Ionizing Radiation Risks to Satellite Power Systems (SPS) Workers in Space DOE/ER-0094 December 1980 Prepared for: U.S. Department of Energy Office of Energy Research Solar Power Satellites Division Under Contract No. W-7405-ENG-48 DOE/NASA Satellite Power System Concept Development and Evaluation Program
NOTICE
DOE/ER-0094 Dist. Category 63,41 Ionizing Radiation Risks to Satellite Power Systems (SPS) Workers in Space December 1980 Prepared for: U.S. Department of Energy Office of Energy Research Solar Power Satellites Division Washington, D.C. 20585 Prepared by: Lawrence Berkeley Laboratory Berkeley, CA. 94720 Under Contract No. W-7405-ENG-48 DOE/NASA Satellite Power System Concept Development and Evaluation Program
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FOREWORD The work reported here was performed as a part of the Satellite Power System Concept Development and Evaluation Program Environmental Assessment. A committee was formed to support Margaret White of Lawrence Berkeley Laboratory, Task 2 manager, in assessing ionizing radiation risks to Satellite Power Systems workers in space. The committee included the following members: Dr. John T. Lyman, Chairman Lawrence Berkeley Laboratory University of California Berkeley, California Dr. E. John Ainsworth Lawrence Berkeley Laboratory University of California Berkeley, California Dr. E. L. Alpen Division Head Biology and Medicine Division Lawrence Berkeley Laboratory University of California Berkeley, California Dr. Victor Bond Associate Director Brookhaven National Laboratories Associated Universities, Inc. Upton, New York Dr. Stanley B. Curtis Lawrence Berkeley Laboratory University of California Berkeley, California Dr. R. J. Michael Fry Biology and Medicine Division Oak Ridge National Laboratory Oak Ridge, Tennessee Dr. Kenneth L. Jackson Radiation Biology Department University of Washington Seattle, Washington Dr. Stuart Nachtwey Biomedical Applications Branch Johnson Space Center Houston, Texas Dr. Charles Sondhaus Department of Radiological Sciences University of California Irvine Medical Center Orange, California Dr. C. A. Tobias Lawrence Berkeley Laboratory University of California Berkeley, California Dr. Jacob I. Fabrikant Lawrence Berkeley Laboratory University of California Berkeley, California
CONTENTS FOREWORD.............................................................. 111 SUMMARY.............................................................. 1 Scope....................................................... 1 Environment................................................. 1 Radiation Dose............................................ 2 Health Risk Assessments................................... 2 Conclusions................................................. 3 Recommendations............................................ 4 1. INTRODUCTION.......................................... 5 Background ................................................. 5 The Space Environment..................................... 5 2. REFERENCE SYSTEM EVALUATION ................................. 7 Estimated Absorbed Doses and Dose Equivalents in SPS Workers.................................... 8 Low Earth Orbit......................................... 8 Transfer Ellipse ....................................... 8 Geosynchronous Orbit .................................. 8 Total 90-Day Dose Equivalent......................... 11 Assessment of Risk for Cancer Induction.............. 11 3. CONCLUSIONS AND RECOMMENDATIONS ............................... 14 Health Risk Assessments ................................. 14 Conclusions............................................... 14 Recommendations ........................................... 15 APPENDIX A, SPACE RADIATION ENVIRONMENT.......................... 17 Trapped Electrons ........................................ 17 Bremsstrahlung............................................ 19 Trapped Protons .......................................... 19 Galactic Cosmic Rays...................................... 20 Solar Particle Events................................... 24 Different Effects of High- and Low-LET Radiations ... 24 Quality Factor............................................ 24 APPENDIX B, RADIATION HEALTH EFFECTS IN THE SPACE ENVIRONMENT 28 Early Effects............................................ 28 Late Effects.............................................. 29 Cancer..................................................... 31 Uncertainties in Dose-Response Relationships for Radiation-Induced Cancer........................ 32 Estimation of Space Radiation Induced Carcinogenic Risk in Man...................................... 34 Genetically Related Ill-Health.......................... 38 Developmental Abnormalities in the Newborn ........... 38
Effects on the Eye............................................ 39 Cataracts.................................................. 39 Fertility..................................................... 40 Female..................................................... 40 Male....................................................... 40 Other Health Effects of Radiation ........................... 41 HZE Particle Radiation Effects ........................... 41 APPENDIX C, GLOSSARY.......................................... 43 REFERENCES..................................................... 48
SUMMARY SCOPE A reference Satellite Power System (SPS) has been designed by NASA and its contractors for the purposes of evaluating the concept and carrying out assessments of the various consequences of development, including those on the health of the space workers. The Department of Energy has responsibility for directing various assessments. Present planning calls for the SPS workers to move from Earth to a low earth orbit (LEO) at an altitude of 500 kilometers; to travel by a transfer ellipse (TE) trajectory to a geosynchronous orbit (GEO) at an altitude of 36,000 kilometers; and to remain in GEO orbit for about 90 percent of the total time aloft. This report deals with the radiation risks to the health of workers who will construct and maintain solar power satellites in the space environment. The charge to the committee was: a. To evaluate the radiation environment estimated for the Reference System which could represent a hazard; b. To assess the possible somatic and genetic radiation hazards; c. To estimate the risks to the health of SPS workers due to space radiation exposure, and to make recommendations based on these conclusions. ENVIRONMENT GEO has the highest ionizing radiation intensity. The Reference System proposes that most workers will