Environmental Risk Assessment Reports - Chapter 25

CHAPTER 25 Radiation Risk Assessment Nava C. Garisto and Donald R. Hart CONTENTS I. Introduction.................................................................................................479 II. Radiation Types and Sources .....................................................................480 A. Types of Radiation........................................................................480 B. Radiation Units.............................................................................481 C. Radiation Sources.........................................................................482 III. Risk Assessment for Radioactive Substances............................................482 A. The Risk Assessment Process ......................................................482 B. Problem Formulation....................................................................483 C. Radiation Exposure Analysis........................................................483 1. Source Term Development ..........................................485 2. Radionuclide Transport Analysis.................................486 3. Food Chain Pathways Analysis...................................486 4. Dose Rate Estimation..................................................489 5. Radiation Response Analysis ......................................490 6. Risk Characterization...................................................492 IV. Conclusion..................................................................................................494 References...................................................................................................494 I. INTRODUCTION* Risk assessment for radioactive substances is a quantitative process that estimates the probability for an adverse response by humans and other biota to radiation * The authors wish to thank Dr. D. Lush, Dr. F. Garisto, Ms. K. Fisher, and Mr. M. Walsh for critically reviewing early drafts of this manuscript. The graphics support of M. Green is greatly appreciated. 479 © 2001 by CRC Press LLC 480 A PRACTICAL GUIDE TO ENVIRONMENTAL RISK ASSESSMENT REPORTS exposure. It has been used for a variety of regulatory purposes such as the derivation of site-specific radionuclide release limits, or the determination of the acceptability of proposed undertakings that may release radionuclides. Radioactive substances, as compared to other chemical substances, have a long history of risk-based regulations. These regulations developed in reaction to early mismanagement of radiation risks. Today, the concept of site-specific risk assessment is fundamental to the regulation of radioactive substances and serves as a model for risk-based regulation of other chemicals. The unique properties of radioactive substances, associated with their emissions of ionizing radiation, require specialized approaches to assessment of exposure, dose, and risk. For example, since a radiation dose can be received without physical contact with the radioactive substance, this external exposure, as well as internal exposure from radionuclides taken into the body, must be considered. Moreover, since radi-ation is the common agent of hazard for all radioactive substances, concentration and dose are usually expressed in radiation units (see below), and doses are additive across radionuclides, in contrast to the situation with chemical toxicants. Whereas the fundamental concepts of risk assessment are the same for radioac-tive and other chemical substances, the unique properties of and approaches to radioactive substances must be understood in order to critically evaluate a consult-ant’s work and integrate it into an overall risk assessment. The purpose of this chapter is to outline these unique properties and approaches to risk assessment of radioactive substances to better enable project managers to work with consultants in this tech-nical area. II. RADIATION TYPES AND SOURCES A. Types of Radiation Radiation consists of energetic particles or waves that travel through space. The less energetic wave types are said to be nonionizing because they do not cause atoms in biological tissue to become electrically charged. Familiar examples of nonionizing radiation are the visible light and heat that reach the earth from the sun. The more energetic wave types, such as ultraviolet rays, X-rays and gamma rays, are said to be ionizing, because they have enough energy to make electrons in biological tissues completely escape their atomic orbitals, forming electrically charged ions. In addi-tion to wave energy, radioactive substances may emit sub-atomic particles such as beta or alpha particles. These particles also have sufficient energy to ionize biological tissues. All types of ionizing radiation (both waves and particles) can produce damage to the biological tissues that they contact. Wave types can easily penetrate biological tissue. Some of the X or gamma rays that are directed towards the body will pass right through without being absorbed (i.e., without transferring energy to cause ionization). Others will be absorbed when they strike atoms in the tissue, forming charged ions. The charged ions are chemically reactive, and often react inappropri- © 2001 by CRC Press LLC RADIATION RISK ASSESSMENT 481 ately. When this happens in the genetic material (DNA) that controls cell function, there is a chance that cell growth may eventually go out of control, causing cancer. If there is sufficient genetic damage in a reproductive tissue, there may also be some loss of reproductive function. Particle radiations, because of their mass and electric charge, are less able to penetrate biological tissue. Their energy is absorbed and damage is concentrated closer to the point of biological contact. For example, if the radiation source is outside the body, most of the beta and alpha radiation will be absorbed in the skin. On the other hand, if the source is a radionuclide that has been incorporated into an internal tissue, most of the beta and alpha radiation will be absorbed inside that tissue. Alpha particles, because of their large mass, high charge, and high energy, produce more localized and intensive ionization effects than either waves or beta particles, and therefore tend to produce a greater amount of genetic damage. They also tend to produce a different spectrum of genetic damage (i.e., a higher proportion of chromosome breaks as opposed to point mutations) which makes accurate repair less likely. Differences in the biological effectiveness of various radiation types are described by “quality factors” (QF). Gamma and beta radiations have quality factors of one (QF = 1), while alpha radiation has a much higher quality factor (QF = 20) based on its greater effectiveness in human cancer induction. Quality factors based on reproductive impairment have not been well defined, particularly for nonhuman species. This is a major source of uncertainty in assessment of ecological risks from alpha-emitting radionuclides. B. Radiation Units A radionuclide is designated by its atomic