It drives the spallation neutron source complex. Moderators are also used in non-reactor neutron sources, such as plutonium-beryllium and spallation sources. Neutrons are usually produced in a nuclear reactor or spallation source. Cosmogenic nuclides such as these are produced by chains of spallation reactions. Cosmic ray spallation was investigated as a possible process to generate deuterium. Inertial confinement fusion has the potential to produce orders of magnitude more neutrons than spallation. Generally the production of neutrons at a spallation source begins with a high-powered proton accelerator. During the 1970s, cosmic ray spallation was proposed as a source of deuterium. This isotope may be produced by cosmic ray spallation of calcium or potassium. This isotope is generated by spallation reactions on magnesium, sodium, silicon, and aluminium. The longest-lived radioisotope is Si, which is produced by cosmic ray spallation of argon. S is formed from cosmic ray spallation of Ar in the atmosphere. Another variation is to use the shock wave for spallation testing of materials. Current accelerators such as the Spallation Neutron Source, incorporate superconducting cryomodules.