganate to generate manganese dioxide in the manner described above. The following results were obtained in the decontamination process. Activity in solution before acid regeneration=2.2.times.10.sup.8 Bq PA1 Activity in solution after acid regeneration=5.times.10.sup.2 Bq/ml PA1 Solution capacity for iron before acid regeneration=22 g/L PA1 Solution capacity for iron after acid regeneration=20 g/L PA1 Rate of metal dissolution before acid regeneration--mild steel=20 .mu.m/hr PA1 Rate of metal dissolution after acid regeneration--mild Steel=25 .mu.m/hr The increased rate of dissolution after acid regeneration can be attributed to some slight concentration of hydrogen peroxide in the regenerated acid.