mass (isotope) number and its chemical element name. As it decays by atomic disintegration, its mass may change and it is transformed to a new element or a series of different “daughter” elements (a decay series). Alpha, beta, or gamma radiation is released with each disintegration over the course of this transition. Under secular equilibrium (i.e., undisturbed) conditions, each element in a decay series has the same activity. Activity is a measure of radiation quantity in terms of atomic disintegration frequency. It is directly related to the amount of a radionuclide and its radiological half-life. Activity is expressed in becquerels (1 Bq = 2.7 ´ 10-11 Ci = one disinte-gration per second). Activity concentration in any medium is expressed in Bq per unit of mass, volume, or surface area. The radiation energy absorbed by an organism is expressed as a dose in grays (1 Gy = 100 rad). The rate of energy absorption is expressed as a dose rate in Gy per unit of time. These units represent absorbed energy without regard to the radiation type or the effectiveness of the absorbed dose (1 Gy of alpha radiation is capable of causing more biological damage than 1 Gy of gamma radiation). Effective dose rates for humans are expressed as gamma dose equivalents in sieverts (Sv) per unit of time (1 Sv = 100 rem) after application of appropriate quality factors to account for radiation type. © 2001 by CRC Press LLC 482 A PRACTICAL GUIDE TO ENVIRONMENTAL RISK ASSESSMENT REPORTS C. Radiation Sources All of us are exposed to ionizing radiation every day. The earth is continually bombarded by protons, X-rays, gamma rays, and ultraviolet radiation from cosmic sources. Approximately 67% of this radiation is absorbed by the earth’s atmosphere and never reaches the earth’s surface. Atmospheric gases such as ozone are partic-ularly important in absorption of ultraviolet energy. In addition to the cosmic sources of ionizing radiation, humans and other biota on earth are exposed to ionizing radiation from the decay of radioactive substances on earth. Ionizing radiation comes from such diverse sources as building materials in houses, glass and ceramics, water and food, tobacco, highway and road construc-tion materials, combustible fuels, airport scanning systems, the uranium in dental porcelain used in dentures and crowns, diagnostic X-ray sources, and many others. Most of these substances contain radionuclides that are naturally present in the earth, although human activity has increased their production and/or the potential for human exposure. Other radionuclides, which are produced in nuclear reactors or accelerators, are geologically unknown or extremely rare. The background radiation dose rate received by the average person from natural sources is approximately 2 mSv/a (UNSCEAR, 1988). Typical dose rates and doses from anthropogenic sources are as follows: · Medical, average of all procedures = 1.0 mSv/a · Fallout from nuclear weapons testing = 0.01 mSv/a · Chernobyl accident, average first year commitment* in Bulgaria = 0.75 mSv · Chest X-ray (one) = 0.1 mSv · Dental X-ray (one) = 0.03 mSv · Barium enema (one) = 8 mSv Natural background varies geographically with altitude, latitude, and local geol-ogy. It is higher at high altitudes where the atmosphere is thinner and there is less atmospheric absorption of cosmic radiation. Fallout from long-range atmospheric transport varies mainly with latitude, due to global air circulation patterns, peaking at 40 – 70° north latitude. III. RISK ASSESSMENT FOR RADIOACTIVE SUBSTANCES A. The Risk Assessment Process Risk assessment of radioactive substances should be conducted whenever radioactive substances are identified as contaminants of potential concern at a site. The process that is recommended by international agencies for risk assessment of radioactive substances (IAEA, 1989, 1992a) is consistent with the more recent U.S. EPA (1989, 1992) paradigms for human health risk assessment (HHRA) and ecological risk assessment (ERA) although there are minor variations in terminology. While the * 50-year dose commitment from exposures over the first year. © 2001 by CRC Press LLC RADIATION RISK ASSESSMENT 483 process has historically been focused on the human receptor, there is increasing attention to nonhuman dose and risk estimation. The radiological risk assessment process is outlined in Figures 1 and 2. The process is iterative as shown in Figure 1, with updating of methodology, models, and data between iterations. The risk assessment process includes the following basic components: · Identification of events and processes which could lead to a release of radionuclides or affect the rates at which they are released and transported through the environ-ment · Estimation of the probabilities of occurrence of these release scenarios · Calculation of the radiological consequences of each release (i.e., doses to indi-viduals and populations and associated human cancer risks or ecological effects) · Integration of probability and consequence over all scenarios to define the overall risk of human cancer or ecological effects · Comparison of maximum doses and/or risks with current regulatory criteria Deterministic estimates of maximum dose from each scenario are often made ini-tially to evaluate whether further analyses are required. Probabilistic estimates are appropriate whenever maximum doses approach effect thresholds or acceptability criteria (IAEA, 1992a). The probabilistic methods explicitly consider the uncertain-ties in key parameters, but use best estimates as central values for each one. This produces a more realistic statement of risk. B. Problem Formulation Problem formulation is the scoping exercise which identifies the radionuclide sources, release scenarios, human and ecological receptors, exposure pathways, and response endpoints to be considered in the subsequent risk assessment. The spatial and temporal scales of analysis must also be defined. Collectively, these elements constitute a conceptual model of the system to be studied. They are included in the first two boxes on the main axis of Figure 1. It is important to ensure, at this stage, that all major stakeholder concerns are represented in the conceptual model. There are few aspects of problem formulation that are unique to radioactive substances, although the gap between realistic concern and public perception is often particularly large for these substances. The scope of an assessment can easily escalate from local site-specific risk issues to encompass national energy policy issues. Without minimizing these public participation challenges, or the importance of problem formulation, this chapter focuses mainly on the subsequent stages of con-sequence analysis and risk characterization. C. Radiation Exposure Analysis Humans and other biota can be exposed to radiation by multiple routes. All envi-ronmental media must be considered as potential routes of exposure. For example, radionuclides may be carried into the atmosphere as aerosols or gases (e.g., radon), © 2001 by CRC Press LLC ... - tailieumienphi.vn