Radiation risks from plutonium recycle According to simulation studies, plutonium released from advanced fuel cycles may increase the risk of lung cancer, but similar risks also exist with coal combustion Richard G. Cuddihy, Roger 0. McClellan Lovelace Biomedical and Environmental Research Institute Albuquerque, N. Mex. 871 15
Mark D. Hoover University of New Mexico Albuquerque, N. Mex. 87131
Virgil L. Dugan Leon D. Chapman, James R. Wayland Sandia Laboratories Albuquerque, N. Mex. 8 71 15 Potential human health risks are major factors in determining the extent to which energy-producing technologies will be applied by society. While this is true for all developing technologies it is probably most evident in the nuclear and fossil fuel-based industries. One of the most widely argued public concerns is for the large quantities of long-lived, alpha-emitting radionuclides of plutonium and other actinide elements that might be handled in advanced breeder fuel cycles. The US. Liquid Metal Fast Breeder Reactor (LMFBR) Program is being debated both here and abroad. The development of plutonium breeding and recycling technologies in this country, though not in other countries, may be delayed as a result of this debate. In the meantime, alternative electricity-generating programs are also being considered. These include the use of 1160
Environmental Science 8. Technology
a thorium-uranium fuel cycle, and the development of clean coal-burning techniques. Nevertheless, whether the energy source is nuclear or fossil fuels, the release of radioactive isotopes may constitute a major part of the overall health risk. Potential sources of these releases in nuclear fuel operations include uranium mining and milling, reactor fuel fabrication, fuel reprocessing and nuclear waste disposal. If plutonium is recycled as it is in the more advanced reactor fuels, then future uranium mining and milling operations can be greatly reduced. Radionuclides, especially elements of the uranium and thorium series, occur naturally in coal. When coal is burned in large power plants many of the trace elements become concentrated in the effluents. The more volatile elements can vaporize and pass through particle-collecting devices in high-temperature streams. These elements can then recondense at lower temperatures further down the effluent streams to be released to the atmosphere. We have used a computer simulation method to estimate the health risks associated with alpha-emitting transuranic radionuclides that could be released to the environment from a nuclear reactor program with plutonium recycle and from burning low-sulfur, Western US. coal. Many of the base assumptions for the model were taken from the LMFBR Environmental Impact Statement (EIS) (see Additional reading). The present application of the simulation model to the proposed plutonium recycle program deals only with the anticipated chronic releases of transuranic elements to the atmosphere from normal operations. The probabilities and consequences of severe nuclear accidents (see Light Water Reactor Safety Study, Additional reading) were projected to be low, and health risks to the general public over a long period of time were projected to result primarily from releases of actinide radionuclidesduring normal operations. The simulation approach, dispersion model Many separate but interrelated pieces of information must be incorporated into an overall examination of the environmental impact of an energy technology. These include models that describe material inventories, fuel processing, effluent releases, environmental contamination, human exposures, dosimetry and potential health effects. As individual models are assembled into an integrated structure, it becomes increasingly difficult to assess the influence of uncertainties inherent to individual models upon the final health-impact predictions. One way to approach this problem is to study the dynamics of the integrated system through simulation and parameter sensitivity analyses. Simulation, as used here, refers to the use of kinetic models to describe the interactions between elements of a system through organized mathematical relationships. Parameter sensitivity analysis is the study of how changes in individual parameters or interaction rates among elements of the model affect the final model predictions. Sensitivity analysis makes it possible to identify the most important model parameters and to determine uncertainties in the overall predictions. The use of simulation analysis in this study