The White House hopes to spur “further steps” at this month’s gathering in Washington.
Inclusion of fissile stockpiles fails to break a deadlock at the Conference on Disarmament.
Officials are crafting an action plan ahead of the nuclear security summit.
Countries with and without nuclear arms are part of the effort.
Designed to make plutonium fuel, the plant is now too expensive, officials say.
Experts outline a plan for converting some reactors now using weapons-grade fuel.
Three men were arrested in Moldova earlier this year for attempting to sell radioactive materials to an undercover police officer posing as a middleman for the Islamic State group.