聚变堆PFC结构缝隙导致的氢同位素加速渗透现象研究

In the PFC design of ITER and other fusion reactors, castellated tile structure with gaps has been utilized to relieve thermal stress and the load from electromagnetic force. Although the angle between the gaps and magnetic lines can prevent charged particle from entering the gaps, high flux neutral tritium (T) atoms from boundary plasma can still hit the structural material at the bottom of the gaps and penetrate into the coolant loop by super permeation. Because of the relatively higher diffusivity and permeability of hydrogen isotopes in structural materials than in plasma-facing material (i.e. W) as well as the thin thickness of coolant tube, the permeation of T into PFC from this “short cut” may be much faster and the flux may be higher than previous estimation, which will lead to further economy and safety concerns of fusion reactors. The accelerated T permeation process is related to the behavior of plasma, the structure of components and the properties of materials. Due to the complexity, this issue has not been well studied in fusion society. In this proposed research, the mechanisms of hydrogen isotopes permeation through PFC will be investigated by performing experiments on PFC modules in the EAST tokamak as well as linear plasma facilities. Using code simulation, T analysis will be extended to the PFC of CFETR and the PFC design will be optimized from the view point of reducing T leakage.

当前ITER以及其它聚变堆的面向等离子体部件（PFC）设计都采用有缝隙的瓦块结构以缓解热应力和电磁力作用。虽然这些缝隙与磁力线有一定夹角，避免了带电粒子沿磁力线进入，但边界等离子体中还有高通量的中性氚原子不受磁场约束，可能通过缝隙直接轰击PFC底部的结构材料，并以超渗透方式进入冷却回路。由于氢同位素在结构材料中的扩散率和渗透率相对于作为面向等离子体材料的钨更高，且管道壁厚度较小，氚从该“捷径”渗透进入PFC的速度和通量都将远高于轰击钨表面的情况，进而影响聚变堆的经济性和安全性。氚在PFC中的加速渗透过程涉及等离子体的行为、部件的结构和材料的性质等，机理较为复杂，国际上尚缺乏充分研究。本项目针对上述影响要素，利用EAST托卡马克、平台装置深入开展部件模块的氢同位素渗透行为机理研究。结合计算模拟，相关研究还将拓展至CFETR部件的氚渗透分析，并从降低氚泄漏角度为PFC的优化设计提供支持。

未来聚变堆中氢同位素（氘、氚）可能通过等离子体驱动渗透穿过第一壁缝隙进入冷却剂，造成冷却剂污染、氚损失、大量放射性废水等一系列问题。聚变堆部件的结构设计和等离子体-材料相互作用过程都会对氚在第一壁的渗透滞留行为产生影响巨大。本项目借助等离子体装置的原位渗透测量平台以及热脱附谱平台，研究了实际部件的设计结构、材料种类、钨的再沉积、杂质粒子的损伤以及同位素效应的作用等。研究发现：毫米级第一壁缝隙将导致氚渗透量的量级提高，低活化钢和钒合金的使用进一步加剧渗透，而沉积层的影响与其厚度有关，氦损伤则从表面形貌和晶格损伤两方面影响渗透行为。相关成果厘清了各种影响因素的作用机制，积累了大量基础数据，为预测ITER以及未来聚变工程示范堆的氚泄漏量提供了有力支持。