[Little Ray]

Tritium. It decays into deturium relatively quickly.

[BeHoldAPaleHorse]

"Tritium. It decays into deturium relatively quickly."

Worse, it decays into helium--and helium captures neutrons very efficiently, so that only a little helium can cause the weapon to not generate a significant yield.

[GingisK]

Tritium

Only in H-bombs, not A-bombs. There are no 2kt H-bombs. All H-bombs start in the megatons. Suitcase nukes are simple a-bombs.