Desorption Kinetics of Radionuclides Fixed in Cement Matrix

Leach characteristics of some typical nuclides such as cesium, strontium, cobalt, I⁻, and CO₃⁻² from ordinary portland cement waste forms have been studied using the ISO test method and radiotracers of the respective nuclides, i.e., ¹³⁴Cs, ⁸⁵Sr, ⁶⁰Co, ¹³¹I, and ¹⁴C. The leach studies suggest a rapid release of radioactivity in the beginning (fast component) followed by slow release for long periods of time (slow component). A mathematical model has been simulated to describe the leaching kinetics of these nuclides from the cement matrix. The effective diffusion coefficient D_eff is computed from the two componental diffusion coefficients, and the retardation factor (α) for a nuclide is evaluated from a knowledge of the radioactivity distribution in the aqueous and solid phase at equilibrium. The product αD_eff for all the nuclides studied has been found to be approximately constant and is equal to the intrinsic diffusion coefficient in the cement matrix. The net fractional release of different radionuclides from cement waste form showed a decreasing pattern, i.e., ¹³⁴Cs > ¹³¹I > ⁸⁵Sr > ¹⁴Cr > ⁶⁰Co indicating the largest diffusion coefficient for cesium as 10⁻² cm²/day and the least for ¹⁴C as 3 × 10⁻⁸ cm²/day.