如图 1所示，COPERNIC和FORWARD计算的包壳-芯块间隙尺寸随燃耗的变化对比情况。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。COPERNIC和FORWARD计算的间隙尺寸变化趋势是一致的，但是两者在发生芯块-包壳间隙闭合的时间有差异，COPERNIC计算出的闭合时间要小于FORWARD。

As shown in Figure 1, the change of cladding pellet gap size calculated by COPERNIC and FORWARD with fuel consumption is compared. 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. The change trend of gap size calculated by COPERNIC and FORWARD is consistent, but there is a difference between them in the time of pellet cladding gap closure. The closure time calculated by COPERNIC is less than FORWARD.