spend most of their tour in space in this environment. The radiation environment in GEO is known only with sufficient accuracy to predict the radiation dose in free space to within a factor of two, and then only for objects in orbit for relatively extended periods for which average values of the radiation environment may be used. The short-term enhancement of the radiation belt intensity caused by geomagnetic substorms may result in significant deviations from the average doses and could increase the risk to health of exposed individuals caught outside shielded or protected areas. Solar particle events, which are rare events occurring most frequently in about eleven-year cycles, could increase the dose by a considerable factor. Shielding is the major factor which influences the dose and thus the potential health risk to the SPS worker. With the exception of the storm cellar, the shielding of the Reference System used in these estimates of dose is the minimum amount of material needed for the
structural integrity of the system. The galactic cosmic ray dose is not readily attenuated by shielding material. This may be a factor for setting a lower limit on the radiation dose if the bremsstrahlung dose is adequately attenuated. RADIATION DOSE Based on the Reference System model and subject to its limitations and assumptions, it is estimated that an SPS worker in GEO may be exposed to a dose of as much as 40 rem during any one 90-day mission. This estimate is based on the following assumptions: a. That between 87 and 98 percent of the worker-years occur in GEO. For risk estimation, we therefore have assumed that the total mission will occur in GEO. b. That effective shielding of the worker will be equivalent to 8 grams per cm2 aluminum (which includes 5 g per cm2 of body self-shielding). c. That the exposure is represented by the conditions in a parking orbit of 160° west longitude. This is considered to be the worst case. d. That no contribution to the total dose is made by solar particle events. e. That the bulk of the dose is produced by trapped electron bremsstrahlung, and that the small contribution to the total dose made by galactic cosmic rays can be neglected. f. That the time spent in GEO is 90 days per space tour. The uncertainty in these assumptions is of an order that the dose estimate could vary by a factor of two. HEALTH RISK ASSESSMENTS Using U.S. life tables of age and sex, the estimated lifetime risk for cancer is 0.8 to 5.0 excess deaths per 10,000 workers per rad of exposure. Thus, for example, in 10,000 workers who completed ten missions with an exposure of 40 rem per mission, 320 to 2,000 additional deaths, in excess of the 1640 deaths from normally occurring cancer, would be expected. These estimates would indicate a 20 to 120 percent increase in cancer incidence in the worker-population. The wide range in these estimates stems from the choice of the riskprojection model and the dose-response relationship. The choice between a linear and a 1 inear-quadratic dose-response model may alter
the risK estimate for some tumors by a factor of at least two. The method of analysis (e.g., relative vs. absolute risk model) can alter the risk estimate by an additional factor of three. Choosing different age and sex distributions can further change tne estimate by another factor of up to three. When decisions have been made about the selection of SPS workers, the precise influence of age and sex distribution can be included in risk estimates. However, the choice of dose-respose relationship and projection models at present is a matter of opinion and will not be resolved scientifically for quite some time. CONCLUSIONS The conclusions of the committee based on the findings above are: a. The risk of excess cancer deaths is assumed to be closer to the lower limit estimated above because the major exposure will be from the low dose rate, low-LET irradiation. This being the case, we consider a reasonable estimate to be one excess death per 10,000 workers per rem of exposure. If this level of risk is applied to the worst case reference system exposure level, namely 40 rem, there would be 400 excess cancer deaths in a work force of 10,000 completing ten missions (accumulative dose equivalent of 400 rem). b. The potential genetic consequences could be of significance, but at the present time, sufficient information on the age and sex distribution of the worker population is lacking for precise estimation of risk. c. The potential teratogenic consequences resulting from radiation are considered significant. Radiation exposure of a pregnant worker could result in developmental abnormalities. d. Based on the Reference System, dose to space workers from low- LET bremstrahlung approaches the cataratogenic level for man. The appropriate quality factor for the HZE particle portion of the dose is unknown at this time. If its Q is greater than 20, the cataract hazard may be significant. More information is needed regarding this hazard. e. In tne absence of a radiation accident or some other unexpected situations (e.g., nuclear detonation) and with adequate shielding to protect against the increased radiation levels during solar particle events, there will be no early or acute radiation health effects occurring in the SPS worker population. f. No other radiation health effects are considered to be of sufficient consequence to be important for risk estimation.
RECOMMENDATIONS This Committee strongly emphasizes the need to reduce the uncertainties in the evaluation of radiation health risks to SPS workers in the space environment. It recommends that tne following be carried out to achieve these goals: a. The short-term variations of the radiation dose rate in space must be better understood so that the range of doses and dose rates to be expected can be established accurately. A radiation environment model should be developed that is appropriate to this SPS mission for study and simulation. The model should include short-term and solar-cycle variations. b. An instrumented research satellite should be placed in GEO to measure absorbed dose rate and particle spectra at depth in phantoms and to measure the temporal variations of the radiation field. c. The differences in the results of dose and dose-rate estimation obtained from the shielding transport codes must be evaluated. The use of different calculational methods with the same set of assumptions should yield the same results. d. When institutional decisions have been made to develop appropriate exposure strategies, engineering decisions for dose control (e.g., improved shielding) should then be made.
