Large amounts of waste contaminated with radioactive substances were generated, posing an urgent problem of its disposal. Techniques to safely and surely contain radioactive substances have been developed, with specific proposals having been made. However, due to the necessity for temporary storage of radioactive substances until final disposal, research is under way regarding containments for that purpose.
Meanwhile, the shielding performance of heavyweight concrete has long been known. Heavyweight concrete is a type of concrete made using heavyweight aggregate with an oven-dry density of over 3.5 g/cm3 to increase its density. Research and development for safe containments made of heavyweight concrete to store radioactive contaminants has been in progress due to its excellent shielding performance. Products developed so far have been mostly thin and small to facilitate transportation. However, such containments are rather required to be robust with high structural capacity and resistance to moisture migration so that they would cause no problem when piled up in layers to store a large amount of waste for a long time in a limited area. When assuming an environment contaminated with radioactivity, quick construction of such containments is also necessary. It is therefore essential to produce them as precast concrete products.
For such precast products, aggregate with an even higher density is used to enhance their shielding performance while keeping a normal member thickness. It is then required to impart segregation resistance to such concrete to prevent segregation during production. It is also preferable to increase the self-compactivity in view of the ease of placing. In consideration of these requirements, the authors developed box culverts made of heavyweight concrete with a high segregation resistance and medium fluidity. These culverts can be made into large-scale containments by jointing multiple units using prestressing steel and waterproofing the joints and internal layers.
Another requirement as important as shielding performance for a containment is the capability of completely eliminating leakage of contaminated waste to the environment for a long time. However, concrete involves a risk of cracking, and the risk is increased by drying-induced paste shrinkage and temperature-induced volume changes, particularly when the containments are placed on the ground, instead of being embedded, as is often the case with temporary storage containers. Water-resistant coating is applied to the inside of such containments to prevent leakage, but water infiltrating through cracks over a long time can swell the coating film until it breaks. To minimize such a risk, cracks in concrete are repaired by chemical grouting, etc., but from the aspect of shielding radioactive contaminants, such containments should preferably be maintenance-free.
With this as a background, the authors conducted research on self-healing performance of heavyweight concrete containing an expansive additive, fly ash, and organic fibers based on water permeability testing and SEM analysis with the aim of self-repairing water leakage through cracking. As a result, the following were found within the range of this study: (1) self-healing led to early reduction in the water permeability, but the internal crack width scarcely decreased, with crack closure being observed only in the surface region; (2) the addition of an expansive additive led to self-healing at an early stage presumably due to chemical prestress and ettringite formation to fill fine cracking; and (3) the inclusion of fly ash and organic fibers appear to exert a self-healing effect slowly over a long time.