LAWRENCE LIVERMORE NATIONAL LABORATORY (U.S. DOE)
[a/k/a LAWRENCE LIVERMORE NATIONAL LABORATORY (USDOE)]
LIVERMORE, ALAMEDA COUNTY, CALIFORNIA
LAWRENCE LIVERMORE NATIONAL LABORATORY (U.S. DOE)
[a/k/a LAWRENCE LIVERMORE NATIONAL LABORATORY (USDOE)]
LIVERMORE, ALAMEDA AND JOAQUIN COUNTIES, CALIFORNIA
THE SAVANNAH RIVER SITE (U.S. DOE)
[a/k/a SAVANNAH RIVER SITE (USDOE))]
AIKEN, AIKEN, BARNWELL AND ALLENDALE COUNTIES, SOUTH CAROLINA
The Agency for Toxic Substances and Disease Registry (ATSDR) is responsible forconducting public health assessments of communities adjacent to hazardous waste sites. During the health assessment of the Department of Energy's Lawrence Livermore NationalLaboratory (LLNL), community members have expressed concerns related to theenvironmental monitoring and dose evaluation of tritium, which is the radioactive isotopeof hydrogen. The community is concerned that existing tritium monitoring procedures focusprimarily on tritium in the form of water (HTO) and gas (HT) and do not measure tritium inan organic form (i.e., as organically bound tritium, or OBT).(1)
In addition to the lack of direct OBT monitoring, there is some uncertainty about the dosefrom tritium as OBT relative to HTO. The uncertainty is due to the longer retentiontime of OBT in the body compared to HTO, and the relative effectiveness of a givendose from tritium in an organic form compared with the same dose from tritium ina water form. These factors, combined with the paucity of direct environmental OBTmeasurements at DOE facilities require ATSDR to address how environmentaltritium exposures are evaluated. To address these concerns, ATSDR has conveneda panel of experts in the field of tritium analysis and dosimetry to evaluate site-specific tritium monitoring programs and determine whether adequate data and doseevaluation models currently exist for assessing the public health implications oftritium exposure at those sites.
The DOE sites chosen for these evaluations are the LLNL because of the community concernand the Savannah River Site (SRS) because it was the primary DOE tritiumproduction site with commensurately large environmental releases and monitoringefforts to document tritium transport and fate in the environment. The technicalquestions addressed by the expert panel concern:
- The extent to which existing tritium measurements provide sufficient information forthe estimation of total doses from all forms of tritium that might have been releasedor created in the environment and,
- Whether current tritium dose models adequately assess total tritium exposures anduncertainties present in estimating total tritium doses and the potential health effectsfrom those doses.
This public health consultation is comprised of two parts, first, the public health consultationsummarizes and presents the ATSDR's interpretation of the expert panel technicalreport in order to address the underlying public health issues. The second part is thefinal report of the tritium panel. The specific public health issues addressed in thehealth consultation are to determine if the potential total tritium exposures at SRSand LLNL, including OBT, present doses of public health concern that warrant thecollection and analysis of additional environmental OBT data, and if such data arewarranted, what media should be sampled and what analytical procedures should beused. The remainder of this consultation will summarize the above topics. Thesetopical summaries will be followed by the panel conclusions and recommendationsand then by the ATSDR conclusions and recommendations.
OBT in Food: Existing studies indicate that OBT can be expected in foods produced inenvironments contaminated with tritium that has been released as HTO or HT. The concernis whether circumstances can arise in which the OBT/HTO ratio is so high that estimates ofdoses from ingested foods that consider only the tritium in the form of HTO significantlyunderestimate the actual dose. We conclude that for foods obtained from environments thathave been contaminated with tritium, the dose from OBT ingested in the food may increasethe dose attributed to tritium by not more than a factor of two, and in most cases by a factormuch less than this.
OBT Residence Time in Body: Most HTO intake is retained in the body with a half-timein the range of 5 to 15 days with the average being about 10 days. Observed OBT retentioncurves may be somewhat longer with half-times of tens of days to several hundred days. Theimportant parameter, however, for intakes of tritium as HTO, is the fraction (about 30%) oforganically-bound hydrogen in the body that can be replaced by tritium from HTO; this putsa bound of about 10% on the relative dose from tritium as OBT that is formed from HTO(Osborne 1972).
There have been only a few studies directly with humans who have been exposed to tritiumas OBT in diet. They indicated that increases in dose could be 12% -106% over that fromtritium as HTO, though a more likely range was 12% - 32%.
