Abstract: Under severe accident conditions,it may cause the containment to fail and release a large amount of fission products into the environment.This paper uses integrated code to study the quality of fission product Cs I in the primary system and the containment and the source term of the fission product released to the environment under the superposition of different break accidents with the total loss of electric power supply.The paper also considers the effectiveness of containment spray in controlling the outward release of fission products.The results show that there is little difference in the quality of fission products released into the environment under small break,medium break and large break accidents.The main difference is the failure time of containment.The release of fission products to the environment can be effectively controlled when the containment spray is turned on.The quality of the aerosol produced in the small breach accident is higher than the other two accidents,and the aerosol is mainly deposited in the containment by diffusion electrophoresis.Opening the cavity injection system can ensure the integrity of the vessel,reduce the quality of fission products entering the containment, and effectively control the release of non-volatile fission products to the environment.The results of this paper can provide reference for the formulation of emergency measures and the evaluation of the consequences of external sources.