Cr涂层对板状燃料元件起泡特性影响数值模拟

Numerical Simulation on Effect of Cr Coating on Blistering Behavior of Plate-type Fuel Element

  • 摘要: 鉴于Cr涂层能够有效地缓解棒状燃料元件包壳在失水事故时的鼓胀现象,本文提出将Cr涂层应用于板状燃料元件以抑制其起泡的方案。为研究Cr涂层对板状燃料元件起泡现象的抑制作用,本文采用有限元分析工具,分别添加Zr和Cr涂层的材料物性,并根据起泡现象的特征,考虑了Zr和Cr的热蠕变和受辐照硬化后的塑性行为。采用开发的工具对起泡退火实验进行了模拟,分析了温度和厚度对Cr涂层抑制作用的影响。研究结果表明:Cr涂层的屈服极限较低,在起泡现象发生时容易进入塑性阶段,从而不能有效地阻止起泡现象的产生;但其在大于700 K的高温下,较低的蠕变率能够降低起泡的高度,且温度越高降低的效果越明显。因此,Cr涂层可以抑制包壳的蠕变以降低起泡的高度。

     

    Abstract: Blistering usually occurs in plate-type fuels because of the absence of the gap between the cladding and the fuel meat. It can narrow coolant channels and deteriorate the local heat transfer, resulting in the increase of cladding temperature and plastic deformation, which threatens the integrity of fuel elements. Much research work mainly focuses on the causes of blistering and its effect on fuel elements, while few measures to reduce blistering were proposed. In view of the effect that Cr coating can effectively reduce the ballooning of fuel rod cladding during loss of coolant accident (LOCA), a design to apply Cr coating to plate-type fuels to restrain their blistering was proposed. In order to study the effect of Cr coating on the blistering of plate-type fuel elements, the simulation of a blistering test was conducted based on the MOOSE framework. The material properties of Zr and Cr were added, and the thermal creep, as well as plastic behavior after irradiation hardening was considered according to the characteristics of blistering. In addition, the effect of temperature and thickness of Cr coating on blistering were also analyzed and some important results were obtained. The simulation of the blistering test indicates that the reduction of blistering for the plate-type fuel cladding by Cr coating is not as significant as that of ballooning for the fuel rod cladding. The reason for the difference and the difference between the two phenomena were analyzed. The results also show that due to its low yield stress, Cr coating is likely to enter the plastic stage when the cladding blisters, which cannot effectively prevent the occurrence of blistering. However, owing to the low creep rate of Cr even at a relatively high temperature, the height of blistering can be reduced, and the higher the temperature, the more obvious the reduction effect. Under the premise of keeping the overall thickness of the cladding and the coating unchanged, even if the presence of Cr coating will reduce the thickness of cladding and thus reduce its mechanical strength, it can still effectively reduce the blistering height at high temperatures, and the thicker the coating, the more obvious the reduction effect. Nevertheless, adding Cr coating to the cladding of the original thickness has the most significant effect. From the results above, the main conclusion can be drawn: The thin coating can hardly affect the overall mechanical properties of the cladding, but it can effectively inhibit the creep, thereby reducing the blistering height.

     

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