Patent Number: 055641059
Section: description

DESCRIPTION OF THE PREFERRED PRACTICES In a preferred practice of the present invention, a radioactive aqueous waste solution is generated in the course of decontaminating the reactor coolant system of a pressurized water reactor by a LOMI process, a CAN-DECON process or a CAN-DEREM process, which is a CAN-DECON improvement that does not employ oxalic acid. The above-mentioned M. E. Pick et al. paper from Nuclear Energy is hereby incorporated by reference for its discussion of these processes and various permanganate oxidation processes which may be employed in alternating steps to decontaminate entire reactor coolant systems or their components. In these decontamination processes, an aqueous solution at about 180.degree. F. to about 210.degree. F. or more and containing a cation-complexing agent is circulated throughout the system to dissolve iron and other metals in radioactive oxide films on the wetted surfaces of vessels, pipes, pumps and the like. Thus, the aqueous solution will contain up to about 1000 ppm or more of one or more carboxylic acids such EDTA, hydroethylenediamine-tetraacetic acid (HEDTA), nitrilotriacetic acid (NTA), oxalic acid, citric acid, picolinic acid, or formic acid or salts thereof. As the aqueous solution circulates through the system, it becomes contaminated with complexes of iron (Fe.sup.+2 and/or Fe.sup.+3), cobalt (Co.sup.+2), nickel (Ni.sup.+2 and/or Ni.sup.+3), chromium (Cr.sup.+3 and/or Cr.sup.+6), manganese (Mn.sup.+2) or other metals from the surface oxides. In the preferred practice of the present invention in the course of decontaminating a nuclear facility, an aqueous solution containing less than about one percent by weight of a carboxylic complexing agent contaminated principally by iron, cobalt and nickel as well as other cations, is pumped out of the reactor coolant system after the solution has circulated throughout the system for up to about thirty minutes to an hour or longer. An oxidizing agent is then added to the contaminated solution to oxidize the complexing agent and thereby to destroy the complex. Accordingly, ozone, hydrogen peroxide or a chlorite solution may be added to the contaminated solution, which reacts with the complexing acid to form carbon dioxide and water. This oxidation step may be enhanced with ultraviolet light. The temperature of the solution is preferably maintained from about 140.degree. F. to about 212.degree. F. Advantageously, the cations tend to precipitate and may then be separated from the solution. Iron may also be magnetically separated from the solution. Centrifugal filtration or micromembrane belt filtration may be employed to separate the cations. The small volume of precipitate may then be buried. The water may be reused or further treated and released for other uses. The present invention was demonstrated by fifteen gallon batch tests performed on surrogate contaminated, CAN-DECON/CAN-DEREM and LOMI solutions from simulated pressurized water nuclear reactor coolant system decontamination processes. In a first test, a contaminated CAN-DECON/CAN-DEREM solution was simulated by an aqueous solution containing 1000 ppm of a well known London Nuclear Company decontaminant agent known as LND 107 (which comprises about 60% EDTA and 40% citric acid with 360 ppm Total Organic Carbon ([TOC]), 650 ppm boron, 113 ppm Fe.sup..sup.+2, 21.5 ppm Ni.sup.+2 and 1.27 ppm Co.sup.+2. The boron is employed in pressurized water reactor coolant systems as a moderator. The temperature of the solution was maintained at about 180.degree. F. and hydrogen peroxide was added in three additions over a period of about 50 minutes. At the end of this test, the solution contained about 3.5% hydrogen peroxide and 0.03 ppm Fe. The cobalt and nickel concentrations were not determined and would, if necessary in a commercial facility, be reduced by pH control. The iron from the destroyed ion complexes precipitated from the solution. FIG. 1 shows that the TOC of the solution was reduced from 360 ppm to less than about 50 ppm in about 50 minutes. In a second test, a contaminated CAN-DECON/CAN-DEREM solution was simulated by an aqueous solution containing 1000 ppm of decontaminant agent LND 107 (which comprised about 60% EDTA and 40% citric acid with 360 ppm TOC), 650 ppm boron, 113 ppm Fe.sup.+2, 21.5 ppm Ni.sup.+2 and 1.27 ppm Co.sup.+2. Hydrogen peroxide was continuously added to the solution over a period of about 20 minutes. At the end of this test, the solution contained 0.1% hydrogen peroxide and less than 1 ppm Fe.sup.+2 and had a final pH of 6.6. The cobalt and nickel concentrations were not determined. The iron from the destroyed complexes precipitated from the solution. FIG. 2 shows that the TOC of the solution was reduced from about 300 ppm to a negligible amount in about ten minutes. A comparison of FIGS. 1 and 2 indicates that the continuous addition of the oxidizing agent is more effective in reducing the TOC than is periodic addition of the agent. In a third test, a contaminated LOMI solution was simulated by an aqueous solution containing 32 ml vanadous formate/liter solution, 103 ppm Fe.sup.+2, 60 ppm Ni.sup.+2 and 2.3 ppm Co.sup.+2. The initial TOC was 2040 ppm and the pH was 4.7. Initially, ultraviolet light was employed to oxidize the formate ions and the hydrogen peroxide concentration of the contaminated solution was less than about 0.25%. FIG. 3 shows that the TOC of the solution was substantially unchanged during this period and substantially no precipitation occurred in this time. Iron precipitation later occurred when the hydrogen peroxide concentration of the solution was raised up to 8%. The present invention is also useful to concentrate heavy metals such as mercury and lead which are washed from equipment or soil by soap or detergent solutions. The present invention is also useful in similar processes where hazardous substances such as dioxins, furans and polyphenols are emulsified by soap or detergent solutions. While a present preferred embodiment of the present invention has been shown and described, it is to be understood that the invention may be otherwise variously embodied within the scope of the following claims of invention.