Model Simulations on the Long-term Dispersal of 137Cs Released Into the Pacific Ocean Off Fukushima

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<ul><li><p>7/29/2019 Model Simulations on the Long-term Dispersal of 137Cs Released Into the Pacific Ocean Off Fukushima</p><p> 1/11</p><p>Model simulations on the long-term dispersal of137</p><p>Cs released into the Pacific Ocean off</p><p>Fukushima</p><p>This article has been downloaded from IOPscience. Please scroll down to see the full text article.</p><p>2012 Environ. Res. Lett. 7 034004</p><p>(</p><p>Download details:</p><p>IP Address:</p><p>The article was downloaded on 09/03/2013 at 05:57</p><p>Please note that terms and conditions apply.</p><p>View the table of contents for this issue, or go to thejournal homepage for more</p><p>ome Search Collections Journals About Contact us My IOPscience</p></li><li><p>7/29/2019 Model Simulations on the Long-term Dispersal of 137Cs Released Into the Pacific Ocean Off Fukushima</p><p> 2/11</p><p>IOP PUBLISHING ENVIRONMENTAL RESEARCH LETTERS</p><p>Environ. Res. Lett. 7 (2012) 034004 (10pp) doi:10.1088/1748-9326/7/3/034004</p><p>Model simulations on the long-term</p><p>dispersal of</p><p>137</p><p>Cs released into the PacificOcean off Fukushima</p><p>Erik Behrens1, Franziska U Schwarzkopf1, Joke F Lubbecke2 and</p><p>Claus W Boning1</p><p>1 GEOMAR | Helmholtz Centre for Ocean Research Kiel, Dusternbrooker Weg 20, 24105 Kiel, Germany2 NOAA/PMEL, 7600 Sand Point Way NE, Seattle, WA, USA</p><p>E-mail:</p><p>Received 16 April 2012Accepted for publication 11 June 2012</p><p>Published 9 July 2012</p><p>Online at</p><p>Abstract</p><p>A sequence of global ocean circulation models, with horizontal mesh sizes of 0.5, 0.25 and 0.1, are used</p><p>to estimate the long-term dispersion by ocean currents and mesoscale eddies of a slowly decaying tracer</p><p>(half-life of 30 years, comparable to that of 137Cs) from the local waters off the Fukushima Dai-ichi Nuclear</p><p>Power Plants. The tracer was continuously injected into the coastal waters over some weeks; its subsequent</p><p>spreading and dilution in the Pacific Ocean was then simulated for 10 years. The simulations do not include</p><p>any data assimilation, and thus, do not account for the actual state of the local ocean currents during the</p><p>release of highly contaminated water from the damaged plants in MarchApril 2011. An ensemble differing</p><p>in initial current distributions illustrates their importance for the tracer patterns evolving during the first</p><p>months, but suggests a minor relevance for the large-scale tracer distributions after 23 years. By then the</p><p>tracer cloud has penetrated to depths of more than 400 m, spanning the western and central North Pacific</p><p>between 25N and 55N, leading to a rapid dilution of concentrations. The rate of dilution declines in the</p><p>following years, while the main tracer patch propagates eastward across the Pacific Ocean, reaching the</p><p>coastal waters of North America after about 56 years. Tentatively assuming a value of 10 PBq for the net137Cs input during the first weeks after the Fukushima incident, the simulation suggests a rapid dilution of</p><p>peak radioactivity values to about 10 Bq m3 during the first two years, followed by a gradual decline to</p><p>12 Bq m3 over the next 47 years. The total peak radioactivity levels would then still be about twice the</p><p>pre-Fukushima values.</p><p>Keywords: Fukushima, nuclear incident, radioactivity, 137Cs release, tracer dispersion, ocean circulation,</p><p>North Pacific Ocean, Kuroshio</p><p>1. Introduction</p><p>As a consequence of the magnitude 9.0 Tohoku earthquake</p><p>and subsequent tsunami hitting the east coast of Japan on</p><p>11 March 2011 the Fukushima Nuclear Power Plants (NPPs)</p><p>Content from this work may be used under the terms</p><p>of the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 licence. Any further distribution of this work must maintainattribution to the author(s) and the title of the work, journal citation and DOI.