As doubly heavy hydrogen, tritium has two extra neutrons, which do not sit well with a single proton. So tritium decays by emitting an electron, a process known as beta-decay. This loss of a negative charge changes one of the neutrons into a
positively charged proton and converts tritium into helium-3, a helium isotope with two protons and a single neutron instead of the usual two. This decay makes tritium radioactive, and it has to be handled carefully in a fusion plant.

Fortunately, the radioactivity is mild. The electron that is emitted has very low energy, about 19 keV. It cannot penetrate the skin, and even in air can go only 6 mm (1/4 in.) [12]. However, it can be harmful if ingested and must be carefully kept out of the water supply. Unlike fission products, tritium has a short half-life of only 12.3 years. This means that 5.47% of it decays into harmless helium each year. Because of its short life, very little tritium exists naturally. Cosmic rays make about 200 g of tritium a year, and there are only about 4 kg of natural tritium at any one time in the earth’s atmosphere. Man-made tritium raises this to about 40 kg. Compared with this, it will take 1 kg of tritium just to get ITER running on DT, and a reactor may use up 100 kg per year.