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Floating nuclear power stations are vessels with nuclear reactors, designed to power offshore oil and gas drilling, island development and remote areas. The safety of the facilities is an important issue. The Leak-Before-Break (LBB) assessment is essential to the design arid evaluation of nuclear power plants against a sudden double-ended guillotine break of pipes. This paper describes the LBB assessment procedure applicable to the nuclear-class pipe of the floating nuclear power station. The loads considered in the analysis include variation of temperature/pressure, swing and underwater impact. Circumferential cracks are postulated at the dangerous positions of the pipe. The fatigue propagation of the surface crack is predicted based on the fracture mechanics and the finite element method to determine the time that pipe leaking happens. The critical length of the through-wall crack is calculated by the J integral tearing modulus approach, and then compared with the minimum detectable crack length for the purpose that unstable fracture would not be happened before the leakage detected. According to the results of the analysis it could conclude that the pipe fulfils the LBB requirements.