Patent Number: 055457940
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) In a first preferred embodiment, a stream of steam is directed through air towards a contaminated metal surface which is to be decontaminated. The solubilization of radioactive contaminants proceeds more rapidly at elevated temperatures; therefore, it is preferred that the stream be applied to the contaminated metal surface for a sufficient period of time to increase the temperature of the contaminated metal surface to a temperature of approximately 90.degree. C., preferably to a temperature of about 100.degree. C. The steam itself is preferentially heated to a temperature between about 110.degree. and about 165.degree. C., and pressurized to between about 10 and about 90 psi preferably between about 10 and about 70 psi. When the contaminated metal surface is sufficiently heated, an aqueous solution of nitric acid and cerium IV is pumped directly into the stream and is thereby carried within the stream whereupon it is applied to the contaminated metal surface. Heat from the steam stream is imparted to the solution, thus it is not necessary to heat the solution prior to its introduction to the stream. Upon contact of the solution-bearing stream with the contaminated metal surface, solubilization of contaminants within the contaminated metal surface is essentially instantaneous. Once solubilized, contaminants are easily removed from the now decontaminated metal surface, by way of example, by washing the part with liquid water. The concentrations of nitric acid and cerium IV, which are useful for decontamination, are well known in the art; nitric acid may range from about 0.1M to about 5M, and cerium IV may range from about 0.01M to about 1M. It will be apparent to one skilled in the art of decontamination that the flow rate and concentration of the aqueous solution of cerium IV and nitric acid, as well as the flow rate of the steam stream, will affect the absolute quantity of cerium IV which will ultimately come in contact, and thereby decontaminate, contaminated metal surfaces. Thus, by increasing either the flow rate or the concentration of the aqueous solution, a contaminated metal surface is exposed more rapidly to a given quantity of cerium IV and nitric acid. Similarly, adjusting the steam stream's flow rate can increase or decrease both the dilution and rate at which the aqueous solution is carried to the contaminated surface. However, it is preferable to expose the contaminated metal surface to only as much nitric acid and cerium IV as is necessary for decontamination, since once decontamination is complete, additional nitric acid and cerium IV simply add to the volume of spent decontamination solution which must be cleaned up or otherwise disposed. Thus, for the present invention, the solution preferably comprises about 2M nitric acid and about 0.5M cerium IV, and is introduced to the steam stream preferably at a rate of approximately 0.5 L/min. In a second preferred embodiment, a stream of steam is injected into a contaminated metal pipe which is to be decontaminated. Again, the solubilization of radioactive contaminants proceeds more rapidly at elevated temperatures. Therefore, it is preferred that the stream be applied to the contaminated metal pipe for a sufficient period of time to increase the temperature of the contaminated metal pipe to approximately 100.degree. C. When the contaminated metal pipe is sufficiently heated, an aqueous solution of nitric acid and cerium IV is introduced into the stream. Again, heat from the steam is imparted to the solution, thus it is not necessary to heat the solution prior to its introduction to the steam. Upon contact of the solution-bearing steam with the contaminated metal pipe, solubilization of contaminants within the contaminated metal pipe is essentially instantaneous. Once solubilized, contaminants are easily removed from the now decontaminated metal pipe, by way of example, by flushing the pipe with liquid water, followed by flushing the pipe with compressed air. EXAMPLE The present invention was utilized to decontaminate a transfer mechanism used for transferring radioactive materials from one radioactive work cell to another. Prior to decontamination, a general survey was performed on the transfer mechanism which measured the dose rate delivered by the transfer mechanism at about 50 R/hr at a distance of about 4 to 6 inches. Operators sprayed the transfer mechanism according to the preferred pressures, temperatures and concentrations of the first preferred embodiment from a distance of about four feet in thirty second bursts. After all exposed surfaces of the transfer mechanism had been sprayed, the transfer mechanism was washed with water and again measured for radiation. The decontaminated transfer mechanism then gave a dose rate of about 25 mR/hr at contact. CLOSURE While a preferred embodiment of the present invention has been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the invention in its broader aspects. The appended claims are therefore intended to cover all such changes and modifications as fall within the true spirit and scope of the invention.