1. INTRODUCTION BACKGROUND The need to develop long-term, baseload, electrical energy sources has initiated search for economically competitive and environmentally acceptable alternatives to our limited supply of fossil fuels and other nonrenewable energy sources. Such a search must consider the widest range of available and potential technologies. Satellite-based solar power generation has emerged as one possible source of electrical energy obtained from inexhaustible and renewable sources. The U.S. Department of Energy (DOE) and the National Aeronautics and Space Administration (NASA) are examining the feasibility of generating baseload electrical power with Satellite Power Systems (SPS) located in geostationary orbit. These systems would collect radiant energy from the Sun, convert it to electrical energy, and then beam it to Earth as microwaves. Ground receiving stations would convert the microwaves to electrical energy to be supplied to power grids. DOE and NASA are studying the large number of factors involved in bringing such a complex undertaking into being. At the present time, initial system definition studies have been completed. A Reference System (DOE/NASA, 1978) has been designed as a basis for further development and evaluation of important areas of uncertainty and for preliminary assessment of environmental impacts and potential health risks to SPS workers. This report directs attention to the radiation risks to the health of workers who will construct and maintain the SPS satellites in the space environment. THE SPACE ENVIRONMENT Man faces substantial risks to health in space. Prolonged periods in space can result in workers being exposed to radiation that can cause adverse health effects such as cancer, genetically related ill health, cataracts, and—with very large doses—even death. Exposure to ionizing radiations is a major factor in the evaluation of potential health risks to workers in space in the SPS program. Assessment of these health risks involves the determination of existing radiation dose levels. Methods must be developed to reduce radiation levels for persons in space to the lowest practical level. Determination must be made of the acceptable radiation levels which may not be absolutely safe but, rather, may be appropriately safe for the special circumstances of the space environment.
Because SPS travel and work will take place in a number of different space environments—primarily low earth orbit (LEO), the transfer ellipse (TE), and geosynchronous earth orbit (GEO)—the worker in space will be exposed to radiation environments with different radiation qualities, varying intensities, and differing difficulty in predicting exposure levels. Assessment of potential health risks in each of the three environments is influenced by many factors, including: (1) the location in space; (2) the type of shielding used in work stations, living quarters, transport systems, and space suits; (3) the types of duties performed; (4) the length of each mission; (5) the age and sex of space workers; and (6) the total number of missions per worker-career. This assessment of the radiation health hazards in space is limited by the lack of adequate information concerning these factors and further complicated by inadequate data concerning the health effects of various types and dose levels of radiation on the human body. The Reference System proposal is based on the following assumptions: 1. The SPS construction would occur mainly in GEO with all material transported from Earth. 2. There would be a two-phase transportation of materials and personnel: movement to LEO, followed by subsequent delivery to GEO. 3. After construction of the LEO base, cargo transportation vehicles for the voyage to GEO would be assembled in LEO. 4. The construction and operation of a fleet of 60 SPS facilities, assuming a 30-year lifetime for each SPS, would require between 22,000 and 57,000 worker-years in space. The range of worker-years required takes into consideration the number of maintenance workers needed per SPS (from 4 to 20). The number of SPS workers required to achieve these goals would range between 10,000 and 20,000, with ten 90-day missions per worker. 5. The time spent in GEO would be between 87 and 98 percent of the total worker-years.
2. REFERENCE SYSTEM EVALUATION In determining potential radiation risks to the health of SPS workers in the space environment, an estimate must first be made of the radiation environment outside the space vehicle. This environment consists of trapped electrons and protons, galactic cosmic rays (GCR), ahd sporadic solar particle event (SPE). The average values of the flux densities of the primary radiation are presently known only to within a factor of two. The spacecraft and the space workers comprise a complex distribution of shielding materials which attenuates the primary radiation and which is also a source of secondary radiations. The transport of the radiation through the shield is fairly well understood, but there is uncertainty in the calculations due to assumptions of shield geometry and composition. Dose estimates for previous space missions have been based on three-dimensional distribution of the shielding materials in the spacecraft and in the astronauts. Other calculations have been based upon solid-angle sectoring of the available shielding. All calculations for an SPS mission thus far have been made assuming simple geometries. Based on a recent analysis (Selzer, 1979), the absorbed dose calculated inside a spherical shell at a given radius is three to four times that at the same depth within a semi-infinite slab with isotropic radiation impinging on it from one hemisphere. For the purposes of this report, spherical shield geometry is therefore used to provide a worst case estimate of dose. However, this SPS Committee recognizes that until calculations for SPS radiation environments are based on more realistic shielding configurations, these calculations remain inadequate for detailed, accurate estimation of dose equivalent. In this report, the shielding assumed is 3 g/cm^ of aluminum for the habitat and work stations and 20-30 g/cm? for the storm cellar to be used during solar particle events (SPE). The assumptions are based on the Boeing contribution to the Reference System (DOE/NASA, 1978). In addition, an average of 5 g/cm^ of aluminum equivalent shielding is assumed for the body self-shielding in the present analysis. Thus, the estimates made below can be considered representative average doses at the center of a sphere of radius 8 g/cm^ of aluminum equivalent material. Because the relative biological effectiveness of the different radiations is variable, a quality factor (Q) is assigned to each radiation, permitting calculations of a total dose equivalent (rem). The use of Q is reasonable for biological endpoints of principal concern. However, there is the possibility that other health effects are caused only by high energy heavy ions (HZE particles). Therefore, at the present time, the use of a Q for the HZE particles does not provide a complete assessment of the risk to health of SPS workers.
