Assuming that theory is true, an American scientist developing technology to be used in nuclear weapons in Ukraine.....that's a lot to answer for.
I hope to God that theory is not true.
Ukraine never owned any nuclear weapons, only hosted them on their territory. Russia inherited all the nuclear weapons in the former Soviets when the breakup took place. This is legally codified in the founding documents of all the former Soviet territories...including Ukraine.
Ukraine never controlled the nuclear weapons on their territory either, they didn't have the nuclear launch codes and didn't have the technical expertise to maintain them independently. The only value to Ukraine would have been to sell the warheads to the highest bidder, which Russia and the other Great Powers wanted to prevent.
The Budapest Memorandum was a diplomatic fig leaf designed in part to prevent Russia from using 'other means' to safe guard the weapons. Both sides have violated its terms by meddling in Ukraine's domestic affairs since then.