The 129-iodine content of subtropical Pacific waters : impact of Fukushima and other anthropogenic 129-iodine sources

© The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Biogeosciences 11 (2014): 4839-4852, doi:10.5194/bg-11-4839-2014.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 m3 potential density surface). The excess inventory of Fukushima-derived 129I in the region (∼150 000 km2) sampled during the cruise is estimated to have been between 0.89 and 1.173 billion Bq (∼136 to ∼179 grams) of 129I. Based on a tight tracer–tracer relation with 134Cs (or 137Cs) and estimates that most of the excess cesium is due to direct discharge, we infer that much of the excess 129I 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 129I. Although this small pulse is dwarfed by the ~90 kg of weapons-testing-derived 129I that was released into the environment in the late 1950s and early 1960s, it should be possible to use Fukushima-derived 129I and other radionuclides (e.g., 134, 137Cs) to study transport and entrainment processes along and across the Kuroshio Current.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344

Iodine-129 AMS for Earth Science, Biomedical, and National Security Applications

This Laboratory Directed Research and Development project created the capability to analyze the radionuclide iodine-129 ({sup 129}I) by accelerator mass spectrometry (AMS) in the CAMS facility at LLNL, and enhanced our scientific foundation for its application through development of sample preparation technology required for environmental, biomedical, and national security applications. The project greatly improved our environmental iodine extraction and concentration methodology, and developed new techniques for the analysis of small quantities of {sup 129}I. The project can be viewed as having two phases, one in which the basic instrumental and chemical extraction methods necessary for general {sup 129}I analysis were developed, and a second in which these techniques were improved and new techniques were developed to enable broader and more sophisticated applications. The latter occurred through the mechanism of four subprojects that also serve as proof-of-principle demonstrations of our newly developed {sup 129}I capabilities. The first subproject determined the vertical distribution of bomb-pulse {sup 129}I ({sup 129}Iv distributed globally as fallout from 1950's atmospheric nuclear testing) through 5 meters in the upper vadose zone in the arid southwestern United States. This characterizes migration mechanisms of contaminant {sup 129}I, or {sup 129}I released by nuclear fuel reprocessing, as well as the migration of labile iodine in soils relative to moisture flux, permitting a determination of nutrient cycling. The second subproject minimized the amount of iodine required in an AMS sample target. Because natural iodine abundances are very low in almost all environments, many areas of research had been precluded or made extremely difficult by the demands of sample size. Also, certain sample types of potential interest to national security are intrinsically small - for example iodine on air filters. The result of this work is the ability to measure the {sup 129}I/{sup 127}I ratio at the 2E-07 level or higher in a sample as small as a single raindrop. The third subproject tested the feasibility of using bomb-pulse {sup 129}I in shallow groundwaters in the Sierra Nevada to determine the source of waters entering into the Merced River. The sources of water and their time (age) within the hydrologic system is crucial to understanding the effects of climate change on California waters. The project is in collaboration with faculty and students at the University of California - Merced, and is now the subject of a follow-on Ph.D. dissertation project funded by the LLNL-URP University Education Participation Program. The fourth subproject examined the requirements for using the decay of {sup 129}I to date pore waters associated with continental shelf methane hydrate deposits. Understanding the age of formation and the historical stability of these hydrates is important in determining their response to climate change. Thawing of the world's methane hydrates would quickly and dramatically increase greenhouse gases in the atmosphere. The calculations and testing performed on this project have led to a follow on project that selectively implants {sup 127}I to the exclusion of {sup 129}I, creating an analytical iodine carrier with a substantially lower {sup 129}I background than is available from natural sources. This will permit measurement of {sup 129}I/{sup 127}I ratios at sub-10-14 levels, thereby providing a method for dating hydrate pore waters that are tens of millions of years old

Accelerator mass spectrometry of Strontium-90 for homeland security, environmental monitoring, and human health