</p><p>were severely damaged and radioactive material was released</p><p>to the atmosphere as well as to the ocean (Buesseler et al</p><p>2011). Samples of sea water collected off the NPPs by the</p><p>Tokyo Electric Power Company (TEPCO) and the Japanese</p><p>Ministry of Technology (MEXT) in the weeks after the</p><p>incident indicated extremely high levels of radioactivity,</p><p>as far as 30 km offshore (Fukushima Nuclear Accident</p><p>Update Log by International Atomic Energy Agency, www.</p><p> Radioisotopes discharged fromthe plant included both short-lived isotopes like 131I (with</p><p>11748-9326/12/034004+10$33.00 c 2012 IOP Publishing Ltd Printed in the UK</p></li><li><p>7/29/2019 Model Simulations on the Long-term Dispersal of 137Cs Released Into the Pacific Ocean Off Fukushima</p><p> 3/11</p><p>Environ. Res. Lett. 7 (2012) 034004 E Behrens et al</p><p>a half-life of 8 days) and long-lived isotopes such as137Cs (half-life of 30 years). Some fraction of these</p><p>long-lived isotopes is associated with particles (review</p><p>provided by Dietze and Kriest (2011)) and accumulates in</p><p>the food chain (IAEA 2004 and Schiermeier 2011), with</p><p>as yet unknown consequences for marine organisms in the</p><p>area (Garnier-Laplace et al 2011). A substantial part of the137Cs input is expected to be carried away by the swift</p><p>near-coastal currents (</p><p>Symphonie/Produits/Japan/SymphoniePreviJapan.htm), and</p><p>dispersed over broader regions.</p><p>Previous studies of the radioactive fallout due to</p><p>the nuclear weapon tests during the 1950s60s and the</p><p>Chernobyl disaster in 1986 suggested that 137Cs behaves</p><p>in seawater approximately like a conservative dissolvable</p><p>tracer (Buesseler et al 1990, Buesseler 1991, Buesseler</p><p>et al 2011), with its distribution on decadal timescales</p><p>primarily governed by physical transport processes (Bowen</p><p>et al 1980, Staneva et al 1999, Tsumune et al 2003).</p><p>The dispersal by ocean currents was identified as a main</p><p>mechanism determining the observed distribution of the137Cs concentrations in the surface waters of the North</p><p>Pacific in the 1970s: a decade after the peak input from</p><p>the weapon tests (1961/62), highest concentrations occurred</p><p>in the eastern basin, off the west coast of North America,</p><p>although reconstructions suggested that the fallout had been</p><p>concentrated in the western basin (Folsom 1980). The</p><p>characteristics of the observed-1970s 137Cs-distribution were</p><p>broadly captured by ocean model simulations, confirming that</p><p>a large amount of 137Cs fallout must have been advected</p><p>from the western to the eastern North Pacific within a decade</p><p>(Tsumune et al 2003).From an oceanographic point of view the main difference</p><p>between the 1960s weapon tests fallout and the recent</p><p>discharge of artificial radionuclides off Japan are the spatial</p><p>and temporal characteristics of the input. Analyses of data</p><p>indicated peak ocean discharges from the stricken power</p><p>plants in early April, rapidly decreasing by a factor of 1000</p><p>during the following weeks (Buesseler et al 2011). Although a</p><p>continued release of radioactivity due to polluted groundwater</p><p>and river discharge contributed to enhanced concentrations</p><p>in the coastal waters over several months (http://ajw.asahi.