Long-term nuclear waste repository strategies in Europe include the developing of safe multi-barrier isolation of spent nuclear fuel. One of the considered schemes is the sealing of nuclear waste into Cu canisters that are then stored in bedrock. Assuring the stability of Cu in a radioactive environment — specifically, exposure to ground water — requires an understanding of the processes happening at the Cu–water interface under radiation and knowledge about compounds that Cu forms with O and H. CuOH is a by-product of the radiation-induced degradation of Cu in water. It is one of the less studied Cu(I) compounds, there is no information about its structure. Neutron diffraction experiments at ISIS will help to determine the positions of light elements and solve the structure of CuOH, and thus help our understanding of the process of radiation-induced degradation of Cu in water.