ESTIMATED ABSORBED DOSES AND DOSE EQUIVALENTS IN SPS WORKERS Low Earth Orbit Dose estimates are most accurate for the LEO phase of the mission. Protons in the South Atlantic Anomaly pose the only major source of external radiation. The dose equivalent in LEO will vary by about a factor of two between solar minimum and solar maximum (Stassinopoulos, 1979). The dose rate estimates at solar minimum, when the doses are higher, range from 0.15 rad per day (Hardy, 1979) to 0.3 rad per day, the latter value corrected from a semi-infinite slab calculation (Stassinopoulos, 1979) to spherical shielding (Seltzer, 1979). Since Q for this radiation is close to unity, these values are good estimates of the dose equivalent rate in rem per day. There will be a negligible contribution to the dose from galactic cosmic rays and solar particle events, due to the large amount of geomagnetic shielding available in the LEO trajectory. The total dose equivalents for a 90-day mission in LEO are therefore estimated to be between 14 and 28 rem at solar minimum and between 7 and 14 rem at solar maximum. Transfer Ellipse Dose calculations have been made for the 5.25 hour transfer ellipse from LEO to GEO. These results vary between 1.0 rad (Hardy, 1979) due primarily to protons, and 0.018 rad (Stassinopoulos, 1979) due primarily to bremsstrahlung. As is the case for LEO, these are the estimates for the dose equivalents in rem as well. The large difference is the result of different assumptions in the trajectory made for the calculations. Geosynchronous Orbit In GEO, a majority of the absorbed dose is due to bremsstrahlung produced by the trapped electrons. An estimate of the dose equivalent for a worst case exposure from predictable radiation is 0.43 rem per day inside an aluminum sphere with radius 8 g/cm^ (Seltzer, 1980). The contribution to the dose equivalent by the galactic cosmic rays (GCR), particularly from the heavy charged particle component, may be important. The fragmentation characteristics of the heavy particle component must be considered for an accurate estimation of the absorbed dose from GCR (Wilkinson and Curtis, 1972). The quality factors (Q) necessary to convert absorbed dose to dose-equivalent are generally unknown for these radiations. Using a Q of 3 for the GCR, independent of depth, yields a very rough estimation of the dose equivalent as a function of depth. Use of an average value for Q of 3 for the galactic cosmic ray contribution is consistent with current recommendations (ICRP 26, 1977). Q values are under continual reassessment, and it is
possible the Q value could be increased for the HZE particle contributions. This Committee believes that an average Q of 3 may not be conservative for carcinogenic or mutagenic effects produced at low doses and low dose rates over long periods. This is a simplified model and more must be learned about the variation of Q with dose and depth for the GCR before a more accurate value for the dose equivalent can be obtained. The shielding of manned space vehicles against the proton and helium ion component of the GCR has been examined (Santoro et al., 1973). The absorbed dose and dose equivalent due to primary particles and to secondary particles arising from the self-shielding of the tissue sphere are little affected by increasing the sphere radius from 5 to 20 g/cm? of aluminum. Protons and helium ions and their associated secondary radiations contribute about 55 to 65 percent to the total galactic cosmic ray dose and a smaller percent to the total dose equivalent. The dose and dose-equivalent rates for the combined proton and helium ion components average 31 mrad per day and 73 mrem per day, respectively, yielding an average Q of 2.4. This value of Q is a lower limit when the HZE particle contribution is very small. If the Q of 2.4 is the value for the proton and helium ion components, the average for all components may be higher than the Q value of 3 assumed above. Solar particle events (SPE) will be a major hazard in GEO; special shielding—a storm cellar—is needed for space workers during a solar particle event. The dose received from a given SPE will depend upon the size of the SPE, the length of time of the warning before the particle buildup, the time required for the workers to get inside the storm cellar and the storm cellar shielding thickness. The size and time of occurrence of an SPE are not currently predictable. It is known that a correlation exists with the sunspot number and that the event frequency has an 11-year cycle. Within this cycle, there is a three- to five-year period that is almost event free. During the remaining six to eight years of the period, there is about a 40 percent probability of a large SPE each year. The total dose from the solar particles within the 11-year cycle is generally dominated by the contribution of the largest event within the cycle. This makes the accuracy of the prediction of size of an event that is about to occur, or is just starting, very important because of the special precautions which must be taken. For a 30 g/cm^ tissue sphere, the Wilson and Denn (1976) calculations provide a dose equivalent of an additional 2.5 rem for an SPE with the size and energy spectrum of the August 1972 event and 25 rem for the February 1956 event (Fig. 1). Webber (1963) made a similar estimate for the 1956 event. Rossi and Stauber (1977) estimated the dose equivalent behind 40 g/cm^ of aluminum (equivalent to about 30 g/cm^ of tissue) to be 25 rem for the August 1972 event, a factor
Figure 1. Dose equivalents from two major solar particle events (SPE) plotted as a function of the radius of tissue equivalent sphere. Adapted from Wilson, 1979.
