Attachment #2 1298 Movement of Radionuclides Through the Environment It can be concluded without doubt, however, that short food chains with high transfer factors can be dangerous and perhaps produce unexpected surprises. These phenomena in the Arctic strengthen further the conclusion that cesium should have been designated as a crucial "radionuclide." 3. Iodine a. General In earlier chapters, we considered the development of ideas concerning iodine releases from nuclear reactors and from chemical processing plants, and the large enterprise that surrounded iodine in fallout. We saw that it was iodine in the environment that probably first focused attention on the importance of food contamination from fallout. We reviewed in some detail the work done in Utah and Nevada regarding milk contamInation from the Nevada tests and the generalized milk networks set up around global fallout. We also traced the gradual shift in emphasis from thinking of iodine as primarily an inhalation hazard in occupational exposure or local releases to primary concern about its presence in milk and dairy products. Yet, there are some aspects of iodine transfer in the environment that need to be considered further before we can close the discussion. These aspects will be reviewed briefly in this section. Appropriate to the fact that much of the work on iodine in the environment began at Hanford is the fact that Hanson wrote a succinct review on iodine for the First National Symposium on Radioecology, described in chapter 11 (Hanson 1963). This review included both stable and radioiodine in many media, the utilization of iodine by plants and animals, and the basis for transfer factors that could be used in later models. At the same symposium, French and Larson, from UCLA, discussed environmental pathways for iodine from nuclear tests in arid regions (French and Larson 1963). These and many others made it quite clear that the chemistry of iodine in reactors, in released clouds, and in the environment can be quite complex, but that surface adsorption can play a prominent role at the forage step. Evidence for a predominant role of ingestion came from two facts: (1) animals existing in a contaminated environment, but prevented from ingesting contaminated material, showed little or no radioiodine in their thyroids; and (2) thyroid radioiodine continues to increase for several days after a contaminating event, a course not expected after inhalation. but consistent with continuing ingestion. Soldat (1963, 1965) added to insight into the passage of 131 in food chains by measuring the kinetics of its behavior in various media, including air, pasture, milk, and cattle thyroids. The work was reported at the Hanford life sciences symposium on the biology of radioiodine in 1963. An average delay of about six days occurred between the peak in vegetation and appearance in milk, about twelve days between peak concentration in grass and in the cow's thyroid. (The British developed slightly shorter times from their investigations around Wind scale.) Average ratios or several components needed to estimate transfer functions were developed: pc/kg of grass = 4,200 pc/g thyroid = 0.45 pc/l milk = 600 pc/m3 of air pc/kg grass pc/m3 air pc/l of milk = 0.15 pc/g thyroid = 3.0 pc/kg grass pc/l milk Chapter 15 1299 Soldat also gave the ratio of doses to the thyroid from milk compared to ___-20 for a 20-g thyroid and 400 for a 2-g thyroid. The UNSCEAR 1972 report has much to say about the complexity and variability of the behavior of iodine in the environment. The amount of activity in the form of particles can vary from 10% to 90%, with the balance in gaseous form. Doses can be calculated only if local deposit and transfer patterns are known or __ milk levels (fresh milk if the principal source among the foods) are known. Also, neither field nor laboratory techniques for measuring iodine separate from gross beta activity were available in the years of most feverish activity in measuring and characterizing fallout. Thus, for 1952, 1953, and 1957, integrated milk levels were derived from beta activities in air, as we saw in chapter 12. As techniques advanced, specific models for iodine isotopes gradually emerged. A major development began in the 1960s at Idaho Falls that had a strong influence on these and on the allowable limits for iodine in the environment. We will review this in the next section. b. The Controlled Environmental Radioiodine Tests at Idaho Falls1 In chapter 14, we reviewed field exposures of animals to fission products from melted reactor fuel elements at the National Reactor Testing Station in Idaho. Iodine predominated in the releases from "green" fuel elements in these tests (called FPFRT for Fission Product Field Release Tests). At the time, we remarked tat further special tests took place at Idaho Falls, but they were so intimately related to the analysis of transfer factors in the environment that we would postpone discussion to chapter 15. These were the Controlled Environmental Radioiodine Tests (CERT), whose stated objectives were: 1. to derive a mathematical model to predict the deposition and retention of radioiodine as a function of (a) vegetation type and state, (b) chemical and physical form of radioiodine, and (c) meteorological conditions 2. to derive a mathematical model to predict the transfer of radioiodine from vegetation to milk 3. to derive a mathematical model to predict the dose from radioiodine to humans. It was determined that a very important transfer factor to be characterized was the "deposition velocity" (Vg) for 131/ from air to pasture grasses or vegetation. To assess it and other relationships for the air-vegetation-cow-milk-human food web, the Idaho Falls group decided to set up a series of tests with 131/ generators and a sample grid in areas of typical pasture grass. There were also some cows, kept in an experimental dairy farm, which could be fed the grass and even some human volunteers who placed themselves in the passing cloud of iodine vapor, or who drank milk from the cows and had their thyroid glands counted at the laboratory. 