Patent Number: 045129215
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In order to decontaminate the coolant system of a water cooled nuclear power reactor, the reactor must first be shut down and the coolant allowed to cool below 100.degree. C. The decontamination solution is prepared by adding concentrated solutions to the coolant to make the coolant about 0.01M in oxalic acid and about 0.005M in citric acid, by adjusting the pH to about 3 with ammonia, and by adding and maintaining about 0.75 ppm dissolved oxygen in the coolant. The decontamination solution is then circulated at a temperature of about 90.degree. C., throughout the coolant system. As the decontamination solution circulates, the metal oxide films on the surface of the system dissolve and are complexed by the oxalic acid and to a lesser extent by the citric acid. In addition to the complexed metal ions, other particulate matter may be loosened and swept along by the decontamination solution. As the decontamination process proceeds, the dissolved metallic ions are continuously removed and the complexing power of the reagents renewed by passing a portion of the coolant containing the metal-ion complexes through a strong-base anion-exchange resin bed which has been presaturated with oxalic and citric acids in about the same ratio and about the same pH as these reagents are present in the circulating coolant. As the contaminated decontamination solution, which contains a mixture of unutilized or metal-ion-free organic anions and complexed divalent and trivalent metal ions, is passed through the presaturated anion-exchange resin bed wherein the metallic-ion complexes are exchanged for the metal-ion-free organic anions on the resin while any unutilized, metal-ion-free organic anions pass through the resin bed unaffected, whereby this portion of the decontamination solution is renewed and is ready for recirculation throughout the cooling system. The coolant may be made from 0.005 to 0.02M in oxalic acid with about 0.01M being the preferred concentration; lower concentrations result in much slower dissolution rates. Citric acid concentration may vary from about 0.002 to 0.01M with 0.005M being the preferred concentration. The ratio of oxalic acid to citric acid may vary from 1:1 to 10:1 with a ratio of about 2:1 preferred. The citric acid acts as a pH buffer and to retard the formation of ferrous oxalate which may otherwise precipitate and may be difficult to resolubilize. The citric acid may also act as a minor complexing agent. The oxalic and citric acids may be injected together or separately into the coolant as concentrated solutions. The pH of the coolant may vary from about 2.5 to 4.0, preferably 2.8 to 3.5 and most preferably about 3.0 and may be controlled by adjusting the pH with ammonia. Control of pH is important to obtain the highest dissolution rate with the minimum amount of corrosion. Coolant temperature during decontamination may vary from about 60.degree. to 100.degree. C. with 90.degree. C. being preferred. Temperatures above 100.degree. C. cause the organic reagents to decompose while below about 60.degree. C. the dissolution rate is very slow. A small amount of dissolved oxygen should also be added to the circulating coolant during decontamination to ensure complete oxidation of the Fe.sup.+2 to Fe.sup.+3. This is important to prevent formation of ferrous oxalate precipitate. The concentration of oxygen may vary from about 0.2 to 4.0 ppm, preferably about 0.5 to 1.0 ppm. The oxygen may be added by any convenient method, such as the addition of hydrogen peroxide to the coolant, or preferably, by gas injection into one of the flowing coolant systems. The anion-exchange resin may be any commercially available strong base anion exchange resin such as Bio Rad AG-1 or Amberlite IR-400. The resin, which is generally received in hydroxide form must be loaded with oxalate and citrate anions so that the anions on the resin are in chemical equilibrium with the reagents in the decontamination solution. This can best be accomplished by first loading the organic anion on the resin bed from a concentrated solution of the reagents. The resin can then be equilibrated in a stepwise matter with a dilute flushing solution of the same composition as the decontamination solution until the effluent from the bed has about the same concentration of reagents and pH as the decontamination solution. For example, a concentrated oxalic acid-citric acid solution is prepared in which the oxalate-citrate ratio is the desired ratio of the two reagents on the resin when it is in chemical equilibrium with the decontamination solution. This concentrated solution is added to the resin at a controlled rate until the pH is about that desired for the decontamination solution. A dilute oxalic acid-citric acid flushing