also provided additional information beyond the analyses in the LMFBR EIS. Here, it was possible to generate time relationships for the buildup of radioactivity in the environment, deposition in human populations and development of health effects. It was also possible to study different patterns for reactor program growth and maintenance and to identify the critical areas where further scientific knowledge would provide a more complete assessment of risk. A Gaussian plume atmospheric dispersion model was used in these studies (see Figure 1). Material was considered to be released to the atmosphere from a point at ground level and carried downwind. Within 100 km of the point of release, the plume was allowed to disperse horizontally and vertically as it passed into each sector. Beyond 100 km, the plume was considered to have constant width, approximtely 90 km, and to turn
FIGURE 1
Atmospheric dispersion model
tical ision
-
Local 100 km behavior
I
dl
Distal I
-
in the direction of the prevailing winds. The ground-level air Concentration of the released particles was reduced with increasing distance and time of travel to account for atmospheric dilution and ground deposition. Material deposited on the ground surface was subject to resuspension back into the moving air volume and to burial or adsorption onto matter less available for resuspension. Material buried in soil was assumed to mix uniformly to a depth of 20 cm. Uptake by piants was simulated by assuming the material would achieve a plant-to-soil concentration ratio of 0.1. Because this factor may cause a considerable Overestimateof the food-chain pathway for transuranic radionuclides and because plant uptake is the subject of much current scientific research, this parameter was among those selected for sensitivity analysis. The pathways for transfers of transuranic radionuclides through animal food chains were incorporated into the early simulation models used in these studies. These had littie influence upon final predictions of radionuclide uptake by humans because of the very low gastrointestinal uptake of transuranic radionuclides by both animals and man. In lieu of including these food-chain pathways in detail, we assumed that all food and water consumed by humans had radionuclide concentrations equal to plants. Radioactivity from plutc constructed in 1000 MWe (megawatt electrical) units: each requiring 50 t of reactor core material with about 17 t reprocessed each year. Releases of transuranic elements to the environment were assumed to be airborne particles of respirable size, 0.3 pn AMAD (activity median aerodynamic diameter). Air filtration
systems typical of those used in the nuclear industry to contain airborne materials are least efficient at this particle size. The major release of transuranics was assumed to occur during reprocessing of spent reactor fuel. The LMFBR EIS estimated that airborne releases from all other fuel cycle steps, such as reactor operation or waste storage, would amount to less than 1% of the total release. The projecte,d composition of transuranics released was approximately the same as the composition of the fuel being reprocessed and w'as estimated to be about lo-' of the yearly reprocessed fuel (T able 1). I:__ No commercial breeder reactors are ------&I ~ I ~ - S ~ . I I uparau~~y. I I ~ UUI for this analysis, two programs for plutonium fuel processing were simulated. As in the LMFBR EIS, these were related in time to 1975. Both programs assumed a near-term need for electrical energy and escalating construction resulting in 2200 operating 1000-MWe reactors by the year 2015. In the long-term use n s r i t v $ ~ m # d rhe ( program, it wasas: ;timed that this nonomtinn n -_r "~ maintained through the year 2475. This permitted simulation of health effects that could result from long-term, continuous release of transuranic alpha emitters. In the short-term use orogram, it was assJmed that by the year 2075 tne LMFBR program would be replaced oy otner SoJrces 01 energy. This short-term LMFBR program permitted predictions of wnether any Jse of the LMFBR woua commit the himan popJlation to a significant perpetual health hazard. The pOstJlated scnemes for reactor constfuction indicated that pea6 generating capac'ty would reqbire about 1 X lo5 Iof fuel in operating reactors with 4 X lo4 t reprocessedeach year. I1has been estimated that one fLei reprocessingfac'liry w II meet the requ.rementsof 80 LMFBR units. Thus. the projected 2200 reactors would require about 30 reprocessing plants. L...
