An atomic battery, nuclear battery, tritium battery or radioisotope generator is a device which uses energy from the decay of a radioactive isotope to generate electricity. Like nuclear reactors, they generate electricity from nuclear energy, but differ in that they do not use a chain reaction. Compared to other batteries, they are very costly, but have an extremely long life and high energy density, and so they are mainly used as power sources for equipment that must operate unattended for long periods of time, such as spacecraft, pacemakers, underwater systems and automated scientific stations in remote parts of the world.[1][2]