WHY DO PLUTONIUM URINALYSIS IN THE MARSHALL ISLANDS?
Very small amounts of plutonium exist in nature. Levels of plutonium in the environment increased significantly during the 1940s–1950s because of global fallout from above-ground nuclear weapons testing and from accidental and/or deliberate releases associated with the nuclear fuel cycle.
People may be exposed to plutonium by breathing air, drinking water, or eating foods. Elevated levels of plutonium were deposited on land and in atoll lagoon sediments during the nuclear test program in the Marshall Islands. Residual levels of plutonium in the terrestrial environment represent potential inhalation and/or ingestion hazards. Early characterization of the terrestrial environment revealed the presence of hotspots containing milligram-sized pieces of plutonium that required some form of remediation. Radioactive debris deposited in lagoon sediments of atoll test sites formed a reservoir and potential long-term source-term for remobilization and transfer of plutonium through the marine food chain and potentially to man.
The main pathway for exposure to plutonium in humans is inhalation of contaminated dust particles in the air that people breathe. Inhaled plutonium deposited in the lungs will be slowly released and may become trapped in other parts of the body, mainly in bone and the liver. The internally deposited plutonium provides continuous exposure of the lungs and other bodily tissues to alpha-particle radiation over a biological half-life of 20 to 50 years. Small quantities (less than 1%) of plutonium may also transfer across the gut wall and be taken up in other parts of the body.
HOW DOES PLUTONIUM URINALYSIS WORK?
Plutonium emits alpha particles (or alpha rays) that may cause harm to living cells once inhaled or swallowed and taken up in the human body. The main isotopes of plutonium include plutonium-239 (239Pu), plutonium-240 (240Pu), and plutonium-238 (238Pu). All 3 isotopes are known to occur in nuclear weapons fallout. The radioactive half-life of plutonium-239 is 24,100 years. This is the time it takes for half of the atoms to undergo radioactive decay and change to a different element/radioisotope.
Alpha rays such as those emitted by plutonium have a short range in tissue—about 40 µm or about half the thickness of a strand of human hair. Internal exposure to alpha radiation cannot easily be measured with whole-body detectors in the same way as used for cesium-137 (137Cs).
Consequently, 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. These in-vitro tests are based on plutonium urinalysis.
This non-invasive process analyzes a urine sample collected over a 24-hour period to determine how much plutonium people have in their bodies. The test turns a urine sample into a powder which scientists analyze by counting the number of plutonium atoms it contains. Early monitoring programs used alpha spectrometry and/or fission track analysis (FTA). These approaches were abandoned as new technologies became available. Alpha spectrometry lacked the necessary level of sensitivity to detect baseline levels of urinary excretion of plutonium. Also, fission track analysis preparation methods were deemed as tedious and susceptible to interferences from uranium.
HOW DO WE USE ACCELERATOR MASS SPECTROMETRY?
Following early efforts using fission track analysis, Lawrence Livermore National Laboratory (LLNL) developed a state-of-the-art technique for routine plutonium urinalysis under the Marshall Islands Program based on Accelerator Mass Spectrometry (AMS). AMS is 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 (239Pu).
Very careful attention is given to avoiding possible sources of contamination during sample collection and analysis. We also routinely measure blind performance test samples prepared by the Oak Ridge National Laboratory (ORNL) to ensure the accuracy and reliability of our plutonium urinalysis data. The methodology has been independently validated the U.S. National Institute for Standards and Technology (NIST).
THE MARSHALL ISLANDS PLUTONIUM URINALYSIS PROGRAM
The Marshall Islands Plutonium Urinalysis Monitoring Program was implemented under the following action plan:
- 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.
- To monitor plutonium exposure in critical populations groups, such as field workers engaged in soil remediation or agriculture.
- 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.
- To ensure that our plutonium bioassay data meet all applicable quality requirements using standardized procedures and performance testing.
- 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.
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. This is especially true 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, remediation activities, resource development activities, and changing land-use patterns.
It should be noted that urinary excretion of plutonium is usually described in activity units, expressed as micro-Becquerel (µBq) of 239+240Pu [the sum of the plutonium-239 (239Pu) and plutonium (240Pu) activity] excreted (lost) per day (d–1); where 1 µBq d–1 = 10–6 Bq d–1 and 1 Bq = 1 t s–1 (transformation per second). Our AMS measurements are primarily focused on analysis of plutonium-239 as the most abundant plutonium radionuclide. These data can be extrapolated to obtain a measure of the total urinary excretion rate of plutonium using measured plutonium-240 to plutonium-239 activity ratios in the environment.
Dose rates are expressed as a 50-year committed effective dose. It should be noted that the annualized dose criteria developed for remediation of radioactively contaminated sites in the United States is usually based on estimates of the Total Effective Dose Equivalent (TEDE) over 50 years. The TEDE consists of the sum of the committed dose due to intakes of fallout radionuclides (of which, plutonium-239 is just one potential radionuclide) and the deep dose equivalent from external exposures experienced during the measurement year.