__..,..-"."
Volume 11, Number 13, December 1977 1161
In the 500-year simulation of the long-term LMFBR program, about 400 Ci of alpha-emitting isotopes were released to the atmosphere. Buildup of these isotopes in the environment was projected to occur predominantly in soil and to be less than 250 Ci of transuranic elements (Figure 2). This can be compared with amounts of transuranic elements currently in our environment. As a result of atmospheric testing of nuclear weapons. an estimated 320 000 Ci of long-lived isotopes of plutonium have been deposited on the surface of the earth, of which about 16 000 Ci were in the U.S. There are also about 1 X I O 7 Ci of alphaemitting radionuclides, including isotopes of uranium, thorium and radium, naturally occurring in the upper 20 cm of soil in the U.S. Simulation of population exposures Human populationsexposed to the radioactive effluents were distributed along sectors radiating from a single source of emissions. Characteristics of the site were considered to be typical of potential future reprocessing facilities. A large city of about 8 million people was located 70 km from the fuel reprocessing facility. Within 10 km the population density was fifteen !imes the U.S. average of 13.2 peoplelkm2.Between 100-3200 ?m,the average U.S. population density was used. Of the U.S. population at risk in 1975, about 40 million people were considered to be potentially exposed to emissions from a single fuel retxocessing facility. Populations in the exposure areas were assumed to grow until the year 2050 when they would stabilize at 1.35 times their present level The model for deposition an(3 retention of transuranic radionuciides in the human body is shown in Figure 3. The major *i^^ ""A :""..a:-" I*:^ A -.,., exposurepatnwaysare oy inhala18vn8a,,u ,v,Yr311urI. IIIIJIIIyycl was developed from recommendations contained in ICRP Pdblicalion 19 (see Additional reading) on the melaoolism of plutonium and other actinide e.ements. For the assume0 particle size of 0.3 Mm AMAO, 30"h of the inhaled particles wdl deposit in the nasopharyngeal region. 5 % in the tracheobroncnia.region and 35% in the deep lung region. Hall the nhaled material was considered mooerarely soluole in the f.ulds of the lung (having a retention hall t me in Lngs of 50 days) and half was cons.dered to oe relatively insoluole (having a retention hall time in lungs of 500 days,. It was assumed that 3 X l o @ of the ingested plutoniLm and 1 X 01 the ogested amer'cim and curium wodld be absorbed from the gastr0intest;nal tract into the olood. This SimLlation assumed thar an indvidual s body burden was zero at birln since cross-placental transfer of Iransuran c ele-
FG.REZ
Fate of alpha-emitting isotopes
I
lo'o
FIGURE 3
Fate of transuranic radionuclides in man Nawphatyngeal region
material. BSoIubilityof material determines its rate of clearance to these sites
ments is very small. Each individual's exposure was assumed to result from breathing 20 m3 of air per day and from eating 900 kg of food per year raised on land having the average soil concentration of transuranic radionuclides. The projected accumulation of transuranic radionuclides in the US. population for the long-term reactor use program is shown in Figure 3. This represents summations of activity present in the bone, liver, lung and gonads of all the people in the U.S. These organ burdens increased from inhalation and ingestion of radioactivity and decreased from biological clearance, radioactive decay and through natural deaths in the population. Simulated population organ burdens for the short-term reactor use program are also shown in Figute 3. After new reactor construction was stopped in 2075, a rapid decline in organ burdens of alpha-emitting radionuclides was projected by the model. The rapid reduction of radioactivity in lungs by more than two orders of magnitude suggested a greater importance for inhalation prior to ground surface deposition compared to the long-term hazard from inhalingmaterial after resuspension from ground deposits. Lung burdens result mainly from inhalation of the radionuclides, whereas liver, bone and gonad burdens are the result of both inhalation and ingestion. Estimated health risks The age at which individuals are exposed to radiation has a large effect on the prediction of resulting health risks. This is especially true for transmitted genetic defects, since exposures after the childbearingage cannot cause defects in subsequent generations. Radiationdoses, received over a 70-y average human lifetime, were used to project the development of neoplastic diseases. Radiation doses received over 30 y were considered to contribute to potential genetic defects. Risk estimators giving the expected number of cancers resulting from radiation exposures were taken from the report of
1162 Environmental Science & Te
No further reactor construction