1300 Movement of Radionuclides Through the Environment The first report (Hawley et al. 1964) was labeled "preliminary" but seems to have been quite complete. A total of 970 mCi of 131/ were released as a gas over a thirty-minute period onto a pasture of crested Wheatgrass. About 13% of the iodine deposited on the sampling grid and 1.5% on the grass. Deposition velocities were calculated as 0.4 to 0.8 cm/sec with an average of 0.6 cm/sec. Effective half-life on the grass was about 3.5 days, and the ratio of activity in milk (pCi/l) to that on the grass (pCi/g) was 240 = 35.2 The average human thyroid uptake of the ingested 131/ was 19% and the predicted dose to the thyroid 0.39 rad. It was a satisfactory test. No further reports appeared for nearly two years. However, the tests continued in the interim, viz., September 1964, December 1964, May 1965, and June 1965. The schedule included two tests over open-range-type vegetation, two tests over irrigated pastures, and one over snow-covered ground. Meteorological conditions varied from lapse conditions (two tests), to inversion conditions (two tests), and neutral conditions (one test). A report edited by Hawley (1966) gives progress through 1965 and summarizes the first five experiments. A sixth test, using methyl iodide, and a seventh designed to provide information on movement under late fall and early winter conditions were described in a report edited by Bunch (1966). A later report by Bunch (1968) gives a summary of the purposes of each test, the location of the release, etc. A copy of this summary of nineteen tests is given as table 15.6. As time wore on, the cows and human thyroid steps were sometimes omitted to allow concentration on some of the more difficult steps at the front end of the chain. Mathematical models for bovine metabolism of 131 3 and other steps began to emerge, and these were compared to other models, such as that proposed by Garner (1967). Further information was contained in a report by Zimbrick and Voilleque (1969) and in a paper presented by Pelletier and Zimbrick (1970) at the Health Physics Society Mid-Year Topical Symposium in Augusta, Georgia. Along with the field work, it was found necessary to develop a laboratory experimental program (late in 1964) with objectives to isolate each variable more than could be done in the field, and to improve techniques. This program was dubbed "CERTLE." Principal results as outlined in 1967, but actually not changed much in subsequent work, included the following: 1. No significant difference was observed in behavior of molecular iodine from a few hundred meters out to two miles from the release point. 2. Deposition velocity figures were usually below 1 cm/sec but varied with a real grass density. 3. A strong linear relationship existed between normalized deposition velocity and friction velocity under moderate wind speeds in an unstable atmosphere. It was not so strong with light winds and a stable atmosphere. 4. Deposition velocity for methyl iodide is on the order of 0.05% of that of molecular iodine. 5. The average uptake fraction in human thyroid from contaminated milk was 0.19. 6. The average uptake fraction in human thyroid by inhalation was 0.30 (three subjects, one release). 7. A reasonably satisfactory model for bovine metabolism of radioiodine was constructed. Chapter 15 1301 Table 15.6. Objectives of Specific CERT Tests Test Release Number Time Date Location Test Objectives 1 1500 5/27/63 Atomic City To check techniques and Area experimental design to be used in establishing three basic relationships under known natural release conditions: (1) The amount of radioiodine in the air relative to that in soil and vegetation. (2) The amount of radioiodine on the vegetation relative to that in milk. (3) The quantity of radioiodine in milk relative to that in the human thyroid after drinking the milk. 2 1344 9/2/64 Experimental To determine the three Dairy Farm relationships in Test 1 (EDF) above using an irrigated pasture typical of present farming practices in this area. 3 1354 12/11/64 EDF To obtain information on the meteorological aspects of iodine deposition using snow as the surface for deposition. 4 0430 5/27/65 South of SL-1 To measure the deposition velocity of l2 on grass during stable atmospheric conditions. 5 0515 6/10/65 South of Sl-1 Same as Test 4. 6 1400 9/14/65 ICPP Stack To evaluate the behavior (Idaho Chemical of CH3l in the milk-food Processing Plant) chain. 7 1410 11/22/65 EDF To determine radioiodine behavior in late fall or winter. 