The current internationally recommended values for the doses from tritium taken in as HTOor OBT are based on modeling assumptions. The ICRP recommended model for HTO hastwo components of retention in the whole body with fractions and halftimes for adultclearance of 0.97 (10 days) and 0.03 (40 days; ICRP 1994, ICRP 1996b). The recommendedmodel for OBT has two components of retention in the whole body with fractions andhalftimes for adult clearance of 0.50 (10 days) and 0.50 (40 days).
Relative Biological Effectiveness: Quantitative estimates of the probability of deleteriouseffects on health in the long term from radiation doses less than a few hundred mSv havebeen the subject of the deliberations of many expert committees over the years. The bestestimates are similar and are believed to be conservative though all emphasize the largeuncertainties inherent in them, the range of uncertainty including zero risk when doses area few millisieverts.
There have been no direct observations in humans of cancer, hereditary effects, or effects in-utero arising as a result of radiation doses from tritium. Hence, the estimation of any suchrisks in humans has to depend on information from laboratory studies of the effectivenessof tritium relative to other radiations involving animals and in-vitro experiments withvarious biological endpoints or effects, supplemented by theoretical studies of the physics of energy distribution.
For radiation protection purposes, the nominal risk coefficient for cancer mortality hasconventionally been taken as 5% per sievert (Section 5.1). Based on laboratory studies andanalyses, this nominal risk coefficient of 5% per sievert should be appropriate for doses from tritium radiation. A value of 10% per sievert for OBT tritium would appear to be very conservative.
SRS: The activity of tritium as OBT in releases, environmental media and foods is notexplicitly addressed in SRS reports of airborne and liquid releases. Recentmonitoring has shown that OBT is a minor component of the total tritium release. Itis not known if earlier discharges from the reactors contained significant quantitiesof OBT in addition to the reported total releases of airborne HTO. It is also unknownif OBT continues to be discharged from various minor sources at SRS. Tritium inliquid effluents has been measured as total tritium and included any OBT.
Special studies of OBT at SRS have been reported in deer, trees, vegetation, soil, and fish(Murphy et al. 1993). Tritium in pine litter and the soil beneath had OBT/HTO specificactivity ratios generally between 1 and 4. Since there is no direct monitoring of OBT inenvironmental media and foods that may have been contaminated by tritium from SRS,estimates of doses to individual members of the public from OBT intakes can only be madebased on computer-modeled concentrations. We conclude that the annual dose from foodis currently less than 0.1 mSv with less than half of this coming from OBT ingested in thefood. The annual dose from drinking water with the tritium levels reported would be about0.4 mSv. If maximum observed concentrations in measured items are used to estimate doses,which is extremely health protective, the annual dose is less than 1 mSv (0.1 mrem).
LLNL: The monitoring methods employed for measuring tritium in effluents are thosecommonly employed elsewhere and should be capable of providing accurate measures oftotal gaseous tritium releases and of HTO and HT. Overall, the technologies currentlyemployed in the monitoring and measurement of tritium appear to be consistent withpractices described in the literature and widely practiced in nuclear establishments elsewhere.
The tritiated water content of vegetation has been sampled for the past 34 years; moreextensively for the past 28 years. The choice of grasses appears to be consistent with theU.S. Department of Energy guideline (USDOE 1991) on sampling indicator materials. Theresults from the grasses appear to provide (at least as reported in the annual reports) the onlydata on tritium (as HTO) currently in vegetation. Given the low levels observed therestriction to tritium as HTO might be justified. The freeze-dry analysis method used forboth vegetation samples and for air moisture samples with low levels of tritium is a standardand reliable technique.
Estimations of doses from tritium released from LLNL, as reported annually, did not includean explicit estimation of the contribution from exposures to tritium in the form of OBTbefore 1997. Without the OBT component, the 1999 annual doses from the consumption ofvegetables, milk and meat were estimated to be 0.072, 0.063 and 0.073 mSv respectively. Foran individual whose diet exclusively comprises vegetables, milk and meat at this level ofcontamination, the annual dose was estimated to be 0.21 mSv. When the OBT componentis included, the doses increase to 0.083, 0.072, and 0.104 mSv (respectively) for an annualdose of 0.26 mSv.
The above annual dose can be compared with an estimate based on the measured OBT/HTOspecific activity ratios from the SRS environment. Using the maximum concentrations oftritium in the water in foods and a total daily hydrogen intake of 0.14 kg with 85% moisturein the food, the annual dose from food would be 0.11 mSv (11 mrem) with 36% coming fromthe OBT intake. This estimate of annual dose is similar to that in the above paragraph withboth approaches having inherently conservative, but different assumptions.
