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HEALTH CONSULTATION

Exposure Assessment of 1965 and 1970 Accidental Tritium Releases at the Lawrence Livermore National Laboratory

LAWRENCE LIVERMORE NATIONAL LABORATORY(U.S. DOE)
[a/k/a Lawrence Livermore National Laboratory (USDOE)]
LIVERMORE, ALAMEDA COUNTY, CALIFORNIA


BACKGROUND AND STATEMENT OF PROBLEM

ATSDR convened an expert panel to assess tritium monitoring and dosimetry issues at the Lawrence Livermore National Laboratory (LLNL) and Savannah River Site (SRS) facilities. ATSDR's summary of the panel's conclusions, which was released as a public health consultation, found that potential community exposures to chronic or long term tritium releases from LLNL (and SRS) are below levels of public health concern and are not likely to produce adverse health effects in the surrounding communities (ATSDR, 2001). However, approximately 80% of the total radiologic releases from the LLNL facility occurred during two accidents in 1965 and 1970. These releases are of similar magnitude (1965- 350 kCi; 1970- 300 kCi(1)). Specific information on the accident scenario and meteorological conditions is available only for the 1970 accident with only general information on the 1965 release. This health consultation will evaluate the tritium dispersion and potential exposures from both accidents with emphasis on the 1970 release.

Myers, et al. (1971) conducted a study of the 1970 release that included real time monitoring and environmental sampling of the release plume and provides general meteorological data. That study also included human dose estimates based on sampling of tritium in air, vegetation, cows milk, and human urine. The results from the Myers, et al. (1971) study will be compared with the modeled dispersion and estimated doses from this evaluation, which implicitly includes the dose contribution from OBT.

Tritium from both accidents was released as hydrogen gas (HT) from a 30 m stack. The 1970 release occurred at 6:14 am on August 6, 1970. During the time of the accident, the wind was from the southwest (~200º) at a speed of 1-2 m/sec (2.2-4.5 mph) with unstable atmospheric conditions (Pasquill-Turner Stability Class B). This stability classification is derived from tables in " Air-EIA: Air quality technical data: atmospheric stability" (http://www.ess.co.at/AIR-EIA/stability.html) using known wind speeds and assuming slight incoming solar radiation based on the time of day (6:00-7:00 AM).

Dispersion of the HT plume was modeled using the RASCAL 3.0 codes developed by the Nuclear Regulatory Commission (NRC, 2000). Input and case summary information is presented in Attachment 1. Although RASCAL can compute deterministic tritium doses, those doses are based on HTO exposures rather than HT. In order to use the uncertainty factors for tritium dosimetry parameters recommended by the expert panel, tritium doses from the 1970 accidental HT release were calculated using a probability-based Monte Carlo procedure.

Modeled HT concentrations for various downwind distances are presented in Table 1. The HT concentration distribution used in the Monte Carlo analysis is a normal distribution around the maximum concentration at the site boundary (5.6 mCi/m-3) derived from the RASCAL3 concentration output. The highest dispersed tritium concentrations occurred at distances less than 0.5 miles from the source which are on-site locations. Maximum modeled concentrations at on site locations are approximately two times larger than the site boundary concentrations.

TABLE 1.

HT Concentrations at various distances and downwind directions from 1970 LLNL Tritium Release source. Distances less than 0.50 mi from source are on-site. 20º column represents centerline of plume.
Ci/m3
10° 20° 30° 40° 50°
0.10 mi 4.33E-03 1.56E-03 5.83E-05 9.84E-08 1.56E-03
0.20 mi 1.37E-02 4.61E-03 1.38E-04 1.53E-07 4.61E-03
0.30 mi 1.08E-02 3.36E-03 7.93E-05 5.53E-08 3.36E-03
0.50 mi 5.59E-03 1.56E-03 2.56E-05 8.70E-09 1.56E-03
0.70 mi 3.31E-03 8.47E-04 1.05E-05   8.47E-04
1.00 mi 1.82E-03 4.21E-04 3.81E-06   4.21E-04
1.50 mi 8.92E-04 1.84E-04 1.13E-06   1.84E-04
2.00 mi 5.34E-04 1.00E-04 4.58E-07   1.00E-04
  1970 accident dispersion from rascal3  
  15 min concentration for 06:29    

Potential doses were calculated for a maximally exposed person at the site boundary using four different inhalation rates (resting, light activity, moderate activity, and strenuous activity). Doses were calculated for one half hour of exposure with concentrations based on a normal distribution of the maximum concentration at the site boundary (0.5 mile downwind of source).

