Japan: Osaka University and a cement company in Hokkaido have co-developed a low-alkaline cement that uses rice chaff to strengthen the durability of final nuclear waste disposal sites, according to Kyodo News.

The cement is essential for the construction of final nuclear waste disposal facilities that need a durability of up to 100,000 years to prevent harm from radioactive materials. In such repositories, nuclear waste is solidified with glass and placed in metallic containers before being covered with clay and buried underground. If normal cements are used underground, they react with groundwater to produce calcium hydroxide, making the groundwater highly alkaline. This causes cracks and deterioration in the clay and bedrock at the facility.

To lower the alkaline levels in cement, professor at Osaka University's Joining and Welding Research Institute, Katsuyoshi Kondo, and Nippon Steel & Sumikin Cement Co mixed silica dioxide extracted from rice chaff with cement. The silica reacts with calcium hydroxide and weakens alkaline.

The team has developed technology to extract highly purified silica with numerous holes measuring 5 – 7nm 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 - 90,000 times larger than that contained in existing cements, enhancing the reaction between silica and calcium hydroxide and thus lowering the alkaline level.

Japan is looking for a place to build a final repository for the 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 utilise the low-alkaline cement at facilities abroad after repeated tests to verify its performance," said a Nippon Steel & Sumikin Cement official.