Media reports are mostly misleading, saying some minor nuclear materials were used only to be able to detect nuclear weapon testing around the world. That technology already exists in spades. In 1996 Bill Clinton terminated nuclear weapons testing to appease the Russians. Three underground tests were ready to commence, and it was decided that it would be safer to leave them in place underground rather than dismantle them. My bet is there are only two now.
Is Biden (Administration, Joe's taking another nap before his next vacation) trying to intimidate Russia and Iran, or is he threatening them with this action?
Also full disclosure, the US has not done atmospheric nuclear weapons testing for decades.