the u.3. ivarionai Hcaoemy or science ~ovisorybommirtee on the Biological Effects of Ionizing Radiation (BEIR Report) (see Additional reading). Cancers were assumed to occur with probabilities of 1.1 X lung tumors per rem, 1.7 X 10-5 bone tumors per rem and 7 X liver tumors per rem. Applying these risk estimators to the projected population radiation doses gave predictions of potential health effects as shown in Figure 3 for the long-term and short-term LMFBR programs. Lung tumors were projected to be the most abundant effect, occurringat a rate of about 0.3 casesly, or a total of 140 cases during the 500-y simulation of the long-term program. This compares to the approximately 80 000 cases per year that presently occur in the US. population. These results were predicted using the conservative assumption that no latent period occurred between the time that a sufficient radiation exposure for tumor development was received and death. It is generally accepted that a latent period of 10-20 y exists between radiation exposure and the appearance of a tumor. In the long-term LMFBR program, assuming a 10-y latent period, we predicted 16% fewer lung tumors and 18% fewer liver and bone tumors than had been predicted assuming no latent period. When a 20-y latent period was assumed, the simulation predicted 32% fewer liver and bone tumors. Two main categories of genetic risk were defined in the BIER Report. The first and best understood involves damage of single genes that produce specific genetic defects of a dominant, recessive or sex-linked recessive character. The risk for development of specific genetic defects owing to ionizing radiation Is given as 50-500 caseslmiiiion-person-rem. The second category of genetic risk involves complex diseases that are poorly understood in themselves, let alone in their relationship to radiation. This risk category has been somewhat arbitrarilv assigned a value of 10-1000 caseslmillion-person-rem. The genetic risk factoi' used in this study was the sum of the upper limits for these tbNO categories or 1500 genetic defectslmil-
iion-person-rem or gonaaai irraaiauon in inaiviauais up IO me age of 30 y. About 10% of the genetic defects for the total U S . population were predicted to occur within the first generation after irradiation. Figure 3 illustrates how all health risks associated with the simulated LMFBR program should decrease rapidly as the program is phased out after 100 y. The predicted low continual development of health effects resulted from the long-term presence of about 15 Ci of transuranic radionuclides in the soil. About 65% of the total deposition of inhaled transuranic radionuclides was predicted to occur wnhin the first few days after release and before ground deposition. More than 95% of the total inhalation exposure was estimatedto occur within one year. The long-term hazard was estimated to result in one extra lung tumor, two ex%a liver tumors, 17 extra bone tumors and 24 extra genetic defects in the 370 y of the simulation following shutdown of the last reactor. Direct inhalation of radionuclides, before deposition on ground of the amount resurfaces, was estimated to be 4.2 X leased both in this study and in the LMFBR EIS. The present of simulation study also projectedthat an additional 2.2 X the radionuclides released would be inhaled after resuspension from ground surfaces. In the LMFBR EIS, it was estimated that 9X of the amount released would be inhaled after resuspension. A lower population exposure was predicted in this Simulation because material resuspended from ground surfaces was not confined to a static column of air but was allowed to disperse with the prevailing winds. About one million reactor-years of operation were projected during the 500-y long-term program. This was estimated to resuit in 1.4 X lung tumors, 1.1 X bone tumors, 2 X liver tumors and 9 X genetic defects for each year of a reactor operation. Projections given in the LMFBR EIS were 4 X 4X 8X and 6 X cases per reactor Year for lung-, bone and liver-related tumors and genetic defects, Volume 11, Number 13, December 1977 1163
respectively. Both studies predicted a similar relative distribution of lung-, bone- and liver-relatedtumors. The higher predictions in the EIS, especially for genetic defects, resulted from the higher estimation of the amount of material inhaled after resuspension and from assuming that the entire lifetime exposure of each individual was delivered at birth. Parameter sensitivity analyses The assumed values of many parameters in this analysis have simple linear relationships to projected health effects. Examples of these included the estimated amount released, the number of people exposed and the risk of disease incidence per unit of radiation dose. Changes in the assumed values of other parameters, however, may have nonlinear effects on projected results. An example of this is the isotopic composition of the radioactive release. Each isotope poses a different degree of hazard because each has a different radioactive half-life, biological haif-life and propensity for organ deposition. Relationships between projected health effects and values selected for four atmospheric dispersion parameters have been simulated. The functions for wind velocity and the vertical diffusion coefficient both displayed maxima. Very low values of each parameter caused the released materials to be confined to the immediate vicinity of the point of release. With high values for these parameters the released materials passed by the population more rapidly, and at higher altitudes than was projected for the mid-range of parameter values. Increasing rainfall or dry deposition velocity caused similar reductions in projected health effects. The greatest number of health effects occurred under conditions that produced minimum ground deposition. again indicating the major importance of direct inhalation compared to the inhalation of resuspended material or uptake through ingestion. Relationships between projected health effects and model parameters that affect material after deposition on ground surfaces have also been simulated. Total projected health effects were decreased by increasing the rate of burial or by reducing the rates of resurfacing and resuspension. Increasing the resurfacing rate by a factor of 100 increased production of lung tumors by 20%. but decreased all other effects. This resulted from an overall reduction of the soil contaminationthrough resuspension of the radionuclides into the moving air stream and their passing beyond the population more rapidly. Increasing the plant-to-soil Concentration ratio above the base value of 0.1 resulted in higher projections for liver- and skeletal-related tumors and genetic defects. Assuming no uptake of transuranic radionuclides by plants, these health effects dropped
