After the USSR collapsed in 1991, U.S. and Russian scientists and governments worked together to dismantle nuclear warheads and weapons on both sides. Paid for with U.S. funds, the bipartisan Nunn-Lugar Cooperative Threat Reduction program removed Russian nuclear missiles from Kazakhstan, Ukraine and Belarus, destroyed thousands of warheads and weapons on both sides, secured nuclear materials in Russia and paid for research projects for Russian scientists who might be otherwise tempted to get high pay to help develop nuclear programs in other countries. (Some did work for other programs, however.)
The Bush administration, convinced that all Clinton-era policies were wrong by definition, at first cut funding for Cooperative Threat Reduction, then later restored some of the money. The Obama administration stepped up the program and neutralized or removed all the bomb-grade highly enriched uranium and plutonium from Kazakhstan, Chile and over a dozen other countries, and secured it within the U.S. or Russia by the end of Obama’s term.
The greatest fear was that some terrorist group would steal or buy poorly-secured highly enriched uranium or plutonium on the black market and send a crude bomb in a shipping container to New York City. However, now that all the bomb-grade material has been secured, it is very difficult to make any more. Even with the financial and industrial resources of an entire country, it takes billions of dollars and years of work. So the idea of terrorists making their own nuclear bomb is far-fetched. However, it would be relatively easy for someone to make a so-called dirty bomb, which uses regular explosives to spread non-bomb-grade radioactive material to contaminate a large area.