48 SHIELDING FACTOR FOR EXTERNAL GAMMA RADIATION

48.1 DEFINITION

This factor is the ratio of the external gamma radiation level indoors on-site to the radiation level outdoors on-site. It is based on the fact that a building provides shielding against penetration of gamma radiation. Therefore, the calculation of the effective dose from the ground pathway should take into account this shielding effect.

The occupancy factor, FO₁, for the ground pathway can be obtained by using the following equation:

where TF₁ is the fraction of time spent outdoors on-site (an input parameter, Section 29), TF₂ is the fraction of time spent indoors on-site (an input parameter, Section 28), and TF₃ is the fraction of time spent off-site (not an input parameter, Sections 28 and 29).

48.2 RESRAD DATA INPUT REQUIREMENTS

This parameter should be input as a fraction, ranging from 0 to 1. The default shield factor used in the RESRAD code for external gamma radiation is 0.7, which assumes that the external gamma radiation level indoors is 30% lower than the outdoor gamma radiation level.

49 ELAPSED TIME OF WASTE PLACEMENT

49.1 DEFINITION

The elapsed time of waste placement parameter is the duration between the placement of radioactive waste on-site and the performance of a radiological survey. It is possible that on-site radioactive wastes originated from different sources and have different placement times. Under these circumstances, an average value or a best representative value should be used.

When using RESRAD for risk assessment, the information obtained during the radiological survey is input to derive soil guidelines for cleanup criteria. This information includes soil/water distribution coefficients, soil radionuclide concentrations, and so forth. The soil/water distribution coefficients are used for calculating the breakthrough and rise times of the groundwater contamination and for predicting the future radionuclide concentration in groundwater. In this case, the elapsed time of waste placement is zero. Nonzero values of this parameter should be input only when the soil/water distribution coefficients are not available and above background level groundwater radionuclide concentrations are measured in a radiological survey. Under such conditions, the input radionuclide concentration in groundwater, together with the elapsed time of waste placement would be used to derive soil/water distribution coefficients and to predict the future radionuclide concentration in groundwater.

49.2 RESRAD DATA INPUT REQUIREMENTS

In the RESRAD code, the elapsed time of waste placement should be entered in units of years (yr). The default value of this parameter is set at zero.

50 SHAPE FACTOR (EXTERNAL GAMMA)

50.1 DEFINITION

A shape factor is used to correct for a noncircular-shaped contaminated area on the basis of an ideally circular zone. The shape factor for a circular contaminated area is 1.0. For an irregularly shaped contaminated area, the shape factor may be obtained by enclosing the irregularly shaped contaminated area in a circle, multiplying the area factor of each annulus by the fraction of the annulus area that is contaminated, summing the products, and dividing by the area factor of a circular contaminated zone that is equivalent in area. The area factors of circular contaminated zones with different radii are listed in Table 50.1. The area factor of an annulus is the area factor from an annular zone bounded by the radii tabulated in Table 50.1.

If an irregularly shaped contaminated zone of 191.4 m² is shaped like that in Figure 50.1, the shape factor can be calculated by surrounding the area with the appropriate annuli as indicated. The contamination fractions within each annulus are 1, 1, 0.97, and 0.22, respectively. The area factor of the irregularly shaped contaminated zone can be calculated as follows on the basis of the values in Table 50.1:

Next, the area factor of a circular contaminated zone equivalent in area must be determined. The radius of a circle with an area of 191.4 m² is 7.8 m. By interpolating the data of Table 50.1, the area factor of a circle equivalent in area can be determined as follows:

The final step in obtaining the shape factor of the contaminated zone is to divide the area factor of the contaminated zone by that of the circular zone with an equivalent area:

Therefore, the shape factor of an irregularly shaped contaminated zone such as that in Figure 50.1 is determined to be 0.97.

50.2 RESRAD DATA INPUT REQUIREMENTS

In the RESRAD code, the default contaminated area is 10,000 m², which is greater than the area of 1,200 m² listed in Table 50.1 for an area factor of 1.0. Because the area factor is already equal to 1, the default value of the shape factor is 1.

If the input shape factor is negative, that is, between 0 and -1, then the SOILD model (Chen 1991) rather than the model mentioned in the original RESRAD report (Gilbert et al. 1989) is used for calculating the effective dose from external radiation. In this case, the shape factor is not actually used in SOILD calculations, it merely serves as a flag for the choice of options. Thus, the user should provide more detailed information about the fractions of annular areas within the contaminated zone for use in the SOILD calculations.

51 INITIAL CONCENTRATIONS OF PRINCIPAL RADIONUCLIDES

51.1 DEFINITION

A principal radionuclide is a radionuclide with a half-life longer than one-half year. Radionuclides with a half-life of one-half year or less are treated as associated radionuclides. The radionuclides "associated" with a principal radionuclide consist of all decay products down to, but not including, the next principal radionuclide in the chain. It is assumed that all associated radionuclides (except radon daughters) are in secular equilibrium with their principal radionuclide in the contaminated zone and also at the location of human exposure. Only the principal radionuclides in the contaminated zone need input values of radionuclide concentrations.

