Tritium (T) is a costly radioactive element that, when retained in plasma-facing materials (PFMs), not only results in fuel loss but also raises issues of radioactive contamination. Hydrogen isotope exchange is a potential method for T removal in future fusion devices. However, in the nuclear fusion environment, PFMs will be subjected to low-energy and high-flux helium (He) plasma irradiation, forming a He bubble layer near the material surface. This greatly impacts the diffusion and retention behavior of hydrogen isotopes in PFMs. In this work, a multi-component hydrogen isotopes exchange model was developed considering the presence of He bubble layers. The influence of He bubbles location on hydrogen isotope retention and exchange behavior was investigated. It is found that when the hydrogen (H) particles implanted outside the He bubble layer, H entering the material would diffuse across the He bubble layer, resulting in a decrease of H amount entering the bulk. In other words, a thicker He bubble layer leads to lower H retention. As to isotope exchange, when there are deuterium (D) retained in the material, implanted H has a possibility to exchange with D, making D release from the materials. However, the D atoms exchanged out by H also need to diffuse across the He bubble layer, further reducing the D release rate. The results showed the barrier effect of the He bubble layer can have two distinct effects on hydrogen isotope exchange. One is that with enhancing the effect of the He bubble layer, the number of H entering the bulk will increase, resulting in an increase in the D removal. The other effect is that with strengthening the barrier effect of the He bubble layer, the D atoms exchanged out will be more inclined to diffuse into the bulk, leading to a decrease in the D removal. As a result, the D removal efficiency exhibits a peak considering the barrier effect of He bubble.

中文翻译：

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氦气泡位置对面向等离子体材料中氢同位素保留和交换行为的影响：数值模拟研究

氚 (T) 是一种昂贵的放射性元素，当它保留在面向等离子体的材料 (PFM) 中时，不仅会导致燃料损失，还会引发放射性污染问题。氢同位素交换是未来聚变装置中去除 T 的潜在方法。然而，在核聚变环境中，PFM将受到低能、高通量氦（He）等离子体辐照，在材料表面附近形成He气泡层。这极大地影响了 PFM 中氢同位素的扩散和保留行为。在这项工作中，考虑到 He 气泡层的存在，开发了多组分氢同位素交换模型。研究了He气泡位置对氢同位素保留和交换行为的影响。研究发现，当氢（H）粒子注入He气泡层外部时，进入材料的H会扩散穿过He气泡层，导致进入材料的H量减少。换句话说，较厚的 He 气泡层会导致较低的 H 保留。对于同位素交换，当材料中保留有氘（D）时，注入的H就有可能与D进行交换，使D从材料中释放出来。然而，被H交换出的D原子也需要扩散穿过He气泡层，进一步降低了D的释放速率。结果表明，He气泡层的阻挡效应对氢同位素交换有两种不同的影响。一是随着He气泡层效果的增强，进入本体的H数量会增加，导致D去除量增加。另一个作用是，随着He气泡层势垒效应的增强，交换出的D原子更容易扩散到本体中，导致D去除量减少。结果，考虑到 He 气泡的势垒效应，D 去除效率呈现出峰值。