Pu-bearing Particles Research Articles

Transport and migration of plutonium in different soil types and rainfall regimes

Leaching and transport of contaminants is a complex interacting system affected by a suite of environmental factors. This study demonstrates the potential significance of weather events and moisture movement when interpreting plutonium (Pu) migration and advective transport in the soil matrix. Using a column transport experiment, two soil types, a sandy soil and clay-rich soil, were spiked with 238Pu as a tracer to observe the effect of simulated tropical and arid rainfall events on Pu mobility.Partition coefficients (Kd) were determined over a period of weeks and under varying rainfall rates to establish the impact of changing weather events on Pu mobility. The variability of these temporal Kds covers six orders of magnitude over a relatively brief time period. This demonstrates the necessity for non-static Kds to accurately describe Pu transport in these systems. The Pu Kds determined by these column transport experiments fall within the bounds of anticipated values (approximately 80–300,000 mL g−1) from immobile (magnitude 106 mL g−1) to moderately mobile (magnitude 101 mL g−1). The overall transport rate, shown by a decrease in calculated Kd, increases in environments where rainfall is more episodic, such as in arid regions as opposed to the consistently abundant rainfall in tropical regions. In contrast to the 238Pu spike, 239+240Pu resulting from contamination from nuclear tests in the sandy soil (aged for >30 years) showed higher mobility; we hypothesise that the ageing of the contamination, in particular Pu-bearing particles, accounts for this significant increase in Pu mobility.Low intensity, high frequency events in tropical sandy soil systems containing Pu particle contamination have the potential to mobilise Pu (>105 decrease in calculated Kd) over shorter periods of weeks, and not years as previously assumed. This increased mobility, when applied to radioecological models using Kd as a site-specific parameter, shows that there is likely to be a continued impact (risk quotient >1) on non-human biota in tropical sandy soil ecosystems.

The nature of Pu-bearing particles from the Maralinga nuclear testing site, Australia

The high-energy release of plutonium (Pu) and uranium (U) during the Maralinga nuclear trials (1955–1963) in Australia, designed to simulate high temperature, non-critical nuclear accidents, resulted in wide dispersion µm-sized, radioactive, Pu–U-bearing ‘hot’ particles that persist in soils. By combining non-destructive, multi-technique synchrotron-based micro-characterization with the first nano-scale imagining of the composition and textures of six Maralinga particles, we find that all particles display intricate physical and chemical make-ups consistent with formation via condensation and cooling of polymetallic melts (immiscible Fe–Al–Pu–U; and Pb ± Pu–U) within the detonation plumes. Plutonium and U are present predominantly in micro- to nano-particulate forms, and most hot particles contain low valence Pu–U–C compounds; these chemically reactive phases are protected by their inclusion in metallic alloys. Plutonium reworking was observed within an oxidised rim in a Pb-rich particle; however overall Pu remained immobile in the studied particles, while small-scale oxidation and mobility of U is widespread. It is notoriously difficult to predict the long-term environmental behaviour of hot particles. Nano-scale characterization of the hot particles suggests that long-term, slow release of Pu from the hot particles may take place via a range of chemical and physical processes, likely contributing to on-going Pu uptake by wildlife at Maralinga.

Detection and identification of Pu particles and HEU particles by nuclear detectors CR-39

Two parameter approach –a new technique has been devised and tested to identify and distinguish Pu–bearing particles from U-bearing particles. This technique can be used to select rare and microscopic Pu-bearing particles from environmental samples, such as in soil, water, air (aerosol) and swipe samples from the wall of buildings and surfaces of machines in nuclear factories. Some amount of Pu and U standard reference materials were respectively added to cellulose nitrate (CN) solution and dropped some quantity of the solution on mica plates and dried to from CN sheets. CR-39 track detectors were used to record tracks of α particles emitted from the Pu or U particles in CN sheets. After etching of CR-39 plates, α track stars were developed and can be seen under optical microscope. Tracks shapes of α particles in the stars were investigated. We found that at certain interval of etching time, the tip part of α tracks of Pu was very sharp, but the tip part of α tracks of U was blunted with spherical tips. The ratio Rt/Ro of the diameter (Rt) of tip point to the diameter (Ro) of the round track in center area of star is a superior parameter to identify Pu from U particles. The Rt/Ro measurements should be carried at a certain distance from the center point. The two parameter approach has been using in nuclear safeguards and nuclear forensics to analyze isotopic compositions and age of Pu samples.

Vertical distributions of 239+240Pu activity and 240Pu/239Pu atom ratio in sediment core of Lake Chenghai, SW China

Due to the different 240Pu/239Pu atom ratios from different sources of Pu in the environment, Pu isotopes have been widely used for source identification of radionuclides in sediments. In this work, using sector-field ICP-MS, we investigated Pu inventory and its isotopic composition in a lacustrine sediment core collected in Chenghai Lake, SW China. The 240Pu/239Pu atom ratios in this sediment core ranged from 0.166 to 0.271 with a mean of 0.195±0.021, which was slightly higher than that of global fallout. The 239+240Pu/137Cs activity ratios ranged from 0.0155 to 0.0411, with a mean of 0.0215, and the 239+240Pu inventory was 35.4 MBq/km2; both 239+240Pu/137Cs activity ratio and Pu inventory were close to those values of global fallout at 20–30 °N. Three peaks were observed for both 137Cs and 239+240Pu activities in the examined sediment core; they most probably indicated the maximum deposition of global fallout between 1963 and 1964, the fallout from a series of Chinese nuclear tests during the 1970s, and the deposition of resuspended Pu-bearing particles from the Chernobyl accident. Therefore, the vertical profile of Pu isotopes should provide useful time markers for rapid dating of recent sediments.

The isotopic signature of fallout plutonium in the North Pacific

Plutonium analyses of a dated coral record from the French Frigate Shoals in the central North Pacific indicate that there are two major sources of Pu in this basin: close-in (tropospheric) fallout from nuclear weapons testing at the Pacific Proving Grounds in the Marshall Islands in the 1950s and global (stratospheric) fallout which peaked in 1962. Furthermore, the 240Pu 239Pu atom ratio of fallout from the Pacific Proving Grounds is characteristically higher (0.24) than that of global fallout Pu (0.18–0.19). Seawater and sediment samples from the North Pacific exhibit a wide range of 240Pu 239Pu values (0.19–0.34), with a trend towards higher ratios in the subsurface waters and sediment. Deep water 240Pu 239Pu ratios are higher in the vicinity of the Marshall Islands relative to stations further from this close-in fallout source. These preliminary data suggest that fallout Pu from the Pacific Proving Grounds is more rapidly removed from the surface waters than is global fallout Pu. Plutonium geochemistry appears to be related to the physical/chemical form of Pu-bearing particles generated by different fallout sources.