Exposure scenarios, inadvertent

The numerator in Equation 1.1 or 1.2 is calculated using generic scenarios for exposure of individual members of the public that arise from waste disposal. Two types of exposure scenarios can be considered (1) scenarios involving release of hazardous substances from a disposal facility and exposure of individuals at locations beyond the boundary of the disposal site or (2) scenarios involving exposure of individuals who inadvertently intrude onto a disposal site, including scenarios involving permanent residence on a disposal site or other unrestricted access after an assumed loss of institutional control. 

An important characteristic of acceptable disposal facilities for high-hazard waste is that inadvertent intrusion into a facility, such as by drilling, must be unlikely. Therefore, assessments of risk or dose to hypothetical inadvertent intruders based on exposure scenarios that are assumed to occur do not provide a suitable basis for determining acceptable disposals in facilities located well below the ground surface. 

The risk index for any hazardous substance in Equation 1.1 or 1.2 (see Section 1.5.1) is calculated based on assumed exposure scenarios for hypothetical inadvertent intruders at near-surface waste disposal sites and a specified negligible risk or dose in the case of exempt waste or acceptable (barely tolerable) risk or dose in the case of low-hazard waste. Calculation of the risk index also requires consideration of the appropriate measure of risk (health-effect endpoint), especially for carcinogens, and the appropriate approaches to estimating the probability of a stochastic response per unit dose for carcinogens and the thresholds for deterministic responses for noncarcinogens. Given a calculated risk index for each hazardous substance in a particular waste, the waste then would be classified using Equation 1.3. 

The use of MLEs of probability coefficients, rather than upper confidence limits (UCLs), to classify waste can be justified, in part, on the grounds that the assumed exposure scenarios for hypothetical inadvertent intruders at waste disposal sites are expected to be conservative compared with likely on-site exposures at future times. However, uncertainties in probability coefficients should still be considered in classifying waste. When risk is calculated using MLEs of... 

In principle, for any type of disposal system that could be assumed for purposes of classifying waste, such as a near-surface disposal facility for hazardous wastes, a multitude of exposure scenarios might be considered. However, NCRP believes that only a single type of exposure scenario should be considered in classifying waste. Specifically, NCRP believes that the concept of a hypothetical inadvertent intruder at waste disposal sites provides a suitable basis for... 

In general, calculation of the risk or dose from waste disposal in the numerator of the risk index in Equation 6.2 or 6.3 involves the risk assessment process discussed in Section 3.1.5.1. As summarized in Section 6.1.3, NCRP recommends that generic scenarios for exposure of hypothetical inadvertent intruders at waste disposal sites should be used in calculating risk or dose for purposes of waste classification. Implementation of models describing exposure scenarios for inadvertent intruders at waste disposal sites and their associated exposure pathways generally results in estimates of risk or dose per unit concentration of hazardous substances in waste. These results then are combined with the assumptions about allowable risk discussed in the previous section to obtain limits on concentrations of hazardous substances in exempt or low-hazard waste. 

Exposure Scenarios for Classifying Exempt Waste. Based on the definition of exempt waste as any waste that would be generally acceptable for disposal in a municipal/industrial landfill for non-hazardous waste, scenarios for inadvertent intrusion appropriate to this type of facility should be used in determining whether a waste would be classified as exempt. 

Establish generic exposure scenarios for inadvertent human intrusion into a municipal/industrial landfill for disposal of exempt waste and intrusion into a dedicated near-surface facility for disposal of low-hazard waste. 

In addition, the reliance on generic scenarios for inadvertent intrusion in classifying waste cannot, by definition, represent site-specific risks. However, this is not a serious shortcoming because such scenarios have been used in establishing subclasses of low-level radioactive waste for disposal in near-surface facilities (NRC, 1982a). Furthermore, as emphasized in this Report, establishment of a risk-based waste classification system using particular exposure scenarios does not obviate the need to perform site-specific risk assessments for the purpose of establishing waste acceptance criteria at each disposal site. 

In implementing the risk-based waste classification system developed in this Report, the selection of exposure scenarios appropriate to waste disposal is an important technical issue that must be addressed. NCRP believes that scenarios for inadvertent intrusion into near-surface disposal facilities are appropriate in classifying waste for purposes of disposal and, further, that scenarios involving permanent occupancy of disposal sites after loss of institutional control would be appropriate (see Section 6.1.3) such scenarios are commonly used in regulating near-surface disposal of low-level radioactive waste and in risk assessments at hazardous waste sites subject to remediation under CERCLA. 

Use of the risk index in classifying waste is illustrated in Figure 6.2. Classification of waste essentially is a two-step process. The first step involves a determination of whether a waste can be classified as exempt, based on an assumed negligible risk and an exposure scenario for inadvertent intruders appropriate to disposal of waste in a municipal/industrial landfill for nonhazardous waste. If the waste is not exempt, the second step involves a determination of whether a waste can be classified as low-hazard, based on an assumed acceptable (barely tolerable) risk and an exposure scenario for inadvertent intruders appropriate to disposal in a dedicated nearsurface facility for hazardous wastes. 

This Section provides example applications of the recommended risk-based waste classification system to a variety of hazardous wastes to illustrate its implementation and potential ramifications. Disposal is the only disposition of waste considered in these examples. In Section 7.1.1, a general set of assumptions for assessing the appropriate classification of hazardous wastes is developed, including a variety of assumed exposure scenarios for inadvertent intruders at waste disposal sites and assumed negligible and acceptable risks or doses from exposure to radionuclides and hazardous chemicals. Subsequent sections apply the methodology to several example wastes. 