</p><p>com/article/0311disaster/fukushima/AJ201111250019), as a</p><p>first approximation it is assumed that a significant portionof the direct oceanic discharge occurred in terms of a</p><p>short-term pulse to the near-coastal areas. The total amount of137Cs directly discharged during the period of 12 March30</p><p>April 2011 in the ocean was estimated to be 4 PBq</p><p>(Kawamura et al 2011), with an additional deposition of</p><p>similar magnitude (5 PBq) for the atmospheric fallout into</p><p>a wider area of the western North Pacific during the first</p><p>days after the NPPs explosion. Other studies suggested</p><p>higher atmospheric deposition values (e.g. 36.6 PBq, Stohl</p><p>et al 2012), reflecting a significant uncertainty of currently</p><p>estimated release rates (Chino et al 2011). Preliminary</p><p>assessments of the initial oceanic spreading were provided by</p><p>operational modelling efforts (</p><p>and hindcast simulations. The numerical simulations sug-</p><p>gested a swift dispersal of both the direct oceanic discharge</p><p>and the near-coastal atmospheric deposition by the intense</p><p>fluctuating currents and tidal motions over the continental</p><p>shelf, with streaks of high tracer concentrations rapidly</p><p>extending beyond the shelf break. Simulations with a coupled</p><p>physicalbiogeochemical ocean model broadly indicated twophases in the initial dispersion: an accumulation on the shelf</p><p>during the first weeks, followed by increasing outflow to the</p><p>open ocean after about 12 months (Dietze and Kriest 2012).</p><p>While the actual pathways are related to the initial ocean state,</p><p>their experiments showed relatively little sensitivity of the net</p><p>export to the open ocean, i.e., a rather robust net evolution</p><p>in the integral characteristics of the tracer patch over the</p><p>following months.</p><p>Building on these findings, a suite of global ocean</p><p>circulation models with different representations of mesoscale</p><p>eddies is used here, to address the question of the long-term</p><p>dispersion, i.e., on timescales of years to a decade, oflong-lived radionuclides in the Pacific Ocean.</p><p>2. Method</p><p>2.1. Ocean circulation model</p><p>The model simulations are based on NEMO (Madec 2008),</p><p>utilizing configurations developed in the Drakkar framework</p><p>(The DRAKKAR Group 2007). The model configurations</p><p>mainly differ in their horizontal resolution (nominally 0.5,0.25 and 0.1; the latter realized by a two-way nesting-</p><p>approach (Debreu et al 2008) in which a 0.1-mesh for the</p><p>North Pacific was embedded in the 0.5-mesh for the global</p><p>ocean), allowing to explore the effect of mesoscale eddy</p><p>variability on the tracer dispersal: typical eddy scales in the</p><p>mid-latitude (3040N) North Pacific are 100 km (Chelton</p><p>et al 2007); these scales are resolved to different degrees by</p><p>the 0.25- and 0.1-configurations (their mesh sizes in the</p><p>North Pacific are 22 km and 9 km, respectively); they</p><p>are not resolved in the 0.5-model (45 km) where the effect</p><p>of eddies is parameterized by a GM-scheme (following Gent</p><p>and McWilliams 1990), with a mixing coefficient related tothe growth of baroclinic instabilities (Treguirer et al 1997)</p><p>but limited to 1000 m2 s1. All model configurations use 46</p><p>vertical levels; layer thickness increases with depth. Turbulent</p><p>vertical mixing is simulated with a TKE scheme (Madec et al</p><p>1998).</p><p>The model experiments were initialized with the</p><p>climatological temperature (T) and salinity (S) fields of the</p><p>World Ocean Atlas (Levitus 1998). The forcing is built on the</p><p>atmospheric reanalysis products (CORE forcing) developed</p><p>by Large and Yeager (2009) covering the period 19482007.</p><p>Since the future atmospheric conditions are unknown, the</p><p>forcing to simulate the tracer distribution is based on the last10 years of the CORE forcing.</p><p>2</p></li></ul>


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