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Plutonium Urinalysis Monitoring

What is Plutonium Urinalysis Monitoring | Routes of Exposure | Purpose of Plutonium Urinalysis Monitoring | Methods of Detection | Validation Methods | Plutonium Urinalysis Monitoring on Enewetak | Plutonium Urinalysis Monitoring on Rongelap | Plutonium Urinalysis Monitoring on Utrōk | Plans for the Future

Diagram  


A schematic diagram of the systems configuration for detection and measurement of plutonium isotopes by Accelerator Mass Spectrometry (AMS). AMS is about 200 to 400 times more sensitive than standard techniques commonly employed in routine internal dosimetry programs, and far exceeds the standard requirements established under the latest United States Department of Energy regulation 10CFR 835 for in vitro bioassay monitoring of alpha-emitting radionuclides such as plutonium-239.



What is Plutonium Urinalysis Monitoring?

Plutonium urinalysis is a very sensitive in-vitro bioassay measurement technique used to determine the amount of plutonium in human urine as a means of estimating the systemic burden (or total amount of plutonium) in the human body. Plutonium urinalysis tests are performed by collecting urine from individuals over a 24-hour period. Under the Marshall Islands Radiological Surveillance Program, we have developed a new state-of-the-art technology for measuring the amount of plutonium in urine based on Accelerator Mass Spectrometry. The test turns a urine sample into a powder which scientists analyze by counting the number of plutonium atoms contained in the sample.

Everybody has a small amount of plutonium in their bodies. Plutonium occurs in nature at very low concentrations but human exposure to plutonium increased dramatically through the 1950s as a result of global fallout from atmospheric nuclear weapons testing. Marshall Islanders are potentially exposed to higher levels of contamination in the environment as a result of exposure to close-in and regional fallout contamination.

 

Routes of Human Exposure

Plutonium is an important radioactive element produced in nuclear explosions. Plutonium emits alpha particles(or alpha rays). Alpha particles have a short range in tissue (about ~40 µm) and cannot be measured by detectors external to the body. However, as heavy, slow moving, charged particles, they have a high relative effectiveness to disrupt or cause harm to biological cells. As a consequence, in-vitro bioassay tests have been developed to test for the presence of systemic plutonium in the human body based on measured urinary excretion patterns and modeled metabolic behaviors of the absorbed radionuclides.

The main pathway for exposure to plutonium in humans is inhalation of contaminated dust particles in the air that people breathe. Inhaled or ingested plutonium may eventually end up in various organs—especially the lung, liver, and bone—resulting in continuous exposure of these tissues to alpha-particle radiation. Plutonium remains in the body for a long time but the systemic uptake of plutonium in people living in the northern Marshall Islands is still expected to be very low (Robison et al., 1980; 1982; 1997).

Inhalation exposure can be estimated from the product of the soil concentration, resuspension enhancement factors, and inhalation dose conversion factors for radionuclides of interest. These estimates show that the projected dose contribution from exposure to plutonium in the Marshall Islands is less that 5% of the total lifetime dose from exposure to residual fallout contamination in the environment (Robison et al., 1980; 1982; 1997a). However, plutonium is a major concern to people living in the northern Marshall Islands because of its long half-life and persistence in the environment. Moreover, radioactive debris deposited in lagoon sediments of coral atolls formed a reservoir and potential long-term source for remobilization and transfer of plutonium through the marine food chain and potentially to man. Elevated levels of plutonium in the terrestrial environment also represent potential inhalation and/or ingestion hazards. Early characterization of the terrestrial environment revealed the presence of hotspots containing milligram-sized pieces of plutonium metal that required some form of remediation (DOE, 1982). Consequently, dose and atoll rehabilitation programs in the Marshall Islands have historically given special consideration to monitoring plutonium uptake in resettled and resettling populations.

 

What is the Purpose of Plutonium Urinalysis Monitoring in the Marshall Islands?

