Fission Product Iodine During Early Hanford-Site Operations: Its Production and Behavior During Fuel Processing, Off-Gas Treatment and Release to the Atmosphere

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FISSION PRODUCT IODINE DURING EARLY HANFORD-SITE FUEL PROCESSING, OFF-GAS TREATMENT AND RELEASE TO THE ATMOSPHERE ITS PRODUCTION AND BEHAVIOR DURING Hanford Environmental Dose the Technical Steering Panel Pacific Northwest Laboratory

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FISSION PRODUCT IODINE DURING EARLY PRODUCTION AND BEHAVIOR DURING FUEL RELEASE TO THE ATMOSPHERE PROCESSING, OFF-GAS TREATMENT AND This document has been reviewed and approved by the Technical Steering Panel. Dose Reconsmction Project

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The appendix to this document isa record o f TSP comments and Bat tell e ' s responses; the TSP has reviewed and approved Bat tell e ' s responses toits comments. . The comment numbers appear i n the left margin next t o the paragraph i n which the corresponding comment was addressed. Changed text is shown in italics. In addition t o responding t o TSP comments, some changes were made t o correct errors or for further clarification.

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The Hanford Environmental Dose Reconstruction (HEDR) Project was established in 1987 to estimate radiation doses that people could have received from nuclear operations at the Hanford Site since 1944. Hanford Site operations began in 1944 to produce plutonium for nuclear weapons. This effort included fabricating fuel elements, irradiating the fuel in nuc'lear reactors, and separating the resulting plutonium from uranium and fission byproducts. During the early years o f Hanford Site plant operation, the quantity of radioactive iodine released to the atmosphere from the fuel processing plants, and its contribution to the total radiation dose to individuals, was greater than that of other radionuclides during this time period. important, therefore, to review the processes that resulted in the release of iodine, the methods developed to suppress its release, and the behavior of iodine in the atmosphere. Three processes were developed for use at the Hanford Site for the separation of p'lutonium, and later uranium, from irradiated uranium metal fuel plutonium compounds; 2) the REDOX process, using '"hexone," an organic solvent, to extract plutonium and uranium; and 3) the PUREX process, using tributyl phosphate as a solvent. All three used nitric acid dissolution of the uranium fuel as the first step; the major release of iodine occurred during this stage. 1) the bismuth phosphate process, using precipitation o f insoluble Because o f the high yield of iodine from the fission process and a knowledge o f its vo7atility, iodine control was expected to be a problem. Unfortunately, little was known o f iodine behavior in the solutions and at the low concentrations involved. known chemistry of iodine and in part on experiments done at the tracer level at Oak Ridge (Site X ) (Dreher 1945). Likewise, there was no experience in removing iodine from large-volume gas streams. actions that could be taken were t o 1) locate the processing plants in the most favorable area with respect to dissipation and dilution by wind, 2) maximize the decay of iodine-131 by storage o f the irradiated fuel before This information was based in part on the Therefore, the only initial

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processing, 3) dilute the off gas at the plant stack, and 4) carry out the dissolver operations during optimal atmospheric conditions. Attention was given to each o f these items. A few months after start - up, disso7ver operat ions were restricted to meteorological conditions that wou7d provide a dilution of at 1 ate matter and that was primar east 500 before the stack plume cou7d reach the ground. species evolved to the gas phase was primarily molecular releases a1 so consisted of considerable amounts o f particumists containing iodides and iodates, and an organic form ly methyl iodide. The dissolver o f f gases and the ventilation off gases were at first released with no treatment. Deep-bed, highefficiency sand filters (99.5%) were developed and installed in late 1948 to remove the radioactive aerosol from the plants' effluent gases. Deep-bed, high-efficiency fiber-glass filters (99.9%) were also developed and type of filter was also used to treat the process vessel vent gases in the REDOX and PUREX plants. 21 At plant startup about 60% to 65% o f the iodine was released to the off gases, roughly 50% from the metal dissolving operation, and another 15% from started in 1951, the amount was closer to 85%. By this time, iodine trappin methods were being greatly improved, and it was much easier t o trap the iodine from the relatively small volume of dissolver off gas than from the very large volume o f gas from the processing facility. filters introduced in 1948, fiber-glass filters in 1950, and the silver reactor filter early in 1951 were found io be capable of removing up to 99.99% o f the iodine. However, the average removal or decontamination fact was considerably less than this because of less than optimum performance o f the control processes. After air sparging of the dissolver solution was The silver nitrate filter, or "silver reactor," became the workhorse for iodine trapping and was applied to all the separations plants. generally satisfactory, but suffered from occasional breakthroughs and deterioration with time, as well as from iodine release due to chlorine The fiber glass filter was the choice for particulate removal

