Abstract: In nuclear reactors, the fuel cladding is exposed to high temperature and high pressure for a long period of time. Chalk River Unidentified Deposits (CRUD) will form on the surface of the fuel cladding during the conventional operation of pressurized water reactors, and the formation of the CRUD will affect the flow heat transfer on the surface of the fuel rods. In order to investigate the effect of surface fouling on the flow characteristics of the CRUD, as well as to explore the influence of cladding surface deposition layer on the active nucleation site density (NSD), the present study was based on the flow-boiling under atmospheric pressure. The flow boiling visualization experiment was conducted to simulate the actual fuel rod cladding with CRUD by using layer-by-layer deposition of SiO₂, under two mass flow rates (0.12 m/s and 0.17 m/s) and three degrees of subcooling conditions (0, 3, 5 K), investigating the flow boiling heat transfer characteristics of fuel cladding Zr-4 non-deposition with two Zr-4 SiO₂ depositions (1 μm and 3 μm). Focus on the relationship between the active nucleation site density Na with wall superheat, and analyze the main reasons for the differences under different operating conditions, and contrast differences in the active nucleation site density on the different of SiO₂ deposited thicknesses, then compare it with existing models for the active nucleation site density. The results show that the deposited Zr-4 has a higher flow heat transfer capacity than the undeposited Zr-4, and this difference is mainly caused to the difference in surface porosity. The active nucleation site density increases on SiO₂ deposited surfaces compared to undeposited surfaces, with a maximum in the 3 μm SiO₂ deposited experimental group. Enhancing wall superheat increases the active nucleation site density, and the increase is more pronounced on surfaces with SiO₂ deposits. For the same sample, under the condition that one of the degree of subcooling and flow rate is the same and the other is different, the difference in flow rate has a greater effect on the active nucleation site density than the degree of subcooling, and the increase in both flow rate and the degree of subcooling decreases the active nucleation site density. Based on the experimental data, nine prediction models were used for calculation and analysis, and the Končar model can better predict the active nucleation site density under the working conditions of this experiment.