9


Radiological Dose Assessment


I. BACKGROUND   §9.1

II. PENETRATING RADIATION MONITORING RESULTS   §9.2 A. Accelerator-Produced Penetrating Radiation    §9.3

Figure 9-1: Environmental Penetrating Radiation Monitoring Stations

Table 9-1: Annual Penetrating Radiation Dose at Site Perimeter
                   Resulting from Accelerators

B. Irradiator-Produced Penetrating Radiation   §9.4

III. DISPERSIBLE AIRBORNE RADIONUCLIDE RESULTS   §9.5

Table 9-2: Summary of Dose Assessment at Location of Maximally
                   Exposed Individual (MEI)

IV. COMBINED DOSE ASSESSMENT   §9.6

Table 9-3: Summary of Radiological Dose Impacts

Figure 9-2: Comparison of Radiological Dose Impacts



§9.1      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 refer 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 type of radiation, distance from the activity, complexity of terrain, meteorological conditions, emission levels, food production and consumption patterns, and length of exposure.

§9.2      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 primarily associated with accelerator and irradiator operations at the Laboratory. Accelerators produce both gamma and neutron forms of radiation when operational. Irradiators are primarily limited to gamma radiation.

Historically, DOE facilities have reported “fence-post doses,” which 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. This chapter provides both maximum fence-post dose estimates and the more realistic estimates of exposures to workplaces or residences of Berkeley Lab’s nearest neighbors.

§9.3      A. Accelerator-Produced Penetrating Radiation

Berkeley Lab operates detection equipment at environmental monitoring stations near the site’s research accelerators, which include the Advanced Light Source (Building 6), Biomedical Isotope Facility (Building 56), and 88-Inch Cyclotron (Building 88).

Figure 9-1      Environmental Penetrating Radiation Monitoring Stations

Berkeley Lab uses two methods to determine the environmental radiological impact from accelerator operations. One method consists of a network of three real-time environmental monitoring stations located around the site’s perimeter that track the instantaneous gamma and neutron radiation impacts. Figure 9-1 shows the location of these stations (i.e., ENV-B13A, ENV-B13C, and ENV-B13H) in relation to the accelerators. Each real-time station contains sensitive gamma and neutron pulse counters, which continuously detect and record direct gamma and neutron radiation. The annual doses to an individual from each form of this radiation are derived from measurements at these stations. For these doses, see Table 9-1.


Table 9-1      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

0.002

0.003

ENV-B13C (Panoramic)

< 0.001

< 0.001

< 0.002

ENV-B13H (ALS)

< 0.001

< 0.001

< 0.002

a1 mSv = 100 mrem
bStandard of comparison is DOE limit of 1 mSv/year.

The second method uses passive detectors known as thermoluminescent detectors (TLDs). Based on a review of historical data, Berkeley Lab reduced the number of TLDs from 27 to 11 in 1999. Seven of these TLDs are located near the site boundary, and four others surround two off-site facilities (Building 903 Warehouse and Building 934). TLDs measure only gamma radiation; they are not sufficiently sensitive to detect environmental levels of neutron radiation. Additionally, because they cannot exclude background gamma radiation from their results, they provide time-average dose results that must be determined by analytical technique rather than real-time instrumentation. Figure 9-1 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 Berkeley Lab operations) and to ensure that public radiation exposure is kept well below allowable regulatory limits. The average gamma radiation recorded by these TLDs for 1999 is about 0.53 mSv (53 mrem). Because this value is near the typical background dose for natural gamma radiation in California (0.72 mSv (72 mrem)),1 the TLD results confirm the low-dose values measured by the real-time monitoring stations. Dose results from the network represent the potential impact to an individual situated at a particular monitoring location. The predicted dose to the surrounding population is estimated through a site-specific model.2 Although no regulatory standard exists for population dose values, Berkeley Lab follows the industry convention of using United States Census3 data, extending outward to a distance of 80 kilometers (50 miles) from a facility, in creating this population model.

In the Laboratory’s model, the population dose due to gamma and neutron radiation is derived from the maximum measured dose at the perimeter, primarily at station ENV-B13A. The predicted population dose to the approximately 5-million people within 80 kilometers (50 miles) of Berkeley Lab was estimated at 2.17 ´ 10–4 person-Sv (2.17 ´ 10–2 person-rem).

§9.4      B. Irradiator-Produced Penetrating Radiation

Used for radiobiological and radiochemical research, a single gamma irradiator with a 1400 curie cobalt-60 source is housed at Berkeley Lab in a massive, interlocked and reinforced-concrete-covered structure built as part of Building 74. Routine surveys performed 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 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.