Alternative Exposure Scenarios. In the scenarios for inadvertent intrusion at a radioactive waste disposal facility considered in Sections 7.1.3.1 through 7.1.3.3, intrusion is assumed to be a onetime event occurring at 100 y after disposal. This Section considers alternative scenarios and their impacts on classification of the Hanford waste. 

The concept of a hypothetical inadvertent intruder at a nearsurface waste disposal site, including permanent occupants of a site after an assumed loss of institutional control, provides a suitable basis for defining exposure scenarios that would be used to calculate risks that arise from waste disposal and the boundaries between waste classes. For other dispositions of waste, alternative scenarios would need to be developed and evaluated. 

The Normal Use (workplace) scenario represents the use of small amounts of radionuclides in industry, etc. in the manner for which they are intended, and involves external exposure and inadvertent intakes of radioactive materials. 

NCRP recommends that generic scenarios for exposure of hypothetical inadvertent intruders at disposal sites should be used in classifying waste. This recommendation is based on two considerations. First, scenarios for inadvertent intrusion can be applied to an assumed type of disposal system at any site, whereas scenarios for exposure of members of the public due to release and transport of hazardous substances to locations beyond the boundary of a disposal facility are highly site-specific and, thus, are not appropriate for use in generally classifying waste. 

The recommendation that generic scenarios for exposure of hypothetical inadvertent intruders should be used in classifying waste is consistent with the approach used by NRC in 10 CFR Part 61 to establish different subclasses of low-level radioactive waste that are generally acceptable for near-surface disposal (Class-A, -B, and -C waste) or are generally unacceptable for near-surface disposal (greater-than-Class-C waste). Such scenarios have not been used to... 

Many details would need to be considered in developing a new waste classification system based on the framework presented in this Report. Assumptions about generic scenarios for exposure of hypothetical inadvertent intruders at waste disposal sites to be used in classifying waste and the time frames for applying the scenarios would be required. Decisions would need to be made about negligible and acceptable (barely tolerable) doses or risks that would be used in classifying waste as exempt or low-hazard, respectively. 

The dominance of the risk to inadvertent intruders at near-surface waste disposal sites allows the use of this type of scenario to develop a risk-based waste classification system. However, NCRP recognizes that exposures of the public and protection of the environment also are of concern in determining acceptable disposal practices at specific sites. The potential for off-site releases of hazardous substances is the primary reason that classification of waste based on risks to hypothetical inadvertent intruders does not obviate the need for site-specific risk assessments to determine waste acceptance criteria in the form of limits on disposal of particular hazardous substances. 

The use of an acceptable (barely tolerable) risk to classify nonexempt waste can be justified, in part, on the following grounds. Disposal facilities for exempt and low-hazard waste both are located near the ground surface, and many scenarios for inadvertent intrusion into municipal/industrial landfills for nonhazardous waste also would be credible occurrences at disposal sites for low-hazard waste. However, these types of scenarios should be less likely to occur at hazardous waste sites, compared with sites for disposal of nonhazardous waste, given the intention to maintain institutional control and records of past disposal activities for a considerable period of time after closure of hazardous waste sites and the possibility that societal memory of disposal activities will be retained long after institutional control is relinquished. Thus, the risk to future inadvertent intruders at dedicated hazardous waste disposal sites, taking into account the probability that exposures according to postulated scenarios would actually occur, should be comparable to the risk at disposal sites for nonhazardous waste. 

In assessing risks based on scenarios for exposure of hypothetical inadvertent intruders at municipal/industrial landfills for non-hazardous waste (i.e., in determining whether a waste would be classified as exempt or nonexempt), scenarios involving permanent occupancy of a disposal site should be assumed to occur beginning at the time of facility closure, based on the expectation that institutional control will not be maintained over this type of facility for a significant period of time after closure. 

Another credible assumption is that permanent access to the site could occur at the end of the 100 y period of institutional control. This assumption has been used in establishing waste acceptance criteria at all DOE low-level waste disposal sites (DOE, 1988c 1999c), including the Hanford site, based on an acceptable dose from chronic exposure of an inadvertent intruder of 1 mSv y Therefore, the waste acceptance criteria for the Hanford site already take into account an acceptable dose to an inadvertent intruder from permanent site occupancy, so the waste is acceptable for near-surface disposal as low-hazard waste according to this scenario without the need for further analysis. 

Approach to Example Analysis. Similar to the previous examples involving radioactive wastes, these residues were assumed to be placed in a typical near-surface disposal facility having a RCRA Subtitle C permit. In this example, it is assumed that an inadvertent intruder excavates an area of the disposal site of approximately 200 m2. This excavation is sufficient to reach the waste, and the exposure pathways considered involve inhalation of resuspended waste, ingestion of waste, and dermal absorption. The intrusion is identified and halted prior to any structures being constructed on the disposal site and before any farming activity can be developed. As in the similar scenarios used in the radioactive waste examples, exposure is assumed to continue for 1,000 h. 

The external dose to an inadvertent intruder who is assumed to be exposed to uncovered waste for a period of 1,000 h at the time of facility closure can be estimated as follows. For a 137Cs source assumed to be uniformly distributed in surface soil with its decay product 137mBa in activity equilibrium, and taking into account the decay branching fraction of 0.946 (Kocher, 1981), the external dose rate per unit concentration is 2.9 X 10 11 Sv s 1 per Bq g 1 (Eckerman and Ryman, 1993). Multiplying this external dose coefficient by the assumed concentration of 137Cs (4.8 Bqg ) and exposure time (1,000 h) gives a total dose for the assumed scenario of 5 X 10 4 Sv. 

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