Caesium-137 is among the most important hazards associated with low-level nuclear waste. Among the most promising materials for scavenging Cs+ ions from waste solutions are the metal ferrocyanides. Although the efficiency of these materials has been demonstrated in industrial conditions, the structural changes that allow them to absorb Cs+ remain unclear. Here I propose an investigation by total neutron scattering of copper ferrocyanide, one of the best scavenging materials. I will collect data from samples as synthesised and after soaking in Cs+ solutions for up to six months, as well as from the analogous copper ferricyanide, which is a poor scavenger, for comparison. The results will reveal the distribution of vacancies and the Cs+ sorption site in this material, yielding the complete sorption mechanism for the first time and helping in the design of related waste sorption materials.