They don't have it anymore.
From Wikipedia: "Ukraine has acceded to the Nuclear Non-Proliferation Treaty. Ukraine inherited about 5,000 nuclear weapons when it became independent from the Soviet Union in 1991, making its nuclear arsenal the third-largest in the world.[60] By 1996, Ukraine had voluntarily disposed of all nuclear weapons within its territory, transferring them to Russia."
and by the way: as far as I know, in return Russia promised Ukraine to respect Ukraine's territorial integrity.
Remember, one doesn't build a nuclear weapon and let it sit. It must be maintained on a regular basis at a great expense, ergo if was more profitable to have them dismantled with other countries footing the bill.