Plutonium urinalysis is a measurement technique that ultimately provides information on the amount of plutonium people have in their bodies. Although plutonium is expected to be a minor contributor to the total manmade dose, it is a concern to people living in the northern Marshall Islands who are potentially exposed to elevated levels of plutonium in the environment from close-in or regional fallout deposition. Consequently, the United States Department of Energy has agreed to monitor resettlement workers and perform a limited number of urinalysis tests on island residents using advanced measurement technologies available at the Lawrence Livermore National Laboratory. The measurement technique currently employed at the Lawrence Livermore National Laboratory is based on Accelerator Mass Spectrometry (AMS). AMS is about 200 to 400 times more sensitive than monitoring techniques commonly employed in internal dosimetry monitoring programs within the United States, and far exceeds the standard requirements established under the latest Department of Energy regulation 10CFR 835 for occupational monitoring of plutonium-239 in the United States.

The Marshall Islands Plutonium Urinalysis monitoring Program was implemented under the following action plan:

  1. To provide more reliable and accurate data to assess baseline and potentially significant incremental uptakes of plutonium within resettled and/or resettling populations in the Marshall Islands.
  2. To monitor plutonium exposure in critical populations groups, such as field workers engaged in soil remediation or agriculture.
  3. To demonstrate and document that occupational and/or public exposures to plutonium in the Marshall Islands are below levels that will impact on human health.
  4. To ensure that our plutonium bioassay data meet all applicable quality requirements through the use of standardized procedures and performance testing.
  5. To document and test the reliability of using environmental data to assess human exposure (and uptake) to plutonium in coral atoll ecosystems, and predict future change.

Methods of Detection of Plutonium in Urine

Researchers from the Brookhaven National Laboratory (BNL) were the first to use whole body counting and plutonium urinalysis techniques to assess intakes of internally deposited radionuclides in Marshallese populations (Sun et al., 1992; 1995; 1997a; 1997b; 1997c; Conard, 1992; Lessard et al., 1984; Miltenberger et al., 1981; Greenhouse et al., 1980). Classical methods for evaluating intakes of plutonium in bioassay samples include alpha spectrometry and fission-track analysis. Alpha spectrometry cannot distinguish between plutonium-239 and plutonium-240, and results are normally reported for the sum of the two isotopes. Moreover, alpha spectrometry lacks the necessary detection sensitivity to accurately assess plutonium exposure in the Marshall Islands, [view full report, UCRL-MI-232208, Hamilton et al., 2004]. Fission-track analysis is limited to the quantification of plutonium-239 but, with a reported detection limit (MDA, Minimum Detectable Amount) of around 1 to 3 microBecquerel (µBq) of plutonium-239, offers greatly improved potential for assessing uptakes associated with low-level chronic exposure to plutonium in the environment.

Under the Marshall Islands Plutonium Urinalysis Program, urine samples were initially sent to the University of Utah for analysis of plutonium using fission-track analysis. Fission is a process where heavy nuclei, such as plutonium and uranium, break up into two large fragments. Fission may occur spontaneously or be induced by collisions with neutrons. During fission- track analysis samples are exposed to a source of neutrons in a reactor in contact with a quartz or plastic slide. Any resulting fission fragments leave behind tracks on the slide that can be counted under an optical microscope to determine the amount of plutonium present. Historically, fission-track analysis has been plagued with a number of deficiencies including the use of less than reliable and tedious preparative methods, low chemical yields, contamination issues, and inaccurate quantification. The University of Utah and the Brookhaven National Laboratory improved on the fission-track process methodology, and adopted a more rigorous approach to data reduction and quality assurance in support of urinalysis testing programs in the Marshall Islands.

More recently, scientists from the Lawrence Livermore National Laboratory have developed a low-level detection technique for determination of plutonium isotopes in bioassay-samples based Accelerator Mass Spectrometry (Brown et al., 2004; Hamilton et al., 2004; 2007). The technique has vastly improved the quality and reliability of assessments of urinary excretion of plutonium from Marshall Islanders, and avoids many of the disadvantages of using conventional atom counting techniques or other competing new technologies.

 

Information Note

There are two main isotopes of plutonium in the environment–namely plutonium-239 (239Pu) and plutonium-240 (240Pu). The isotopic composition of plutonium (i.e., the relative amounts of 239Pu and 240Pu) may vary significantly depending on the source of plutonium. For example, the 240Pu/239Pu content of nuclear fallout from high-yield atmospheric nuclear tests in the Marshall Islands produced 240Pu/239Pu atom ratio signatures of ~0.35 compared with that present in integrated global fallout deposition (~0.18) or unfissioned nuclear fuel (~0.05). Consequently, it may be possible to use urinalysis testing and plutonium isotope measurements as an investigative tool to assess source-specific exposures to Bravo fallout as well as from other specific nuclear events.

