Abstract: Chinese helium-cooled solid breeder (CH HCSB) test blanket module (TBM) with helium cooling system and secondary cooling water system was modeled and thermal-hydraulic behavior and safety performance of the system were assessed using the RELAP5/MOD3.4 code. According to the accident sequences of ITER accident analysis specification for TBM, the transient analysis of the design basis ex-vessel coolant pipe break accident was carried out. The influences of different break locations, leak areas and plasma shutdown processess on the first wall of TBM were compared. The results indicate that it is much more danger when the pipe break occurs at the downstream side of the helium circulator compared with that at upstream side. The results also show that the accident consequence is worse in case of smaller area break than that in case of larger area break. In case of much more severe accident that the ex-vessel break leads to the break of TBM the first wall, the results reveal that the decay heat can be removed to cool down TBM by natural circulation and radiation. The first wall melting can be avoided if the method to shutdown plasma within 3 seconds in case of ex-vessel break is adopted.