Incinerating the radioactive sludge that has piled up at sewage facilities across eastern Japan and burying it after mixing it with cement could increase the risk of cesium seeping into the environment, scientists say.
Disposal of the sludge, which is thought to have been created after radioactive materials from the Fukushima nuclear disaster were carried by rainwater into sewage pipes and then condensed during sewage treatment, has become a major headache for local governments.
One solution, being pursued by local governments, is incinerating the sludge and then mixing it with cement before burying it in landfill sites.
But the results of a research team led by Hideo Yamazaki, a professor of environmental analysis at Kinki University, suggest the process may increase the risk of the transfer of radioactive cesium into the environment. Instead, the scientists found that adding clay to the incinerated material may offer a safer way of processing it.
The researchers added a small amount of rainwater to radioactive soil from Minami-Soma, Fukushima Prefecture, as well as the ash generated by incinerating the soil. They measured the extent of radioactive cesium transfer into the water after stirring the mixtures for two hours.
No cesium was transferred from the radioactive soil into the water when raw soil was used, but 0.11 percent of the cesium passed into the water when water was added to incinerated soil.
The transfer rate soared to 2.87 percent when cement was added. The addition of clay suppressed the transfer.
When contained in unprocessed soil and sewage sludge, the cesium appears to bond strongly with the soil’s clay component and there is little seepage. Incineration allows more cesium to drift away because it breaks up the clay. The addition of alkaline cement apparently further assists the flow of cesium, the scientists said.
Currently, many local governments across eastern Japan are storing ash at sewage disposal facilities because they have nowhere else to put it. The Tokyo metropolitan government is burying it in landfill sites after mixing the ash with cement and water.
According to central government safety standards, incinerated sludge and soil can be put in landfill sites if it contains 8,000 becquerels or less of radioactive cesium per kilogram.
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