Properties of Low-ph Cement Grout as a Sealing Material for the Geological Disposal of Radioactive Waste

The OECD NEA (Nuclear Energy Agency) anticipates that by 2050 the amount of nuclear-generated electricity worldwide will increase by a minimum of 1.6 times and a maximum of 3.9 times its 2008 value of 372 GWe [1]. At the same time, the amount of radioactive waste that will be produced is expected to increase substantially. High-level radioactive waste is usually derived from the burning of uranium fuel in a nuclear reactor. Fission products and transuranic components such as spent fuel can be included in the definition of high-level waste. Although various radioactive waste disposal concepts, including deep-seabed disposal, disposal in the polar ice sheets, and rocketing of the waste into space, have been investigated, deep geological disposal is generally considered to be the best option to deal with high-level radioactive wastes [2]. In South Korea, a total of twenty nuclear reactors are in operation; the cumulative amount of spent fuel is estimated to be 10,490 MTU (2009). The full capacity of the country’s storage is anticipated to be saturated around 2016, and choosing a long-term disposal policy is an urgent matter. However, a national strategy for spent fuel management has not yet been set down, and high-level waste (HLW) such as spent fuel will have to be stored atreactor (AR) by re-racking or in interim storage vessels such as dry storage casks until 2016, in accordance with the decision of 256 Korea Atomic Energy Commission (2004). Even if the decision on what kind of disposal policy to adopt in Korea is answered in the near future, the use of cementitious materials is inevitable at least to deal with the constraints of water ingress, access tunnel floors, plugging of backfill, and sealing of tunnels from water flowing into a HLW repository. Regular/conventional cement (in general of high-pH) can accelerate the release of uranium; increase the mobility of uranium, fission products, and actinides. It can also change the solubility of bentonite, which aggravates the function of buffer materials. To minimize the environmental impact and influence of cementitious materials, the development of optimized cement is indispensable; research into field applications for and long-term performance of such materials is needed. As part of an effort to optimize the properties of cementitious materials, studies related to The current solution to the problem of using cementitious material for sealing purposes in a final radioactive waste repository is to develop a low-pH cement grout. In this study, the material properties of a low-pH cement grout based on a recipe used at ONKALO are investigated by considering such factors as pH variation, compressive strength, dynamic modulus, and hydraulic conductivity by using silica fume and micro-cement. From the pH measurements of the hardened cement grout, the required pH (< pH 11) is obtained after 130 days of curing. Although the engineering properties of the low-pH cement grout used in this study are inferior to those of conventional high-pH cement grout, the utilization of silica fume and micro-cement effectively meets the long-term environmental and durability requirements for cement grout in a radioactive waste repository.