I
to 75% of the numoer projected in the base simulation example. Thus. all the lung tumors and 75% of the other projected health effects were directly related to inhalation. Biological retention of inhaled materia s influencedby aerosol panicle size and solubility. Particle size influences the deposition of inhaled part.cles in the respiratory tract and so ubility of tne depositeo partic es influepces pu monary retention and redistribLtion to other interna organs. The relationship of pro;ected health effects to assume0 particle solubility is g'ven in Figure 4. More Soluole aerosols Caused an increase 'n projected Done and liver tLmors and genetic defects and a decrease in projected tung 1Lmors. The relationsnip of the aerosol particle s'zes of the released materials IO projected health effects s also shown in Figure 4. The oase-simulation case used a particle size of 0.3 urn AMAD: however, two or tnree times greater depos'II on can resLlt w th larger or smaller aerosols. Health effects were much lower for part cles greater inan 10 p m AMAD. since tnese are too arge to be deposited in the lower respiratory tract. Lung oeposition is at a m nimum for partic es of 0.5 p m AMAD. but increases again for very small particles. naividual y. no parameter values were found that increased the predicted health consequences by more than three times. however. many comoinat'ons of parameters might oe varied in concert IO proddce more significant changes in the predictions. For example, confirming tne releasea material IO the lowest meter aoove the grouno by eliminating all ground depos'rion and vertical dispersion of the released aerosols WoJld produce a maximum numoer of projected health effects. alfhough it would not be at all realistic. Such a CalcLlation can. however. illustrate the extent to which atmospheric dilJtion reduces the exposure of large human populations to airoorne emiss ons. For example. with a wind speed of 1 mls and a geographic Sector width of 90 km, as used in the simulation. abobt 9 X lo4 m3 of air passes out of the sector per second from the lowest meter. If there are 2.5 X 10' people in the Sector oreaihing 20 m3 of air per aay. then 0.6% of the sector's lowest meter of air wou U be inhaled by the sector population. The amount inhaled oy the population, as estimated from the simulation study. was 0.00064"~of 11 1000 of the amount inhaled. assuming no deposit on or vert cal mixing. Lung tumors would then oe projected to occur at a rate of 3001y. which is still small compared to the normal incidence of aoout 80 000 casesly.
Radioactivity from coal burning Predictions of health-effectsmodels, oy themselves. cannot justify an "acceptaole" or "insignificant" level of disease incidence in h e exposed population. Greater meaning can be gained
FIGURE4
Influence of solubility, size of particle on its fate inI body
Lung Wnon
r
1164
Environmental Science & Technology
by using the same population exposure methods to compare similar health hazards from alternative energy industries. The magnitudes of radioactive effluents from a pulverized coal-fired power plant were reported by Kaakinen et al. (see Additional reading). After passage through mechanical dust collectors and an electrostatic precipitator, the effluent contained 3.51 mCi of 226Ra,166 mCi of 210Poand 150 mCi of 210Pb/1000 MWe-y. Polonium-210 is a daughter product from 210Pbdecay and both 210Po and 226Raare alpha-emitting radionuclides. Population risks for exposure to these effluents of coal power plants were also simulated. This analysis projected incidences of 5.4 X lo-* lung tumors, 1.6 bone tumors, 3 X liver tumors and 1.86 genetic defects in the exposed population for each year of operation of a 1000 MWe coal plant. Comparedto the operation of an equivalent size LMFBR, this was 400 times the incidence of lung tumors, 15 000 times the incidenceof bone tumors, 150 times the incidence of liver tumors and 21 000 times the frequency of genetic defects. The greater numbers of projected health effects from coal burning result from larger releases of alpha-emitting radionuclides, their higher gastrointestinal absorption and their greater deposition in internal organs after absorption. Uptake of transuranic radionuclides by plants was assumed to result in a plant-to-soil concentration ratio of 0.1 and the same factor was used for lead, polonium and radium. This was considered to overestimate plant absorption of all of these elements. In several published studies, values of this ratio are similar for lead, polonium and plutonium but range from 1 X to 3 X depending upon plant and soil conditions. Alpha-emitting radionuclides in effluents from power generators is only one risk to which human populations are exposed. There are also hazards from mining operations, from transport of materials and a variety of mechanical operations as well as from handling of by-products and wastes that may be radioactive or chemically toxic. In studies of the health histories of uranium miners, the most harmful exposures involve a combination of radiation, tobacco smoke and vehicle exhausts. In the future, more attention must be given to the effects of multiple insults from a broader range of toxic materials.