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22,32 Once the gases were released from the plant, the iodine compounds were subject to oxidation and photochemical reactions. Direct deposition and washout by rainfall were the removal pathways. residence time in the atmosphere is only a few minutes for the iodine molecule and about 200 hours for methyl iodide. The final species, which may include iodine dioxide, iodides and iodates as particulates, and perhaps hypoiodous acid and hydroiodic acid in aqueous aerosols, are then deposited by the previously described mechanisms. Stable iodine, iodine-127, is a normal component in the atmosphere, originating largely from sea water, and is therefore present in the atmosphere at all times. This iodine dilutes any radioactive iodine that is released. Accompanying the stable iodine is about one part in I E+12 o f iodine-129, which is produced by natural fission o f uranium and by cosmic rays. Present data suggest that the

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73.1 THE BISMUTH PHOSPHATE PROCESS ............... 4.0 OPERATION OF THE BISMUTH PHOSPHATE PLANTS ...........11 5.1 DISSOLUTION OF URANIUM................... 13 5.4 IODINE BEHAVIOR I N THE REDOX PLANT ............. 19 5.5 IODINE BEHAVIOR IN THE PUREX PLANT ............. 20 6.0 FILTRATION OF GAS STREAMS .................... 21 6.2 THE SILVER REACTOR ..................... 23 6.3 METHYL IODIDE TRAPPING ................... 28 6.4 THE AMMONIA PROBLEM .................... 29

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6.5 NEWER METHODS OF IODINE TRAPPING .............. 29 7.3 TRANSPORT OF IODINE. .................... 36 7.4 CHEMICAL REACTIONS IN THE ATMOSPHERE ............ 37

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3 PUREX Dissolver Off-Gas Treatment ................ 24 4 Distribution of Natural Iodine .................. 33 5 Global Iodine Model ....................... 35 1 Total Daily Iodine-131 Curie Emission for Several Cooling Times and Filter Efficiencies .................... 25 2 Free Energies for Silver Iodide and Silver Iodate Formation 3 Distribution of Natural Iodine .................. 34

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The Hanford Environmental Dose Reconstruction (HEDR) Project was estab1 ished to estimate the radiological dose impact that Hanford Site operations may have made on the local and regional population. This impact is estimated by examining operations involving radioactive materials that were conducted at the Hanford Site from the startup of the first reactor in 1944 to the present. HEDR Project work is divided among several technical tasks. One o f these tasks, Source Terms, is designed to develop quantitative estimates of all significant emissions of radionuclides by Hanford Site operations since for specific radionucl ide transport conditions and population demography. This document provides technical information to assist in the evaluation o f Radiation doses can be estimated from these emissions by accounting The first full-level irradiated uranium was discharged from the Hanford Site reactors late in 1944 as the reactors were gradually brought up to the design power level. After a cooling period the discharged fuel elements, essing facilities for the separation o f plutonium. There the uranium fuel elements were dissolved, the plutonium precipitated, and the fission product waste, along with the excess uranium, was neutralized and sent to storage tanks. A portion of the volatile fission products, primarily iodine, krypton and xenon, was released from solution, and, after dilution with large volumes of air, was discharged to the atmosphere through high stacks. The first complete charge of irradiated uranium (from B Reactor) was dissolved 33,34 knowledge of its volatility, iodine control was expected t o be a problem. Unfortunately, little was known o f iodine behavior in the solutions and a t t h e low concentrations involved. known chemistry o f iodine and in part on experiments.done a t the tracer level a t Oak Ridge (SiteX) (Dreher 1945). Likewise, there was no experience i n removing iodine from large-volume gas streams. Therefore, the only initial actions that could be taken were t o 1) locate the processing plants in the Because ofthe high yield of iodine from the fission process and a This information was based in part on the