The probability distributions and parameter statistics for each variable are presented in Attachment 2. Variables used in the Monte Carlo analysis are:

  1. the energy of the tritium beta decay derived from the National Nuclear Data Center, Brookhaven National Laboratory
  2. the effective ½ life of tritium in the body
  3. body mass using a standard human model
  4. tritium weighting factor as determined by the expert panel
  5. the tritium dose and dose rate reduction factor as determined by the expert panel
  6. the maximum HT concentration at the site boundary from the RASCAL code
  7. breathing rates for rest, light, moderate, and strenuous activity based on the USEPA Exposure Factors Handbook

The mean and 95th percentile doses from the probability analysis are listed in Table 2. These doses are based on exposure to HTO with 1% of the atmospheric HT converted to HTO. The dose from HT exposure is ~10,000 times less than the dose from HTO exposure due to the very limited human intake of hydrogen gas or tritiated hydrogen gas (HT; ICRP, 1996 - Age-dependent doses to members of the public from intake of radionuclides: Part 4 inhalation dose coefficients. ICRP Publication 71).

The assumption that 1% of the HT gas is converted to tritiated water (HTO) is supported by field studies at the Chalk River Laboratory, Canada. Brown, et al. (1988; 1990) found that there is no direct atmospheric conversion of HT to HTO. The HT:HTO conversion process is mediated by soil microbial activity with a rate of conversion less than 1% per hour. The conversion rate is dependent on the deposition of HT to the soil surface, microbial conversion, and re-emission of the converted HTO back into the atmosphere. Considering that the 1970 accidental release occurred over a time span of one half hour, the assumption of the 1% HT to HTO conversion is conservative and health protective. The assumption of 1% HT:HTO conversion is also consistent with Myers, et al. (1971).


EVALUATION OF MODELED RADIATION DOSES

The air dispersion and tritium dosimetry values used in this exposure analysis are estimates for the parameters. In those cases where a more conservative value could have been used, the Monte Carlo parameters were adjusted to the more conservative (or health protective) data range (Attachment 2). Ranges and most likely values of dosimetry parameters were obtained from an expert panel review of tritium issues (ATSDR, 2001).

The estimated doses ranged from approximately 1 millirem (mrem; 0.01 millisieverts, mSv) to 19 mrem (0.19 mSv). As a comparison, exposure to background radiation in the United States is on the order of 360 mrem (3.6 mSv) including atmospheric radon. Therefore, the most probable doses represent less than 6% of the typical exposure one receives from background radiation.

The current radiation dose limits to members of the public vary depending on source. Generally, the overall limit is 100 mrem per year (1 mSv/y) [10 CFR 20]. The Clean Air Act [40 CFR 61 subpart H] limits the exposure of members of the public to radioactive materials released to the atmosphere resulting from routine DOE activities to an effective dose not to exceed 10 mrem (0.1 mSv) in a year for radionuclides other that tritium. No limit was given for tritium [DOE Order 5400.5 - Radiation Protection of the Public and the Environment, February 8, 1990]. The regulations of the Clean Air Act are not pertinent to the 1965 and 1970 releases from LLNL as they were not routine but accidental releases.

Table 2.

Estimated outdoor tritium doses to maximally-exposed community member from 1970 accidental tritium (HT) release. Dose is based on exposure
Exposure While Mean Dose (mrem) 95th Percentile Dose (mrem)
Resting 1.1 2.8
Light Activity 2.9 7.1
Moderate Activity 5.6 13.3
Strenuous Activity 7.7 19.0

The estimated doses in Table 2 do not include an explicit contribution from OBT. However, the dosimetry parameters included in the Monte Carlo analysis do include an implicit acknowledgment of OBT contributions. For example, the effective ½ life (biological retention) term varies from 1 to 40 days with a most likely value of 10 and an average value of 17 days. This distribution reflects the potential longer retention of OBT relative to HTO. Similar acknowledgment of potential OBT contributions are reflected in the tritium weighting factor and the dose and dose rate reduction factor (Attachment 2).

The estimated doses in Table 2 reflect only acute exposures (30 minute) to the HT plume. The RASCAL3 model predicts that exposure at any downwind location will only occur for 30 minutes. After 30 minutes the wind will have dispersed the plume past any fixed location. Any long term consequences of the acute releases will be included in environmental tritium monitoring. Again there is no explicit OBT monitoring, but the ATSDR expert panel report indicates that potential OBT contributions can be extrapolated from known environmental HTO concentrations (ATSDR, 2001).