The single-radionuclide soil guidelines do not depend on the radionuclide concentrations in soil. Even if the radionuclide concentrations are not known, values for these guidelines can be obtained by entering any nonzero radionuclide concentration. The calculated doses, however, depend on the radionuclide concentrations; thus, doses calculated by RESRAD are valid only if the soil radionuclide concentrations are known. When the radionuclide concentrations in soil and groundwater are used with the elapsed time of waste placement to derive the soil/water distribution coefficient, the values of the initial concentrations of the principal radionuclide must be known to obtain accurate results.

A DOE-approved statistical approach (DOE 1991a, Section 7) should always be considered as the first priority regarding the estimation of the soil concentration. When such an application is impossible, then the following approach will serve as a default procedure in determining the average soil concentration; however, this approach will result in a conservative estimate of the effective dose.

For a site-specific case, the distributions of radionuclides are nonuniform. The potential annual individual dose received through a particular pathway is an average of the nonuniform residual radioactivity over an area determined by the scenario activities; for example, the area of daily activities for external radiation or the size of the garden for the plant food pathway. For the purpose of deriving soil guidelines, it is assumed that this area is 100 m² for all pathways. The effect of vertical nonuniformities is taken into account by averaging the radionuclide concentrations in a 0.15-m-thick layer over the 100-m² area.

The initial concentration of a principal radionuclide is determined by the following procedures. For a contaminated zone with an area greater than 100 m², the average radionuclide concentration for any subzone with a 100-m² area and 0.15-m thickness is determined. If one or more soil samples within this subzone have radionuclide concentrations greater than three times the average radionuclide concentration, then the average radionuclide concentration of this subzone is replaced by one-third the maximum measured soil radionuclide concentration. The initial concentration of a principal radionuclide in the contaminated zone is the maximum value of the average subzone radionuclide concentration. For a contaminated zone with an area less than 100 m², the initial concentration of a principal radionuclide is the maximum average radionuclide concentration of the 0.15-m-thick subzones.

51.2 RESRAD DATA INPUT REQUIREMENTS

In the RESRAD code, the initial concentrations of principal radionuclides in the contaminated zone are expressed in units of picocuries per gram (pCi/g). RESRAD treats the contaminated zone as a uniformly contaminated area with a single radionuclide concentration at every point.

52 DRINKING WATER INTAKE RATE

52.1 DEFINITION

The EPA uses 2 L/d as the average amount of water consumed by an adult; this includes juices and beverages containing tap water (e.g., coffee). However, this value was established by the U.S. Army in determining the amount of water needed per person in the field and is believed to be an overestimate.

The National Academy of Sciences (NAS) (1977) calculated the average consumption rate of water to be 1.63 L/d per person. It is reasonable to assume that people in physically oriented occupations or living in warmer regions may have an intake rate exceeding this level. Although the consumption rate of 1.63 L/d seems to have a more scientific basis than the 2 L/d rate, the NAS (1977) still adopted the larger volume (i.e., 2 L/d) to represent the intake rate for the majority of people.

Several other drinking water intake rates have been suggested. The National Cancer Institute (NCI) in an investigation of the possible relationship between bladder cancer and drinking water, interviewed approximately 9,000 individuals by using a standardized questionnaire and suggests that the overall average tap water consumption rate is 1.39 L/d (Cantor et al. 1987). According to the NCI's distribution data, 1.3 L/d is the approximate value of the 50th percentile and 2.0 L/d is the approximate value of the 90th percentile.

Gillies and Paulin (1983) suggest an average rate of 1.256 (+ 0.39) L/d and a 90th percentile rate of 1.9 L/d on the basis of a survey conducted in New Zealand. On the basis of data from the U.S. Food and Drug Administration's total diet study, Pennington (1983) reported an average daily fluid consumption rate for water and water-based foods of 1.2 L/d. The ICRP has summarized the intake levels for adults as ranging from about 0.4 L/d to about 2.2 L/d under normal conditions.

The EPA (1984c) used data collected by the USDA in its 1977-1978 nationwide food consumption survey to determine daily beverage intake levels by age. The daily beverage intake level for adults ranged from 1.24 to 1.73 L. The EPA (1990) has suggested that the average adult drinking water consumption rate is 1.4 L/d; the reasonable worst-case value is 2.0 L/d on the basis of the above studies. These values correspond to 510 and 730 L/yr, respectively, if a 365 d/yr is used. Further evidence to support these values is provided by Pennington (1983) and Cantor et al. (1987) who report average total fluid intake rates of 1.7 and 1.87 L/d among adults. Thus, the average water consumption rate should be less than the 2.0 L/d commonly used. Although data are available for the intake rate for the reasonable worst case, from the reported value of 1.90 L/d for the 90th percentile by Gillies and Paulin (1983) and 2.0 L/d by Cantor et al. (1987), it is reasonable to assume a worst-case value of 2.0 L/d in risk assessment.

The drinking water intake rate used by RESRAD does not differentiate the contaminated fraction from the uncontaminated fraction. A separate input parameter, that is, fraction of drinking water from site, is used to adjust the contaminated fraction, and site-specific data can be used to reflect more realistic conditions.

52.2 RESRAD DATA INPUT REQUIREMENTS

In the RESRAD code, the drinking water intake should be entered in units of liters per year (L/yr). The default value currently used in RESRAD is 510 L/yr.