The total activity of tritium released from LLNL has been reported as approximately 28 PBq (760 kCi) during LLNL's more than 40-year history (Tate et al. 1999). Most of this activity was released in two events; 13 PBq (350 kCi) in 1965 and 11 PBq (300 kCi) in 1970. Both releases were cited as being in the form HT. The unplanned releases of HT in 1965 and 1970 (350 kCi and 289 kCi, respectively) will have contributed to the doses for those years. For the 1970 release, Myers et al. (1973) estimated that the "maximum credible dose" to a child was 0.7 mSv.
The dose from tritium that is ingested as OBT in foods obtained from environments widelycontaminated with tritium will add to the dose received from tritium ingested in the form ofwater in food and in drinking water. Enough is known about the behavior of tritium in theenvironment that reasonable bounds can be placed on likely levels, given the observed levelsof HTO. For an environment where water is generally contaminated with tritium, theadditional dose from tritium ingested as OBT is unlikely to result in a total dose from tritiumthat is more than twice that from tritiated water alone.
For the current levels of contamination around the SRS and the LLNL site, even if ultra-conservative estimates were to be made of the organically-bound tritium in foodstuffs, theincrease in the dose so estimated using current dosimetric models would still be very smallcompared with any that would have any appreciable impact on the health of the localcommunities.
The technologies currently employed in the monitoring and measurement of tritium at LLNLand SRS appear to be consistent with practices described in the literature and widelypracticed in nuclear establishments elsewhere. Additional comparisons between modeledand measured dose estimates at these or other sites may be helpful.
Ideally, this conclusion concerning the current significance would be supported by directmeasurements of OBT in representative samples of foods in the Livermore area. However, measurements of OBT are expensive and the magnitude of current dosesfrom tritium in general does not warrant there being precise knowledge of theamounts of tritium as OBT compared to that as HTO in each food - a more generalset of upper bounds suffices. It might be worthwhile providing some assurance ofthe validity of the conclusion about the upper bounds to current levels of OBT by 3Hemass spectrometry.
The relative biological effectiveness (RBE) of tritium in its various chemical forms is thesubject of on-going studies, both experimental and theoretical, as are the biokinetics andmetabolism of tritium by humans in various organic forms. Efforts to resolve both themetabolic and RBE questions for tritium and OBT in particular need to be continued toprovide the needed quantification should instances arise with higher levels of contaminationin the environment.
Releases in the past may not have been as well measured and reported as is the current case. If the conclusions about past routine releases from LLNL made in this review are correct,then the doses from those releases, although more than an order of magnitude higher onaverage than the current annual releases, are still below levels that should prompt publichealth concerns. A broader review of documentation might clarify some of the uncertaintiesnoted.
Although site documents identify uncertainties for many of their individual tritiummeasurements, they do not appear to provide uncertainties on their dose estimates. Giventhe information available to them, it seems that the uncertainties could be identified and propagated so that realistic best-estimate uncertainties would be provided for the dose estimates.
- Based on information presented and reviewed in the expert panel report, total tritiumdoses to the communities surrounding SRS and LLNL, including potential contributions from OBT, HTO, and HT are below levels of public health concern.
- Although there are some uncertainties associated with tritium monitoring and doseestimation procedures, use of health-protective (conservative) assumptions relatingto areas of uncertainty provides adequate and appropriate information for assessingtritium doses and potential health effects.
- Estimated tritium doses for communities adjacent to SRS and LLNL do not warrantthe collection and analysis of additional OBT data. However, such data may be helpful in reducing uncertainties in estimating tritium exposure doses at other sites.
- SRS and LLNL should continue existing tritium monitoring programs to verify thatfuture tritium doses remain below levels of public health concern.
- The production, release, and potential exposure to special tritium compounds, suchas stable tritiated particulates, should be evaluated. As with the OBT issuesaddressed in this evaluation, it is not known if existing monitoring techniquesadequately measure, nor if current dosimetric procedures account for the biologicaluptake of these compounds (see footnote, page 1).
1 In addition to HTO, HT, and OBT, tritium may also occur chemically bound to metal particulates (stabletritiated particulates). Tritium in this form may have a significantly higher biological uptake and retentionrelative to either OBT or HTO. This type of special tritium compound is not addressed in this evaluation, however, the health consultation will recommend further evaluation of the potential release and exposure to those special tritium compounds.