of ten greater than the Wilson and Denn (1976) calculations. These differences in dose calculations for the 1972 event are still to be resolved. Total 90-Day Dose Equivalent Daily dose equivalents in the three phases of the SPS mission are summarized in Table 1. From the table, the best estimate of dose equivalent from predictable radiation sources at the worst parking orbit is approximately 40 rem for 90 days in GEO, assuming a Q value of unity. This dose equivalent value is derived from the calculations of Seltzer (1980) for a geostationary orbit with an altitude of 35,790 km, an inclination of 0°, and a parking longitude of 160°W, the worst condition for radiation exposure to trapped electrons (Stassinopoulos, 1980). The incident electron spectrum was integrated for the epoch 1979.0 using the AEI7-HI environmental model. This model is based on a recent compilation of trapped electron data yielding conservatively high average values for the flux densities. A total spherical shield of 8 g/cm^ aluminum equivalent material (3 g/cm^ of spherical spacecraft shielding plus 5 g/cm^ effective body selfshielding) is assumed to obtain the above value of the absorbed dose in water. The shape of the dose vs depth curve is such that a 50 percent change in the total shield assumed will affect the volume of the dose by a factor or two. The largest contribution to the dose equivalent is the bremsstrahlung. In addition, there is about a 10 percent probability of an additional 2.5 rem from solar particle events, and a smaller probability that the SPE dose equivalent might be as high as 25 rem. A smaller contribution will be made by GCR. Therefore, the worst case 90-day mission dose equivalents will most likely be within the range of 40 rem (no SPE) to 65 rem (with large SPE). The Committee emphasizes that these are preliminary dose-equivalent estimates with large uncertainties. ASSESSMENT OF RISK FOR CANCER INDUCTION This SPS committee considered radiation-induced cancer the major health risk associated with exposure to ionizing radiations at dose levels most likely to be encountered in the SPS space environment* The risks can be calculated based on the whole-body exposure and a linear-quadratic dose-response model (NAS-BEIR, 1980), and average career dose-equivalent values may be used. As an illustrative example,
Table 1. Estimated dose equivalents at the center of an aluminum sphere of 8 g/cm^ radius for the three phases of SPS*.
among 10,000 SPS workers from the general population of all age groups, about 1,640 persons would be expected to die of cancer, in the absence of any additional radiation exposure. Based on the dose-equivalent estimate in GEO for the Reference System design (40 rem per 90-day mission) and ten missions per career (accumulated dose equivalent of 400 rem), the 1 inear-quadratic dose response model predicts increases of between 20 and 60 percent depending on the projection model (about 160 to 1,000 excess cancer deaths). The linear dose-response model predicts values about two times larger. Such cancer risk predictions are subject to a large number of uncertainties, as outlined in this report. In spite of these uncertainties, this SPS Committee concludes that the increased potential in cancer-induction risk due to radiation exposure in the SPS environment, as presently envisaged with the present Reference System design, can be substantially reduced. There is also the possibility that this radiation might increase genetically-related ill health, developmental abnormalities in the newborn, lens cataracts, and temporarily decrease fertility. If the radiation dose is substantially reduced, as suggested above for the purpose of reducing cancer incidence, the probability of these other health risks will also be reduced. Additional details regarding the space radiation environment and radiation health effects are given in Appendices A and B.
3. CONCLUSIONS AND RECOMMENDATIONS HEALTH RISK ASSESSMENTS For ionizing radiation, the major concern will be late or delayed health effects, particularly the increased risk of radiation-induced cancer. The estimated lifetime risk for cancer is 0.8 to 5.0 excess deaths per 10,000 workers per rad of exposure. Thus, for example, in 10,000 workers who completed ten missions with an exposure of 40 rem per mission, 320 to 2,000 additional deaths in excess of the 1640 deaths from normally occurring cancer, would be expected. These estimates would indicate a 20 to 120 percent increase in cancer deaths in the worker-population. The wide range in these estimates stems from the choice of the risk-projection model and the dose-response relationship. The choice between a linear and a linear-quadratic dose-response model may alter the risk estimate by a factor of about two. The method of analysis (e.g., relative vs absolute risk model) can alter the risk estimate by an additional factor of three. Choosing different age and sex distributions can further change the estimate by another factor of up to three. When decisions have been made about the selection of SPS workers, the precise influence of age and sex distribution can be included in later risk estimates. However, the choice of dose-response relationship and projection models is a matter of opinion and will not be resolved scientifically for quite some time. CONCLUSIONS The conclusions of the committee based on the findings above are: 1. The risk of excess cancer deaths will be doser to the lower limit estimated above because the major exposure will be from low dose rate, low-LET irradiation. This being the case, we consider a reasonable estimate to be one excess death per 10,000 workers per rem of exposure. If this level of risk is applied to the worst case reference system exposure level, namely 40 rem/mission, there would be 400 excess cancer deaths in a work force of 10,000 completing ten missions (accumulated dose equivalent of 400 rem). 2. The potential genetic consequences, which vary with the population age distribution, could be of significance. At the present time, sufficient information on the age and sex distribution of the worker population is lacking for precise estimation of impact. 3. Similarly, the radiation exposure of a pregnant worker could lead to developmental abnormalities in the embryo.
