WATCHMAN - a kiloton-scale, gadolinium-doped water Cherenkov detector - will demonstrate for the first time the feasibility of detecting the antineutrino signal from a hidden reactor against another reactor background, for application to nuclear non-proliferation. Such a detector will out of necessity operate at long range, often at the very limit of its sensitivity, and so minimising backgrounds is essential. Accidental coincidences from the decay of radioactive isotopes in PMT glass mimic the correlated signal in gadolinium water and are one of the principle sources of backgrounds for the experiment. The poster will give a brief overview of the application of antineutrino detection technology to nuclear threat reduction, followed by the presentation of a glass radioactivity study for WATCHMAN, with a discussion of the relative benefits of different PMT types and the optimisation of analysis for minimising accidental coincidences due to radioactivity in PMTs.