When tritium decays, it undergoes beta decay, transforming into helium-3.

Here's why:

* Tritium is an isotope of hydrogen with one proton and two neutrons.

* Beta decay involves a neutron transforming into a proton, emitting an electron (beta particle) and an antineutrino.

* After the decay, the tritium atom has lost a neutron and gained a proton. This results in an atom with two protons and one neutron, which is the composition of helium-3.

So, the nuclear reaction is:

³H (tritium) → ³He (helium-3) + e⁻ (beta particle) + ν̅ₑ (antineutrino)