4. Based on the Reference System (estimated 400 rem from ten missions), dose to space workers from low-LET bremstrahlung approaches the cataractogenic level for man. The appropriate quality factor of the HZE particle portion of the dose is unknown at this time. If its Q is greater than 20, the cataract hazard may be significant. More information is needed about the cataractogenic risk of exposure to high—LET radiations. 5. With adequate information on the radiation environment, with well-designed areas protected from the increased levels during solar particle events, and in the absence of a radiation accident or some other unexpected situations (e.g., nuclear detonation), there will be no early or acute radiation health effects occurring in the SPS worker population. 6. No other radiation health effects are considered to be of sufficient consequence to be important for risk estimation. RECOMMENDATIONS The SPS Committee strongly emphasizes the need to reduce the uncertainties in the evaluation of radiation health risks to SPS workers in the space environment. It recommends that the following be carried out to achieve this goal: 1. A radiation environment model, appropriate to this SPS mission, should be developed for study and simulation. The model should include short-term and solar-cycle variations. The short-term variations of the radiation dose rate in space must be better understood so that the range of doses and dose rates to be expected can be established accurately. 2. An instrumented research satellite should be placed in GEO to measure absorbed dose rate and particle spectra at depth in phantoms and to measure the temporal variations of the radiation field. 3. The differences in the results of dose and dose-rate estimation obtained from the shielding transport codes must be evaluated. The use of different calculational methods with the same set of assumptions should yield the same results. 4. When institutional decisions have been made to develop appropriate exposure strategies, engineering decisions for dose control should then be made. 5. Radiation shielding of transport vehicles, work stations, habitats, and space suits should be designed to achieve minimal radiation exposure levels. The use of laminar layering, where
possible, to reduce the dose and dose rate from bremsstrahlung to the lowest practical values provides a practical example. 6. Studies should be initiated promptly to define the space worker population profiles, particularly the age and sex distributions. Age-specific deaths rates from all causes, in general, and cancers, in particular, should be obtained for a worker population composition similar to that which will work in space. These studies on the SPS worker populations will then provide the data base for calculations of potential health risks of ionizing radiations in the space environment. 7. The biological effects of the HZE particle radiations must be investigated in detail in order to determine appropriate quality factors and risks from lesions unique to HZE. Interactions between high- and low-LET radiation that could occur in the space radiation environment should be considered, particularly the possibility of synergistic effects on carcinogenesis or mutagenesis. 8. The health effects of exposure to low-level ionizing radiations must be considered in the context of the potential health effects of other physical and chemical agents in the space environment. Such competing effects may interact with other host or constitutional factors to mask, enhance, or diminish any radiation health effects, such as cancer. Environmental factors to be considered include cabin atmosphere, temperature and pressure, nutrition, non-ionizing radiation and weightlessness. This SPS Committee concludes that the radiation environment estimated for the Reference System represents a health risk to SPS workers. However, the Committee emphasizes that none of the problems identified are considered sufficiently intractable to preclude achieving a minimal risk to SPS workers. A number of areas have been considered which we believe impact on the potential health effects on workers in space. These include, for example: the biological effects of HZE particles; the effects of environmental agents in space, other than ionizing radiation, which may affect the radiation health effects; and the RBE/LET relationships. However, much more needs to be known about these factors before they may be used to improve the accuracy of quantitative estimation of health risks in space. The Committee has chosen not to include these uncertainties in its estimations at the present time but urges increased studies in these areas to provide greater precision for future SPS health assessments.
APPENDIX A SPACE RADIATION ENVIRONMENT The preliminary SPS Reference System (DOE/NASA, 1978) calls for workers to move from Earth to low Earth orbit (LEO) (altitude 500 km) for stays of varying lengths of time. Workers then travel to geosynchronous Earth orbit (GEO) (altitude about 36,000 km), following an elliptical trajectory (transfer ellipse, TE). These three SPS environments—LEO, TE, and GEO—have ionizing radiations of different quality, time dependence, and predictabi1ity of dose levels. The various components of the radiation environment are described in this section, and those important to each SPS stage are identified. TRAPPED ELECTRONS Large flux densities of electrons, trapped in the earth's magnetic field, are contained in an inner and outer zone separated roughly by the magnetic shell parameter,* L = 2.8 earth radii. The low energies of electrons in the inner zone are important only if persons are protected by very thin shielding (<0.5 g/cm^ aluminum) (Stassinopoulos, 1979). The outer zone contains flux densities of electrons which are greater in magnitude than those of the inner zone and have a larger fraction of high-energy particles. Maximum flux densities occur in the region of about 3.5 earth radii (approximately 21,300 km); the trapping region extends out to about 12 earth radii (76,540 km). Space vehicles transporting workers and materials from LEO must travel through the heart of the outer electron zone to reach GEO which, at 6.6 earth radii, is well within the outer zone of trapped electrons. Figure A-l illustrates the spatial variation of trapped electrons as a function of altitude and geographic latitude, the positions of LEO and GEO, and a representative pass for the transfer trajectory (TE). Two large temporal variations of the outer zone electron flux densities at the position of a GEO satellite have been identified (Stassinopoulos, 1980): 1. Diurnal variations—At GEO, electron flux densities vary over factors between 6 and 16 (depending on L value) between day and night. The maxima occur at about 1000 to 1100 hours, and the minima at about 2200 to 2300 hours, local time. The extent and times of these extremes also depend on electron energy and, to a small degree, on position in the eleven-year solar cycle. *The magnet shell parameter, L, denotes roughly a geomagnetic field line. The value gives the approximate geocentric distance, in earth radii, of the intersection of the field line with the geomagnetic equatorial plane.
