TEHRAN, Iran — Iran's nuclear agency began studies Saturday to build an experimental nuclear fusion reactor, something that has yet to be achieved by any nation. Iran is not known to have carried out anything but basic fusion research, but it does have a nuclear fission program that the U.S. and its allies believe is a front to build weapons – a charge Tehran denies. Nuclear fusion, the process powering the sun and stars, has so far only been mastered as a weapon, producing the thermonuclear explosions of hydrogen bombs.