Plutonium-239 (239Pu) and plutonium-240 (24°Pu) are important alpha emitting radionuclides contained in radioactive debris from nuclear weapons testing. 239Pu and 24°Pu are long-lived radionuclides with half-lives of 24,400 years and 6580 years, respectively. Concerns over human exposure to plutonium stem from knowledge about the persistence of plutonium isotopes in the environment and the high relative effectiveness of alpha-radiation to cause potential harm to cells once incorporated into the human body. In vitro bioassay tests have been developed to assess uptakes of plutonium based on measured urinary excretion patterns and modeled metabolic behaviors of the absorbed radionuclides. Systemic plutonium absorbed by the deep lung or from the gastrointestinal tract after ingestion is either excreted or distributed to other organs, primarily to the liver and skeleton, where it is retained for biological half-times of around 20 and 50 years, respectively. Dose assessment and atoll rehabilitation programs in the Marshall Islands have historically given special consideration to residual concentrations of plutonium in the environment even though the predicted dose from inhalation and/or ingestion of plutonium accounts for less than 5% of the annual effective dose from exposure to fallout contamination. Scientists from the Lawrence Livermore National Laboratory (LLNL) have developed a state-of-the-art bioassay test to assess urinary excretion rates of plutonium from Marshallese populations. This new heavy-isotope measurement system is based on Accelerator Mass Spectrometry (AMS). The AMS system at LLNL far exceeds the standard measurement requirements established under the latest United States Department of Energy (DOE) regulation, 10CFR 835, for occupational monitoring of plutonium, and offers several advantages over classical as well as competing new technologies for low-level detection and measurement of plutonium isotopes. The United States National Institute of Standards and Technology (NIST) has independently verified the accuracy and precision of the AMS detection system for low-level bioassay measurements of plutonium isotopes through participation in an intercomparison exercise whereby performance evaluation samples were prepared in a synthetic urine matrix and submitted to participating laboratories for blind analysis. The results of the analyses were then sent to the NIST to independently evaluate the performance of laboratory participants. At LLNL, the AMS measurements of 239Pu and 24°Pu met ANSI 13.30 criteria for both precision and accuracy at all sample test levels. Livermore scientists continue to test the performance of the Marshall Islands Plutonium Urinalysis Program by routine blind analysis of externally prepared quality control test samples, and through the rigorous implementation of standardized methods and procedures. Although not addressed directly in the report, AMS measurements show that the urinary excretion of plutonium by selected Marshallese populations fall into a low and reproducible range. Moreover, there appears to be no evidence of small incremental intakes of plutonium associated with resettlement activities - past or present. The improved quality, reliability and detection sensitivity of AMS for low-level plutonium isotope measurements will enable DOE to develop high-quality, baseline urinary excretion data for Marshallese populations, and accurately assess and track potential uptakes of plutonium. associated with resettlement activities and/or from long-term changes in plutonium exposure conditions in the Marshall Islands.

Lawrence Livermore National Laboratory (LLNL) has been performing accountability measurements of plutonium (Pu) -bearing items with the 30-gallon drum neutron multiplicity counter (NMC) since August 1998. A previous paper focused on the LLNL experience with Pu-bearing oxide and metal items. This paper expands on the LLNL experience with Pu-bearing salts containing low masses of Pu. All Pu-bearing salts used in this study were measured using calorimetry and gamma isotopic analyses (Cal/Iso) as well as the 30-gallon drum NMC. The Cal/Iso values were treated as being the true measure of Pu content because of the inherent high accuracy of the Cal/Iso technique, even at low masses of Pu, when measured over a sufficient period of time. Unfortunately, the long time period required to achieve high accuracy from Cal/Iso can impact other required accountability measurements. The 30-gallon drum NMC is a much quicker system for making accountability measurements of a Pu-bearing salt and might be a desirable tradeoff. The accuracy of 30-gallon drum NMC measurements of Pu-bearing salts, relative to that of Cal/Iso, is presented in relation to the mass range and alpha associated with each item. Conclusions drawn from the use of the 30-gallon drum NMC for accountability measurements of salts are also included.