Abstract: To quantify and estimate the risk of fission products release from Xi’an Pulsed Reactor(XAPR), the special technical essential of XAPR Level 2 Probabilistic Safety Assessment (PSA) was studied, and accident progression and confinement response were analyzed. Then the source terms analysis of XAPR for internal initial event during 2 MW full power operation condition was performed. The results show that in all release categories (RC), the contribution of confinement intact RC01 which is representative of success sequence is about 97.52%. In all abnormal release categories, the confinement seal failure RC02 gives a high contribution of 75.81%. The results also demonstrate that the highest frequency of all categories is from RC01 which is about 4.80×10^-6/(reactor·year）, but its release quantity is the smallest which is 10⁹~10¹³ Bq. The largest release quantity is about 10¹⁴ Bq from RC06 which is described as confinement seal failure and confinement by-pass sequence, and the release frequency of RC06 is about 1.38×10^-8/(reactor·year）. Based on the results, suggestions are given that the ventilation systems should be shut down to prevent fission products from leaking through the stack and the reactor building.