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

[摘要] Radium has four naturally occurring isotopes that have proven useful inconstraining water mass source, age, and mixing rates in the coastal andopen ocean. In this study, we used radium isotopes to determine the fate andflux 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 alongthe eastern shelf of northern Japan, from Fukushima south, to the edge ofthe Kuroshio Current, and in an eddy ~ 130 km from the FNPPsite. Locations with the highest cesium also had some of the highest radiumactivities, suggesting much of the direct ocean discharges of Cs remained inthe coastal zone 2–3 months after the accident. We used a short-lived Raisotope (²²³Ra, t_1/2 = 11.4 d) to derive an average water massage (T_r) in the coastal zone of 32 days. To ground-truth the Ra agemodel, we conducted a direct, station-by-station comparison of water massages with a numerical oceanographic model and found them to be in excellentagreement (model avg. T_r = 27 days). From these independent T_rvalues and the inventory of Cs within the water column at the time of ourcruise, we were able to calculate an offshore ¹³⁴Cs flux of3.9–4.6 × 10¹³ Bq d⁻¹. Radium-228 (t_1/2 = 5.75 yr) was used to derivea vertical eddy diffusivity (K_z) of 0.7 m² d⁻¹ (0.1 cm² s⁻¹); from this K_z and ¹³⁴Cs inventory, we estimated a¹³⁴Cs flux across the pycnocline of 1.8 × 10⁴ Bq d⁻¹ for thesame time period. On average, our results show that horizontal mixing lossof Cs from the coastal zone was ~ 10⁹ greater thanvertical exchange below the surface mixed layer. Finally, a mixing/dilutionmodel that utilized our Ra-based and oceanographic model water mass agesproduced a direct ocean discharge of ¹³⁴Cs from the FNPP of 11–16 PBqat the time of the peak release in early April 2011. Our results can be usedto calculate discharge of other water-soluble radionuclides that werereleased to the ocean directly from the Fukushima NPP.

[学科分类] 地球化学与岩石