Figure A-l. The spatial variation of trapped electrons, plotted as a function of altitude and geographic latitude. The positions of LEO, GEO, and a representative pass for the transfer trajectory are illustrated. Adapted from Wilson, 1979.
2. Short-term enhancements—Electron flux densities can increase markedly due to intermittent changes in the solar plasma associated with small substorms. Flux densities may rise two to three orders of magnitude in several hours followed by a decay lasting several days (Stassinopoulos, 1980). BREMSSTRAHLUNG In general, the absorbed dose from bremsstrahlung is not dominant behind thin shielding (e.g., <3 g/cm? aluminum) because of the overriding preponderance of primary electrons. However, at greater depths, the dose from electrons drops sharply, dependent largely on the shape of the incident electron energy spectrum. This causes the bremsstrahlung to dominate the absorbed dose at large shielding thicknesses, (e.g., >3 g/cm^ aluminum). For inner zone electrons at LEO, the bremsstrahlung dose is completely dominated at all thicknesses by the dose from trapped protons (see below). Here, the bremsstrahlung dose is sufficiently small to be negligible. For outer zone electrons at GEO, on the other hand, the bremsstrahlung dose dominates behind shielding of 3 g/cm? aluminum or greater thickness (Stassinopoulos, 1980). Thus, the bremsstrahlung dose is an important component of the radiation environment in GEO and in the transfer ellipse between LEO and GEO, taking into account certain assumptions of trajectory and vehicle speed. TRAPPED PROTONS Protons are also trapped in large numbers by the geomagnetic field. A region of the trapped proton zone dips close to tne earth in the southern Atlantic Ocean, southeast of the Brazilian coast. This region, called the South Atlantic Anomaly, is the most important contributor to the radiation environment for space workers in LEO. A spacecraft in LEO, however, will pass through this region on only 60 percent of its revolutions. Stassinopoulos (1979) has estimated that 86 percent of the time in LEO is flux-free, i.e., in a radiation environment of less than one particle/cm2-sec of electrons with energies greater than 0.5 MeV and protons with energies greater than 5 MeV. Each day, for a maximum duration of ten hours, there are about six consecutive flux-free revolutions without danger of increased radiation exposure. Trapped protons may be important in the TE between LEO and GEO, depending on the trajectory and vehicle speed selected. Two different calculations of the proton environment encountered by a transfer vehicle have been made, assuming proton environments varying by several orders of magnitude (see "Transfer Ellipse," p. 8). In GEO, the trapped protons are of such low energy that they will not present a health risk to workers at nominal shielding thicknesses.
GALACTIC COSMIC RAYS Galactic cosmic radiation consists primarily of high-energy nuclei with origins outside our solar system; these particles appear to pervade regions outside our magnetosphere isotropically. Approximately 88 percent are protons, 10 percent are helium nuclei, 1 percent are electrons, and 1 percent are heavy nuclei (Z > 2). Ratios of abundances relative to carbon (Z = 6) are shown for the heavy nuclei as a function of nuclear charge number or Z in Fig. A-2. If each nuclear species has a similarly shaped energy spectrum, the relative absorbed dose in free space in a small sphere of tissue from each species would be the GCR abundance ratios scaled by Z?. Due to differences in the spectral shapes at low energies, there are variations noted from the simple Z? scaling. However, the difference at low energies is important only behind thin shielding. Table A-l presents three calculations of the estimated absorbed dose rate in a small tissue sphere in free space and the relative composition from particles grouped in broad ranges of Z (Curtis, 1974). The first two calculations (Curtis and Wilkinson, 1968; Schaefer, 1968) were obtained by integration of the appropriate energy spectra. The third calculation was based on experimental data from a balloon at high altitude (Anderson, 1968). The daily dose rate of between 30 and 40 mrad/day is the maximum expected, since the spectra used were appropriate for solar minimum when the galactic flux densities are known to be the highest. The daily absorbed dose within the body will be less than this value because, although the dose from the protons will not decrease with depth, there will be a decrease in the contribution from the heavier components (see Section 2). Curtis (1973) calculated that the flux densities of HZE particles with LET > 100 keV/pm decrease from 8 to 3 cm? hr as the shielding is increased from 1 to 10 g/cm? of aluminum. Approximately 65 percent of the particles with charge greater than two have charges in the range of 20-26, and this percentage does not vary significantly with thickness of shielding. Measurements made on Soviet satellites Prognoz 1 and Prognoz 2 over an eight month period in 1972, during quiet radiation conditions outside the magnetosphere, yield an estimated dose-rate of about 24 mrad/ day (Logachev et al., 1974). The estimated dose rate is about 10 mrad/ day at solar maximum. Data from U.S. satellites Mariner II and Mariner IV from late 1962 to 1964 provide estimated dose-rate values of 30 to 45 mrad/day at solar minimum. Pioneer IV measurements correspond to dose rates of about 9.6 and 24 mrad/day, the latter at solar minimum (Janni and Holly, 1969). Pioneer V data yield an estimated dose rate value of about 14.4 mrad/day at solar maximum. These estimates are summarized in Table A-2.
Figure A-2. Comparison of the abundances of the elements in galactic cosmic rays with the solar system abundances. Adapted from Wefel, 1979.
Table A-l. Primary galactic cosmic-ray dose rates at solar minimum*
Table A-2 Comparison of measured radiation dose-rates in space satellites.
