Special Case Exposure Scenarios
While characteristics that define the standard and historical MEI are selected to define a high-exposure
scenario that is unlikely to occur, they do not necessarily represent the scenario with the highest conceivable
radiation dose. Low-probability exposure scenarios exist that could conceivably result in somewhat higher
doses. Two potential scenarios include an individual who could spend time at the Site boundary location with
the maximum external radiation dose rate, and a sportsman who might obtain contaminated wildlife that
migrated from the Site. These special cases are discussed below, as is the potential dose from consumption of
drinking water at the FFTF Visitors Center.
Maximum "Boundary" Dose Rate
The "boundary" radiation dose rate is the external radiation dose rate measured at publicly accessible
locations on or near the Site. The "boundary" dose rate was determined from radiation exposure
measurements using TLDs at locations of expected elevated dose rates onsite and at representative locations
offsite. These boundary dose rates should not be used to calculate annual doses to the general public because
no one can actually reside at any of these boundary locations. However, these rates can be used to determine
the dose to a specific individual who might spend some time at that location.
External radiation dose rates measured in the vicinity of the 100-N, 200, 300, and 400 (FFTF) Areas are
described in Section 5.7, "External Radiation Surveillance." The 200 Areas results were not used because
these locations are not accessible to the general public. Radiation measurements made at the 100-N Area
shoreline (Figure 6.0.1) were consistently above the background level and represent the highest measured
boundary dose rates. The Columbia River provides public access to an area within a few hundred meters of
the N Reactor and supporting facilities.
The annual average dose rate at the location with the highest exposure rate along the 100-N shoreline during
1994 was 0.03 mrem/h (
), or about 0.02 mrem/h () above the average
background dose rate of 0.01 mrem/h () normally observed at offsite shoreline locations.
Therefore, for every hour someone spent at the 100-N Area shoreline during 1994, the external radiation dose
received from Hanford operations would be about 0.02 mrem (
). This dose would be in
addition to the annual dose calculated for the hypothetical MEI. The public can approach the shoreline by
boat, but they are legally restricted from stepping onto the shoreline.
The FFTF Visitors Center, located southeast of the FFTF Reactor building (Figure 6.0.1), was opened to the
public during the first 9 months of 1994. Dose rates measured at this location continued to be essentially
equal to normal background radiation levels in the vicinity of Hanford (0.01 mrem/h []).
Sportsman Dose
Wildlife have access to areas of the Site that contain contamination and could thereby become contaminated.
The potential also exists for contaminated wildlife to move offsite. For this reason, sampling is conducted
onsite to estimate maximum contamination that might possibly exist in animals hunted offsite. This is a
unique and relatively low probability scenario that is not included in the MEI calculation.
Listed below are examples of the estimated radiation doses that could have resulted if wildlife containing the
maximum concentrations measured in onsite wildlife in 1994 migrated offsite, were hunted, and were
consumed. These are very low doses and qualitative observations suggest that the significance of this
pathway is further reduced because of the relatively low migration offsite and the inaccessibility of onsite
wildlife to hunters. Not all of the maximum values were observed in the same animal of each species
sampled. However, the maximum values were compounded to arrive at an upper limit to the potential
concentrations. These doses would be in addition to the MEI dose.
- The dose from eating 1 kg (2.2 lbs) of meat containing the maximum concentration of cesium-137 measured
in a deer collected onsite is estimated to be
(
).
- The dose from eating 1 kg (2.2 lbs) of meat containing the maximum concentration of cesium-137 and
cobalt-60 measured in any game bird collected onsite is estimated to be
(
).
- The dose from eating 1 kg (2.2 lbs) of meat containing the maximum concentration of cesium-137 measured
in a rabbit collected onsite is estimated to be ().
- The dose from eating 1 kg (2.2 lbs) of meat containing the maximum concentrations of cesium-137 and
cobalt-60 measured in bass, whitefish, or carp collected from the Hanford Reach of the Columbia River
is estimated to be ().
- The dose from eating 1 kg (2.2 lbs) of meat containing the concentrations of uranium isotopes measured in a
composite sample of small asiatic clams collected from the Columbia River downstream of the 300 Area
is estimated to be ().
The methodology for calculating doses from consumption of wildlife are addressed in more detail in Soldat et
al. (1990).
FFTF Visitors Center Drinking Water
During 1994, ground water was used as a drinking water source at the FFTF Visitors Center (Figure 6.0.1).
This water is sampled and analyzed throughout the year in accordance with applicable drinking water
regulations. Radionuclide concentrations during 1994 were well below applicable drinking water standards
but concentrations of iodine-129 and tritium were detected at levels greater than typical background values.
Based on these measurements, the potential dose received by a member of the public from drinking 1 L (~1
qt) of drinking water during a visit to the FFTF Visitors Center was calculated to be
(). The maximum organ dose (thyroid) was calculated to be ().
These doses are very small percentages of the DOE limit of 4 mrem effective dose equivalent (0.04 mSv)
from drinking water.
Comparison with Clean Air Act Standards
Limits for radiation dose to the public from airborne emissions at DOE facilities are provided in 40 CFR 61,
Subpart H, of the Clean Air Act Amendments. The regulation specifies that no member of the public shall
receive a dose of more than 10 mrem/yr (0.1 mSv/yr) (EPA 1989) from exposure to airborne radionuclide
effluents (other than radon) released at DOE facilities. It also requires that each DOE facility submit an
annual report that supplies information about atmospheric emissions for the preceding year and their potential
offsite impacts. The following summarizes information that is provided in more detail in the 1994 air
emissions report (Diediker et al. 1995).
The 1994 air emissions from monitored Hanford facilities including radon releases from the 300 Area resulted in a
potential dose to an MEI across from the 300 Area of 0.01 mrem (), which is 0.1% of the limit. Of
this total, radon emissions from the 300 Area contributed 0.007 mrem and nonradon emissions from all stack
sources contributed 0.005 mrem. Therefore, the estimated annual dose from monitored stack releases at the
Hanford Site during 1994 was well below the Clean Air Act standard. The Clean Air Act requires the use of
CAP-88-PC or other EPA models to demonstrate compliance with the standard, and the assumptions embodied in
these codes differ slightly from standard assumptions used at the Hanford Site for reporting to DOE via this
document. Nevertheless, the result of calculations performed with CAP-88-PC for air emissions from Hanford
facilities agrees reasonably well with that calculated using the GENII code (0.01 mrem or ).
The 1990 amendments to the Clean Air Act (40 CFR 61, Subpart H) also require DOE facilities to estimate
the dose to a member of the public for radionuclides released from diffuse and unmonitored sources as well as
from monitored point sources. The EPA has not specified or approved methods for estimating emissions from
diffuse sources, and standardization is difficult because of the wide variety of such sources at DOE sites.
Estimates of potential diffuse source emissions at the Hanford Site have been developed using environmental
surveillance measurements of airborne radionuclides at the Site perimeter.
During 1994, the dose to the MEI across the river from the 300 Area was 0.05 mrem (), which
was greater than the estimated dose at that location from stack emissions (0.01 mrem or ).
Doses at other locations around the Hanford Site perimeter ranged from 0.02 to 0.08 mrem (
to
). Based on these results, the combined dose from stack emissions and diffuse and
unmonitored sources during 1994 was much less than the EPA standard.
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