Thermonuclear reaction. Computer simulation showing a heavy fluid (red) mixing with a lighter fluid (blue) during a thermonuclear reaction. When a heavy fluid layer rests on top of a lighter fluid, the two layers attempt to interchange, creating turbulence and releasing potential energy. This is known as the Rayleigh-Taylor (RT) instability and occurs in situations such as supernovae explosions, salt domes, mushroom clouds and oil floating on water. A high level of RT instability is undesirable in a nuclear fusion reactor as it prevents ignition and maximum energy output. This simulation was produced on the BlueGene/L supercomputer at the Lawrence Livermore National Laboratory, USA. In June 2007, this was the world's fastest computer.