I have written before about what happened in Fukushima in March 2011, and about measuring radiation levels in Japan. Given the situation there, a major question is: how can the contamination been cleaned up?
Removing the top 5cm
Most of the longer lasting radioactive material released was caesium-137, which is a metal that boils at a low temperature (641° C). Large quantities of caesium vapour were released into the atmosphere, and this condensed into very fine particles, which were spread by wind and rain and deposited on over a thousand of square kilometers. Each atom of caesium-137 is unstable, and within about 30 years, half of them will decay, releasing gamma rays. Caesium can form salts (similar to sodium chloride, or table salt) which are absorbed into plants and by animals and humans.
Investigations have shown that the majority of the caesium is in the top 5cm – 10cm of soil. It has been found that removing this top layer can reduce the radiation level to 1 mSv/y, which is internationally considered a safe level. There are a few complications however.
The first challenge it what to do with the layer of highly radiative soil. 5cm may not sound like much, but a square of 8m x 8m would result in more than a 3 ton truck could carry. A single full size sports field would result in over 170 truckloads. Wherever this soil is dumped, it concentrates the caesium and results in a radiation level much higher than the original level. And if it is dumped in the open, wind in the dry season will blow it around, and in the wet season the caesium will get washed into gutters, rivers, lakes and dams. If a really big hole is found or dug, and the soil is buried, the risk is that groundwater is directly contaminated.
Currently, this soil is being taken within the 20km exclusion zone, and dumped.
The second challenge emerged when ‘cleaned’ properties were re-tested. It was found than in cases where all the surrounding properties had not been cleaned, the radiation level gradually increased almost back to the original level. So attempts to clean up ‘critical’ areas, such as school grounds, were found to have limited success, unless all the surrounding properties were also decontaminated. Of course, the same applies to the properties around those properties…
Some surfaces such as roads and pathways, as well as structures such as buildings were contaminated with radioactive caesium and needs to be cleaned before the soil is removed. High pressure water cleaners are used, which creates spray. This spray can be blown onto surrounding structures, recontaminating them. Spraying down every surface in a whole town is a huge amount of work, and to do it without spreading the radiation back into cleaned areas, and without contaminating storm water drains and rivers, is a logistical nightmare.
When the wind containing the caesium was onshore, it blew into the mountain slopes, which are now some of the most contaminated areas outside of the 20km zone. The rain and melting snow will wash the caesium downhill, and again into rivers, lakes and dams. This means that at some time in the future, lower lying areas previously safe could exceed the legal maximum doses, forcing further evacuations or decontamination operations.
Cleaning up the contaminated areas is a logistical impossibility. The only viable solution seems to be to abandon them for a few hundred years until the caesium has mostly decayed. Since the human race began using nuclear power, there has been on average one major disaster every 17 years. If unexpected disasters continue at this rate (as is likely especially if many more nuclear plants are built) then it will not take many decades before we render a significant portion of our planet uninhabitable. Is nuclear power worth that risk?