Background: Unlike the traditional solid fuel reactor with fixed boundary conditions

the inlet and outlet pipe and the core of molten salt reactor fuel is connected so that the flowing liquid fuel can travel freely between the pipe and core.Purpose: This article has made systematic study of the influence of different molten salt fuel regions on reactor physics

including the top and bottom fuel of the core vessel and the pipe fuel.The core physics was researched under different boundary conditions

and the region of the effective core was indicated subsequently.Methods: MSRE was taken as the reference reactor and the calculation was completed based on the Monte Carlo Code MCNP.Results: Results show that the fuel in the top and bottom of vessel impacts on keff and energy spectrum obviously.The influence of outlet pipe on keffwas negligible when pipe radius less than 25 cm

and the perturbation of outlet pipe on the keffcould be neglected when its length more than 20 cm.Conclusions: Results provide rational theory for the design of MSR and the development of computation code.