solution is prepared having the same composition and pH as the decontamination solution. The resin is then flushed in a column with large quantities of the flushing solution until the effluent is the same pH as the decontamination solution. At this time the resin is presaturated and ready for use in regenerating the reagents in the coolant. When the coolant decontamination solution, containing a mixture of the unutilized metal-ion free organic anions and the metallic-ion complexes, is passed through the presaturated anion-exchange resin, the metallic-ion complexes are exchanged for the metal-ion-free organic ions in the resin. The unutilized reagents pass through the resin bed unaffected. Using oxalic acid as an example, the exchange reactions for the Fe.sup.+3 oxalate and Co.sup.+2 oxalic complexes are: EQU Fe(C.sub.2 O.sub.4).sub.3.sup.-3 +3RHC.sub.2 O.sub.4 .fwdarw.R.sub.3 Fe(C.sub.2 O.sub.4).sub.3 +3HC.sub.2 O.sub.4.sup.-, and EQU Co(C.sub.2 O.sub.4).sub.2.sup.-2 +2RHC.sub.2 O.sub.4 .fwdarw.R.sub.2 Co(C.sub.2 O.sub.4).sub.2 +2HC.sub.2 O.sub.4.sup.-, where R stands for the cationic species affixed to molecular structure of the resin. Although these are reversible, equilibirum reactions, they are driven to the right by the thermodynamic preference of the resin for the multicharged metallic-complex ion over the single-charged binoxalate anion and by the multistage sorption effect of the anion-exchange column. The decontamination reagents can be easily removed from the reactor coolant system by passing the coolant containing the reagents, either complexed or uncomplexed through a mixed ion-exchage resin bed, i.e. both anion- and cation-exchange resins, until the conductivity of the solution drops to about 1 .mu.mho. At this point, the coolant is essentially free of reagent and reactor start-up can be commenced. While the method of the invention as described, is applied only to the regeneration of oxalic acid and citric acid systems, the technology could potentially be applied to the regeneration of solutions of a variety of other metal complexing organic chemicals which might be used as decontaminating agents. These include nitrilotriacetic acid (NTA) and hydroxyethylethylenediaminetriacetic acid (HEDTA). The addition of a small amount (5 to 10% by volume) of cation-exchange resin to the presaturated anion-exchange resin could potentially provide a margin of additional capacity for the removal of divalent ions. Since the cation-exchange resin does not sorb appreciable amounts of Fe.sup.+3 from oxalate and citrate solutions, its capacity for removing the divalent metallic ions from the decontaminating solution is essentially independent of the Fe.sup.+3 concentration. Thus, even though it is less efficient initially than the anion-exchange resin for the removal of divalent ions, the cation-exchange resin may become more efficient as the anion-exchange resin reaches saturation with the Fe.sup.+3 complexes. EXAMPLE I To compare the effectiveness of the anion- and cation-exchange resins on the removal of iron and cobalt from a decontamination solution, a typical laboratory ion-exchange column (1 cm.times.30 cm) was loaded with strong-base anion-exchange resin (Bio Rad AG-1) to a height of 20 cm. To presaturate the resin, a solution of 0.02M oxalic acid, adjusted to a pH of 3.0 with NH.sub.4 OH, was passed through the column until the column effluent had the same concentration and pH as the feed. A simulated decontamination solution, consisting of 0.02M oxalic acid with 1.51.times.10.sup.-3 M Fe.sup.+3 and 4.0.times.10.sup.-5 M Co.sup.+2 (6.8.times.10.sup.-3 .mu.Ci/ml Co-60), was passed through the column, and the effluent was sampled periodically. The effluent samples were analyzed for the concentrations of Fe.sup.+3 and Co-60. The Fe.sup.+3 and Co-60 concentrations in the effluent are shown in FIG. I. The anion-exchange resin, presaturated with binoxalate anions, was essentially 100% efficient at removing Co-60 for about 500 bed-void volumes and at removing Fe.sup.+3 for about 550 bed-void volumes. This demonstrates the efficiency of the anion-exchange process for removing the metallic-ion complexes from the solution. Similar experiments to evaluate the cation regeneration process were conducted with hydrogen-ion-form cation-exchange resin and a pH 3 solution of 0.02M oxalic acid with 1.55.times.10.sup.-3 M Fe.sup.+3 and 5.7.times.10.sup.-5 M Co.sup.+2 (7.7.times.10.sup.-3 .mu.Ci/ml Co-60). The results of the Fe.sup.+3 and Co-60 are plotted in FIG. II. These data readily indicate that the Fe.sup.+3 -oxalate complex is not efficiently removed by cation-exchange resin (breakthrough after about one bed-void volume) and efficiency of the cation-exchange resin for removal of Co.sup.