10


Radiological Dose Assessment


I. BACKGROUND   §10.1

II. PENETRATING RADIATION MONITORING RESULTS   §10.2

A. Accelerator-Produced Penetrating Radiation   §10.3

Figure 10-1: Environmental Penetrating Radiation Monitoring Stations

Table 10-1: Annual Penetrating Radiation Dose at Site Perimeter Resulting from Accelerators

Table 10-2: Summary of Environmental TLD Monitoring Results

B. Irradiator-Produced Penetrating Radiation   §10.4

III. DISPERSIBLE AIRBORNE RADIONUCLIDE RESULTS   §10.5

Table 10-3: Summary of Dose Assessment at Location of Maximally Exposed Individual (MEI)

IV. COMBINED DOSE ASSESSMENT   §10.6

Table 10-4: Summary of Radiological Dose Impacts

Figure 10-2: Comparison of Radiological Dose Impact



§10.1      I. BACKGROUND

This chapter presents the estimated dose results from Berkeley Lab’s penetrating radiation and airborne radionuclide monitoring programs. The doses projected from each monitoring program are given separately, before being evaluated cumulatively at the end of the chapter to summarize the overall impact of the Laboratory’s radiological activities on the surrounding region.

Earlier chapters referred to monitoring and sampling results in terms of concentrations of a substance. The health effect of exposure to a concentration over a period of time is referred to as "dose." An important measure for evaluating the impact of any radiological program, dose can be estimated for individuals as well as populations. Factors affecting either type of dose (individual or population) include the distance from the activity, complexity of terrain, meteorological conditions, emission levels, food production and consumption patterns, and length of exposure.

§10.2      II. PENETRATING RADIATION MONITORING RESULTS

Radiation-producing machines (e.g., accelerators, x-ray machines, irradiators) and various radionuclides are used at Berkeley Lab for high-energy particle studies and biomedical research. Penetrating radiation is mainly associated with accelerator and irradiator operations at the Laboratory. Accelerators produce both gamma and neutron forms of radiation. Irradiators are primarily limited to gamma radiation.

Historically, Department of Energy (DOE) facilities have reported "fence-post doses." These are measured or computed values reflecting the exposures to an individual assumed to be living 100% of the time at the perimeter or fence-line of the facility. In order to present realistic assessments of exposures to actual individuals (not overly conservative and unrealistic estimates), this chapter provides both maximum fence-post dose estimates and more realistic estimates of exposures to workplaces or residences of Berkeley Lab’s nearest neighbors.

§10.3      A. Accelerator-Produced Penetrating Radiation

Berkeley Lab operates detection equipment at environmental monitoring stations near the site’s research accelerators that generate penetrating radiation when operational. These accelerators are the Advanced Light Source (Building 6), Biomedical Isotope Facility (Building 56), and 88-Inch Cyclotron (Building 88).

Berkeley Lab uses two methods to determine the environmental radiological impact from accelerator operations. One method utilizes a network of three real-time environmental monitoring stations located around the site’s perimeter to track the instantaneous gamma and neutron radiation impacts from accelerator operations. Figure 10-1 shows the location of these stations. Each real-time station contains sensitive gamma and neutron pulse counters, which continuously detect and record direct gamma and neutron radiation—both of which are forms of penetrating radiation. The gamma and neutron doses to an individual are derived from measurements at the three monitoring stations and result from accelerator operations for the year. These doses are listed in Table 10-1.


Figure 10-1      Environmental Penetrating Radiation Monitoring Stations

Table 10-1      Annual Penetrating Radiation Dose at Site Perimeter Resulting from Accelerators


Monitoring station

Net gamma dose
(mSv/yr)
a

Net neutron dose
(mSv/yr)

Total doseb
(mSv/yr)

ENV-B13A (Bldg. 88)

0.001

0.003

0.004

ENV-B13C (Panoramic)

< 0.001

< 0.001

< 0.002

ENV-B13H (ALS)

< 0.001

< 0.001

< 0.002

a 1 mSv = 100 mrem
b Standard of comparison is DOE limit of 1 mSv/year.

 

Table 10-2      Summary of Environmental TLD Monitoring Resultsa


Location

Number
of sites

Average annual dose (mSv)b

Laboratory gate entrances

3

0.59

Environmental monitoring stations

5

0.66

Laboratory perimeter

19

0.58

Off-site facilities

6

0.55

Average dose

33

0.59

a Results include background dose. Average background in Bay Area is typically 0.72 mSv. Background dose rate can vary between locations and years.
b 1 mSv = 100 mrem


The second method uses 27 passive detectors known as thermoluminescent detectors (TLDs) located near the site boundary and six additional TLDs located around two off-site facilities (Building 903 Warehouse and Building 934). TLDs measure only gamma radiation because they do not have sufficient sensitivity to detect environmental levels of neutron radiation. TLDs are not able to exclude background radiation from their results and give time-average dose results that must be determined by an analytical technique rather than real-time instrumentation. Figure 10-1 shows the locations of TLD sites near the main facility.