 

Validation Method

Method validation is the process used to monitor and document the quality of the measurement data. Methods validation testing under the Marshall Islands Urinalysis Monitoring Program has included participation in an independent interlaboratory exercise organized by the United States National Institute of Standards and Technology (NIST). The results of this exercise clearly demonstrate that Accelerator Mass Spectrometric is well suited for detection of µBq concentrations of plutonium-239 and plutonium-240 in urine (Figure 12) [view full internal report: UCRL-ID-147972, Marchetti et al., 2001]. An independent report on the results of this intercomparison exercise was recently published in the open scientific literature (McCurdy et al., 2005).

 

Figure 12:   

Click image to view larger size


Results of a NIST interlaboratory exercise on determination of plutonium-239 in synthetic urine in the microBecquerel (µBq) range.



Quality Controls

We also continue to test the performance of the technique by analyzing externally prepared quality control natural urine samples artificially spiked with known amounts of plutonium. These external quality control performance test samples are prepared under contract with the Oak Ridge National Laboratory and analyzed along with routine bioassay samples collected from the Marshall Islands. The activity concentration of plutonium-239 in the quality control samples is kept below 200 µBq in order to avoid possible cross-contamination problems, and the plutonium-240/plutonium-239 atom ratio approximates that observed in integrated worldwide fallout deposition, i.e., ~0.2. The results of the quality control sample analyses are sent to Oak Ridge National Laboratory researchers for review and, in return, they prepare a data quality assurance report. All quality control data must pass ANSI 13.30 performance criteria for accuracy and precision before acceptance of any routine bioassay measurement data. The average combined measurement bias and precision based on spiked quality samples analyzed under the Marshall Islands Program (2001-2006) were 1.1% and ±6.8% for plutonium-239, and 4.6% and ±11.1% for plutonium-240, respectively. The results of the plutonium-239 measurements are shown in Figure 13. Based on the results from these performance tests we consider that the methodologies employed under the Marshall Islands Urinalysis Monitoring Program represent the current state-of-the-art in the field for a routine plutonium bioassay program.

 

Figure 13:   

Click image to view larger size


Results of plutonium-239 measurements in externally-prepared natural matrix spiked quality control samples.



Plutonium Urinalysis Monitoring on Enewetak

Individual measurement data from the Plutonium Urinalysis Monitoring Program on Enewetak Atoll are available on this web site [view individual measurement data].

The bioassay sampling program on Enewetak Atoll has involved 5 periodic sample collections of 40 to 50 volunteers between 2001 and 2005, and a small number of subsequent follow-up collections. At the request of the Enewetak-Ujelang Atoll Local Government priority was given to collecting bioassay samples from three main cohort groups; (1) agricultural workers, (2) Enewetak residents born during the 1940-50s; and (3) Enewetak residents born during the early 1980s and who have mostly lived at Enewetak Atoll. Some additional bioassay samples were collected through 2006 in order to investigate occurrences related to measurement data that either failed outlier tests and/or other internal quality control criteria  or whose value exceeded the dose criterion investigation threshold (see under ‘follow-up’). Where investigations have been performed and the results are significantly different, we have typically used the re-analysis results in developing the summary statistics outlined on this web site. Of the 275 bioassay tests performed on Enewetak through 2006, there are 4 bioassay measurement results still under investigation. The Enewetak bioassay collection program also included collections of sample replicates to study inter- and intra-variability in the bioassay collection process as well as control samples (N=7) and full procedural field blanks (N=41). These quality control samples were all prepared and analyzed with routine bioassay samples over the same time period.

The urinary excretion rates of plutonium from the resident population group on Enewetak Atoll ranged from <1 to 8 µBq per day (including all outliers) and are all well below the action level established under the latest Department of Energy regulation 10 CFR 835 for occupational monitoring of plutonium-239 in the United States (Hamilton et al., 2007). Moreover, the vast majority of the individual bioassay samples collected from Enewetak Island residents contained less than the critical level needed to accurately determine  if plutonium was actually present in the sample or not (Lc ~0.25 µBq). As a consequence, the bioassay measurement data are characterized by high relative measurement uncertainties and are not conducive to performing detailed individual dose assessments. Nonetheless, we are able to provide an assessment of the systemic uptake of plutonium and the associated dose delivered to the Enewetak Atoll resident population group based on statistical analyses of the combined data.

