Radium-based estimates of cesium isotope transport and total direct ocean discharges from the Fukushima Nuclear Power Plant accident

Abstract

Radium has four naturally occurring isotopes that have proven useful in constraining water mass source, age, and mixing rates in the coastal and open ocean. In this study, we used radium isotopes to determine the fate and flux of runoff-derived cesium from the Fukushima Dai-ichi Nuclear Power Plant (FNPP). During a June 2011 cruise, the highest cesium (Cs) concentrations were found along the eastern shelf of northern Japan, from Fukushima south, to the edge of the Kuroshio Current, and in an eddy ~ 130 km from the FNPP site. Locations with the highest cesium also had some of the highest radium activities, suggesting much of the direct ocean discharges of Cs remained in the coastal zone 2–3 months after the accident. We used a short-lived Ra isotope (²²³Ra, <i>t</i>_1/2 = 11.4 d) to derive an average water mass age (<i>T</i>_r) in the coastal zone of 32 days. To ground-truth the Ra age model, we conducted a direct, station-by-station comparison of water mass ages with a numerical oceanographic model and found them to be in excellent agreement (model avg. <i>T</i>_r = 27 days). From these independent <i>T</i>_r values and the inventory of Cs within the water column at the time of our cruise, we were able to calculate an offshore ¹³⁴Cs flux of 3.9–4.6 × 10¹³ Bq d⁻¹. Radium-228 (<i>t</i>_1/2 = 5.75 yr) was used to derive a vertical eddy diffusivity (<i>K</i>_<i>z</i>) of 0.7 m² d⁻¹ (0.1 cm² s⁻¹); from this <i>K</i>_<i>z</i> and ¹³⁴Cs inventory, we estimated a ¹³⁴Cs flux across the pycnocline of 1.8 × 10⁴ Bq d⁻¹ for the same time period. On average, our results show that horizontal mixing loss of Cs from the coastal zone was ~ 10⁹ greater than vertical exchange below the surface mixed layer. Finally, a mixing/dilution model that utilized our Ra-based and oceanographic model water mass ages produced a direct ocean discharge of ¹³⁴Cs from the FNPP of 11–16 PBq at the time of the peak release in early April 2011. Our results can be used to calculate discharge of other water-soluble radionuclides that were released to the ocean directly from the Fukushima NPP

Short-term dispersal of Fukushima-derived radionuclides off Japan: modeling efforts and model-data intercomparison

“…However, it agrees well with the estimate of 11 to 16 PBq reported by Charette et al (2013), is consistent with the 14.8 PBq estimate obtained by Masumoto et al (2012) based on the JCOPE model results, and is within the 10-34 PBq range reported by Bailly du Bois et al (2012). As a consistency check, combining the total 137 Cs inventory of 2 PBq reported by Buesseler et al (2011) with our model-based percentage of 137 Cs left in the KOK area from the oceanic (17.5 %) and atmospheric (4.5 %) sources, and making use of the notion that about 95 % of the measured 137 Cs inventory comes from the oceanic source, one can obtain an estimate of the total source amplitude of about 11 PBq for the oceanic and 2.2 PBq for the atmospheric sources.…”

“…This result agrees with Charette et al (2013) who concluded, based on the radium isotope ratios, that the water sampled during the KOK cruise shows a strong coastal signature. This conclusion is further supported by the 90 Sr measurements at the KOK stations (Casacuberta et al, 2013).…”

“…Our simulations revealed that the bulk of the fallout atmospheric 137 Cs had moved out of the survey region by mid-June, so that most (> 95 %) of the 137 Cs left in the KOK area at the time of the cruise came from the oceanic source. This conclusion is supported by the radium-based analysis of Charette et al (2013), who reported a strong coastal signature in waters sampled during the KOK cruise. The result is further supported by the similarity between the measured 90 Sr and 137 Cs patterns, which suggests that both radionuclides came from the oceanic source (Casacuberta et al, 2013).…”

Cesium, iodine and tritium in NW Pacific waters – a comparison of the Fukushima impact with global fallout

“…The relatively short-lived isotope of cesium ( 134 Cs, T 1/2 = 2.06 yr) can clearly identify cesium of the Fukushima NPP origin, as there is no remaining contribution from global fallout and the 1986 Chernobyl accident. Chino et al (2011), g Stohl et al (2012), h JG (2011), i , j Morino et al (2011), k Dietze and Kriest, (2012), l Tsumune et al (2013), m Kawamura et al (2011), n Estournel et al (2012), o Miyazawa et al (2012b), p Charette et al (2013), r Rypina et al (2013), s Bailly du Bois et al (2012), t atmospheric deposition (Kawamura et al, 2011), u liquid discharges into the sea plus atmospheric deposition (this work), v liquid discharges into the sea (this work), x atmospheric deposition (this work), y Povinec et al, 2012a, z Casacuberta et al (2013 Anthropogenic tritium and 129 I have been recognized as ideal short-term ( 3 H half-life T 1/2 = 12.32 yr) and long-term ( 129 I half-life T 1/2 = 15.7 Myr) oceanographic tracers, important for investigation of water circulation in the ocean (Schlosser et al, 1999;Raisbeck and Yiou, 1999;Hou et al, 2000Hou et al, , 2013Povinec et al, 2000Povinec et al, , 2010Povinec et al, , 2011Shima et al, 2006;He et al, 2013;Suzuki et al, 2010Suzuki et al, , 2013.…”

“…Both the Estournel et al (2012) and Miyazawa et al (2013) estimations could be higher due to evaluating larger 137 Cs data sets (not only around the Fukushima Dai-ichi NPP, but also covering larger areas that could be influenced by the coastal current system). Charette et al (2013), using a different approach based on Ra isotopes measured in seawater samples collected in June 2011 during the Ka'imikai-o-Kanaloa (KOK) cruise, which was carried out in the region 34-37 • N and 142-147 • E (Buesseler et a., 2012), estimated the 137 Cs source term due to direct liquid discharges to be 11-16 PBq. Recently, Rypina et al (2013), using numerical simulations and the 137 Cs data of Buesseler et al (2012) from the same KOK sampling cruise, estimated the oceanic source to be 9.1-17.8 PBq, with the mean and standard deviation of 16.2 ± 1.6 PBq.…”