8 2040 5/31/66 NE of ICPP To check techniques and experimental design to be used in evaluating the deposition of radioiodine under various meteorological conditions. 9 0245 6/7/66 NE of ICPP Same as Test 8. 10 1040 6/14/66 Test Grid 3 To determine the behavior of elemental iodine over travel distances typical of postulated reactor accidents. 11 1323, 7/21/66 EDF To evaluate the behavior 1311 of Ch3l at night. 12 2108 7/26/66 EDF To measure the deposition of CH3l at night. 13 1020 8/3/66 EDF To measure the deposition of elemental iodine as a function of meteorological conditions. 14 0234 8/5/66 EDF Same as Test 13. 15 0540 8/5/66 EDF Same as Test 13. 16 0230 8/24/66 EDF Same as Test 13. 17 0530 8/24/66 EDF Same as Test 13. 18 1955 9/8/66 EDF Same as Test 13. 19 1024 11/7/66 EDF To evaluate the behavior of radioiodine in late fall. 1302 Movember of Radionuclides Through the Environment In addition to the iodine tests, the Idaho Falls group did work with 137Cs on a Michigan dairy farm (referenced in chapter 12), onsite at the National Reactor Testing Station, and more general radioecology and ecology programs.4 The significance and uses made of some of this work will become apparent in the next section. c. The Factors of 700 and 1,000 In the work of Soldat (1963) described above, we saw a factor of 600 between pCl/l of milk and pCi/m3 of air. This, and similar figures from other sources (e.g., the fallout studies in Utah and other work on the iodine-milk pathway), led the regulators to considerable concern about the validity of the then- current maximum permissible air concentration [(MPC)a] of iodine for population exposure. Table II, Appendix 8 of Title 20 of the Code of Federal Regulations (10 CFR 20) had set a "legal" (MPC)a in the environment for 131/ of 1 x 10-10 ęCicc. The fact that such values did not take any account of the "reconcentrations"5 and food-cycling phenomena we have been examining disturbed the regulators. They had already urged that all releases be "As Low as Reasonably Achievable," but this did not seem to be enough. In a new appendix (Appendix 1)6 to Part 50 of the Code of Federal Regulations, which covered licensing of production and utilization facilities, some numbers were attached to the "As Low as Reasonably Achievable" recommendations. These were designated to apply specifically to the effluents from light-water-cooled reactors. This placed a limit on annual total body exposure of individuals in unrestricted areas of 5 mrem and a calculated annual total radiation dose or dose commitment from all radioactive iodine and radioactive material in particulate form in effluents released to the atmosphere of 15 mrem to any organ from all pathways. These were a factor of one thousand lower than the extant occupational whole-body limit of 5 rem/yr and individual organ doses of 15 rem/yr, respectively, then in effect. It is commonly understood that this factor represented a rounding off upward of a factor of seven hundred derived from the environmental transport work, much of it in the CART tests. The exact calculations are not given in any official derivation, but they were common corridor conversation at environmental and regulatory meetings of the day and were fortunately summarized for posterity by Burnett (1970) at Oak Ridge. The 10 CFR 20 table II Appendix B limit for 1311 in population exposure, i.e., 1 x 10-10,ęCi/cc, can be compared to an air concentration limit derived from use of a deposition rate of 1 cm/sec and other transfer factors in the milk pathway of 1.42 x 10-13, ęCi/cc. Conversely, we can take the Part 20 figure and divide it by the effective renewal constant and get an allowable concentration on vegetation of 0.625 ęCi/m2 if there were no environmental cycling. The figure with recycling using the numbers derived by the Federal Radiation Council (FRC 1964) (80 pCi per day and drinking l of milk per day was 8.9 x 10__ ęCi/mi. The ratio of these is about 700 in each case (1 x 10-10/1.42 x 10-13 and 0.625/8.9 x 10__. This ratio came from the iodine figures; it could have been derived in other ways also. If we add the smaller contribution of other radionuclides in the (a) Bibliographies are available in Reports IDO-12078 (November 1973) and DOE/ID12096 (June 1983). (b) The term is put in quotes since, while it is the one commonly used, it is not accurate. "Recycling" or simply cycling leading to increase in concentration is the process. (c) Capital l, not the number one. Footnotes - 1 The author is greatly indebted to Dr. Eddie W. Chew, Chief Environmental Sciences Branch. Radiological and Environmental Sciences Laboratory, Idaho Operations Office, DOE, for copies of the pertinent IDO reports. 2 The units for this ratio are PCi/l of milk to pCi/g or grass, whereas those on page _____ use pCi/kg grass. The ratios in common terms would be o.16 versus 0.24 or 150 versus _______ depending on the units chosen and if the agreement between the studies is satisfactory. 3 Interestingly, this included a compartment for radioiodine in body fluid not available for secretion in milk. 4 Bibliographies are available in Reports IDO-12078 (November 1973) and DOE/ID12096 (June 1983). 5 The term is put in quotes since, while it is the one commonly used, it is not accurate. "Recycling" or simply cycling leading to increase in concentration is the process. 6 Capital l, not the number one.