In addition, the complex phase diagram of plutonium makes it difficult to grow the single crystal materials necessary to characterize surface properties such as the surface energy. In particular, hydriding experiments are difficult since a PuO 2 layer of nonuniform thickness is present on plutonium surfaces that have been exposed to air, providing variable protection to the underlying plutonium and introducing an element of randomness into the observed hydriding induction times. Lastly, experiments alone cannot provide a detailed atomicresolution hydriding mechanism nor identify ultimately how it can be controlled.