We report on development from Japan this week with the creation of a low-alkali cement for use at nuclear waste sites. Professor Katsuyoshi Kondo, Joining and Welding Research Institute at Osaka University, and Nippon Steel & Sumikin Cement Co have prepared a process that mixes silica dioxide extracted from rice chaff with cement.
As press reports explain, 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.
The stated application for this new research is for underground nuclear waste disposal sites. At these locations extremely high durability is required for long periods of time, potentially for tens of thousands of years.
Normally the concern with alkali-silica reactivity is between alkali in the cement and a sensitive aggregate over a shorter time period. Under high moisture and high alkali content the resulting concrete can crack leading to reduced-performance. However, the issue with nuclear waste storage is that it has to be stored underground and for long periods of time. This means that the cement can potentially react with groundwater producing calcium hydroxide making the groundwater alkaline. This can then react with aggregates in the clay and bedrock at the storage site. Clearly this is undesirable for a long-term storage site of hazardous materials.
In the wake of the Fukushima disaster, this kind of development will be of high interest in Japan. It will also have applications around the world wherever radioactive waste sites are built.
One example of the demanding construction conditions facing builders in these environments is the original sarcophagus used to encase the Chernobyl Nuclear Power Plant reactor in 1986. Building it used more than 7,000t of steel and 410,000m3 of concrete. Erected in a hurry under horrendous conditions, the container was never sealed properly and the structure was only given a design lifespan of 20 to 30 years. Currently a replacement, New Safe Confinement, is being built at a projected cost of Euro2bn for completion in 2017. The structure will be up to 100m tall and 165m long with a lifetime of at least 100 years.
One of the issues raised in the documentary film 'Into Eternity' is what exactly should one daub on the entrance to a long-term waste dump? Given that the Onkalo spent nuclear fuel repository in Finland is planning to stay sealed for 100,000 years, how should its planners communicate to people, who potentially rediscover it in the future, that they should stay away? One suggestion quoted here is to put Edvard Munch's The Scream on the door. However, we have difficulty today in reading and interpreting Ancient Egyptian writing and art from 5000 years ago. What this means for any of our descendants unlucky enough to stumble upon a buried nuclear waste site is anyone's guess. At the very least though using a low-alkali cement that will last as long as possible is a good start.