[en] The steam generator in a PWR primary coolant system is one of the pieces of equipment made in China for the Qinshan nuclear power plant, Zhejiang. It is a crucial unit belonging to the category of nuclear pressure vessel. The purpose of this research work is to carry out an examination of the weld quality of the steam generator under fatigue loading and to assess its reliability by using experimental results of a fatigue test of the nuclear pressure vessel steel S-271 (Chinese Standard) and of qualified tests of welded seams of a simulated prototype of the bottom closure head of the steam generator. A guarantee of weld quality is proposed as the quality assurance of safety for the China National Nuclear Safety Supervision Bureau. The results of the reliability analysis reported in this work can be taken as supplementary material for a Probabilistic Risk Assessment (PRA) of the Qinshan nuclear power plant. According to the requirement of Provision 2-1500 CYCLIC TESTING, ASME Boiler and Pressure Vessel Code, Section 3, Rules for Construction of Nuclear Power Plant Components, a simulated prototype of the bottom closure head of the steam generator was made in this work for the qualified tests. Qualified tests with small sample size present a problem which is difficult to solve in reliability analysis, and are therefore of interest. Here, the authors offer proposals attempting to solve this problem