The microscopic effects of He irradiation on the H isotope retention in W were quantitatively evaluated using two types of high-function transmission electron microscopes: an ion-gun directly coupled TEM equipped with a high-resolution quadrupole mass spectrometer and an aberration-corrected scanning TEM combined with electron energy-loss spectrometer (STEM-EELS). He pre-irradiation induced fine and dense bubbles near the surface of the samples, thereby significantly increasing the deuterium (D) retention. The STEM-EELS analysis revealed that most of the D atoms were trapped and distributed homogeneously in the bubbles and released at relatively low temperatures without microstructural changes with increasing temperature. In contrast, some of the He was dissociated from the bubble by D post-irradiation in the matrix with a metastable state and retrapped by the bubble as the temperature was increased.
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