The objectives of the TLD measurement are to record the gross penetrating radiation exposures (from background and from LBNL operations) and to ensure that public radiation exposure is kept well below allowable regulatory limits. TLDs use aluminum oxide, which can measure low-level gamma and photon radiation with a minimum detection level of 0.001 mSv (0.1 mrem). Table 10-2 summarizes the calculated annual average TLD gamma radiation dose equivalents from the environmental TLD monitoring program, organizing the 33 monitoring locations into similar groups.

Annual TLD radiation dose measurements at each individual monitoring locations are similar and near the typical background dose for natural gamma radiation in California: 0.72 mSv (72 mrem).1 These TLD results confirm the low dose values measured by the real-time monitoring stations. See Table 10-1.

Another measure of the potential impact of accelerator-produced penetrating radiation is the population dose equivalent. For many years, Berkeley Lab has used a site-specific model to estimate the population dose equivalent resulting from penetrating radiation.2 Population data from the 1980 United States Census3 are used in this calculation. Although the population within 80 kilometers (50 miles) of Berkeley Lab increased by about 20% during the 1970s and 1980s from 5 to 6 million, the populations of Berkeley and Oakland (the two cities immediately adjacent to the site) declined. Population statistics from the 1990 census have not produced noticeable differences in dose.

In the Laboratory’s model, population dose equivalent is computed from the maximum measured value of perimeter dose. For 1998, this maximum dose was collected at monitoring station ENV-B13A. See Table 10-1. The collective effective dose equivalent to the approximately 5-million people within 80 kilometers (50 miles) of Berkeley Lab attributable to penetrating radiation from Laboratory accelerator operation during 1998 was estimated at 4.34 ´ 10–4 person-Sv (4.34 ´ 10–2 person-rem).

§10.4      B. Irradiator-Produced Penetrating Radiation

Used for radiobiological and radiochemical research, Berkeley Lab has a single gamma irradiator, with a 1400 curie cobalt-60 source. This unit is housed in a massive interlocked, reinforced-concrete-covered structure built as part of Building 74. Routine surveys taken when the irradiator was in operation confirmed that no area exceeded 0.01 mSv/hr (1 mrem/hr) at 1 meter from the outside walls or ceiling of the labyrinth. The Building 74 irradiator is about 80 meters (260 feet) from the site’s perimeter fence and more than 700 meters (2,300 feet) from the nearest residence.

The projected annual dose equivalent to any member of the public is less than 0.01 mSv/yr (1.0 mrem/yr) at the perimeter fence and less than 2 ´ 10–4 mSv/yr (0.02 mrem/yr) at the nearest residence. The remaining smaller, well-shielded gamma irradiators pose considerably less environmental impact than the Building 74 irradiator and do not increase the cumulative dose level. Because the locations of the maximum doses are different for each radiological-producing activity, the type of maximum cumulative dose is not additive. See §10.6.

§10.5      III. DISPERSIBLE AIRBORNE RADIONUCLIDE RESULTS

Dose due to dispersible contaminants represents the time-weighted exposure to a concentration of a substance, whether the concentration is inhaled in air, ingested in drink or food, or absorbed through skin contact with soil or other environmental media. Dispersible radionuclides that affect the environmental surroundings of Berkeley Lab, and consequently the projected dose from Laboratory activities, originate as emissions from building exhaust points¾generally located on rooftops. Once emitted, these radionuclides may affect any of several environmental media: air, water, soil, plants, and animals. Each of these pathways represents a possible pathway of exposure affecting human dose. Determining the dose to an individual and the population is accomplished using multipathway dispersion models. The basic radionuclide inputs for this modeling are the airborne emissions presented in chapter 4.

The US/EPA National Emission Standards for Hazardous Air Pollutants (NESHAPs) regulation requires that any facility that releases airborne radionuclides, like Berkeley Lab, must compute the impact of such releases using an approved computer program.4 Berkeley Lab uses CAP88-PC for this purpose.

CAP88-PC is a radionuclide dispersion and dose-assessment predictive model supplied and approved by US/EPA. It computes the cumulative dose from all significant exposure pathways such as inhalation, ingestion, and skin absorption. The methods and parameters used to calculate the dose are quite conservative, taking an approach that reports dose calculations as "worst case" doses to the population exposed. For example, the model assumes that some portion of the food consumed by the individual was grown within the assessed area, that the individual resided at this location continuously throughout the year, and that all the radioactivity released was the most hazardous form. Consequently, this worst-case dose is not a dose likely to be received by anyone, but merely an upper-bound estimate.

In addition to the emissions information, dose-assessment modeling requires the meteorological parameters of wind speed, wind direction, and atmospheric stability. Before 1995, Berkeley Lab’s dose-assessment modeling efforts used Oakland airport data that US/EPA distributed with the model. Berkeley Lab started using on-site data with the 1995 NESHAPs assessment after it completed a project to upgrade its local meteorological network.