In conclusion A major conclusion in the LMFBR EIS was that the anticipated breeder reactor fuel cycle releases of transuranic radionuclides to the environment did not pose a significant health risk to humans in comparison to the many other risks that people assume in the normal activities of society. We do not disagree with this conclusion so long as environmental releases of radionuclides can be maintained reasonably close to the projected levels. In both the LMFBR EIS and our study, inhalation of radioactive particles was judged to be the most significant hazard. Our study also projected that 90% of all anticipated health effects will occur in the population consuming the related electric power. No assumptions concerning the environmental transport of transuranic radionuclides or their deposition in human tissues were identified as having sufficient uncertainty to cause more than a factor of two or three change in the overall projected health effects. The most significant remaining needs are for better information on the physical and chemical nature of released material, environmental weathering of particles, the physical size of particles resuspended from ground surfaces, uptake of radionuclides by plants and the validity of low-dose health-risk extrapolations from high-level biological dose-response studies. In estimating the potential health effects of burning coal, bone tumors and genetic defects were projected to be the major health impact rather than lung tumors, as in the LMFBR simulation. This was the result of the increased importance of the ingestion pathway, although this is probably the most uncertain aspect of the composite model. Still, even the projection for lung tumors was 400 times greater for operation of a coal-fired plant compared to that of an equivalent size LMFBR. This evaluation is limited in scope compared to all the hazards associated with
either the total nuclear or coal fuel cycle. Nevertheless, it applies the same criteria, degree of conservatism and scientific base of information in making this one health risk comparison.
Additlonal reading Liquid Metal Fast Breeder Reactor Program Environmental Impact Statement, US. Atomic Energy Commission, WASH-1535 (1974). Reactor Safety Study. An Assessment of Accident Risks in U.S. Commercial Nuclear Power Plants, WASH-1400, US.Nuclear Regulatory Commission (1975). InternationalCommission on RadiologicalProtection, Publication 19, The Metabolism of Compounds of Plutonium and Other Actinides, Pergamon Press, New York, 1977. Task Group on Lung Dynamics, Committee II of the International Commission on Radiological Protection, Health Phys., 12, 173 (1966). US. National Academy of Sciences-National Research Council, The Effects on Populations of Exposure to Low Levels of Ionizing Radiation, Washington, D.C., 1972. Kaakinen, J. W., Jorden, R. M.,Lawasani, M.H., West, R. E.,Environ. Sci. Technol.,9, 862 (1975).
Richard Cuddihy (left),.a research scientist at the Inhalation Toxicology Research Institufe, brings his background in radiation biology and biophysics to evaluations of inhalation dosimetry problems in large population exposures. Roger McClellan (middle) is the director of the Inhalation Toxicology Research Institute, and has research interests in a wide range of inhalation toxicology problems related to environmental and occupational exposures of people to toxic substances. Mark Hoover (right) is in a doctorate program at the U. of New Mexico, Department of Nuclear Engineering, and is on the research staff of the Inhalation Toxicology Research Institute.
Virgil L. Dugan (left) is manager of the Systems Analysis Department of Sandia Laboratories. He is active in the analysis of fossil, solar and nuclear energy systems and in nuclear material safeguards evaluations. Leon Chapman (middle), supervisor of the Systems Analysis Division of Sandia Laboratories, is currently involved with modeling methodology dealing with socioeconomic, environmental and security problems related to developing energy techniques. Robert Wayland (right) is a staff member in the Fuel Cycle Risk Analysis Division of Sandia Laboratories and is involved in analysis of light-water reactor operational safety problems. Volume 11, Number 13, December 1977 1165
