Yuanzineng kexue jishu (Oct 2023)
Experimental Study of Flow Boiling in Layer-by-layer Deposition of SiO2 to Simulate CRUD Layer on Surface of Fuel Cladding
Abstract
The fuel cladding in nuclear reactors is subjected to long periods of high temperature and pressure operation, and a certain degree of reaction control is accomplished by adding chemicals such as boric acid to the coolant, which puts the fuel cladding in a chemically corrosive environment; in addition, in the pressurized water reactor first circuit, the cooling mass flows through the steam generator and stainless steel coolant piping, which are mainly made of nickel-based alloys, and even though these metal vessels or pipes have been passivated to produce a dense oxide film, which greatly reduces the rate of corrosion reactions, metal ions are still inevitably released into the coolant. Therefore, in practice, Chalk River unidentified deposits (CRUD) are naturally formed on the surface of the fuel cladding. It has been found that the CRUD layer is a porous hydrophilic layer with certain surface characteristics such as porosity, good wettability, as well as certain capillary characteristics. Considering the presence of CRUD layer on the fuel cladding, it is important to clarify the mechanism of the CRUD layer on the heat transfer characteristics of the flow in the reactor to guide the routine operation and maintenance of pressurized water reactors as well as the development of thermal safety design benchmarks. It is difficult to obtain CRUD layer directly. Some studies have shown that the SiO2 deposited layer has some similarity to the CRUD layer. Therefore, SiO2 deposited layer can be used to simulate CRUD layer. Based on the above analysis, zirconium-4 alloy was used in this paper as a substrate and deposited SiO2 deposition layers of different thicknesses on its surface by layer-by-layer deposition method to simulated CRUD through the SiO2 deposited layers. The surface morphological parameters of the SiO2 deposition layers, including contact angle, surface porosity and surface roughness, were subsequently measured. Experimental studies were carried out on a flow boiling experimental platform to compare and analyzed the effects of different thicknesses of SiO2 deposited layer on the flow heat transfer characteristics, and provided a reference for clarifying the heat transfer characteristics of the deposited layers on the fuel envelope surface. The SiO2 deposited surface shows better flow heat transfer characteristics and higher critical heat flux than the non-deposited surface, which is mainly due to the difference in surface wettability. The enhancement of heat transfer capability also differs between different thicknesses of SiO2 deposited surface, with the enhancement effect of 3 μm SiO2 deposited surface being more obvious compared to 1 μm SiO2 deposited surface, especially at the subcooling degree of 0 K, the critical heat flux increased by 77.4%.
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