POTENTIAL EXPOSURE TO RADIOACTIVE IODINE
This chapter lists sources of radioiodine that potentially can affect public health and safety. It presents information on the routes by which radioiodine could reach members of the public after a nuclear incident and general guidelines for protection against radioiodine exposure. It evaluates the safeguards built into US nuclear power plant designs to demonstrate the multiple barriers that must be breached before radioiodine is released to the environment and to contrast these plants with the Chornobyl type of nuclear power plants. Radioiodine used in medical diagnosis and therapy has limited potential for use in weapons of mass destruction (dirty bombs), and nuclear-powered submarines represent a potential for affecting public health and safety only when they are involved in accidents in port.
Sources of Radioactive Iodine
Radioactive iodine1 is a byproduct of the fission of uranium atoms. Two processes that lead to the creation of radioiodine are the fission of uranium as fuel in nuclear reactors and its use as an explosive material in atomic bombs. Under normal circumstances, minimal radioiodine is released to the environment from operating nuclear reactors, whether such reactors are operated for power production, for production of radioisotopes for use in medical diagnosis or treatment, or in materials testing or teaching. In the case of nuclear power plants (NPPs), the uranium fuel is contained in sealed metal tubes (fuel rods), the fuel rods are placed inside an eight inch thick steel reactor vessel, the reactor vessel is contained inside a thick (several feet), reinforced-concrete reactor building. In addition, all aspects of the reactor operation are carefully monitored with sensitive instruments, and highly effective filtration systems are used to remove radioiodine from air or water released from the reactor facility. For the smaller, teaching-type, reactors, many of these same safety features are used. The quantities of radioactive iodine normally found at locations related to medical use are small enough to be unlikely to be used in radioactive dispersal devices (so-called dirty bombs).
Before radioactive iodine from the nuclear fuel of a NPP can reach the environment, extensive damage must occur to the fuel elements in the reactor core with additional damage to the containment structure enclosing the reactor. Radioactive iodine (131I) has a short half-life (8 days) and is of concern only with respect to the fuel in an operating NPP or with fuel from a reactor core that has recently been shut down. Spent nuclear fuel stored at NPPs is free of significant radioiodine within weeks after shutdown of the reactor and cessation of the fissioning process that produces the radioiodine. Spent nuclear fuel that is stored outside a nuclear-reactor building is old fuel that has been kept so long that it contains minimal quantities of short lived radioiodines. Spent nuclear fuel in transportation does
not present a potential for release of radioiodine. NPPs in the United States are designed and built to withstand severe accidents and to prevent the escape of radionuclides that could threaten public health. Severe damage to the reactor core coupled with extreme damage to the reactor facility is required for radioiodine to be released to the environment. US NPP design features ensure that in the worst-case situations there will be a delay, usually days, before radionuclides can escape to the environment in amounts that would be of a public-health significance. Additional steps are taken in reactor design to ensure that such unexpected events as earthquakes and tornadoes do not lead to damage of nuclear fuel or release of radionuclides to the environment. These rigid design features coupled with heightened security measures make radioiodine releases from terrorist activities highly unlikely.
Nuclear Power Plant Radioiodine Inventory
The approximate inventory of 131I and other iodine radioisotopes in the core of a NPP depends on the size of the reactor core (megawatt-electrical (MWe)) and on how long the reactor has been in operation. The table in Appendix B can be used to estimate the approximate inventory (actual inventory will depend on how long the reactor has been in operation) of the predominant radionuclides of public-health concern that are present in a reactor core. By multiplying the “Ci/MWe” value indicated in the appendix by the operational power level of the plant, one can obtain an approximation of the total activity (in curies) of each radionuclide. For example, in the case of Three Mile Island, a 740 MWe reactor, the 131I inventory at the time of the accident was equal to about 740 MWe × 85,000 Ci/MWe or 63 megacuries (MCi). Less than one-millionth of the inventory of radioiodine in the Three Mile Island core was postulated to have escaped to the environment, that is, about 15 Ci (Kemeny et al., 1979). In the United States, nuclear power plants are designed to isolate the reactor building in the event of an accident. In the case of Three Mile Island, initial isolation of the reactor building was not totally successful. As accident-generated water began to flood the lower levels of the reactor building, sump pumps began pumping this
water into a holdup tank outside the reactor building, in the Auxiliary Building. The tank was already three-fourth full at the time of the accident and quickly overflowed, discharging radioactive materials into the Auxiliary Building. If radioiodine did escape to the environment after the Three Mile Island accident, the most likely pathway would have been discharge from the Auxiliary Building. However, the determination of radioiodine in environmental samples immediately after the accident was uncertain because the overwhelming activity of radioactive noble gases in collected samples that led to erroneous interpretations of radioiodine. Later analysis of samples showed that what was first thought to be radioiodine was xenon-133 and xenon-135. Of 57 samples collected in the first 3 days of the accident, 40 were analyzed later using gamma spectroscopy and shown to be free of detectable radioiodine (USNRC, 1979).
