Thematic Articles

Five models have been used to estimate the oceanic dispersion of 137Cs from the Fukushima Daiichi nuclear power plant during March and April 2011, following the accident on March 11, 2011. The total discharged activity of 137Cs is estimated to be 2 to 15 petabequerels. A weak southward current along the Fukushima coast was responsible for the initial transport direction, while mesoscale eddy-like structures and surface-current systems contributed to dispersion in areas beyond the continental shelf. Most of the discrepancies among the models in April are caused by differences in how the mesoscale current structures off the Ibaraki coast are represented.

Radionuclides, such as 134Cs, 137Cs, and 131I, were released during the Fukushima Daiichi nuclear power plant accident in March 2011. Their distribution was monitored by airborne surveys and soil sampling. The most highly contaminated areas are to the northwest of the plant and in the Naka-dori region of Fukushima Prefecture; this contamination was mainly the result of wet deposition on March 15. Radionuclides were also released on March 21, and they were dispersed up to 200 km south of the plant. The Cs/I ratios are different for these two events, probably because of differences in the initial ratios in the airborne plumes and the amount of wet deposition. Numerical simulations of the dispersion process and vertical profiles of radionuclides in soils are used to describe the contamination of soils.

On March 11, 2011, an earthquake and tsunami hit the northeast coast of Japan and damaged the Fukushima Daiichi nuclear power plant, leading to the release of radioactive material into the atmosphere. We trace the evolution of radioactivity release to the atmosphere and subsequent dispersion as simulated by models, and we compare these to actual measurements. Four main release periods are highlighted. The first event had limited consequences to the north of the power plant along the coast; the second had no impact on Japanese territory because the plumes travelled toward the Pacific Ocean; the third was responsible for significant and longterm impact, especially northwest of the plant; and the last had consequences of lesser impact on the Tokyo area.

The major nuclear accident at the Fukushima Daiichi nuclear power plant more than one year ago was the result of a combination of four interrelated factors: site selection, external hazard assessment and site preparation, the utility’s approach to risk management, and fundamental reactor design. The reactor accident was initiated by a magnitude 9 earthquake, followed by an even more damaging tsunami. An insufficient tsunami defense-in-depth strategy led to significant core damage in three units and radioactive release to the environment. This paper provides a summary of the sequence of events that led to the accident and current efforts to contain and manage the released radioactivity.

Rapid seismological analyses, carried out within minutes of the March 11, 2011, Tohoku earthquake, were crucial in providing an earthquake ground shaking and tsunami early warning and in hastening the evacuation of the population along Japan’s northeastern coast. By 20 to 30 minutes after fault rupture began, these analyses had established that the event had a moment magnitude of Mw = 9 and involved shallow thrust faulting on the plate boundary megathrust. Preparation for future large earthquakes on megathrusts in Japan and elsewhere should include onshore and offshore geodetic monitoring of strain accumulation, implementation of rapid earthquake and tsunami warning systems, and public training and education for shaking and tsunami response.

This thematic issue of Elements describes the events at the Fukushima Daiichi nuclear power plant on March 11, 2011, and the aftermath.