Graphite:
Since they commonly have high surface areas, graphite samples adsorb large amounts of hydrogen gas (4 x 1018 molecules/g for a graphite pellet used in gas-cooled reactors). Methane, protium, and (possibly) water are produced from beta irradiation of the graphite surface.
The surface of the graphite will be contaminated along with chemically-bound tritium, and decontamination might be possible through baking the graphite at 500º C in the presence of a hydrogen exchange medium, such as H2, H2O, or NH3. Except for possible surface erosion, graphite will possibly not be degraded mechanically even over a period of several years, as bulk diffusion and solubility are very low.