3,803,295 United States Patent Office Patented Apr. 9, 1974 using a diluent carrier isotope as taught by the prior art 3 803 295 was unacceptable for achieving satisfactory iodine re- METHOD FOR REMOVING IODINE FROM moval. NITRIC ACID SUMMARY OF THE INVENTION George I. Gathers and Calvin J. Shipman, Knoxville, Tenn., assignors to the United States of America as 5 It is thus an object of this invention to provide a proc- represented by the United States Atomic Energy Com- ess whereby radioactive iodine may be effectively and mission efficiently removed from nitric acid solutions to concen- No Drawing, Continuation-in-part of application Ser. No. trations which are acceptable for reprocessing require- 231,820, Mar. 6, 1972. This application Dec. 21, 1972, ments. Ser. No. 317,455 10 Int. CI. COlb 7114, 21/44 This object as well as other objects is accomplished U.S. CI. 423—390 8 Claims according to this invention by separating radioactive iodine from nitric acid solutions using isotopic dilution, ozone sparging, and distillation while sparging with a gas ABSTRACT OF THE DISCLOSURE which will reduce nitric acid to an oxide of nitrogen. 15 A method for removing small amounts of radioactive DETAILED DESCRIPTION iodine from nitric acid solution by isotopically diluting the iodine, sparging the solution with ozone, and distilling According to this invention it has been found that the iodine from solution. sparging an isotopically diluted solution with an ozone mixture prior to distillation, produces a solution in which 20 the radioactive iodine is effectively removed upon distil- CROSS REFERENCE TO RELATED APPLICATIONS lation. The reason for this phenomenon is not entirely This is a continuation-in-part of application S.N. 231,- understood; however, the following non-limiting theory 820, filed Mar. 6, 1972. is presented as a possible explanation. The small amount of radioactive iodine remaining in solution after a con- BACKGROUND OF THE INVENTION 25 ventional distillation is probably in a highly oxidized The invention described herein was made in the course state, i.e., either in the iodic or periodic state. When the of, or under, a contract with the United States Atomic nonradioactive iodine diluent is added to the solution, it Energy Commission. is in a lower oxidation state than the radioactive iodine. This invention relates to the art of removing radioac- The differences in the oxidation states prohibit isotopic tive iodine from nitric acid solutions. ^ exchange from occurring. When the solution is sparged In reprocessing fuels from a liquid metal fast breeder with an ozone mixture, the diluent isotope is oxidized to •reactor (LMFBR) it is desirable to dissolve the entire a higher state similar to that of the radioactive isotope, fuel element in nitric acid and to then separate the desired thus facilitating isotopic exchange. radioactive elements one at a time from the solution. The Some of the radioactive iodine is also present as an principal purpose of the reprocessing is to recover urani- organic iodide, such as methyl iodide. The organic iodides um and plutonium for reuse in another fuel element. One are extremely stable and do not release iodine upon distil- of the radioactive elements in solution is iodine as l29I lation. The ozone sparge apparently oxidizes the organic and 131I. The proportions of the activities of these two iodides to leave the iodine in the iodic or periodic state, isotopes vary with time in accordance with their half which will then release iodine upon distillation in accord- lives. In order to protect the environment and operating ance with the method of this invention. personnel from possible exposure to radioactive iodine, As a part of the process of this invention, the iodine it is desirable to separate and isolate the iodine isotopes nitric acid solution is sparged with a gas which will re- from solution prior to further processing. Radioactive duce nitric acid to release an oxide of nitrogen (NO*) iodine is particularly dangerous because of its volatility, during distillation. The preferred sparging gas is N203; and its ability to be carried by vegetation and transmitted however other such gases include N02, S02, H2S, and or- through milk for human ingestion. ganic vapors such as CH3OH. The improved results One prior art method for removing small quantities of which are achieved by using this sparge are consistent a radioactive isotope from solution involves the use of a with the theory presented above. In order for the iodine carrier. A carrier is a material which the radioactive ele- to distill as I2, it is necessary for the oxidized iodine ment will tend to follow. The best carrier for a radioac- 50 species to be reduced. The nitrogen oxide sparge pro- tive element is a nonradioactive isotope of the same ele- motes the reduction of the oxidized iodine species thus