In general, urinary excretion of plutonium from Marshallese populations will consist of a long-term baseline component from residual systemic burdens acquired from all previous exposures plus any prompt (new) contributions (and eventual long-term excretion) resulting from recently acquired systemic burdens of plutonium. It is reported that people living in the Northern Hemisphere have acquired sufficiently high systemic burdens of plutonium from exposure to global fallout contamination to produce urinary excretion rates of plutonium of around 2-4 µBq per 24-h void (Boecker et al., 1991). Based on fission-track analysis of urine samples collected by scientists from Brookhaven National Laboratory, the systemic uptake of plutonium from exposure to global fallout contamination in the Marshall Islands is estimated to produce background urinary excretion rates of 1-2 µBq of plutonium per 24-h void (National Research Council, 1994) or about an order of magnitude higher than levels observed in our studies. Consequently, we believe that the more precise, lower background, and higher quality bioassay data based on Accelerator Mass Spectrometry detection and measurement will provide a much more accurate basis for assessing small incremental uptakes of plutonium associated with resettlement of the northern Marshall Islands. Similarly, the sensitivity of the method is such that we may be able to track long-term changes in the availability and transfer of plutonium through the marine and/or terrestrial pathways to humans.

In general, the concentration of plutonium observed in bioassay samples collected from Enewetak residents is well within the normal range expected for people exposed to world-wide fallout contamination in the environment. A more interesting finding is that urinary excretion of plutonium is significantly positively correlated with volunteer age. For example, the estimated error-weighted average urinary excretion of plutonium-239 from Enewetak residents in the <35 y, 35 to <45 y, and > 45 y age-groups is 0.09 µBq, 0.16 mBq and 0.23 µBq per 24-h void, respectively.

The population average urinary excretion of plutonium from Enewetak residents (median age = 36.1 years) of 0.14 µBq per 24-h void compares with a measurement background of ~0.01 µBq observed in compatible sets of field blank samples. A more detailed statistical analysis of plutonium bioassay data from Enewetak will be given elsewhere (Hamilton et al., 2007) using statistical techniques developed by Bogen et al., (2006) to take into account the large measurements uncertainty as well as inter-individual and intra-individual sampling variability. However, the age-related trend is supported heuristically based on Fisher exact, extended Fisher exact and Bartholomew’s trend tests without regard to measurement error (updated after Bogen et al., 2006). This is illustrated in Table 1. The proportion of bioassay samples containing plutonium-239 concentrations >0.35 µBq per 24-h void increasing systematically from 22 % in the <35 year age-group to 53% in the >45 years age group. By comparison, the proportuon of field blank samples (N=41) containing plutnoium-239 concentration >0.35 µBq was less than 5%.

Table 1. Fraction of bioassay samples from Enewetak Atoll containing >0.35 µBq of plutonium-239.

Atoll

Sample group

N

>0.35 µBq

Enewetak Atoll
(median age = 36.1 years)

field blanks

41

5%

<35 y

130

22%

35<45 y

57

39%

> 45y

84

52%

N = number of field blank measures or the number of volunteer in each age group.

As previously discussed, urinary excretion rates of plutonium from Enewetak Atoll residents are at or below worldwide background levels. As such, there appears to be no discernible evidence of elevated levels of plutonium uptake associated with resettlement of Enewetak Atoll. However, for completeness, we attempt to assign a dose to all the measurement data posted on this web site using default assumptions (refer to an associated Technical Basis Document, Daniels et al., 2007).

Based on the error-weighted average values in the urinary excretion of plutonioum-239, the population average committed effective dose equaivalent delivered to Enewetak Atoll agricultural workers and residents from internally deposited plutonium is around 1.7 mrem (or 17 mSv). The maximal estimated dose delivered to Enewetak Atoll residents from internally deposited plutonium occurs in adult males from the >45 year age-group and is estimated to be around 2.8 mrem (or 28 mSv). Please note that the annualized dose criteria developed for remediation of radioactively contaminated sites (NCRP, 2004) is usually based on estimates of the committed effective dose equivalent (TEDE) over 50 years and consists of the sum of the dose equivalent due to intakes of radionuclides (of which, plutonium is just one potential component) and the deep dose equivalent from >external exposures experienced during the measurement year.

