The use of cement-based materials for radioactive waste confinement and storage must rest on precise measurements of their physical and chemical properties. An important property is the diffusivity of tritium in its liquid form (tritiated water) through a sample considered as representative. Here, we report quantitatively the effect of radioactive decay compared to the effective diffusion coefficient on tritium diffusion. The numerical model was validated by comparing it to experimental data. We found that, in the worst case scenario, the calculated effective diffusion coefficient of tritiated water based on the classical analytical solution to Fick's law is underestimated by more than 20 %, compared with the results provided by the numerical model, which accounts for the radioactive decay within the material.