Osaka University and a cement company in Hokkaido have co-developed a low-alkaline cement utilizing rice chaff that will strengthen the durability of final nuclear waste disposal sites.
The development of low-alkaline cement is essential for the construction of nuclear disposal facilities that will need to remain durable for as long as 100,000 years to prevent radioactive materials from escaping.
In nuclear repositories, waste will be solidified with glass and placed in metallic containers before being covered with clay and buried deep underground.
If normal cement is used underground, it reacts with groundwater, producing calcium hydroxide and making groundwater highly alkaline. This causes cracking and deterioration in the nearby clay and bedrock.
To lower the alkaline levels in cement, Katsuyoshi Kondo, a professor at Osaka University's Joining and Welding Research Institute, and Nippon Steel & Sumikin Cement Co. based in Muroran, Hokkaido, mixed silica dioxide extracted from rice chaff with cement. Silica reacts with calcium hydroxide and weakens alkaline.
More than 100 million tons of rice chaff, a highly siliceous crop residue, is produced worldwide every year.
The team succeeded in developing technology to extract highly purified silica with numerous holes measuring 5 to 7 nanometers in diameter by washing rice chaff with organic acid and burning it.
The surface area of the silica extracted from rice chaff is 50,000 to 90,000 times larger than that contained in existing cements, enhancing the reaction between silica and calcium hydroxide and thus lowering the alkaline level.
The central government is searching for a site to build a final repository for highly radioactive nuclear waste generated from power plants. In Finland, a disposal site for high-level radioactive waste is already under construction.
"We aim to utilize the low-alkaline cement at facilities abroad after repeated tests to verify its performance," a Nippon Steel & Sumikin Cement official said.