The departure from nuclear boiling (DNB) is an essential point for the heating channel with water flowing through, because when it happens, the water near the heated wall can be dried out, which will cause the material temperature exceeding the upper limits. Presently, the blanket is under design phase, and some essential parameters are uncertain, including the heat flux on the first wall (FW). For the sake of thermal safety, the occurrence conditions of DNB are analyzed solely, regardless whether material temperature exceeds the limits or not, in the FW of water-cooled ceramic breeder blanket for Chinese Fusion Engineering Reactor. This paper is useful for further blanket thermal design. Based on the lookup table that is summarized by experiments for the uniformed heating circular tube of 8 mm diameter, the local critical volumetric fraction of vapor, which acts as the evaluation criteria for the U-shaped FW of square channel with one-sided heating, is obtained using a computational fluid dynamics approach under different DNB operating conditions. After the comparison of phase distribution between different turbulence models, the K-Omega model is selected to do the calculation. The effects of coolant inlet temperature, mass flow rate, and operating pressure on DNB value are evaluated. Under the same operating pressure, either increasing mass flux or decreasing inlet temperature can reduce the critical vapor fraction. On the contrary, the DNB is enlarged effectively. Under the same inlet conditions, the lower operating pressure can increase DNB because of the higher latent heat of vaporization.