Plutonium (Pu) adsorption to and desorption from mineral phases plays a key role in controlling the environmental mobility of Pu. Here we assess whether the adsorption behavior of Pu at concentrations used in typical laboratory studies (≥10(-10) [Pu] ≤ 10(-6) M) are representative of adsorption behavior at concentrations measured in natural subsurface waters (generally <10(-12) M). Pu(V) sorption to Na-montmorillonite was examined over a wide range of initial Pu concentrations (10(-6)-10(-16) M). Pu(V) adsorption after 30 days was linear over the wide range of concentrations studied, indicating that Pu sorption behavior from laboratory studies at higher concentrations can be extrapolated to sorption behavior at low, environmentally relevant concentrations. Pu(IV) sorption to montmorillonite was studied at initial concentrations of 10(-6)-10(-11) M and was much faster than Pu(V) sorption over the 30 day equilibration period. However, after one year of equilibration, the extent of Pu(V) adsorption was similar to that observed for Pu(IV) after 30 days. The continued uptake of Pu(V) is attributed to a slow, surface-mediated reduction of Pu(V) to Pu(IV). Comparison between rates of adsorption of Pu(V) to montmorillonite and a range of other minerals (hematite, goethite, magnetite, groutite, corundum, diaspore, and quartz) found that minerals containing significant Fe and Mn (hematite, goethite, magnetite, and groutite) adsorbed Pu(V) faster than those which did not, highlighting the potential importance of minerals with redox couples in increasing the rate of Pu(V) removal from solution.

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We evaluate two dominant nuclear reaction rates and their uncertainties that affect 44 Ti production in explosive nucleosynthesis. Experimentally we develop thick-target yields for the 40 Ca(α,γ) 44 Ti reaction at E α = 4.13, 4.54, and 5.36 MeV using γ-ray spectroscopy. At the highest beam energy, we also performed an activation measurement which agrees with the thick target result. From the measured yields a stellar reaction rate was developed that is smaller than current statistical-model calculations and recent experimental results, which would suggest lower 44 Ti production in scenarios for the α−rich freeze out. Special attention has been paid to assessing realistic uncertainties of stellar reaction rates produced from a combination of experimental and theoretical cross sections. With such methods, we also develop a re-evaluation of the 44 Ti(α,p) 47 V reaction rate. Using these two rates we carry out a sensitivity survey of 44 Ti synthesis in eight expansions representing peak temperature and density conditions drawn from a suite of recent supernova explosion models. Our results suggest that the current uncertainty in these two reaction rates could lead to as large an uncertainty in 44 Ti synthesis as that produced by different treatments of stellar physics.

Abstract.Results obtained from a dedicated radiochemistry cruise approximately 100 days after the 11 March 2011 Tohoku earthquake and subsequent disaster at the Fukushima Daiichi Nuclear Power Plant show that Fukushima derived radionuclides in the nearby ocean environment had penetrated, on average, to ≤ 250 m depth (1026.5 kg m 3 potential density surface). The excess inventory of Fukushima-derived 129 I in the region (∼150 000 km 2 ) sampled during the cruise is estimated to have been between 0.89 and 1.173 billion Bq (∼136 to ∼179 grams) of 129 I. Based on a tight tracer-tracer relation with 134 Cs (or 137 Cs) and estimates that most of the excess cesium is due to direct discharge, we infer that much of the excess 129 I is from direct (non-atmospheric deposition) discharge. After taking into account oceanic transport, we estimate the direct discharge, i.e., that directly released into the ocean, off Fukushima to have been ∼1 kg 129 I. Although this small pulse is dwarfed by the ∼90 kg of weaponstesting-derived 129 I that was released into the environment in the late 1950s and early 1960s, it should be possible to use Fukushima-derived 129 I and other radionuclides (e.g., 134,137 Cs) to study transport and entrainment processes along and across the Kuroshio Current.