Determination of the effective diffusion coefficient for 125I- in Beishan granite

With the policy of active development of nuclear energy in China in the following 20 years or more, the high-level radioactive wastes and spent fuels will increase considerably. In order to dispose of these wastes safely, China plans to construct a geological repository in the middle of the 21st century. Through more than 20 years of research and discussion in China, granite has been regarded as one of the most favorable candidate host rocks for a potential geological repository and I-129 has been recognized as one of the most important nuclides for the safety of the potential repository. In this paper, the diffusion of I-125 (as a surrogate of I-129) in deep granite taken from Beishan, Gansu Province is investigated with the help of a small scale diffusion apparatus. The granite core sample was taken at the depth of about 300 m and cut into sections of 5 similar to 20 mm. The granite section samples were weighted and the pore volume was determined with water immersion method. The chemical and elemental compositions of the granite are presented. Experimental results were treated with different methods, producing an effective diffusion coefficient of I-125(-) in the granite of (2.44-2.72) x 10(-12) m(2) s(-1).