Environmental Risk Assessment Reports - Chapter 25 pdf

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 aut

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 LA4111 ch25 new Page 479 Wednesday, December 27, 2000 2:51 PM

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, concentrradi-ation 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-LA4111 ch25 new Page 480 Wednesday, December 27, 2000 2:51 PM

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

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

LA4111 ch25 new Page 481 Wednesday, December 27, 2000 2:51 PM

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.

LA4111 ch25 new Page 482 Wednesday, December 27, 2000 2:51 PM

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), LA4111 ch25 new Page 483 Wednesday, December 27, 2000 2:51 PM

484 A PRACTICAL GUIDE TO ENVIRONMENTAL RISK ASSESSMENT REPORTS

and may fall onto the land and/or be leached into surrounding water bodies As they disperse from the area of release, in either air or water, they are generally diluted and concentrations tend to decrease with distance from the source Humans and biota near the source may take in larger quantities of radioactivity in the air they breathe, the water they drink, and the food they eat than organisms farther away Figure 2 illustrates the major steps in exposure estimation within the overall risk assessment framework These steps include source-term development, radionuclide transport analysis, food chain pathways analysis, and dose-rate estimation

Figure 1 Overall process of radiological risk assessment.

LA4111 ch25 new Page 484 Wednesday, December 27, 2000 2:51 PM

RADIATION RISK ASSESSMENT 485

1 Source Term Development

The source term development will determine, through measurement or theoretical calculation, the type and quantity of radionuclides released in terms of activity per unit time The chemical and physical form of the released radionuclides must also

be considered In the past, little emphasis was placed on accurately estimating source terms and considerable uncertainty still exists in this area for many assessments Source term models that have been developed specifically for radioactive waste management applications include, e.g., the AREST model (Liebetrau et al., 1987),

Figure 2 Major steps in radiological risk assessment as related to the framework for

eco-logical risk assessment.

LA4111 ch25 new Page 485 Wednesday, December 27, 2000 2:51 PM

486 A PRACTICAL GUIDE TO ENVIRONMENTAL RISK ASSESSMENT REPORTS

the VAULT model (Johnson et al., 1994), and the RAMSIM model (BEAK, 1996a) These models take into account the evolution of geochemical and hydrological conditions in the source matrix, and the corresponding changes in radionuclide release rates over time

2 Radionuclide Transport Analysis

The radionuclide transport calculations trace radionuclide movements through air, surface water, and groundwater The objective here is to predict the activity concen-trations of radionuclides to which humans and other biota are exposed The contam-inant transport models simulate physical transport due to processes such as advection and dispersion The mechanisms of radionuclide movement through the natural environment are not dependent on the activity level of the radionuclide, except in a few cases (e.g., radiolysis of groundwater, the decomposition of groundwater caused

by high levels of radiation, affects the oxidation states of radionuclides in ground-water and thereby affects radionuclide mobility) Since radioisotopes have chemical properties identical to those of their stable homologs, their movements will parallel those of stable elements From the point of view of release and mobility, therefore, the important parameters are the physical state, the type of aggregation if any (e.g., colloidal), the chemical form, solubility, oxidation states, sorption properties, and volatility The key product of a transport model is an estimate of radionuclide activity per unit volume of air, water, or soil as a function of time

Processes that affect radionuclide transport through the atmosphere are shown schematically in Figure 3 In addition to the conventional dispersion processes, which are considered for all contaminants, radioactive decay and buildup have to be taken into account for radionuclides For example, in modeling the transport of radon gas,

it is important in some cases to consider its radioactive decay products and their deposition, especially within confined environments The transformations that occur with degradation of some organic compounds add a similar level of complexity to their transport analyses Atmospheric transport models include a whole range of models, from screening-level analytical (Gaussian plume) models to sophisticated numerical models that can take into account complex terrain, shoreline effects, building wake effects, and long-range transport The more sophisticated models require more extensive input data This often limits their usefulness

Processes that affect contaminant transport through surface waters and ground-water are shown schematically in Figure 4 As with atmospheric transport, radioac-tive decay and buildup have to be taken explicitly into account Numerous mathe-matical models, from simple to complex, have been developed to simulate the flow

of water and the transport of radionuclides in surface waters and groundwater It is important to understand the simplifying assumptions inherent in the simple models,

in order to recognize the complex situations in which they are not applicable

3 Food Chain Pathways Analysis

The food chain analysis traces radionuclide movements from surface water, soil, and atmosphere through a variety of internal exposure pathways to humans and other LA4111 ch25 new Page 486 Wednesday, December 27, 2000 2:51 PM

RADIATION RISK ASSESSMENT 487

biota, in order to calculate radiation doses due to inhalation of air and ingestion of food, drinking water, and soil Processes typically considered in food chain models include: atmospheric deposition to vegetation and soil, bioaccumulation from water

Figure 3 Atmospheric processes that affect radionuclide transport.

Figure 4 Radionuclide transport processes in surface waters and groundwater.

LA4111 ch25 new Page 487 Wednesday, December 27, 2000 2:51 PM

© 2001 by CRC Press LLC