3:00 PM - 3:15 PM
[MAG24-18] Behaviour of oceanic 137Cs from the Fukushima Dai-ichi Nuclear Power Plant for four and a half years
Keywords:Fukushima Dai-ichi Nuclear Power Plant Accident, Oceanic despersion, Cesium-137, Regional Ocean Model, Ocean General Circulation Model
We reconstructed spatiotemporal variability of 137Cs activity in the regional ocean for four and a half years by numerical models, such as a regional scale (horizontal resolution is about 1 km) and the North Pacific scale (horizontal resolution is about 10 km) oceanic dispersion models, an atmospheric transport model and river runoff model.
Direct release rate of 137Cs were estimated for four and a half years after the accident by comparing simulated results and observed activities very close to the site. The estimated total amounts of directly release was 3.7±0.7 PBq. Directly release rate of 137Cs was the order of magnitude of 1014 Bq/day and decreased exponentially with time to be the order of magnitude of 109 Bq/day by the end of September 2015. Estimated direct release rate have exponentially reduced with constant rate since November 2011. Apparent half-life of direct release rate was estimated to be 346 days. Simulated 137Cs activities attributable to direct release were in good agreement with observed activities, a result that implies the estimated direct release rate was reasonable, while there is no observed data of 137Cs activity in the ocean from 11 to 21 March 2011. Observed data of marine biota should reflect the history of 137Cs activity in this early period. The comparisons between simulated 137Cs activity of marine biota by a dynamic biological compartment and observed data also suggest that simulated 137Cs activity other than attributable to dirent release was underestimated in this early period. We reconstructed the history of 137Cs activity in this early period with direct release, atmospheric deposition, river input, runoff from 1F NPP site with precipitation. River runoff process is still unknown in the early period because there were no observed data. We assumed that 10% of deposited 137Cs on each river basin run off thorough rivers along the Miyagi, Fukushima and Ibaraki coasts. The simulation with overestimated river runoff rate (10 %) suggests that the river flux of 137Cs to the ocean was not effective to the 137Cs activity in the ocean in this early period. We estimated the release rate of 137Cs with rain water runoff from the 1F NPP site from the observed 137Cs activity before 26 March 2011 and precipitation data close to 1F NPP site. Simulation with additional release of 137Cs from the 1F NPP site suggests that additional release from 1F NPP site was effective to the 137Cs activity adjacent to 1F NPP and 2F NPP. Simulated atmospheric depositions of 137Cs on a reginal ocean by 9 regional atmospheric transport models still have huge uncertainties. It is also important to estimate the deposition process on a regional ocean to understand contamination process of marine biota.
In the North Pacific scale, 137Cs activity in the intermediate water increased due to the Subtropical Mode Water (STMW) formation. 137Cs is a useful tracer to detect the STMW formation. Not only the direct release but also the atmospheric deposition are essential for the distribution of 137Cs activity in the North Pacific. Five-member, ensemble simulation with high resolution can represent the increase of 137Cs activity in the intermediate water.