 

Plutonium Urinalysis Monitoring on Rongelap

Monitoring Program on Rongelap Atoll are available on this web site [view individual measurement data].

The bioassay sampling program was originally designed to monitor the systemic uptake of plutonium in resettlement workers who were either actively involved in soil remediation or who lived on Rongelap Island for extended periods of time during the initial phase of the resettlement program. A total of 139 bioassay samples, 9 control samples, and 21 procedural field blanks were analyzed under this program. The vast majority of these samples were collected between 2001 and 2004.

The urinary excretion of plutonium from the Rongelap resettlement workers ranged from <1 to 4 µBq per 24-h void (including outliers) (2001-2004). All the individual measurement data falls below the action level established under the latest Department of Energy regulation 10 CFR 835 for occupational monitoring of plutonium-239 in the United States (Hamilton et al., 2007). Moreover, the vast majority of the individual bioassay samples collected from the resettlement workers contained less than the critical level needed to accurately determine if plutonium was actually present in the sample or not (Lc ~0.25 µBq). As a consequence, the bioassay measurement data are characterized by high relative measurement uncertainties and are not conducive to performing detailed individual dose assessments. Nonetheless, we are able to provide an assessment of the systemic uptake of plutonium and the associated dose delivered to Rongelap resettlement workers based on statistical analyses of the combined data.

In general, urinary excretion of plutonium from Marshallese populations will consist of a long-term baseline component from residual systemic burdens acquired from all previous exposures plus any prompt (new) contributions (and eventual long-term excretion) resulting from recently acquired systemic burdens of plutonium. It is reported that people living in the Northern Hemisphere have acquired sufficiently high systemic burdens of plutonium from exposure to global fallout contamination to produce urinary excretion rates of plutonium of around 2-4 µBq per 24-h void (Boecker et al., 1991). Based on fission-track analysis of urine samples collected by scientists from Brookhaven National Laboratory, the systemic uptake of plutonium from exposure to global fallout contamination in the Marshall Islands is estimated to produce background urinary excretion rates of 1-2 µBq of plutonium per 24-h void (National Research Council, 1994) or about an order of magnitude higher than levels observed in our studies. Consequently, we believe that the more precise, lower background and higher quality bioassay data based on Accelerator Mass Spectrometry detection and measurement will provide a much more accurate basis for assessing small incremental uptakes of plutonium associated with resettlement of the northern Marshall Islands. Similarly, the sensitivity of the method is such that we may be able to track long-term changes in the availability and transfer of plutonium through the marine and/or terrestrial pathways to humans.

The concentration of plutonium observed in bioassay samples collected from Rongelap resettlement workers is well within the normal range expected for people exposed to world-wide fallout contamination in the environment. Plutonium excretion is also monotonically related to volunteer age although the trend is less evident than that observed for Enewetak. The error-weighted average urinary excretion of plutonium from Rongelap resettlement workers (median age = 35.4 years) of 0.11 µBq per 24-h void compares with a measurement background of 0.00 µBq observed in compatible sets of field blank samples. A more detailed statistical analysis of plutonium bioassay data from Rongelap Atoll will be given elsewhere (Bogen et al., 2006) taking into account the measurement uncertainty. However, the age-related trend is supported heuristically based on Fisher exact, extended Fisher exact and Bartholomew’s trend tests without regard to measurement error. This is illustrated in Table 2. The proportion of plutonium values >0.35 µBq per 24-h void increases with increasing age of the program volunteers. As shown, the proportion of values >0.35 µBq per 24-h void was 22% in the <35 year age-group compared with 32 % for workers in the >45 years age-group. By comparison, none of the field blank samples (N=21) contained >0.35 µBq of plutonium-239.

Table 2. Fraction of bioassay samples from Rongelap Atoll containing >0.35 µBq of plutonium-239.

