如图1所示，包壳-芯块间隙尺寸随燃耗的变化情况。由于在芯块向外热膨胀、芯块向外肿胀以及包壳向内蠕变的综合作用过程中，芯块与包壳的间隙尺寸逐渐减小。在寿期初，芯块-包壳间隙尺寸有个先增大后减小的过程，分析原因间隙尺寸增大是芯块密实化的影响，后续间隙尺寸减小则是由于芯块热膨胀、芯块辐照肿胀以及包壳蠕变的综合作用大于芯块密实化的作用造成的。在寿期末，间隙尺寸存在突变，分析原因是功率的突降导致温度降低，使得热膨胀效应减弱，从而使得间隙尺寸增大。

As shown in Figure 1, the change of cladding pellet clearance size with fuel consumption. Due to the combined effect of the outward thermal expansion of the pellet, the outward swelling of the pellet and the inward creep of the cladding, the gap size between the pellet and the cladding gradually decreases. At the beginning of the service life, the pellet cladding gap size has a process of increasing first and then decreasing. It is analyzed that the increase of the gap size is due to the influence of the pellet densification, and the subsequent decrease of the gap size is due to the combined effect of the thermal expansion of the pellet, the radiation swelling of the pellet and the creep of the cladding is greater than the effect of the pellet densification. At the end of the service life, there is a sudden change in the gap size. It is analyzed that the reason is that the sudden drop in power causes the temperature to decrease, which weakens the thermal expansion effect, and thus increases the gap size.