Strontium-90 is one of the most hazardous materials managed by agencies charged with protecting the public from radiation. Traditional radiometric methods have been limited by low sample throughput and slow turnaround times. Mass spectrometry offers the advantage of shorter analysis times and the ability to measure samples immediately after processing, however conventional mass spectrometric techniques are susceptible to molecular isobaric interferences that limit their overall sensitivity. In contrast, accelerator mass spectrometry is insensitive to molecular interferences and we have therefore begun developing a method for determination of {sup 90}Sr by accelerator mass spectrometry. Despite a pervasive interference from {sup 90}Zr, our initial development has yielded an instrumental background of {approx} 10{sup 8} atoms (75 mBq) per sample. Further refinement of our system (e.g., redesign of our detector, use of alternative target materials) is expected to push the background below 10{sup 6} atoms, close to the theoretical limit for AMS. Once we have refined our system and developed suitable sample preparation protocols, we will utilize our capability in applications to homeland security, environmental monitoring, and human health

Further improvement in the precision of 233U measurements by accelerator mass spectrometry at the Lawrence Livermore National Laboratory.

In response to sponsor interest in October 2005 we proposed two methods for enhancing the precision of {sup 233}U accelerator mass spectrometry (AMS) capabilities at the Lawrence Livermore National Laboratory (LLNL). In a previous report we evaluated the first method and demonstrated that by increasing sample loading by a factor of four coupled with performing four replicates of each sample, we could achieve measurement precision of {approx}1%. Recent modifications to our system have enabled us to test the second proposed method. By changing our setup to normalize {sup 233}U ions counted in a gas ionization chamber to {sup 238}U measured as a current in an off-axis Faraday cup we were able to attain 1% precision without the need for replicate analysis. This method could be further refined to achieve 0.5% precision in samples of interest

Editorial

The Tenth International Conference on Accelerator Mass Spectrometry (AMS-10) was held from September 5-10 at the University of California, Berkeley campus. The conference attracted 305 attendees from 26 countries who gave 144 platform presentations and presented a total of 170 posters. The conference opened with a special tribute to the late Roy Middleton, which was followed by a companion session on 'ion sourcery'. A plenary talk by Wally Broecker on his '53 years in the Radiocarbon Trenches', provided thought-provoking challenges to commonly accepted paradigms. A workshop on issues in the estimation of isotopic ratios and evaluations of activities from AMS measurements preceded the conference and a workshop on AMS in low-dose bioscience concluded it. Conference attendees had ample opportunity to sample local sights and mid-week excursions to the Napa Valley wine region and the Monterey Bay Aquarium were well attended. The social highlight of the conference was a dinner cruise on San Francisco Bay aboard the San Francisco Belle, which toured the bay on a clear evening and afforded spectacular views of the city front as well as the Bay and Golden Gate bridges. The proceedings of AMS-10 contain 140 peer-reviewed papers that detail recent developments in AMS technology and a broad range of scientific applications. The editors worked to ensure that these contributions represent original research that has not been published elsewhere. We are grateful to the many outside reviewers who provided thoughtful consideration and suggestions in their reviews of these manuscripts. The staff of the Center for Accelerator Mass Spectrometry at the Lawrence Livermore National Laboratory wishes to thank the many members of the international AMS community in allowing us to organize this conference. We are particularly grateful to the University of California's Toxic Substances Research Program, which provided key assistance with conference administration

Reaction rate sensitivity of 44Ti production in massive stars and implications of a thick target yield measurement of 40Ca(alpha,gamma)44Ti

We evaluate two dominant nuclear reaction rates and their uncertainties that affect 44Ti production in explosive nucleosynthesis. Experimentally we develop thick-target yields for the 40Ca(alpha,gamma)44Ti reaction at E(alpha) = 4.13, 4.54, and 5.36 MeV using gamma-ray spectroscopy. At the highest beam energy, we also performed an activation measurement that 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 44Ti production in scenarios for the alpha-rich freeze out. Special attention has been paid to assessing realistic uncertainties of stellar rates produced from a combination of experimental and theoretical cross sections, which we use to develop a re-evaluation of the 44Ti(alpha,p)47V reaction rate. Using these we carry out a sensitivity survey of 44Ti 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 44Ti synthesis as that produced by different treatments of stellar physics.Comment: Comments: 45 pages, 19 postscript figures Minor corrections from Referee and Proof Editors Figs 9 & 10 now in colo

