Preventing radioactive leaks at Fukushima Daiichi
Japan’s nuclear catastrophe, triggered by an earthquake and tsunami, now faces challenges from a relentless flow of subterranean freshwater.
A full cleanup and decommissioning of the Fukushima plant could take four decades.
On March 11, 2011, a
9.0-magnitude earthquake and 46-foot tsunami knocked out power and backup generators at the Fukushima Daiichi nuclear power plant on the east coast of Japan.
Reactors 4, 5 and 6 were shut down at the time of the earthquake. Fuel-core meltdowns occurred in reactors 1, 2 and 3, which were active during the earthquake.
A cascading series of events sent radioactive particles into surrounding areas. Radioactive nuclides escaped in several ways: by steam from overheating reactors; water leaking from damaged reactor vessels; smoke; and explosions that rocked reactors 1, 3 and 4.
Since the accident, a circulating water system has been installed to cool the molten fuel.
Water leaving the reactors passes through a filtration plant that removes radioactive cesium, then proceeds through a reverse-osmosis filter before flowing into storage tanks that feed water back into the reactors.
The tanks can store 400,000 tons of water. Tokyo Electric Power Co. (Tepco) anticipates needing twice that amount of storage in the future.
Radioactive water continues to escape into the environment:
In August, 80,000 gallons of radioactive water leaked out of a storage tank.
Toxic water collects
in underground electrical conduits and seawater cooling-system trenches close to shore, where high levels of radioactivity have recently been detected in the soil.
Trenches and conduits
Reactors sit in permeable soil, through which about 800 tons of freshwater flows every day on its way to the ocean. In the process that water mixes with on-site contaminants and becomes toxic. Half of that water makes its way into the sea. The other half leaks into
buildings, from where it is pumped out, filtered and stored.
Because of the inflow of groundwater, the amount of
water stored on site constantly accumulates. But
some of the water is recycled, taken from the tanks and
fed back into the reactors to cool the molten fuel.
To prevent water from flowing through the accident site, engineers are pumping out groundwater with a dozen wells and piping it to a
series of holding tanks, where it is being held until TEPCO, the
Japanese government, local residents and fishermen agree
that the water is safe enough to empty into the ocean.
Groundwater storage tanks
To prevent contaminated water from leaving the site, engineers are pumping out and filtering water from soil, conduits and trenches, and are building a new filtration plant that can remove more than 60 different radionuclides.
Glass walls are being installed near the shoreline to isolate contaminated soil.
In the future, impervious walls will seal the shoreline.
A mile-long array of pipes will be sunk 100 feet into the ground, where circulating refrigerants will create a watertight wall of frozen soil surrounding the reactors.
Water collecting inside the ice barrier will be pumped out and filtered.
The accident has discharged about a 10th of the amount of radiation released by the 1986 Chernobyl disaster.
Because some fish samples contain radioactive elements above allowable limits, fishing of many species is banned in the area.
SOURCE: Tepco; International Atomic Energy Agency; Japan Ministry of Economy, Trade and Industry; Institute of Electrical and Electronics Engineers; Bulletin of the Atomic Scientists; staff reports
Drawings are diagrammatic.