Berkeley Lab set up 15 individual CAP88-PC modeling runs to predict the impact from groupings of the Laboratory’s release points. Table 10-3 lists the attributes of these groupings. Details on these groupings and modeling runs are included in the Laboratory’s annual report under the NESHAPs program. The location of the maximally exposed individual was determined from the complete set of modeling runs. The source groupings listed in Table 10-3 give the orientation of their release points relative to the location of the maximally exposed individual (distance and direction). The combined dose from airborne radionuclides for 1998 was less than 0.003 mSv (0.3 mrem).

 

Table 10-3      Summary of Dose Assessment at Location of Maximally Exposed Individual (MEI)



Building



Building description

Distance to MEIa (meters)


Direction to MEI
a


Dose at MEI (mSv/yr)
b


Percent of MEI dose

75

National Tritium Labeling Facility

110

NW

2.7 ´ 10–3

97.2%

55/56

Research Medicine/BIF

490

E

5.0 ´ 10–5

1.8%

85

New Hazardous Waste Handling Facility

730

WNW

8.6 ´ 10–6

.3%

75A/75

Old Hazardous Waste Handling Facility

150

NW

2.1 ´ 10–6

0.1%

88

88-Inch Cyclotron

670

ENE

3.6 ´ 10–6

0.1%

70/70A

Nuclear / Life Sciences

510

NE

1.9 ´ 10–6

0.1%

74/74B/83

Buildings 74/74B/83 Research Medicine

730

WNW

7.2 ´ 10–10

0.1%

1

Donner Laboratory (UC Berkeley)

980

ENE

1.2 ´ 10–5

0.4%

2/6

Advanced Material Laboratory/ALS

370

NE

2.5 ´ 10–7

<0.1%

26/76

RAML/Counting Laboratory

240

N

6.2 ´ 10–10

<0.1%

934

Molecular and Cell Biology
(off site)

4,900

ENE

1.8 ´ 10–9

<0.1%

71/72

HILAC/NCEM

220

E

0.0

0%

3

Calvin Lab (UC Berkeley)

1,070

NE

2.9 ´ 10–11

<0.1%

75C

EHS Calibration Sources

150

NW

0.0

0%

903

Receiving Warehouse

N/A

N/A

0.0

0%

Total

2.78 ´ 10–3

100%

aDistances and directions are relative to the cumulative MEI from all contributing sources.
b1 mSv = 100 mrem

Collective population dose is calculated as the average radiation dose in a specified region, multiplied by the number of individuals in that area. The region is defined by regulation as a circular area around the site with a radius of 80 kilometers (50 miles). Berkeley Lab divided this region into 208 sectors (i.e., 13 increasingly smaller circles, each divided into 16 equally spaced sectors) and again used CAP88-PC to estimate the average dose to each sector. Input parameters for the model used those from the Building 75 dose assessment, with the exception that the source term was expanded from tritium to include all the radionuclides used at the Laboratory. Population data for each area from the 1980 census were then used to estimate the population dose within each area. The total collective population dose represents the summation of the population doses from all the areas. This approach projected a total collective population dose from all airborne radionuclides at 2.52 ´ 10–2 person-Sv (2.52 person-rem).

§10.6      IV. COMBINED DOSE ASSESSMENT

The total radiological impact from accelerator operations and airborne radionuclides is well below applicable standards and nominal background radiation. As presented in Table 10-4 and Figure 10-2, the maximum effective dose equivalent to an individual from all Berkeley Lab operations in 1998 is about 0.007 mSv (0.7 mrem) per year. This value is about 0.3% of the nominal background5 in the Bay Area and less than 1% of the DOE annual limits.6 The estimated dose to the population within 80 kilometers of Berkeley Lab from these same activities was 2.56 ´ 10–2 person-Sv (2.56 person-rem) in 1998. With the dose from natural background sources alone to this same population base estimated at 13,000 person-SV (1,300,000 person-rem) for the same period, the Laboratory’s collective population dose is a mere 0.0002% of the background level.

 

Table 10-4      Summary of Radiological Dose Impacts

 

 

Maximally
exposed individual
(direct radiation)

Maximally
exposed individual
(airborne nuclides)

Maximally
exposed individual
(direct and airborne)

Annual EDEa

0.004 mSv/yrb

0.003 mSv/yr

0.007 mSv/yr

MEI location

Residence
(110 meters
west of Bldg. 88)

Workplace
(110 meters
northwest of
Bldg. 75 at Lawrence
Hall of Science)

Residence
(110 meters west of
Bldg. 88)

Standard of comparison

1 mSv/yr
(DOE)

0.10 mSv/yr
(US/EPA)

1 mSv/yr
(DOE)

Impact as % of standard

0.4%

3%

0.7%

Annual background

1 mSv/yr

1.6 mSv/yr

2.6 mSv/yr

Impact as % of background

0.4%

0.2%

0.3%

a EDE = Effective Dose Equivalent
b 1 mSv = 100 mrem




Figure 10-2      Comparison of Radiological Dose Impact