The in-vessel deuterium retention in Alcator C-Mod from a run campaign of about 1090 plasmas is estimated to be about 0.1 gram. This estimate is based on nuclear reaction analysis of deuterium retained on 22 molybdenum tiles from the inner wall and divertor. Most of the retained deuterium is on the walls of the main plasma chamber, only about 2.5% of the deuterium is in the divertor. The D coverage is consistent with a layer saturated by implantation with ions and charge-exchange neutrals from the plasma. Tritium coverage was below the detection limit of 10/sup 8/ atoms/fm/sup 2/. This contrasts with tokamaks with carbon plasma-facing components (PFCs) where long-term retention of tritium and deuterium is large and mainly in the divertor due to codeposition with carbon eroded by the plasma. The low deuterium retention in the C-Mod divertor is mainly due to the absence of carbon PFCs in C-Mod and the low erosion rate of Mo.
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