Other Radionuclides in Nuclear Fuel
Radioiodine is but one of many radionuclides presents in nuclear fuel or released when an atomic bomb detonates. Some of the radionuclides that are important from a public radiation dose standpoint are given in Appendix B. A review of this list indicates that radioactive fission products are typically either gasses (such as the radioactive noble gasses of xenon and krypton), particulates (such as strontium) or volatile (can be readily evaporated) materials, such as radioiodine. Radioactive noble gasses are, in general, not considered a threat to public health because they do not stay in the body if breathed in and they do not concentrate in the environment. While particulate radionuclides are of concern from a public health standpoint, they have a very low probability of being released to the environment as a result of an accident at a NPP. Under normal operating conditions, or even accident conditions, they do not escape nuclear reactor facilities in quantities to be of concern from a public health standpoint because they do not mix readily with air and because they are easily removed from a reactor facility’s water and air by filters. Furthermore, the rigid NPP design features discussed above are designed to prevent particulate radionuclides from reaching the environment, even under accident conditions. Radioiodine is of
particular concern because of the ease with which it can convert to a vapor state and because radioiodine taken into the body will concentrate in the thyroid gland if the thyroid is not saturated with nonradioactive iodine before the radioiodine reaches it. Under normal operating conditions, radioiodine is easily removed from a NPP’s air and water with highly effective filtration systems. A review of the nuclear accidents at Three Mile Island and Chornobyl, see Chapter 4, illustrates how the design of reactors in the United States specifically addresses the nature of radioactive fission products and makes releases to the environment unlikely. In contrast, poor design of the Chornobyl reactor facility led to the release of a considerable portion of its entire fission-product inventory, including radioiodine.
Nuclear Weapons Radioiodine
Radioactive iodine is produced in the detonation of a nuclear weapon just as it is produced in a NPP. However, in addition to radioiodine, many other fission products are produced when a nuclear weapon detonates. For those far enough away to survive the blast from such a weapon, protection from the radionuclides in fallout from the ensuing plume, including radioiodine, would be required. Such actions are outlined in National Council on Radiation Protection and Measurements (NCRP, 2001) and include:
Evacuation when possible and safe to do so.
Remaining inside and the minimizing opening of doors and windows.
Turning off fans, air conditioners, and forced-air heating units that bring fresh air in from the outside.
Avoiding consumption of fruits, vegetables, and milk from the area until shown to be free of contamination.
Routes of Exposure
Radioiodine can exist in particulate form, such as cesium iodide (Cs 131I) or sodium iodide (Na 131I), as a radioiodine vapor, or
as a solution with the radioiodine dissolved in water. In the environment, the different physical and chemical states of radioiodine provide several pathways for it to reach humans.
Exposure from inhalation. Radioiodine in the gaseous state can be inhaled into the lungs where it can dissolve and enter the blood. The blood then circulates through the body, including the thyroid gland. Under the condition of normal nutritional iodine supply, a normally functioning thyroid gland will take up and store between 15-30% of the iodine to which it is exposed, whether it is radioiodine or the chemically identical nonradioactive iodine (the thyroid does not recognize any difference between radioactive and nonradioactive iodine). Studies have shown that about 55% of radioiodine breathed in is absorbed into the blood in the lungs and transported throughout the body (Costa et al., 1982). However, the inhalation-exposure route was not the major contributor to the thyroid doses of the populations exposed to radioiodine released in the Chornobyl accident; rather, as previously discussed, it was estimated that most of the radioiodine taken into the thyroid entered the body in contaminated food or drink.
