Pb-Bi-cooled direct contact boiling water fast reactor (PBWFR) can produce steam from the direct contact of feedwater and lead bismuth eutectic (LBE) in the chimney of 3 m high, which eliminates the bulky and flimsy steam generators. Moreover, as the coolant LBE is driven by the buoyancy of steam bubbles, the primary pump is not necessary in the reactor. The conceptual design makes the reactor simple, compact and economical. Owing to the large thermal expansion coefficient of LBE and good performance of steam lift pump, the reactor is expected to have excellent passive safety. A computer code was developed to investigate the thermal-hydraulic behaviors and safety performance of PBWFR. In order to test and verify its safety performance, the following accidents without scram were simulated with the code: Unprotected rod run-out transient over power (UTOP), unprotected loss of heat sink (ULOHS), simultaneous UTOP and ULOHS accident. The results show that PBWFR has very good inherent safety due to the satisfactory neutron and thermal-physical properties of LBE. Cladding materials turn to be the key factor to restrict its safety performance and UTOP is more risky for PBWFR than the others.

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