[en] Plans are to convert the ²³⁷Np that is currently stored as a nitrate solution at the Savannah River Site to NpO₂ and then ship it to the Y-12 National Security Complex in Oak Ridge for interim storage. This material will serve as feedstock for the ²³⁸Pu production program, and some will be periodically shipped to the Oak Ridge National Laboratory (ORNL) for fabrication into targets. The safe storage of this material requires an understanding of the radiolysis of moisture that is sorbed on the oxides, which, in turn, provides a basis for storage criteria (namely, moisture content). A two-component experimental program has been undertaken at ORNL to evaluate the radiolytic effects on NpO₂: (1) moisture uptake experiments and (2) radiolysis experiments using both gamma and alpha radiation. These experiments have produced two key results. First, the water uptake experiments demonstrated that the 0.5 wt % moisture limit that has been typically established for similar materials (e.g., uranium and plutonium oxides) cannot be obtained in a practical environment. In fact, the uptake in a typical environment can be expected to be at least an order of magnitude lower than the limit. The second key result is the establishment of steady-state pressure plateaus as a result of the radiolysis of sorbed moisture. These plateaus are the result of back reactions that limit the overall pressure increase and H₂ production. These results clearly demonstrate that 0.5 wt % H₂O on NpO₂ is safe for long-term storage--if such a moisture content could even be practically reached