Abstract: The fuel cladding is the first barrier of nuclear power plant. Once it failed, fission products would release to reactor coolant system (RCS) through the crevasse. So determining the general position and size of the crevasse would help a lot to know the seriousness of the accident. This paper builds four models, which are models for cases of fission products accumulated in fuel pellets, diffused to the gap between pellet and cladding and released from the gap to RCS, and the balance model in RCS. By detecting the activities of fission products in RCS, we can deduce the initial products released from the crevasse and subsequently the specific situation of the crevasse. These models have been proved applicable by an accident of fuel rod failure in the Daya Bay Nuclear Power Plant.