| With the aggravation of energy crisis and the increasing demand of human life,the demand for clean energy such as nuclear power is becoming more and more urgent.Nuclear fuel generates heat in a nuclear reactor.By heating the water outside the fuel rod cladding,the steam generated turns a turbo generator to generate electricity.As an important part of a nuclear reactor,the heat transfer performance(HTP)and structural stability of fuel rod cladding in high temperature and high pressure environment are the key to the safe and stable operation of a nuclear reactor.At present,zirconium(Zr)alloy is the most widely used nuclear fuel cladding material,but Zr alloy cladding material still faces some problems such as corrosion,hydrogen embrittlement and deterioration of HTP.Surface treatment of Zr alloy is a method to improve the properties of Zr alloy.In particular,if a kind of hydrophilic coating can be formed on the surface of Zr alloy,it is expected to improve the water absorption capacity of the cladding layer and prevent the formation of gas film on the surface of the cladding and the film boiling,which is of great significance for preventing the explosion caused by the deterioration of heat transfer in the cladding.In order to improve the HTP of fuel rod cladding materials for nuclear reactors,concave chromium(Cr)coatings and micron Cr coatings were synthesized on stainless steel and Zr alloy substrates by electrochemical method.The surface morphology,structure and wettability of the samples were characterized by SEM,XRD and contact angle analyzer.The HTP of the samples with good wettability and large specific surface area was also studied.The HTP of two kinds of Cr coating surface and smooth surface was compared,and the reason of improving the HTP of the coating was analyzed.The main research results are as follows:1 Cr coating was grown on the surface of stainless steel substrate by electroplating technique under the condition of current density of 5 A dm-2.It was found that the sample electroplated for 2 h had the best appearance and the most smooth microstructure,and the thickness of the Cr coating was 37?m.A concave Cr coating was formed on the surface of stainless steel by anodizing the 2 h Cr coating.The diameter of the coating is about 7.4?m,and the contact Angle is 41.8°,which is about20°lower than that of the substrate.2 The HTP of concave Cr coating surface and original surface was tested by self-designed micro-channel flow boiling test system.The results show that under the conditions of mass flow rate of 33.3 kg m-2s-1,50 kg m-2s-1and inlet temperature of40?,the heat transfer coefficient(HTC)of stainless steel can be improved by 22.84%and 21.28%on the surface of concave Cr coating.The improved HTC is due to the enhanced wettability and specific surface area of the concave structure,which provides more core sites for vaporization.The analysis of pressure and pressure drop fluctuation shows that concave Cr coating can effectively reduce pressure drop fluctuation in microchannels.3 Cr microparticle coating was prepared on the surface of Zr alloy by electrochemical method.The effect of different current densities on the coating morphology was studied.It was found that the morphology of Cr coating was all granular under different current densities.When the current density is higher,the Cr micron particle coating is uniformly distributed and the particle density is larger,and the particle diameter is about 3.6?m.When the current density is low,the coating of Cr micron particles is sparse,which exposes a large area of Zr alloy substrate to the air.It may be that the current density is too small to cause the phenomenon of leakage plating.At this time,the particle diameter is about 6.1?m.4 The HTP of the samples prepared with the current density of 5 A dm-2with good wettability and large specific surface area was tested.Under the conditions of mass flow rate of 33.3 kg m-2s-1and inlet temperature of 60?,the two-phase HTC can be improved by 14.5%on the surface of Cr micron particles.The HTP of Cr coating surface is enhanced due to its unique rough structure,which increases the specific surface area and improves the surface wettability with fluid.In this paper,two kinds of Cr coatings with different morphologies on stainless steel and Zr alloy were synthesized by simple electrochemical method,and their applications in HTP were preliminariously explored.These results have important reference value for improving the HTP of cladding materials and the application of cladding materials in nuclear reactors. |
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