Experimental research on influencing factors of temperature stratification in suppression pool
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摘要: 对于小型模块式反应堆，可采用安全壳抑压装置限制失水事故引起的安全壳快速升温升压。然而随着排放质量流率及水池水温的变化，水池中可能出现温度分层现象，进而降低传热传质效果。建立了抑压排放水池温度分层实验装置，开展了蒸汽质量流率、鼓泡器淹没深度及气水容积比对水池温度分层特性影响的实验研究。结果表明：在较宽的蒸汽质量流率范围内，水池内均发生了温度分层现象，随着质量流率增大，对分层的影响程度减弱，冷热交界面下移，带动更多水体参与热量交换；鼓泡器淹没深度增加导致热交界面位置下移，水体搅动作用增强，从而提高水体冷却利用率；气水容积比增大，冷热交界面下移，受扰动区域增大。Abstract: For small modular reactors, containment suppression devices can be used to limit the rapid increasement of temperature and pressure in the containment caused by loss of coolant accidents (LOCAs). However, temperature stratification may occur in the suppression pool with the change of the mass flow flux and water temperature during the process of discharge, which reduces the heat and mass transfer. In this paper, an experimental facility for temperature stratification of suppression pool is established. The effects of steam mass flow flux, submerged depth of sparger and gas-water volume ratio on the temperature stratification characteristics in the pool are studied. Results show that there is temperature stratification in the pool at a wide range of steam mass flow flux. With the increase of mass flow flux, the influence on stratification is weakened, and the thermal interface moves down, leading to more water participating in heat exchange. With the increase of the submerged depth of sparger, the position of thermal interface moves down and the stirring effect of water is enhanced. As the gas-water volume ratio increases, the thermal interface moves down and the disturbed area enlarges.
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Key words:
- suppression pool /
- mass flow flux /
- thermal interface /
- temperature stratification
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随着科学技术的发展，核技术具有零碳排放、能源独立、安全等诸多优势，在人类社会中的地位越来越重要。然而，核辐射事故却为核技术发展迅速蒙上了一层阴影。1986年，苏联切尔诺贝利核电站发生了迄今为止人类历史上最严重的核辐射事故[1]。2011年，日本东北海岸发生了里氏9.0级的强烈地震和海啸，造成了福岛第一核电站的1～3号机组反应堆熔毁[2]。由于反应堆内部高温和高辐射等极端环境，人类无法直接进入进行勘察和处置工作，因此在福岛事故中使用了多种类型和功能的机器人。光纤激光器具有高功率、高光束质量，光束可以远距离柔性传输等优点，可以用于无人区开展激光切割救援等工作[3]。比如Shin等人研究了用10 kW光纤激光器拆除核设施的150 mm厚的厚钢板和大型管道的切割性能[4]。当然，光纤激光器在辐射环境中也会受到影响[5]，高能射线会导致增益光纤产生色心等各类缺陷，这些缺陷引起的额外光吸收增加了传输损耗，降低了光纤激光器性能。
课题组基于光纤激光器存在的自漂白效应，利用60CO辐照源探索不同辐照剂量率下的光纤激光器暗化与自漂白的平衡关系。实验先采用低功率光纤振荡器进行不同辐照剂量率下激光器输出功率演化和去辐照后自漂白研究。使用的光纤激光振荡器实验结构如图1所示，谐振腔由常规商业掺镱光纤（YDF）、高反射光纤光栅（HR-FBG）、低反射光纤光栅（OC-FBG）构成，中心波长为976 nm的泵浦源（LDs）通过前向（2+1）×1泵浦信号合束器（FPSC）注入到谐振腔中，激光经过包层光滤除器（CLS）后由光纤端帽（QBH）扩束输出。
首先，利用较高辐照剂量率研究在去辐照后的自漂白效应，结果如图2（a）所示。图2（a）的(I)为未辐照阶段，持续时间为680 s，由于水冷机周期性制冷使得功率计温度周期变化导致测试激光功率也存在周期变化，激光器功率起伏为1.44%；需要注意的是，这个是主要功率测量误差导致，并不是激光器本身功率起伏。图2（a）中(II)为辐照阶段，在总辐照时间298 s内，辐照总剂量为14 900 rad，激光器输出功率从150 W下降至105 W。图2（a）的(III)为去辐照后的自漂白阶段，在光纤激光器的泵浦光子与热效应的共同作用下，激光器输出功率从118 W恢复di至145 W，与初始功率相差仅5 W，表明自漂白效应可以较为有效地恢复由于辐照导致的激光功率下降。
图 2 光纤激光器在辐照环境下的输出特性Figure 2. Output characteristics of the optical fiber laser in an irradiated environment然后，为了探索不同剂量率的自漂白与在线辐照相互作用是否可以达到平衡，开展了不同剂量率的对比研究，结果如图2（b）所示。图2（b）中，总辐照剂量为2 400 rad，红色、蓝色曲线分别对应辐照剂量率为50 rad/s、1 rad/s时激光器归一化输出功率演化情况；在辐照剂量率为50 rad/s时，激光输出功率下降了3%；在辐照剂量率1 rad/s时，功率起伏1.22%，考虑到这里的周期性起伏主要由于水冷机周期性制冷导致，可以认为在低辐照剂量率下，光纤激光器自漂白导致的功率提升与辐照导致的功率下降基本达到平衡。
进一步地，基于图2（b）的实验结果，我们验证了1 kW级光纤激光器中自漂白与辐照平衡的实验现象。在辐照剂量率为0.1 rad/s时，激光器输出激光功率曲线演化如图2（c）所示。从实测功率曲线来看，在总辐照剂量为190 rad的整个辐照过程中，光纤激光器的输出功率都稳定在1 050 W以上，即使考虑前述由于水冷机导致的功率变化，激光器的功率起伏在1.79%以内。如果不考虑水冷机周期性制冷影响，激光器的功率起伏在0.66%以内。
实验首次验证了在一定辐照剂量率下，光纤激光器自漂白效应导致的激光功率提升可以平衡辐照效应导致的功率下降，为相关场景应用的光纤激光器设计提供了有效支撑。后续，我们将继续深入相关研究，探索不同类别、不同结构激光器辐照与自漂白平衡的机理、阈值和可能的应用。
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图 2 不同蒸汽质量流率下水池温度瞬态图
Figure 2. Temperature transient diagram of pool at different steam mass flux
图 3 冷热交界面机理形成示意图
Figure 3. Schematic diagram of formation mechanism of thermal interface
图 4 淹没深度2.2 m时水池温度瞬态图
Figure 4. Temperature transient diagram of pool at submerged depth of 2.2 m
图 5 不同淹没深度下水池高度方向温度分布
Figure 5. Temperature distribution in pool height direction at different submerged depths
图 6 气水容积比0.56时水池温度瞬态图
Figure 6. Temperature transient diagram of pool while gas-water volume ratio equals 0.56
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