<P> Fusion power is a theoretical form of power generation in which energy will be generated by using nuclear fusion reactions to produce heat for electricity generation . In a fusion process, two lighter atomic nuclei combine to form a heavier nucleus, and at the same time, they release energy . This is the same process that powers stars like our Sun . Devices designed to harness this energy are known as fusion reactors . </P> <P> Fusion processes require fuel and a highly confined environment with a high temperature and pressure, to create a plasma in which fusion can occur . In stars, the most common fuel is hydrogen, and gravity creates the high temperature and confinement needed for fusion . Fusion reactors generally use hydrogen isotopes such as deuterium and tritium, which react more easily, and create a confined plasma of millions of degrees using inertial methods (laser) or magnetic methods (tokamak and similar), although many other concepts have been attempted . The major challenges in realising fusion power are to engineer a system that can confine the plasma long enough at high enough temperature and density, for a long term reaction to occur, and for the most common reactions, managing neutrons that are released during the reaction, which over time can degrade many common materials used within the reaction chamber . </P> <P> As a source of power, nuclear fusion is expected to have several theoretical advantages over fission . These include reduced radioactivity in operation and little nuclear waste, ample fuel supplies, and increased safety . However, controlled fusion has proven to be extremely difficult to produce in a practical and economical manner . Research into fusion reactors began in the 1940s, but to date, no design has produced more fusion power output than the electrical power input; therefore, all existing designs have had a negative power balance . </P>

What is the primary fuel that would be used in fusion reactors