Background: A large quantity of fission products would be produced when the nuclear reactor runs. These fission products would be released to the pellet-rod gap and then to the primary circuit while the fuel rod is broken. The fission products in the primary circuit become the main source term of the reactor. Purpose: This study aims to analysis the source term accurately by calculating the release to birth ratio (RBR) of volatile fission products precisely. Methods: After researching American NRC ANSI/ANS-5.4 methods and French methods for calculating the RBR of volatile fission products

we calculated the RBR of volatile fission products of a fuel rod at different power and different burn-up by these methods

and made some comparison and analysis of these results. Results: Computation results showed that the RBR of volatile fission products would increase with the power and burn-up of the fuel rod. Compared with the ANSI/ANS-5.4-2011

the French method is more conservative whilst the ANSI/ANS-5.4-1982 is the most conservative among these methods. Conclusion: ANSI/ANS-5.4-2011 is the best method among these three methods and it can be used to calculate the release to birth ratio of volatile fission products with good results. Abstract