+2 decreases after only about 150 bed-void volumes of solution is passed through the column. This premature cobalt breakthrough occurred even though the ion-exchange column was not saturated with the metallic ions. EXAMPLE II An anion exchange resin was presaturated with oxalate and citrate anion in the following manner: A concentrated solution of oxalic acid and citric acid was prepared by dissolving 83.6 g oxalic acid and 33.5 g in citric acid in 1070 ml H.sub.2 O to form a solution 0.62M in oxalate and 0.149M in citrate. The concentrated solution was added to a beaker containing 780 ml of a strong base anion resin in the OH.sup.- form at a controlled rate of 12 ml/min and stirred, until a pH of 3 was achieved. This required about 625 ml of solution. The resin was then loaded into a standard ion exchange column and flushed with a solution of 0.012M oxalic acid and 0.005M citric acid at pH3 until the column effluent had about the same pH and oxalate-citrate concentration as the flushing solution. Table I below shows the correlation between solution volume and oxalate-citrate concentration. TABLE I ______________________________________ Total Solution Oxalate-Citrate Volume ml pH Concentration ______________________________________ 800 4.11 Not Determined 6200 4.10 Not Determined 18200 3.43 Oxalate-0.0085 M Citrate-0.008 M 29900 3.19 Not Determined 40900 2.98 Oxalate-0.0116 M Citrate 0.0049 M ______________________________________ The resin was then presaturated and ready for regeneration of the decontamination solution. EXAMPLE III An ion-exchange column breakthrough experiment was conducted to evaluate the elution sequence and the capacity of a mixed-bed of cation and presaturated anion resin used for the regeneration process. A solution of 0.01M oxalic acid and 0.005M citric acid at pH 3 containing 0.003M Fe.sup.+3 and 0.0001M Cr.sup.+3, Ni.sup.+2, Co.sup.+2, Zn.sup.+2, Mn.sup.+2, Cu.sup.+2, and Fe.sup.+2 was passed through a 90/10 mixture of anion and cation resins until the effluent and feed concentrations were similar. The effluent was sampled periodically and analyzed for metal ion concentrations by plasma spectrometry; the Fe.sup.+2 concentrations were determined spectrophotometrically. The elution sequence was (Fe.sup.+3,Cr.sup.+3), Cu.sup.+2, (Ni.sup.+2, Zn.sup.+2, Co.sup.+2), Mn.sup.+2 and Fe.sup.+2, which is in agreement with the oxalate complex stabilities for the various ions. These data indicate the quantity of cation resin added to the pre-saturated anion resin to provide back-up Co-60 capacity for the regeneration process must have sufficient capacity to adsorb all of the divalent corrosion products except Cu. In addition, these data indicated the Fe.sup.+3 and Cr.sup.+3 oxalate complexes have similar affinities for the anion-exchange resin since their breakthroughs occurred simultaneously and their final concentrations on the resin were proportional to their solution concentrations. The capacity of the presaturated anion-exchange resin for trivalent ions was determined to be 0.47 moles/liter, which is equivalent to the theoretical capacity. The capacity of the cation-exchange resin for divalent-ions was determined to be 0.33 moles/liter, which is approximately 40% of the theoretical capacity. EXAMPLE IV A circulating test loop was prepared to study the effects of the decontamination reagents on the removal of iron oxides and cobalt from reactor coolant system piping and to determine the efficiency of the reagent regeneration. A solvent consisting of 0.01M oxalic acid and 0.005M citric acid at pH3 was circulated through the loop. The dissolved oxygen content of the solvent was maintained within the specification of 0.75.+-.0.25 ppm. No ferrous oxalate precipitation was observed. Approximately 85% of the Co-60 activity was removed over the 12-hour dissolution cycle. Within a few minutes after the initial injection of the reagents, the loop chemistry and operating conditions were stabilized. All of the operating parameters were maintained within specification for the remainder of the run. The Co-60 activity in solution increased to a maximum during the first few hours of the test and then it decreased as the oxide film dissolution rate decreased and was exceeded by the solvent regeneration rate. The maximum Co-60 concentration obtained was equal to approximately 6.7% of the total Co-60 dissolved during the test. The oxalic acid and citric acid concentrations were maintained within specification during the decontamination cycle. The iron concentration in solution was maintained below 6% of the total iron dissolved. As can be seen from the preceeding discussion and Examples, the process of this invention provides an effective method for the decontamination of the coolant systems of water cooled nuclear power reactors by providing an efficient and effective method for continuously regenerating the reagents used for the decontamination process.