SOLAR PARTICLE EVENTS Giant solar particle events (SPE) are caused by large upheavals on the solar surface which accelerate protons and, to a lesser extent, heavier nuclei to high energies. These particles are then transported through the solar magnetic field and can increase radiation levels to high values for several hours or days in the vicinity of the earth outside our magnetosphere. GEO, at 6.6 earth radii, is in such minimal geomagnetic shielding that particles down to very low energy arriving in the vicinity of the earth will be able to reach it. Large events occur with highest probability during the rising or falling portions of the eleven-year solar activity cycle. A very small number of events have dominated the total fluence of particles arriving in an eleven-year period. Figure A-3 shows that the events occurring in August 1972 dominated the total fluence arriving in cycle 20 and compares these events with others which occurred in 1967, 1969, and 1971. In Figure A-4, two different estimates of the dose equivalent at the center of a sphere of radius r from the August 1972 event are plotted as a function of the radius; a discrepancy exists between the two calculations, and this is still to be resolved. The figure shows, however, that the sphere radius must be at least 10 g/cm^ of tissue before the dose equivalent drops below 100 rem. The occurrence of such large events, therefore, results in high dose-rate exposures which will be a considerable radiation hazard to SPS workers in GEO. For workers in LEO, on the other hand, enough shielding is provided by the geomagnetic field to make dose equivalents from solar events negligible. DIFFERENT EFFECTS OF HIGH- AND LOW-LET RADIATIONS In the evaluation of the potential health effects caused by space radiations, the radiations may be divided into general categories based on linear energy transfer (LET) or collision stopping power. Low-LET radiations, such as nigh energy electrons and protons, have ionizations which are relatively far apart, with only a small probability of interaction between the ionization products created by a single ionizing particle. Those interactions which do occur are primarily the result of multiple particle tracks. High—LET radiations, such as heavy charged particles, are characterized by ionizations which are normally more closely spaced, and there is therefore a correspondingly greater probability of interactions between the ionization products created by the passage of a single particle. This difference in the microscopic dose distribution generally causes the higher LET radiations to have a greater biological effectiveness. QUALITY FACTOR A quality factor, Q, has been defined to account for the varying degrees of potential adverse health effects in man caused by different
Figure A-3. Degree of domination of total fluence arriving in cycle 20 by solar events which occurred on 4-9 August 1972. Adapted from Crawford et al., 1975.
Figure A-4. Dose equivalents from the August 1972 solar event at the center of a sphere as calculated by Rossi and Stauber (1977) and Wilson and Denn (1976).
ionizing radiations of different LET (ICRP 26, 1977). Currently recommended quality factors vary from 1 (for radiations with LET equal to or less than 3.5 keV/ym) to 20 (for radiations with LET equal to or greater than 175 keV/ym). These values have been selected on the basis of relevant low dose and/or low dose-rate RBE values. They have guided our estimation of a quality factor for the galactic cosmic radiation. Q values are used for assessing radiation health effects for radiation protection purposes, and not to assess the potential health effects of severe accidental high-dose exposures. There may be some health effects caused only by HZE particles and not by other radiations, and here the conventional use of a quality factor does not provide an assessment of potential health risks.
APPENDIX B RADIATION HEALTH EFFECTS IN THE SPACE ENVIRONMENT Radiation doses and dose rates which may be encountered in space present potential health risks to space workers. This section discusses the biological and health effects of the different radiations encountered in the space environment and is based upon data currently available from human epidemiological surveys, laboratory animal studies, and space experimentation. The effects on health are considered in two general categories: early or acute effects, and late or delayed effects. Early effects are those occurring within hours, days, or a few weeks following high-dose, whole-body exposure. Late or delayed effects usually occur months to years following exposure and include cancer-induction, developmental abnormalities in the newborn, genetically related ill-health, lens cataracts, shortened lifespan, and impairment of fertility. The special problems of HZE-particle induced health effects are also discussed. EARLY EFFECTS Early radiation health effects assume clinical significance only with whole-body dose equivalent greater than 150 rem received in relatively short time periods (minutes to hours). Such exposure levels are likely to be encountered only during major solar particle events or during nuclear detonations in space. Radiation is similar to other potentially hazardous physical or chemical agents in that high doses can produce tissue and organ injury, illness, and possibly death. The principle site of biological action of ionizing radiation is the proliterating cells of the renewal system of the organism, such as the bone marrow and intestinal epithelium and spermatogonia. Within these cell renewal populations, the most sensitive cell is the progenitor or stem cell. When the supply of functional cells is temporarily disrupted, the result is impaired function of that tissue or organ and potentially serious injury to the individual. If the therapeutic measures are inadequate or regeneration of the depleted cell population does not occur soon enough, the individual may die. Damage to bone marrow and the intestine may cause death within 7 to 60 days after acute exposure to whole-body dose of radiation greater than a few hundred rad of low-LET radiation. Cellular depletion in tissues and organs that contain large numbers of dividing cells, e.g., in the bone marrow, lymphopoietic tissues, and testis, can be detected at doses as low as 50 rad delivered in short time periods, and are readily evident at doses of 100 to 150 rad. Death does not occur at these dose levels, although severe cellular depletion can be observed in some non-vital renewing organs, such as the testis. Whole-body radiation doses in the 200- to 400-rad
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