The fission product 110mAg released from coated particles can be the dominant source of radioactivity from the core of a high-temperature reactor in the early stages of the reactor life and possibly limits the accessibility of primary circuit components. It can be shown that silver is retained in oxide fuel by a diffusion process (but not in carbide or carbon-diluted fuel) and that silver is released through all types of pyrocarbon layers. The retention in Triso particles is variable and seems to be mainly connected with operating temperature and silicon carbide quality.