​In the Fukushima region, around 22 million cubic metres of radioactive soil was removed from the surface of the soil following the nuclear power plant accident with a view to making the zone accessible to its inhabitants as soon as possible. This soil was stored in 'big bags' at several dedicated sites. The Japanese authorities are looking for the best technology to eradicate or reduce the radioactivity from the soil. The aim is to reduce the volume of this waste by concentrating the radioactivity, composed primarily of caesium, into a small volume and recovering the decontaminated part. The specific feature of the process tested by CEA, Orano and Veolia is that it directly separates the soil particles contaminated by the radioactive caesium from the particles that are barely contaminated or not contaminated at all.

Accidental pollution remediation technologies 

In France, the participants in this project, named Demeterres (1), have, since 2013, been developing several innovative remediation technologies for use on contaminated soil and effluents. These technologies, based on biological or physicochemical processes, are designed to decontaminate soil using "eco-compatible" methods with a view to rehabilitating and re-using it.

A real-life test campaign in Japan

One of the physicochemical processes developed within the framework of this project by CEA, Orano and Veolia, known as "froth flotation", was tested in Japan during the week of 13 to 17 November 2017. The process entails frothing a soil suspension in a flotation column. First the contaminated soil is mixed with water to form a suspension and a frothing agent is added. The mixture is introduced into the middle of a vertical column, and air is injected into the bottom of the column to produce bubbles. The caesium-charged clay particles attach to the surface of the air bubbles and form a froth, which builds up and is sucked to the top column, producing a concentrate of the radioactive particles. The following is thus recovered:

• In the "frothy" part at the top of the column, the fine particles containing a significant proportion of radioactivity;
• At the bottom of the column, the soil that is free of the fine particles, and thus free of a significant proportion of the radioactivity.

The flotation tests carried out on two big bags by the teams from the CEA/Orano/Veolia in Japan in November 2017 achieved the objectives set. 70 to 85% of the initial mass of soil was recovered. This mass contained only 33% to 50% of the initial amount of radioactivity.

This decontaminated soil displays a level of radioactivity that complies with the limit value of 8 kBq/kg authorised for potential re-use under Japanese law.

In order to further increase the caesium extraction yield, soil preparation optimisations (drying, crumbling, pre-sifting and dispersion in water) were proposed.

Prospects

If the technology presented is selected by the Japanese authorities, the next stage will be to develop the process on a larger scale so that it can be used in the Japanese municipalities that house storage centres.  

This process is subject to a patent belonging to the CEA and used at "pre-industrial pilot" level by Orano and Veolia.

From the pilot to the on-site demonstration phase

• 2016: first pilot column (2 meters high, 20 cm in diameter) was tested in France at CEA Marcoule using various non-contaminated soils. Parameters: air flow, frothing agent, residence time... were optimised in order to maximise clay particle extraction. These tests served to collect data relating to the functioning of the process.
• April 2017: the technology was proposed via a Japanese subsidiary of Orano (Anadec) within the framework of the call for projects relating to the "Demonstration of new decontamination techniques" launched by the Japanese Ministry of the Environment
• July 2017: the technology was selected for on-site demonstration by the Japanese authorities along with nine other technologies (out of 19 projects presented). These demonstrations of innovative technologies are based on brief test runs in a real environment. They enable the local stakeholders to broaden the range of soil decontamination possibilities.
  
• November 2017: the demonstration tests were carried out successfully from 13 to 17 November in the presence of representatives of the MoE in the municipality of Okuma, which houses storage centres for big bags containing contaminated soil.  On most of the soil tested, 70 to 80% of the fine particles previously released through agitation in the water were separated by flotation, allowing for a radioactivity volume concentration of a factor of 3 to 7.