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most favorable area with respect to dissipation and dilution by wind, 2) maximize the decay of iodine-131 by storage of the irradiated fuel before processing, 3) dilute the off gas at the plant stack, and 4) carry out the dissolver operations during optimal atmospheric conditions. Attention was given to each of these items. A few months after start-up, dissolver operations were restricted to meteorological conditions that would provide a dilution of at least 500 before the stack plume could reach the ground. The quantity of radioactive iodine released to the atmosphere from the fuel processing plants, and its contribution to the total radiation dose to individuals, was greater than that of other radionuclides during the early years of plant operation at the Hanford Site. summary of the sources of radioactive iodine, the Hanford Site operations that released iodine to the environment, the chemistry involved in the processes, the methods used to trap the iodine at the source, and the behavior of iodine species in the atmosphere. Emphasis is on the first fifteen years of operation at the Hanford Site. This report gives a brief This report does not quantify the release of radioactive iodine from the Hanford Site operations. Hanford Site operations wi 1 1 be addressed in separate reports. Quantities of radioactive materials released by The work described in this report was conducted in accordance with the requirements of ANSI/ASME NQA-1 1986 Edition, Quality Assurance Program Requirements for Nuclear Facilities, as interpreted by the PNL Quality All procedures used to support this report were written and controlled in accordance with PNL QA program requirements. Records that support the data i n this report were created and stored in accordance with applicable HEDR Project record control requirements. Drafts of this document underwent internal independent technical review. Review comments were satisfactorily resolved, and there were no controversial resolutions to the comments.

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Iodine has one stable isotope, of mass 127. It is widely distributed and occurs in all organisms, in rocks, and in large amounts in the oceans. There are 23 other isotopes, all radioactive. O f those produced from fission reactions, the more significant ones are shown in Figure 1 along with the decay half lives and the yields (Holland 1963). Iodine-127 is also produced, with a yield o f about 25% that of iodine-129 (Soldat 1973). 3 7 . 3 8 fuel after a few weeks' decay time. Iodine-129 is the most abundant iodine isotope in irradiated nuclear It is o f some concern in present day Hanford Site reactors, however, only about 1% as much iodine-129 was produced per ton of uranium as is produced in power reactor fue7. small amount produced and its long half life, iodine-129 was considered of minor importance. As shown in Figure 1, the fission yie7d for iodine-129 is by a factor o f one bi77ion. Actually, during the ear7y years of the Hanford Site operation, the true value of the half life of iodine-129 was not known. Ear7y estimates from the Meta77urgica7 Laboratory were 10,000 to 16,000 days, and a 7ater estimate based o n samp7es taken at Hanford was "greater than 3.8 Because of the The correct value is 1.6 E+07 years. Iodine-129 also occurs in nature, being the result of natural fission o f uranium as well as the product of high-energy particles in the upper atmosphere interacting with xenon. is in the range 1 E-15 to 1 E-12 (Soldat 1974). At the present time, with the contributions from nuclear testing, weapons production, a n d nuclear power, it may be in the range 1 E-12 to 3 E-11 (Burger 1980). The natural ratio of iodine-129 to iodine-127

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12 From the environmental point of view, iodine-131 was the important iodine isotope resulting from Hanford processing activities. It decays to stable xenon, with a moderate energy B particle. The fission yield from uranium is about 2.9%, and from plutonium about 3.9% (Holland 1963; Walker et a l . 1977). Because o f its short half life of 8.05 days, it reaches a steady-state concentration in the reactor which i s determined by the power level. This concentration, which i s reached in about 40 days, is about 26 Ci (2.1 E-04 grams) per kW (Holland 1963), which would then be the level at discharge from the reactor. For the early Hanford Site reactors, the concentration at discharge was roughly 1 E+05 Ci per ton of uranium. At 70 days cooling (a typical value), the iodine had decayed to about 240 Ci per ton or about 0.002 g. Unlike iodine-131, the isotopes o f mass 129 and 127 continue to build up in the reactor. Their corresponding concentrations in the typical fuel described here would be 1.6 and 0.4 g/ton, respectively. At this concentration, the activity o f the iodine-129 would be 280 pCi/ton.

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Plants were built at the Hanford Site to separate plutonium, and later uranium, from irradiated fuel elements using three processes: the bismuth phosphate process, the REDOX process, and the PUREX process. 3.1 THE BISMUTH PHOSPHATE PROCESS Three plants, the B, T, and U Plants, were built to use the bismuth phosphate process to separate plutonium from the irradiated fuel. Only the B and T Plants were used for the design purpose. three dissolvers and two holding tanks for the dissolved metal solution. From these, the dissolver solution was jetted into the processing cells for by Ballinger and Hall (1989). Each building contained A more detailed description of the facilities i s given 16 and buildings were under negative pressure relative to the atmosphere. Air - Of particular interest here is the ventilation system. All equipment was pulled from the cells to the stack via a long ventilation tunnel. At the time the plants were built, the process vessels vented directly to the cells. Dissolvers and ventilation air systems exhausted separately to the stack. (Zahn 1953a; 1953b). The dissolvers discharged their off gases, diluted with i n - leakage air, through separate 3-inch stainless steel lines. Vacuum was maintained in the lines by steam jets. Air removal for the entire plant was then provided by two electric fans rated …