ment. making it possible for I2 to be released from the solution In the case of radioactive iodine it was generally during distillation. It is preferred to carry out the sparg- thought that the iodine could be removed from nitric acid gg ing and distillation step at a temperature below the nor- solution by first using conventional distillation followed mal boiling point of the solution. At the normal boiling by isotopic dilution with 127I as a carrier and repeated point of 4 M HNO3 (108° C.) the solubility of NO, further distillation and isotopic dilutions until an ac- gases is very low. The solubility, however, increases ceptably low radioactive iodine concentration was rapidly as the temperature is reduced. The desired chemi- achieved. However, it was discovered that this technique 60 cal reaction is very dependent upon the NOx concentra- was generally ineffective for removing radioactive iodine tion; hence operating at about 105° C. is preferred. How- from nitric acid solutions at molarities of less than about ever, the vapor pressure of the solution is reduced as the 10-6. Apparently upon reaching a low .iodine concentra- temperature is lowered so a lower limit is about 100° C. -6 tion of about 10 M, the remaining radioactive iodine More specifically, according to this invention radioac- is in a nondistillable form, with the result that only the gg tive iodine is removed from nitric acid solutions by first diluent isotope is distilled off while the radioactive isotope distilling about 10 to 20 and preferably about 15% of remains in solution. the volume of a 3 to 6 molar aqueous solution of nitric Reprocessing safety requirements will probably necessi- acid. Under actual plant conditions this original solution tate the removal of radioactive iodine to less than about may have a radioactive iodine molarity of about 10-4. -10 10 molarity for fuels aged less than 30 days and to r,Q After the first distillation, the solution is isotopically di- less than 10-8 molarity for fuels aged more than 30 days. luted with carrier iodine, preferably by adding potassium In view of the above safety requirement, the process of iodide which readily oxidizes to I3. For the sake of con- 3,803,295 3 venience, the carrier iodine is added in an amount such N203 at 300 ml./minute. After a 10 percent cut, the re- that the original iodine concentration is restored. The sidual 131I in solution was 0.0021 percent of the original amount of iodine added, however, must be sufficient to concentration, giving an overall decontamination factor provide a distillable level of iodine, i.e., an iodine molari- of 4.8X104. This leaves a total radioactive iodine molarity ty of at least 2.5 Xl0~5. In order to maintain the nitric of only about 2x 10~18. acid concentration at about 4 M, the original solution In the above example the process of this invention yields volume is preferably restored by adding water to the an overall decontamination factor of 4.8 xlO4 after three solution. distillations. When using the process as taught by the prior At this point, the process of this invention comprises art under similar conditions using only isotopic dilution contacting the isotopically diluted solution with ozonized 10 and distillation without any gas sparging, a decontamina- oxygen to promote isotopic exchange. Preferably, the tion factor of about 100 is achieved after three distilla- oxygen-ozone mixture comprises about 2 volume percent tions. If only the N203 sparge is used, a decontamination ozone; however about 1 to 3 volume percent ozone may factor of about 1200 is achieved. be used. The sparge is conducted at a rate of about 200 It is thus seen that the process of this invention not only 4 ml./minute and preferably 2x 10~ Oa moles per min- overcomes the problems presented by the prior art iso- g 15 ute for 250 ml. of solution, i.e., 10- moles 03/mimite- topic exchange technique but provides a superior method ml., and at a temperature of 70 to 80° C. for a time by which radioactive iodine may be removed from nitric period of from 20 to 30 minutes. acid solutions. Conventional distillation equipment such as that de- What is claimed is: 1 scribed in copending application Ser. No. 231,820 may be 20 1. A method for removing radioactive iodine from used in the distillation steps of this invention. As men- nitric acid solution comprising the steps of isotopically tioned above, improved results are achieved by sparging diluting said solution with nonradioactive iodine to pro- the solution with a nitrogen oxide, preferably N2O3, dur- vide an iodine molarity of at least 2.5 XlO-5, sparging ing distillation. The sparge is carried out at a rate of said solution with ozone, and at a temperature below the about 250 ml./minute and preferably about 4x 10—5 moles 25 normal boiling point of the solution distilling iodine from of N2O3 per minute per ml.