External radiation is defined as radiation originating from a source outside the body. External radiation fields consist of a natural component and an artificial or manmade component. The natural component can be divided into 1) cosmic radiation, 2) primordial radionuclides in the earth's crust (primarily potassium-40, thorium-232, and uranium-238), and 3) an airborne component, primarily radon and its progeny. The manmade component consists of radionuclides generated for or from nuclear medicine, nuclear power, nuclear research, nuclear waste management, and consumer products. Environmental radiation fields may be influenced by the presence of radionuclides deposited as fallout from past atmospheric testing of nuclear weapons or those produced and released to the environment during the production or use of nuclear fuel. The interaction of radiation with matter results in energy being deposited in matter. Ionizing radiation energy deposited in a mass of material is called radiation absorbed dose. A special unit of measurement called the rad was introduced for this concept in the early 1950s, and more recently, an International System (SI) unit called the gray (Gy) has been defined.
External radiation exposure rates were measured at locations on and off the Hanford Site using thermoluminescent dosimeters (TLDs). External radiation and contamination surveys were also performed with portable radiation survey instruments at locations on and around the Hanford Site. This section describes how external radiation was measured, how surveys were performed, and the results of these measurements and surveys.
To convert to SI units of Gy and Sv, divide rad and rem by 100, respectively.
An environmental TLD comprises three plastic cards that each hold four LiF (TLD 700) chips and one calcium fluoride:dysprosium (TLD 200) chip. TLDs are positioned 1 m (3.3 ft) above the ground at various locations both on and off the Hanford Site. The TLDs are collected and read quarterly; those located along the Columbia River shoreline at the 100-N Area are processed monthly. The 12 TLD 700 chips at each location are used to determine the average total environmental dose at that location. The average dose rate is computed by dividing the average total environmental dose by the length of time the TLD was in the field. The three TLD 200 chips are included to determine doses in the event of a radiological emergency.
The TLDs are positioned at numerous locations onsite (Figure 5.7.1), around the Site perimeter, in nearby and distant communities, (Figure 5.7.2), and along the Hanford Reach of the Columbia River (Figure 5.7.3). All community and most of the onsite and perimeter locations are collocated with air monitoring stations. These locations were selected based on historical determinations of the highest potentials for public exposures (access areas, downwind population centers) from past and current Hanford operations.
Dose rates were also measured using both TLDs and survey instruments at three community-operated stations located at Edwin Markham Elementary School north of Pasco, Basin City Elementary School in Basin City, and Leslie Groves Park in Richland (Figure 5.7.2).
Twenty-eight TLD locations have been established on the Columbia River shoreline, from upstream of the 100-B Area to just downstream of Bateman Island at the mouth of the Yakima River. The general public has access to most of this shoreline. Historically, dose rates measured along the shoreline have been higher than typical background rates. Sula (1980) attributed these elevated rates to cobalt-60 and europium-154 deposited in shoreline sediments as a result of liquid releases to the Columbia River during past reactor operations in the 100 Areas.
Perimeter dose rates for 1994 were similar to those
observed in 1993. In 1994, the average perimeter
external radiation dose rate was
while in 1993, the average
was . Variations in natural
background radiation can occur as a result of
changes in annual cosmic radiation (up to 10%) and
terrestrial radiation (15 to 25%, NCRP 1987).
Other factors possibly affecting annual dose rates
reported here may include variations in the
sensitivity of individual TLDs zero-dose readings,
fading, random errors in the readout equipment or
procedures (Rathbun 1989), and changes in TLD
station locations.
The average background external radiation dose rate
(at distant locations) was 
as
compared to the perimeter average of
. This difference in average
dose rates may be due to natural geographic
variations in terrestrial radiation (the soils at many
of the perimeter locations are rich in potassium-40
and thorium isotopes [Rathbun 1989]) and
variations resulting from human activity. Human
activities affecting the average dose rates may
include landscape modifications such as buildings
and other
construction, which may shield a portion of the
terrestrial component. Figure
5.7.4
graphically displays a comparison between, and
trends of, onsite, perimeter, and distant TLD
locations during 1989 through 1994. Year-to-year
variability is possible for these reasons, and 10%
variability is possible (NCRP 1987).
Figure 5.7.3 shows locations of TLDs positioned along the Columbia River shoreline, and Table 5.7.2 shows the maximum and average measured dose rates for shoreline locations. Dose rates were highest near the 100-N Area shoreline, two times higher than typical shoreline dose rates. The high rates measured in the 100-N Area are attributed to past waste management practices in that area. The public does not have legal access to the 100-N Area shoreline, but does have access to the adjacent Columbia River. The dose implications associated with this access are discussed in Section 6.0 "Potential Doses from 1994 Hanford Operations."
Figure 5.7.1 displays the 28 onsite locations of TLDs in 1994. Table 5.7.3 summarizes the results of 1994 measurements, which are grouped by operational area. The average dose rates in all operational areas were higher than dose rates measured at background locations. The highest average dose rate onsite was seen in the 600 Area and was due to waste disposal activities at US Ecology Inc., a non-DOE facility.
Hand-held survey instruments were used to perform radiation surveys at certain Columbia River shoreline TLD locations. These surveys provided a coarse screening for elevated radiation fields. The surveys showed that radiation levels were comparable to levels observed at the same locations in previous years. The highest levels were seen along the Columbia River shoreline in the 100-N Area and ranged from 4 to 40 mrem/h. Survey results are not included in the 1994 data volume (Bisping 1995), but are maintained in the Surface Environmental Surveillance Project files at the Laboratory.
Figure 5.7.1
Figure 5.7.2
Figure 5.7.3
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