Abstract:
Radioactively contaminated material is cleaned by contacting the material with a decontaminating liquid comprising an aqueous solution of nitric acid containing an NOx generating agent. The NOx generating agent may be a nitrite, for example, sodium nitrite, or a ferrous metal. The material to be cleaned may comprise a plastics material contaminated with uranium or other actinides. Cleaning is effected by placing the material in a rotatable, apertured vessel in which the material is subjected to a leaching cycle by contact with the decontaminating liquid and then a washing cycle in which the material is contacted with a washing liquid.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the cleaning of radioactively contaminated material and, more particularly to the cleaning of radioactively contaminated plastics material. 
     BACKGROUND OF THE INVENTION 
     Before consignment of radioactively contaminated material to a waste disposal site, for example, a waste landfill site, it must be ensured that the contamination of the material is below the specified disposal limits of the site. It is therefore often necessary to treat the contaminated materials before disposal in order to ensure that the contamination levels are within the disposal limits. 
     Products, for example gloves and sheets, made of plastics material are widely used in the processing and handling of radioactive material. Difficulties have been experienced in cleaning such material sufficiently to enable it to be disposed of safely. 
     Attempts to clean contaminated plastics material by simply subjecting the material to a nitric acid leaching operation, followed by at least one washing cycle have proved to be unsatisfactory. It was found that the material had not been cleaned sufficiently to enable safe disposal. 
     A known process for cleaning contaminated waste plastics material is described in International Publication No. 95/16997. This process comprises washing the material in water which contains a strong base, such as soda or potash in aqueous solution. Optionally, a wetting agent, preferably non-foaming, may also be added to the water. During the washing operation a saponification reaction occurs so that the material is subjected to a selective chemical treatment whereby certain surface agents, for example, plasticisers, which contain most of the contaminants are attacked. The washed material is then rinsed in water. 
     A disadvantage of this process is that the contaminants, such as uranic substances, are not rendered soluble and this presents certain difficulties in their recovery. Recovery must be effected by a solid-liquid separation process, such as filtration, followed by either direct leaching of the uranic substances from the filter, or by physical removal of the solids from the filter and then leaching the uranic substances from the removed solids. 
     It is an object of the present invention to provide a method of cleaning radioactively contaminated plastics material which is efficient and enables the treated material to be disposed of safely. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a method of cleaning plastics material contaminated with radioactive substances comprises the step of contacting the plastics material with a decontaminating liquid comprising an aqueous solution of nitric acid which contains a NOx generating agent. 
     Preferably the NOx generating agent comprises a nitrite. 
     Advantageously, the NOx generating agent comprises sodium nitrite. 
     Alternatively, the NOx generating agent may comprise a ferrous metal. 
     Preferably the method includes the step of agitating the decontaminating liquid. 
     The method may include the further step of washing the plastics material following contact thereof with the decontaminating liquid. 
     The method may comprise the steps of placing the contaminated plastics material in a rotatable vessel having one or more apertures, subjecting the material to a leaching cycle comprising supplying the decontaminating liquid to the inside of the vessel, and rotating said vessel whereby said decontaminating liquid is agitated and mixed with the contaminated material, terminating the rotation of the vessel and discharging the decontaminating liquid therefrom. 
     The method may further comprise subjecting the material to a washing cycle comprising supplying a washing liquid to the inside of the vessel, rotating said vessel to enable the washing liquid to mix with the material, terminating the rotation of the vessel and then discharging the washing material therefrom. 
     Preferably the material is subjected to at least one further washing cycle. 
     The material may be subjected to three washing cycles. 
     Suitably the contaminated material may be held in a container having one or more perforations. 
     The decontaminating liquid may have a nitric acid molar concentration having a value within a range of 3M to 5M, the preferred value being 4M. 
     The material to be cleaned may be contaminated with uranic substances. 
     An advantage of the method according to the present invention is that it is compatible with processes used in the nuclear industry for the recovery of uranium and for its reincorporation in the uranium fuel cycle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a graph showing the effect of sodium nitrite addition on the sample cleaning time; 
     FIG. 2 is a diagrammatic cross-sectional plan view of an apparatus for cleaning radioactively contaminated plastics material; and 
     FIG. 3 is a schematic layout of a cleaning apparatus incorporating the apparatus as shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     During decontamination trials for cleaning plastics material contaminated with uranic substances using an aqueous nitric acid solution, it was found that there was a reaction between a ferrous metal component of the apparatus and the nitric acid which had a beneficial effect in the cleaning operation. This reaction between the ferrous metal, specifically, mild steel, and the nitric acid produces both ferric nitrate and nitrogen oxide (NOx) gases within the nitric acid. Laboratory tests showed that the presence of ferric nitrate in the nitric acid had no effect on the ability of the acid to clean the plastics material. However, the addition of iron filings to the acid resulted in an almost immediate effect on the contaminated plastics material submerged in the nitric acid. Within seconds the material was totally clean with no evidence of any discoloration. Further tests, where the NOx gas from the iron/nitric acid reaction was bubbled into the nitric acid showed that it was the presence of the NOx gas which was an important factor in the successful cleaning of the material. 
