Abstract: Based on the Leak-Before-Break （LBB） Analysis Method, this paper performs a fracture mechanics analysis on the intermediate loop of the decay heat removal system in China Experimental Fast Reactor（CEFR）. Assuming that before the operation of the reactor a throughwall defect had existed where the loads were most serious, according to J-integral tearing modulus inter-section method, the critical length of crack in the worst condition （namely synthesized enveloped operation conditions） was obtained. With Paris formula, the stability expansion of the crack was calculated under alternating loads. The open area of the critical crack as well as that of the life-end crack were obtained by a mechanical model established by CASTEM2000 code. It is proved that the biggest crack open area of the tube is less than DT/4 （D means pipeline diameter,and T tube wall thickness） at the end of its life.