Atoll

Sample group

N

>0.35 µBq

Rongelap Atoll
(median age =
35.5 years)

field blanks

21

0%

<35 y

65

22%

35<45 y

45

30%

> 45y

28

32%

N = number of field blank measures or the number of volunteer in each age group.

As previously discussed, urinary excretion rates of plutonium from resettlement workers on Rongelap Atoll are at or below worldwide background levels. As such, there appears to be no discernible evidence of elevated levels of plutonium uptake associated with cleanup/resettlement activities on Rongelap Island. However, for completeness, we attempt to assign a dose to all the measurement data posted on this web using >default assumptions (refer to an associated Technical Basis Document, Daniels et al., 2007).

 

Plutonium Urinalysis Monitoring on Utrōk Atoll

 

Individual measurement data from the Marshall Islands Plutonium Urinalysis view individual measurement data].

We have only recently initiated a bioassay program to assess urinary excretion rates of plutonium from Utrōk Atoll residents. The bioassay program was formally established under a working agreement between the United States Department of Energy, the Utrōk Atoll Local Government and the Republic of the Marshall Islands. The aim of the bioassay program is to develop a statistically meaningful, high-quality baseline dataset comparable to what has been done on Enewetak Atoll. Predicted dose assessments based on environmental data indicate that the 50-y committed effective dose from plutonium on Utrōk Atoll will be around 12 mrem (0.12 mSv) (Robison et al., 1999) but these estimates have never been substantiated by individual bioassay testing due largely to technical limitations in measuring very low level of plutonium-239 in urine. Moreover, we question the quality and reliable of historical bioassay data developed for the Utrōk Atoll population group. These data indicate that the systemic burden of plutonium in Utrōk Atoll residents is relatively high and variable and, for the most part, these data are inconsistent with plutonium bioassay measurement data developed for other atoll populatio groups using Accelerator Mass Spectrometry.

Preliminary data on urinary excretion data of plutonium from Utrōk Atoll residents range from –0.20 to 0.47 µBq per 24-h void with anm error-weighted average of ~0.11 µBq per 24-h void. This compares with an error-weighted average of –0.01 µBq of plutonium-239 observed in a compatible set of field blanks. All the plutonium bioassay measurement data from Utrōk Atoll are well below the action level established under the latest Department of Energy regulation 10 CFR 835 for occupational monitoring of plutonium-239 in the United States (Hamilton et al., 2007). Moreover, the individual bioassay samples all contained less than the critial level needed to accurately determine if plutonium was actually present in the sample or not (Lc ~0.25 µBq). As a consequence, the bioassay measurement data are characterized by high relative measurement uncertainties and are not conducive to performing detailed individual dose assessments. Nonetheless, we are able to provide a preliminary assessment on systemic uptake of plutonium and the associated dose delivered to Utrōk residents based on statistical analyses of available data.

Based on the error-weighted average values in the urinary excretion of plutonioum-239, the committed effective dose equivalent delivered to Utrōk Atoll residents from internally deposited plutonium is around 1.3 mrem (or 13 µSv). The maximal dose observed on Utrōk Atoll from internally deposited plutonium was 5.6 mrem (or 56 µSv). Please note that the annualized dose criteria developed for remediation of radioactively contaminated sites (NCRP, 2004) is usually based on estimates of the total effective dose equivalent(TEDE) over 50 years and consists of the sum of the committed dose due to intakes of radionuclides (of which, plutonium is just one potential component) and the deep-dose equivalent from external exposures experienced during the measurement year.

 

Plans for the Future

Some of the early urinary excretion data on plutonium in the Marshall Islands is of questionable quality because of the poor quantification sensitivity of the methods employed and/or general lack of quality control. Consequently, we plan to expand the plutonium urinalysis program over the next year to include additional volunteer collections from the Utrōk and Rongelap Atoll population group.

Such provisions should help provide assurances to resettled and resettling populations concerned about long-term exposure to residual fallout contamination in the Marshall Islands. Additionally, by establishing a well documented baseline for urinary excretion of plutonium in the Marshall Islands, we will be better able to track and monitor potential long-term changes in exposure conditions on the atolls, especially in relation to assessing the remobilization and transfer of plutonium through the aquatic food chain or from potential increases in inhalation exposure associated with resettlement, remediationactivities, commercial development and changing land-use patterns.

 

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