In a commentary on the long-term storage of SNF in the March 2015 issue of Nature Materials, Professor Rod Ewing writes that, "to design reliable and safe geological repositories it is critical to understand how the characteristics of spent nuclear fuel evolve with time, and how this affects the storage environment. Globally, about 10,000 metric tonnes of heavy metal (MTHM) are produced each year by nuclear power plants, and a cumulative inventory of approximately 300,000 MTHM is stored either in pools or dry casks at reactor sites around the world1. Most of this inventory is destined for long-term storage and eventual geologic disposal. Thus, the behaviour of UO2 in spent fuel as a waste form must be understood and evaluated under the extraordinary conditions of geologic disposal, which extends to hundreds of thousands of years. The behaviour of nuclear fuel under the conditions of long-term disposal in a geologic repository depend specifically on the chemical changes that have occurred to the fuel during service life in the reactor."