Evacuation and sheltering are the preferred methods of protecting populations from direct exposure to radioactive materials (including radioiodine) in a nuclear emergency. When plumes (radioactive clouds) are passing, considerable protection is provided by staying indoors and shutting off air-handling systems that bring outside air into buildings. Additional protection can be gained by moving to areas of buildings that provide the most shielding from radiation coming from radionuclides in outside air (for example, moving inward and downward within the building). Evacuation before or as soon as possible after plume passage is the preferred way of getting people out of harm’s way and ensuring that they are not exposed to released radionuclides.
Radioiodine in food and water. The ability of radioiodine to exist in a particulate form or as a solution in water makes it possible for it to contaminate drinking water, soil, and plants used as food. Radioiodine on pasture grasses or on feed for cattle or goats can contaminate their milk. After Chornobyl, most of the dose to people’s
thyroids was due to consumption of contaminated water and food, including milk. During a nuclear incident, specific guidance would be given on foods and drinks that should not to be consumed by humans because of the presence of radioiodine or other radionuclides. It is unlikely that contaminated foods would be a major source of exposure to radioiodine in the United States.
The accident at Chornobyl provided considerable information on reactor design, the types and quantities of radionuclides that can escape the nuclear fuel (source term), environmental pathways and potential risks of released radionuclides to public health and safety. That accident was precipitated by an explosion and a fire in the graphite-moderated core. The fire carried large quantities of fission products, including radioiodine and noble gases, into the environment.
Reactor designs in the United States are different from the Chornobyl design in that:
The choice of moderators (material used to slow down neutrons) is different for US NPPs. In the United States, water is used, whereas the Chornobyl type reactors use graphite (graphite is combustible, water is not).
US NPP designs prevent sudden, difficult to control increases in power level (sudden increases in the fissioning process).
US NPPs employ multiple layers of ("defense-in-depth") barriers to ensure that nuclear fuel and fission products cannot escape from the core. In the United States LWRs have pressure vessels with walls that are about 187 mm (7.4 in) thick (NUREG-1250) (USNRC, 1987). The Chornobyl Reactor had no such reactor containment vessel.
US NPPs are designed on the basis of "full containment" or the complete enclosure of all reactor and primary support systems for the reactor in the event of a design basis accident (DBA) (NUREG-1250) (USNRC, 1987). In the United States,
full primary containment is achieved by a thick steel reactor vessel and heavily reinforced concrete reactor building that surrounds all primary reactor systems. The containment can contain the peak pressure reached in DBAs or has sufficient pressure-suppression capacity to contain the worst-case peak pressure.
In the case of the accident at Three Mile Island, ignition of accumulated hydrogen gas in the reactor containment building caused a pressure pike of 28 psi but, did not lead to a breach of the containment building or any apparent increase in the escape of radioactivity to the environment (USNRC, 1988). On the other hand, the thin metal of the Chornobyl reactor’s building was easily breached by the explosion and fire that accompanied that accident. The breaching of the reactor building allowed the fire that consumed much of the reactor core to spew radionuclides, including radioiodine, directly to the atmosphere.
In NPP licensing, the US Nuclear Regulatory Commission subscribes to the "defense-in depth" (multiple layers) safety strategy, which includes: accident prevention, redundant safety systems, containment, accident management, siting, and emergency planning. US NPPs have considerable redundancy in their design to prevent consequential releases of radioactive material to the environment. Thus, fission products in the reactor core of US NPPs have to pass through several distinct fission-product barriers, including the fuel matrix, the fuel cladding, the reactor vessel and coolant system and the reactor building before reaching the environment.
Design Basis Accidents
A design basis accident (DBA) is a hypothetical accident that is assumed to include substantial meltdown and then release of appreciable quantities of fission products. It is used in the design of nuclear power facilities in the United States and in emergency planning.
When a NPP is proposed, NRC regulations require the site and reactor-design combination must be such that the consequences of DBAs at the site boundary are below the plume-exposure dose limits of 250 mSv (25 rem) total effective dose equivalent (TEDE) to the whole body and 3 Gy (300 rad) thyroid dose. The design basis loss-of-coolant accident (DBA-LOCA) has been typically the most severe DBA because it usually results in the largest calculated offsite doses. The DBA-LOCA is not a realistic accident scenario because of the engineering assumptions made to cause the damage used in the dose modeling. These assumptions result in release magnitudes that are much more severe than would be expected in a real incident. A realistic assessment of the release after a LOCA is much smaller than that from the DBA-LOCA used for siting purposes. DBA-LOCA assessments initially conducted by the NRC in evaluating the size requirement of the plume emergency planning zone concluded that plume exposures of 250 mGy (25 rad) thyroid dose would not be exceeded beyond 16 km (10 miles). Even under the more restrictive protective-action recommendations for thyroid protection recently published by the FDA considered in this report (50 mGy (5 rad) thyroid dose), over 70% of the DBAs modeled would not require any consideration of emergency responses due to inhalation radioiodine from a plume beyond 16 km (10 miles).