The potential average tritium doses presented in Table 2 range from about 1 to 8 mrem (95th percentile doses from 3 to 19 mrem). Because these doses are based on the maximum potential exposure to outside air at the site boundary it is unlikely that any member of the public was exposed at those concentrations. Myer, et.al. (1971) provided similar estimates of potential exposure (3 mrem at the site boundary). That study also included urine analyses from potentially exposed people and did not detect any elevated tritium body burdens corresponding with a potential tritium exposure of 0.025 mrem. Those results indicate that the modeled exposures may over-estimate the measured human exposures.

Although information on the 1965 release is limited, potential maximum doses estimated using the 350 kCi source term and the health protective meteorological default values in RASCAL3 are in the same range as the 1970 release (1-4 mrem). The 1965 release occurred on January 21, 1965 at approximately 3:30 PM. The default meteorological conditions assume relatively low wind speeds, no plume rise, and a relatively stable atmosphere which will conservatively result in maximum transport with minimum dispersion (Attachment 3). Monte Carlo analyses of the 1965 release were not conducted. The 1-4 mrem maximum dose values are deterministic estimates based on RASCAL3 dispersion and a 1% HT to HTO conversion rate.


CONCLUSIONS AND RECOMMENDATIONS

Based on current peer-reviewed scientific literature, the one-time exposure to tritium resulting in a committed effective dose of 19 mrem (0.19 mSv) is not considered a public health hazard nor are any adverse health effects expected. While some public exposure to tritium probably did occur as a result of the accidental releases of tritium gas (HT), estimated maximum exposures are not a public health hazard and measured human exposures were not detectable. Because the 1965 accidental release involved a similar quantity and form of tritium (~350 kCi HT) and was also released from the 30 meter stack, the 1965 release is also considered to be no apparent health hazard. As the accidental releases are not an apparent health hazard, no recommendations are required.


REFERENCES

ATSDR, 2001. Public Health Consultation, Tritium Releases and Potential Offsite Exposures at the Lawrence Livermore National Laboratory and the Savannah River Site. Agency for Toxic Substances and Disease Registry, Atlanta GA.

Brown, R.M., et.al. 1988. Field studies of HT behavior in the environment. Fusion Technology 14:1165-1175.

Brown, R.M., et.al. 1990. Oxidation and dispersion of HT in the environment: the August 1986 field experiment at Chalk River. Health Physics 58(2): 171-181.

International Commission on Radiological Protection, 1996. Age-dependent doses to members of the public from intake of radionuclides: Part 4 Inhalation dose coefficients. Oxford: Pergamon Press, ICRP Publication 71; Ann. ICRP 25(3/4).

Myers, D.S., et.al., 1971. Health Physics Aspects of LRL Tritium Release. Lawrence Radiation Laboratory. University of California, Livermore CA, UCRL-73310, July 16, 1971.

NRC, 2000. RASCAL Version 3.0. Radiological Assessment System for Consequence Analysis, Incident Response Operations, Nuclear Regulatory Commission, Rockville MD, March 2001.


PREPARERS OF REPORT

Mark W. Evans, Ph.D.
Environmental Geologist
Federal Facilities Assessment Branch
Division of Health Assessment and Consultation

Paul Charp, Ph.D.
Health Physicist
Federal Facilities Assessment Branch
Division of Health Assessment and Consultation


Reviewers of Report

Burt J. Cooper, M.S.
Supervisory Environmental Health Scientist
Energy Section Chief,
Federal Facilities Assessment Branch
Division of Health Assessment and Consultation

Sandra G. Isaacs
Chief,
Federal Facilities Assessment Branch
Division of Health Assessment and Consultation


Regional Representative

William Nelson
Senior Regional Representative
Regional Services, Region IX
Office of Regional Operations


Case Summary of RASCAL3 Analysis for 1970 LLNL Release


ATTACHMENT 2. Summary of Parameters used in Monte Carlo Analysis

Forecast: Total Dose-rest

Summary:
    Certainty Level is 94.95%
    Certainty Range is from -Infinity to 3.33 mrem
    Display Range is from 0.00 to 4.00 mrem
    Entire Range is from 0.01 to 8.13 mrem
    After 2,000 Trials, the Std. Error of the Mean is 0.02
Percentile:
Percentile mrem
0.0% 0.01
2.5% 0.07
5.0% 0.12
50.0% 0.81
95.0% 3.34
97.5% 4.07
100.0% 8.13

