During high-temperature gas refrigeration reactor (HTGR) operation, SiC in the fuel particles is the key structure for stopping radionuclides diffusing from the particles. Researchers put forward the concept of breakthrough time, which is the time of a large amount of radionuclides released through SiC from the particles, for the consideration of HTGR safety. However, this concept is not so accurate. This paper gives a strict expression of cumulative fractional release interims of classic and improved methods, and also analyses the relevant physical content of breakthrough time in the use of Fick’s law of diffusion. By computing the cumulative fractional release of three kinds of important radionuclides 137Cs, 90Sr and 110mAg, and using the above two methods, we find that the traditional meaning of the physical quantity is not the time of radionuclides released from SiC. The improved method is more accurate for describing the release and transport condition of radionuclides in nuclear fuel particles.