Cleaning up Fukushima Daiichi

By ThinkReliability Staff

The nuclear power plants at Fukushima Daiichi were damaged beyond repair during the earthquake and subsequent tsunami on March 11, 2011.  (Read more about the issues that resulted in the damage in our previous blog.)  Release of radioactivity as a result of these issues is ongoing and will end only after the plants have been decommissioned.  Decommissioning the nuclear power plants at Fukushima Daiichi will be a difficult and time consuming process.  Not only the process but the equipment being used are essentially being developed on the fly for this particular purpose.

Past nuclear incidents offer no help.  The reactor at Chernobyl which exploded was entombed in concrete, not dismantled as is the plan for the reactors at Fukushima Daiichi.  The reactor at Three Mile Island which overheated was defueled, but the pressure vessel and buildings in that case were not damaged, meaning the cleanup was of an entirely different magnitude.  Lake Barrett, the site director during the decommissioning process at Three Mile Island and a consultant on the Fukushima Daiichi cleanup, says that nothing like Fukushima has ever happened before.

An additional challenge?  Though the reactors have been shut down since March 2011, the radiation levels remain too high for human access (and will be for some time).  All access, including for inspection, has to be done by robot.

The decommissioning process involves 5 basic steps (though the completion of them will take decades).

First, an inspection of the site must be completed using robots.  These inspection robots aren’t your run-of-the-mill Roombas.  Because of the steel and concrete structures involved with nuclear power, wireless communication is difficult.  One type of robot used to survey got stuck in reactor 2 after its cable was entangled and damaged.   The next generation of survey robots unspools cable, takes up slack when it changes direction and plugs itself in for a recharge.  This last one is particularly important: not only can humans not access the reactor building, they can’t handle the robots after they’ve been in there.  The new robots should be able to perform about 100 missions before component failure, pretty impressive for access in a site where the hourly radiation dose can be the same as a cleanup worker’s annual limit (54 millisieverts an hour).

Second, internal surfaces will be decontaminated.  This requires even more robots, with different specialties.  One type of robot will clear a path for another type, which will be outfitted with water and dry ice, to be blasted at surfaces in order to remove the outer level, and the radiation with it.  The robots will them vacuum up and remove the radioactive sludge from the building.  The resulting sludge will have to be stored, though the plan for the storage is not yet clear.

Third, spent fuel rods will be removed, further reducing the radiation within the buildings.  A shielded cask is lowered with a crane-like machine, which then packs the fuel assemblies into the cask.  The cask is then removed and transported to a common pool for storage.  (The fuel assemblies must remain in water due to the decay heat still being produced.)

Fourth, radioactive water must be contained.  An ongoing issue with the Fukushima Daiichi reactors is the flow of groundwater through contaminated buildings.  (Read more about the issues with water contamination in a previous blog.)  First, the flow of groundwater must be stopped.  The current plan is to freeze soil to create a wall of ice and put in a series of pumps to reroute the water.    Then, the leaks in the pressure vessels must be found and fixed.  If the leaks can’t be fixed, the entire system may be blocked off with concrete.

Another challenge is what to do with the radioactive water being collected.  So far, over 1,000 tanks have been installed.  But these tanks have had problems with leaks.    Public sentiment is against releasing the water into the ocean, though the contamination is low and of a form that poses a “negligible threat”.  The alternative would be using evaporation to dispose of the water over years, as was done after Three Mile Island.

Finally, the remaining damaged nuclear material must be removed.  More mapping is required, to determine the location of the melted fuel.  This fuel must then be broken up using long drills capable of withstanding the radiation that will still be present.  The debris will then be taken into more shielded casks to a storage facility, the location of which is yet to be determined.  The operator of the plant estimates this process will take at least 20 years.

To view the Process Map laid out visually, please click “Download PDF” above.  Or click here to read more.