Oak Ridge National Laboratory (U.S.)
Have you ever had one of those friends who's back at the dealership the moment the new-car smell wears off his or her last vehicle? That's pretty close to what Oak Ridge National Laboratory's (ORNL) rankings look like; you can fully expect that today's hot new system will either be upgraded or replaced by the new model in a couple years.
Aside from the aggressive turnover, another interesting fact about ORNL is its use of Cray-built systems (lately, many of the top U.S. players run IBM-built Blue Gene systems). In 2012, its Jaguar XT5 system was getting upgraded to the new XK6 Titan system. Those fancy name changes will help the system leapfrog from 1.94 petaflops to somewhere between 10 and 20 petaflops, which could land it near the top of the world rankings.
The Titan uses a combination of CPUs by AMD and GPUs by NVIDIA to conduct research for the U.S. Department of Energy, including studying the effect of extending the life cycles of nuclear power plants, the viability of new biofuels, population dynamics, development of solar technology and climate change modeling. When Japan's Fukushima Daiichi plant was damaged in the earthquake and ensuing tsunami of 2011, ORNL researchers put in long hours to simulate some of the emergency scenarios at the nuclear plant [source: Munger].