     It will be appreciated that, although the use of iron as an additive to the process has beneficial cleaning results, its presence may cause problems in the subsequent processing of the recovered uranium. A NOx generating additive which would be acceptable at the downstream processing stage is a suitable nitrite, such as sodium nitrite. 
     An investigation to discover the effect of sodium nitrite on the time required to decontaminate plastics material was carried out as follows: 
     EXAMPLE 
     Plastics material contaminated with approximately 2.5 to 3.5% w/w of uranium dioxide was shredded using a heavy duty office paper shredder. Different masses of sodium nitrite were added to batches of an aqueous solution of 4M nitric acid in which a small swatch, or sample, of approximately 0.2 grams material was submerged. The time taken for the swatch to become visibly clean in the unstirred solution was noted. The results of the tests are shown graphically in FIG. 1 in which the swatch time in minutes is plotted against the mass, in grams, of sodium nitrite added per 100 ml of 4M nitric acid. 
     Swatch time represents the time taken for the sample of plastics material to be rendered clean. Since it was recognised that the inherent instability of the nitrite in an acid medium, the use of the terms nitrite concentration would be meaningless, hence the sodium nitrite addition is expressed as in terms of mass added per 100 ml of 4M nitric acid. 
     In one embodiment of the invention the plastics material to be cleaned is placed in a vessel containing a decontaminating liquid comprising an aqueous nitric acid solution to which sodium nitrite has been added. The vessel is equipped with a suitable agitator or stirrer which is operated to agitate the solution. The sodium nitrite reacts with the nitric acid solution to generate NOx gases in the solution which is effective to clean the plastics material. 
     A machine and associated equipment suitable for cleaning contaminated plastics material on a commercial scale is shown diagrammatically in FIGS. 2 and 3, to which reference is now made. The machine comprises a housing  2  having an access opening  3  normally closed by a door  4  which is pivotably mounted at  5  and has a lockable fastening device  6 . Seals are provided to ensure that the door  4  is watertight when closed. Interlocks ensure that the door cannot be opened when the machine  1  is in operation. Inside the housing  2  is a cylindrical vessel, preferably a drum  7 , having a cylindrical wall perforated by a plurality of holes and arranged for rotation about a horizontal axis within a stationary cylindrical casing  8 . Preferably, the drum  7  and the casing  8  are made from stainless steel. The drum  7  has an open end adjacent to the door  4  and is fixedly mounted on a shaft  9  which extends rearwardly through the outer casing  8 . A driven pulley  10 , mounted on the end of the shaft  9 , is rotated by a driving belt  11 . Movement of the driving belt  11 , and hence rotation of the drum  7 , is derived from a drive assembly  12  which may comprise an electric motor and gearbox having a variable speed output. It will be appreciated that other types of variable speed driving arrangements for the drum could be used. A radiation measuring instrument  13 , for example, a gamma radiation monitor, may be fitted to the outside of the housing  2 . 
     A schematic layout of a simplified pipework system is shown in FIG. 2 in which the cleaning machine  1  is connected to a tank  14  containing an aqueous nitric acid solution, and a tank  15  containing a washing liquid, preferably water. Suitably, the molar concentration of the nitric acid may be within the range of 3M to 5M, the preferred value being 4M. The machine  1  is equipped with a supply pump  16  and a discharge pump  17 . Each of the pumps  16 ,  17  is preferably of the type comprising a stainless steel, double-diaphragm pump operated by compressed air supplied through lines  18 . The supply pump  16  is connected by a pipe  19 , provided with a valve  20 , to the nitric acid tank  14  and by a pipe  21 , equipped with a valve  22 , to the water tank  15 . Similarly, the discharge pump  17  is connected by a pipe  23 , provided with a valve  24 , to the nitric acid tank  14  and to the water tank  15  by a pipe  25  having a valve  26 . Nitric acid can be supplied to the tank  14  through a pipe  27  and water can be supplied to the tank  15  through a pipe  28 . A dispenser  29  is provided for supplying a suitable NOx generating agent, preferably sodium nitrite to the interior of the machine. 
