Stress Mitigation Design of a Tubesheet by Considering the Thermal Stress Inducement Mechanism
Stress Mitigation Design of a Tubesheet by Considering the Thermal Stress Inducement Mechanism
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DOI：
10.1115/1.4024618
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发表时间：
2013-12
影响因子：
1
通讯作者：
M. Ando;Hideki Takasho;Nobuchika Kawasaki;N. Kasahara
中科院分区：
文献类型：
--
作者：
M. Ando;Hideki Takasho;Nobuchika Kawasaki;N. Kasahara
日本计划在下一代快堆(FBR)中采用Mo.9Cr-1Mo钢制双壁直管蒸汽发生器(SGS)。SG的主要问题之一是管板的结构完整性。在瞬变过程中，管板内的温度分布可能会引起最大热应力，而应力的大小取决于管板的形状。为此，通过有限元分析研究了管板的应力产生机理。首次研究了半球型管板模型的热应力机理。半球形管板模型的计算结果表明，在最外面的孔附近存在广泛的峰值应力。公认的半球形管板热应力机理如下：(1)穿孔区域与周围区域温差引起的环向应力是主要的热应力。(2)热应力对特定部位的大小不敏感，但主要受穿孔区域与周围区域的相互作用机制控制。(3)孔边应力集中产生峰值应力。(4)峰值应力的大小与管板的穿透角度有关，在最外面的孔处应力集中最大。基于上述应力产生机理，我们提出了一种应力缓和管板--中心扁球管板(CFST)作为一种改进的结构形式。计算的钢管混凝土管板的峰值应力小于半球形管板的峰值应力。进一步的研究揭示了钢管混凝土在热瞬变过程中的详细应力产生机理。事实上，对于钢管混凝土，观察到了两种不同的可比较的温度峰值应力机制。最大峰值应力的位置和大小取决于热瞬变过程中的蒸汽温度历史。结构不连续引起的径向应力位于最外面的孔处，与蒸汽温度变化率(dt/dt)有关。在第一内层孔(相对于最外层孔)处，穿孔区域与周围区域相互作用产生的环向应力取决于蒸汽温度变化的范围(ΔT)。
Adoption of double-wall straight-tube steam generators (SGs) made of Mod.9Cr-1Mo steel is planned for next-generation fast breeder reactors (FBRs) in Japan. One of the major concerns with the SG is the structural integrity of the tubesheet. During a transient event, a maximum thermal stress may be induced by the temperature distribution in the tubesheet, and the magnitude of the stress depends on the configuration of the tubesheet. Therefore, the stress generation mechanism of a tubesheet was studied through finite element (FE) analysis. Semispherical tubesheet models were investigated for the first survey of the thermal stress mechanism. The calculated results of the semispherical tubesheet model indicated an extensive peak stress around the outermost hole. The recognized thermal stress mechanism of a semispherical tubesheet is as follows: (1) The dominant thermal stress is hoop stress caused by the temperature difference between the perforated and surrounding regions. (2) The thermal stress is insensitive to the size of the specific portion, although it is dominated by an interaction mechanism between the perforated and surrounding regions. (3) The stress concentration around the edge of the holes generates a peak stress. (4) The amplitude of the peak stress depends on the tubesheet penetration angle, and the stress concentration becomes greatest near the outermost hole. Based on the above stress generation mechanism, we proposed a stress-mitigated tubesheet, a center-flattened spherical tubesheet (CFST), as an improved configuration. The calculated peak stress of the CFST was smaller than that of the semispherical tubesheet. Further investigation revealed the detailed stress generation mechanism of the CFST during a thermal transient. There were, in fact, two different comparable thermal peak stress mechanisms observed for the CFST. Both the location and magnitude of the maximum peak stress depended on the steam temperature histories during the thermal transient. The radial stress caused by structural discontinuity, which was located at the outermost hole, depended on the rate (dT/dt) of the steam temperature change. The hoop stress caused by the interaction between the perforated and surrounding regions, which occurred at the first inner layer hole (with respect to the outermost layer holes) depended on the range (ΔT) of the steam temperature change.