Berkeley Lab also uses other smaller, well-shielded gamma irradiators, which pose considerably less environmental impact than the Building 74 irradiator. This class of smaller irradiators does not increase the cumulative dose level, because the maximally exposed individual (MEI) locations for each of the small irradiator activities are significantly different. See §9.6.

§9.5      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 media 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 primary radionuclide inputs for this modeling are the airborne emissions presented in Chapter 4. The National Emission Standards for Hazardous Air Pollutants (NESHAPs), the governing regulations for dispersible radionuclides, require that any facility that releases airborne radionuclides must assess the impact of such releases using a computer program approved by the Environmental Protection Agency.4 Berkeley Lab satisfies this requirement with the use of CAP88-PC.

CAP88-PC is both a 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 very 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 (i.e., a single, specific point) continuously throughout the year, and that all the radioactivity released was the most hazardous form. Consequently, this worst-case dose is an upper-bound estimate and not one likely to be received by anyone.

In addition to the emissions information, dose-assessment modeling requires the meteorological parameters of wind speed, wind direction, and atmospheric stability. Berkeley Lab uses on-site data from its local meteorological network for the dispersion modeling module of CAP88-PC.

Berkeley Lab performed 15 individual CAP88-PC modeling runs to predict the impact from groupings of the Laboratory’s release points. Table 9-2 lists the attributes of these groupings. Details on these groupings and modeling runs are included in the Laboratory’s annual NESHAPs report. The location of the maximally exposed individual was determined from the complete set of modeling runs. The source groupings listed in Table 9-2 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 1999 was less than 0.001 mSv (0.1 mrem).


Table 9-2     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

6.7 ´ 10–4

83.0%

55/56

Research Medicine/BIF

490

E

8.2 ´ 10–5

10.2%

85

New Hazardous Waste Handling Facility

730

WNW

1.0 ´ 10–5

1.3%

75A/75

Old Hazardous Waste Handling Facility

150

NW

5.8 ´ 10–6

0.7%

88

88-Inch Cyclotron

670

ENE

3.8 ´ 10–6

0.5%

70/70A

Nuclear / Life Sciences

510

NE

2.3 ´ 10–6

0.3%

74/83

Buildings 74/83 Research Medicine

730

WNW

5.4 ´ 10–8

<0.1%

1

Donner Laboratory (UC Berkeley)

980

ENE

3.3 ´ 10–5

4.1%

2/6

Advanced Material Laboratory/ALS

370

NE

2.3 ´ 10–7

<0.1%

26/76

RAML/Counting Laboratory

240

N

2.0 ´ 10–8

<0.1%

934

Molecular and Cell
Biology (off-site)

4,900

ENE

7.4 ´ 10–8

<0.1%

71/72

HILAC/NCEM

220

E

0.0

0%

3

Calvin Lab (UC Berkeley)

1,070

NE

1.8 ´ 10–9

<0.1%

75C

EHS Calibration Sources

150

NW

0.0

0%

903

Receiving Warehouse

N/A

N/A

0.0

0%

Total

8.07 ´ 10–4

100%

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

As with penetrating radiation, the dose from airborne radionuclides to the surrounding population is estimated for a region that extends out from the site for 80 kilometers (50 miles). This region is divided into 208 sectors (i.e., 13 increasingly smaller circles, each divided into 16 equally spaced sectors). CAP88-PC is used to estimate the average dose to each sector for each radionuclide used at the Laboratory. Combining this dose with United States Census data for each sector gives a population dose to that sector. The total population dose represents the summation of the population doses from all sectors. This approach projected an annual population dose from all airborne radionuclides of 0.007 person-Sv (0.7 person-rem).

§9.6      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 9-3 and Figure 9-2, the maximum effective dose equivalent to an individual from all Berkeley Lab operations in 1999 is about 0.003 mSv (0.3 mrem) per year. This value is approximately 0.1% of the nominal background radiation5 in the Bay Area and less than 0.3% of the DOE annual limits.6


Table 9-3     Summary of Radiological Dose Impacts


 

 

Maximally
exposed individual (direct radiation)

Maximally
exposed individual (airborne nuclides)

Maximally
exposed individual (direct and airborne)

Annual EDEa

0.002 mSv/yrb

0.001 mSv/yr

0.003 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

0.2%

1%

0.3%

Annual background

1 mSv/yr

1.6 mSv/yr

2.6 mSv/yr

Impact as % of background

0.2%

0.1%

0.1%

aEDE = Effective Dose Equivalent
b1 mSv = 100 mrem

Figure 9-2      Comparison of Radiological Dose Impacts

The estimated dose to the population within 80 kilometers of Berkeley Lab from these same activities was 0.0074 person-Sv (0.74 person-rem) for the same period. From natural background sources alone, this same population faces an estimated dose of 13,000 person-Sv (1,300,000 person-rem). The Laboratory’s population dose is a mere 0.00006% of the background level.