Severe accidents are accidents that would involve sequences of successive failures more severe than those postulated for the purpose of establishing the design basis for protective systems and engineered safety features. Severe accidents cover the spectrum of releases involving life-threatening, environmental releases of large fractions of the available radioactive materials in a reactor (tens of millions of curies). The lower range of the spectrum involves accidents in which a core "melt-through" of the containment would occur. The upper range includes catastrophic containment failure. Emergency response plans for severe accidents have as their highest priority the reduction of early severe health effects. Evacuation of the area impacted by the resulting plume from the power plant would
significantly reduce early injuries and deaths from even the most severe atmospheric releases (USNRC, 2002). For a severe accident to occur, multiple safety systems would have to fail and extreme reactor and atmospheric conditions would have to exist, circumstances which are extremely unlikely.
The fission-products that could be released from reactor fuel to the containment are known as the source term. The source term is characterized by the composition and magnitude of the radioactive material, the chemical and physical properties of the material, and the timing of the release from the reactor core. The source term is used in evaluating the radiological consequences of DBAs and severe accidents. Some fission products tend to form more often than others during the fission process. In 1962, the Atomic Energy Commission adopted the analysis in Technical Information Document TID-14844 as the licensing model source term. This "hypothesized source term" was postulated to appear instantaneously in the containment atmosphere and to consist of 100% of the noble gases, 50% of the halogens (particularly radioiodine), and 1% of other fission products; half the released halogens were assumed to be deposited on reactor building surfaces. It was thought, at that time, that a major radioiodine release was possible, and that radioiodine was considered to pose a major risk because it was subject to inhalation and not just external exposure. The simplistic critical-organ dose model used at that time supported that conclusion. The 1% of fission product particulates was dropped from the source term because without massive failure of the containment structure, release of particulates was seen as negligible in comparison with iodine and noble gases. That source term was offered for conservatism and calculational convenience.
The release of radioactive material into the atmosphere creates the greatest potential for offsite consequences. Meteorology is
important because it determines where the offsite release (the plume) goes, and the concentration of the radionuclides to which the downwind public is exposed.
Meteorological information includes wind speed, direction, persistence and variability, and vertical dispersion. Those factors describe the stability of the atmosphere and indicate how fast, far and wide radionuclides would be transported in air. Under very stable atmospheric conditions, there is little dispersion of the plume and the radionuclide concentration is much greater than under very unstable atmospheric conditions that would disperse the plume and lower radionuclide concentrations in it. Stable conditions (unfavorable meteorology) are usually chosen for performing DBA calculations, rather than the prevailing meteorological conditions.
Under normal meteorological conditions, the plume from a site moves away from the release point much as smoke moves away from a chimney. Only those in the direct path of the plume would be in immediate danger. Just as smoke dissipates as it moves away from a source, so does a radioactive plume. This dissipation quickly decreases the concentration of radioactive materials within the plume. If the quantities of radionuclides in the plume have public-health significance for people in the path of the plume, such protective actions as sheltering (remaining indoors) and evacuation would be considered. Weather conditions that might prevent immediate evacuation, such as blizzards or torrential rainfalls, would remove radioactivity from the plume close to the point of release and actually help to protect people who are downwind.
Reactor-Accident Exposure Pathways
In a reactor accident, there are three principal ways for radioactive materials to deliver radiation doses to people: external exposure to radiation emitted by radionuclides in the passing plume or from radionuclides deposited on surfaces, including the ground, internal exposure from inhalation of airborne radioactive material, and internal exposure from the ingestion of radioactively contaminated food or water. Absorption of radioactive material through the skin, either by direct absorption or absorption from contaminated wounds is
possible for some radionuclides but this method of exposure is of much less concern.
