Weather and dispersion modeling of the Fukushima Daiichi nuclear power station accident

Abstract

Contamination due to the surface deposition of radioactive material from the accident at the Fukushima Daiichi nuclear power station was investigated for 11 March to 17 March 2011. A coupled weather and dispersion modeling system was developed and simulations of the accident performed using two independent source terms that differed in emission rate and height and in the total amount of radioactive material released. Results show significant differences in the distribution of cumulative surface deposition of 137Cs due to wet and dry removal processes. Parameterizations for precipitation scavenging by rain, snow, and graupel were implemented to investigate surface wet deposition fields . Results show aerosols from a source term with emission heights at 50, 300, and 1000 meters above ground level (AGL) were scavenged by rain (70%) and graupel (30%) compared with a source term with emission heights at 20 and 120 meters AGL in which material was scavenged preferentially by rain (95%). A sensitivity study was performed that broadened the particle size distribution (PSD) of the source terms during explosive events of the accident. Results for the source term with elevated emission heights show enhanced wet deposition due to scavenging by snow (5%, 35%, 51%) compared with scavenging by rain and graupel as the effluent PSD was increased (0.5, 1.0, and 10m, respectively). In contrast, the source term with relatively lower emission heights remained preferentially scavenged by rain (90%). A second study that investigated the complexity of the cloud microphysics scheme showed that precipitation scavenging of radioactive material was not very sensitive to the choice of single-vs. double-moment cloud microphysics parameterization. A comparison of 137Cs deposition predicted by the model with aircraft observations of surface-deposited gamma radiation showed reasonable agreement in surface contamination patterns during the dry phase of the accident. During the wet phase the pattern is not as well predicted. It is suggested that this discrepancy is because of differences between model predicted and observed precipitation distributions. Dry deposition was the dominant removal process, accounting for the majority of surface contamination (12 orders of magnitude over that due to wet deposition near the source).