     In use, the door  4  is opened and the permeable bag  30  containing shredded, contaminated plastics material  31  is inserted through the access opening  4  into the drum  7 . Several bags  30  may be treated simultaneously. The door  4  is then closed and it is ensured that the valve  20  is open and that the valves  22 ,  24  and  26  are closed. A leaching cycle is then initiated by supplying compressed air through the line  18  to the diaphragm pump  16  which operates to pump the nitric acid from the tank  14  through the pipe  20  into the machine  1 . The nitric acid is directed into the casing  8  and passes through the perforated wall of the drum  7 . Sodium nitrite is introduced from the powder dispenser  29  into the drum  7  of the machine  1 . Alternatively, the sodium nitrite can be held in a perforated container which is placed directly into the drum  7  when inserting the bags  30 . Typically, the amount of sodium nitrite used is 1000 g for a 10 kg load of plastics material. 
     The sodium nitrite functions to generate NOx gases in the nitric acid to form a decontaminating liquid. When there is sufficient decontaminating liquid in the machine  1 , the drive assembly  12  is operated to cause rotation of the drum  7  at, say 30 rpm. The permeability of the bag  30  allows the decontaminating liquid to act on the plastics material  31 , but will prevent the material from blocking the apertures in the drum  7 . Rotation of the drum  7  agitates the leaching liquid and promotes intimate mixing of the decontaminating liquid and the plastics material. 
     Evidently, the chemical process which effects the cleaning of the plastics material is extremely complex. However, it is believed that the NOx gases attack the material surrounding the uranic substances so that these substances are dislodged and released into the leaching liquid. It is apparent that the rate of decontamination is determined by the initial conditions within the washing machine and not by the instantaneous conditions during the leaching process. During the first few moments of the leaching process, it is possible that the plastics material adsorbs the ‘active species’ which carry out the process of decontamination. The amount of ‘species’ adsorbed is a function only of the initial conditions within the washing machine. 
     If desired, the drum  7  may be rotated for a period in the opposite direction, or in successive clockwise and anti-clockwise directions, to enhance the mixing of the leaching liquid with the plastics material. After a period of time, say 15-90 minutes, rotation of the drum  7  is stopped and the pump  17  is operated to pump the decontaminating liquid from the machine  1  to the tank  14  through the pipe  25  and the valve  26 , which had been opened previously. Optionally, the drum  7  may then be rotated at a high speed, for example at 400 rpm to subject the material to a spin-drying operation by ejecting further decontaminating liquid from the material, the ejected decontaminating liquid then being pumped to the tank  14 . A washing cycle is then started by operating the pump  16  with the valve  20  closed and the valve  22  open. Water is thus delivered from the tank  15  through the pipe  21  to the machine  1 . By operation of the drive assembly  12  the drum is rotated at, say 30 rpm so that the water mixes intimately with the plastics material  31  and washes out the dissolved uranium substances which have remained in the medium following the leaching cycle. After a period of time, typically 10 to 15 minutes, rotation of the drum  7  is stopped and, with the valve  24  open and the valve  26  closed, the pump  17  is operated to return the water to the tank  15  through the pipe  23 . If required, the washing cycle may be repeated. We have found, in practice, that three washing cycles produces satisfactory results. 
     For a nominal load of contaminated plastics material weighing 10 kg and using 1000 kg sodium nitrate a typical acid leaching cycle has a duration of 60 minutes, followed by three water washing cycles, each of 10 minutes duration. 
     The drum  7  may then be rotated at a high speed, typically 400 rpm, so as to subject the material  31  to a spin-drying process whereby excess moisture is ejected from the medium. Preferably the drum  7  is rotated at a speed sufficient to subject the material  31  to a centrifugal force in the region of 150 g. Following the spin-drying operation the bag  30  containing the dried, treated and cleaned material  31  can be removed from the machine  1 . 
     The radioactivity of the contents of the machine  1  can be measured by the gamma monitor  13 . Before removal of the bags  30  from the machine the gamma monitor  13  can be used to check whether the treated filter medium has been cleaned sufficiently to permit safe disposal. If desired, a separate monitoring station can be provided for checking the contamination level of the treated material. It has been found that decontamination factors in excess of 100 can be achieved. 
     In practice, the operating sequence and duration of the operation of the pumps, valves and drive means are carried out automatically in accordance with a predetermined programme. Variations in the cycle times can be effected by modifying the programme.