During the plume phase of a reactor-accident release, the thyroid might be exposed externally to gamma radiation from radionuclides in the plume or it might be exposed internally if radioiodine is present and inhaled. The thyroid can also be exposed internally through the intake of radioiodine by the consumption of contaminated milk, water or foods, such as leafy vegetables. Consideration of the ingestion of milk is particularly of concern because radioiodine deposited on pasture grass is reconcentrated in the milk of grazing animals (particularly cows, goats, sheep and reindeer). It takes a day or two for the radioiodine to appear in milk. To reduce exposure via the ingestion pathway, including thyroid exposure, officials would recommend that dairy animals be fed uncontaminated stored feed or recommend the interdiction of local milk supplies and contaminated foods (USNRC, 2002).
Other Types of Incidents
Facilities that use radioactive iodine in research, medical diagnosis, and medical treatment and vehicles that transport the material could also be sources of exposure as a result of an incident. However, such facilities or vehicles would not involve the quantities of radioiodine present in an operating NPP. The objectives of emergency response to incidents that involve these types of facilities are the same as the objectives of response to NPP incidents: to ensure public safety and to minimize the effects of radiation exposure.
The principles of evaluating the consequences of and developing response plans for incidents associated with other types of nuclear plants (other than NPPs) are the same as with NPPs. An analysis (Hotspot 2.01, 2002) that simulated release of large quantities of 131I from a large-structure fire showed that the evacuation zone for a 50 mGy (5 rad) dose to the thyroid was within the normal area of evacuation for a fire of the size that would be needed to release all the radioactive iodine typically in such a facility. A radiological dispersion device (dirty bomb) scenario associated with a transportation incident also indicated that the evacuation zone for a 50
mGy (5 rad) dose to the thyroid was in the normal area of evacuation (DOT, 2000). In general, the large-fire scenario results in more dispersion of the radioactive iodine than a dirty bomb containing approximately the same amount. Therefore, existing emergency-response actions of local first responders to these incidents constitute a sound basis of an emergency response.
Radioactive iodine has been used in medicine since 1946 when the first thyroid cancer treatment was conducted with 131I, dubbed an “atomic cocktail”. 131I continues to be a favored treatment for overactive thyroid glands and thyroid cancer. Typically, the procedures are performed with a few thousandths of a curie (several millicuries) to a few tenths of a curie (several hundred millicuries) of 131I (see Chapter 2 for a more detailed description of the medical aspects of radioiodine use). As was seen after the Three Mile Island accident, if several curies of radioiodine were released to the environment, as was postulated, radioiodine was barely detectable in the environmental samples collected later. Thus, it is unlikely that the radioiodine used by a hospital for patient treatment, if used in a dirty bomb, would pose any significant public health threat. The National Council on Radiation Protection and Measurements (NCRP) has estimated that the harm from such a device would be primarily psychosocial (NCRP, 2001).
Naval Nuclear Propulsion Plants
Naval nuclear propulsion plants are smaller than commercial NPPs, operate at low power or are shut down in or near port, and are operated by highly trained crews. Less than 1% of the radioactivity contained in a typical commercial NPP could be released from a naval reactor, and this limits the possible dose to the general public and the size of the area of potential concern. In addition, naval nuclear propulsion plants are ruggedly designed to withstand battle shock conditions, sit in an unlimited source of water that can be used for
emergency cooling, and can be moved away from a nearby population if necessary.
In our discussions with the Naval Nuclear Reactor Program and our knowledge of the program, we know that 131I exposure is possible for the crew and base personnel near the vessel. Base personnel are not considered the public in this situation just like employees at a reactor are not considered the public. However, significant thyroid doses at significant distances for the public located off the naval base are not likely if one takes into account all protective actions. The primary protective action would be to evacuate near the base if necessary; 131I will not be the only radionuclide released. Due to this smaller release, dilution of the plume prior to reaching the base boundary, and relatively small area required for protective actions, evacuation of the public would be completed before a 50 mGy (5 rad) thyroid dose would be received. All of this is in addition to the physical restrictions on the release previously mentioned: these reactors are very small compared to commercial reactors, operate at low power, and built to withstand battle damage.
Because of those design and operational features, the occurrence of a reactor accident on a Navy submarine is highly unlikely, and the radiological impacts of any credible event would be localized and not severe. The public would not be required to take any immediate protective action, and the thyroid dose of radiation received by any member of the public would be less than the threshold dose established by FDA and EPA for administering KI. Stockpiling or distributing KI to the public surrounding naval bases due to operation of naval nuclear-powered warships is not necessary.