Variable Levels Of Drift In Tunicate Cardiopharyngeal Gene Regulatory Elements

Background: Mutations in gene regulatory networks often lead to genetic divergence without impacting gene expression or developmental patterning. The rules governing this process of developmental systems drift, including the variable impact of selective constraints on different nodes in a gene regulatory network, remain poorly delineated. Results: Here we examine developmental systems drift within the cardiopharyngeal gene regulatory networks of two tunicate species, Corella inflata and Ciona robusta. Cross-species analysis of regulatory elements suggests that trans-regulatory architecture is largely conserved between these highly divergent species. In contrast, cis-regulatory elements within this network exhibit distinct levels of conservation. In particular, while most of the regulatory elements we analyzed showed extensive rearrangements of functional binding sites, the enhancer for the cardiopharyngeal transcription factor FoxF is remarkably well-conserved. Even minor alterations in spacing between binding sites lead to loss of FoxF enhancer function, suggesting that bound trans-factors form position-dependent complexes. Conclusions: Our findings reveal heterogeneous levels of divergence across cardiopharyngeal cis-regulatory elements. These distinct levels of divergence presumably reflect constraints that are not clearly associated with gene function or position within the regulatory network. Thus, levels of cis-regulatory divergence or drift appear to be governed by distinct structural constraints that will be difficult to predict based on network architecture

Investigating Uranium Isotopic Distributions in Environmental Samples Using AMS and MC-ICPMS

Major, minor, and trace uranium isotopes were measured at Lawrence Livermore National Laboratory in environmentally acquired samples using different instruments to span large variations in concentrations. Multi-collector inductively-coupled plasma mass spectrometry (MC-ICPMS) can be used to measure major and minor isotopes: {sup 238}U, {sup 235}U, {sup 234}U and {sup 236}U. Accelerator mass spectrometry (AMS) can be used to measure minor and trace isotopes: {sup 234}U, {sup 236}U, and {sup 233}U. The main limit of quantification for minor or trace uranium isotopes is the abundance sensitivity of the measurement technique; i.e., the ability to measure a minor or trace isotope of mass M in the presence of a major isotope at M{+-}1 mass units. The abundance sensitivity for {sup 236}U/{sup 235}U isotope ratio measurements using MC-ICPMS is around {approx}2x10{sup -6}. This compares with a {sup 236}U/{sup 235}U abundance sensitivity of {approx}1x10{sup -7} for the current AMS system, with the expectation of 2-3 orders of magnitude improvement in sensitivity with the addition of another high energy filter. Comparing {sup 236}U/{sup 234}U from MC-ICPMS and AMS produced agreement within {approx}10% for samples at {sup 236}U levels high enough to be measurable by both techniques

Micro mechanical testing of candidate structural alloys for Gen-IV nuclear reactors

Ion irradiation is often used to simulate the effects of neutron irradiation due to reduced activation of materials and vastly increased dose rates. However, the low penetration depth of ions requires the development of smallscale mechanical testing techniques, such as nanoindentation and microcompression, in order to measure mechanical properties of the irradiated material. In this study, several candidate structural alloys for Gen-IV reactors (800H, T91, nanocrystalline T91 and 14YWT) were irradiated with 70 MeV Fe9+ ions at 452 °C to an average damage of 20.68 dpa. Both the nanoindentation and microcompression techniques revealed significant irradiation hardening and an increase in yield stress after irradiation in austenitic 800H and ferritic-martensitic T91 alloys. Ion irradiation was observed to have minimal effect on the mechanical properties of nanocrystalline T91 and oxide dispersion strengthened 14YWT. These observations are further supported by line broadening analysis of X-ray diffraction measurements, which show a significantly smaller increase in dislocation density in the 14YWT and nanocrystalline T91 alloys after irradiation. In addition, good agreement was observed between cross-sectional nanoindentation and the damage profile from SRIM calculations