End of Forecast

Forecast: Total Dose- light work

Summary:
    Certainty Level is 95.05%
    Certainty Range is from -Infinity to 8.20E+0 mrem
    Display Range is from 0.00E+0 to 1.00E+1 mrem
    Entire Range is from 2.47E-2 to 2.25E+1 mrem
    After 2,000 Trials, the Std. Error of the Mean is 6.08E-2
Percentile:
Percentile mrem
0.0% 2.47E-02
2.5% 1.88E-01
5.0% 2.93E-01
50.0% 2.06E+00
95.0% 8.18E+00
97.5% 1.01E+01
100.0% 2.25E+01

End of Forecast

Forecast: Total Dose- Moderate Activity

Summary:
    Certainty Level is 95.00%
    Certainty Range is from -Infinity to 1.61E+1 mrem
    Display Range is from 0.00E+0 to 2.00E+1 mrem
    Entire Range is from 4.91E-2 to 4.99E+1 mrem
    After 2,000 Trials, the Std. Error of the Mean is 1.18E-1
Percentile:
Percentile mrem
0.0% 4.91E-02
2.5% 3.56E-01
5.0% 5.69E-01
50.0% 4.08E+00
95.0% 1.60E+01
97.5% 1.94E+01
100.0% 4.99E+01

End of Forecast

Forecast: Total Dose- Strenuous Activity

Summary:
    Certainty Level is 95.05%
    Certainty Range is from -Infinity to 2.20E+1 mrem
    Display Range is from 0.00E+0 to 2.75E+1 mrem
    Entire Range is from 5.82E-2 to 5.75E+1 mrem
    After 2,000 Trials, the Std. Error of the Mean is 1.62E-1
Percentile:
Percentile mrem
0.0% 5.82E-02
2.5% 5.08E-01
5.0% 7.81E-01
50.0% 5.66E+00
95.0% 2.19E+01
97.5% 2.82E+01
100.0% 5.75E+01
Assumptions

Assumption: H-3 Energy
Distribution

Uniform distribution with parameters:
Uniform distribution with parameters:
Min. 5.92E-3
Max. 5.922E-3
Mean value in simulation was 5.92E-3 KeV

Assumption: T 1/2 (effective half life)

Triangular distribution with parameters:
Minimum 1.00
Likeliest 10.00
Maximum 40.00
Selected range is from 1.00 to 40.00 days
Mean value in simulation was 17.05

Assumption: body mass

Normal distribution with parameters:
Mean 70.00
67% - tile 73.08
Selected range is from -Infinity to +Infinity
Mean value in simulation was 70.08 kg

Assumption: weighting factors

Triangular distribution with parameters:
Minimum 1.00
Likeliest 1.30
Maximum 3.00
Selected range is from 1.00 to 3.00
Mean value in simulation was 1.76

Assumption: dose and dose
rate effectiveness factor (ddref)

Triangular distribution with parameters:
Minimum 0.10
Likeliest 0.40
Maximum 1.00
Selected range is from 0.10 to 1.00
Mean value in simulation was 0.50

Assumption: Inhalation rate-resting (m-3/hr)

Normal distribution with parameters:
Mean 0.43
67% - tile 0.45
Selected range is from -Infinity to +Infinity
Mean value in simulation was 0.43

Assumption: Inhalation rate-light activity (m-3/hr)

Normal distribution with parameters:
Mean 1.10
67% - tile 1.14
Selected range is from -Infinity to +Infinity
Mean value in simulation was 1.10

Assumption: Inhalation rate-moderate activity (m-3/hr)

Normal distribution with parameters:
Mean 2.14
67% - tile 2.23
Selected range is from -Infinity to +Infinity
Mean value in simulation was 2.14

Assumption: Inhalation rate-strenuous. activity (m-3/hr)

Normal distribution with parameters:
Mean 2.94
67% - tile 3.07
Selected range is from -Infinity to +Infinity
Mean value in simulation was 2.94

Assumption: HT Concentration (Ci/m-3)

Normal distribution with parameters:
Mean 5.60E-03
67% - tile 5.00E-04
Selected range is from -Infinity to +Infinity
Mean value in simulation was 5.62E-3

Case Summary of RASCAL3 Analysis for 1965 LLNL Release


1. Myers, et.al. (1971) indicates that 289 kCi of tritium was released in the 1970 accident. This health consultation conservatively rounds this value up to 300 kCi.

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