Working with the United States, Japan has removed all of the highly enriched uranium (HEU) and the separated plutonium from one of its nuclear reactor facilities, to minimize the risk of theft and use by terrorists. The two countries have now pledged to convert a second research reactor to use safer, low-enriched uranium. These are among the latest in a series of accomplishments that have stemmed from US President Barack Obama’s biennial Nuclear Security Summit, which wrapped up on 1 April. More than 50 countries attended, most represented by heads of state, making a variety of commitments to reduce the risk of nuclear terrorism.

These projects are also a reminder of just how slow progress has been — and how much remains to be done.

Obama launched the agenda in a 2009 speech in Prague, calling on governments to secure or eliminate all vulnerable nuclear materials in four years. His speech underscored the fact that the threat of a nuclear attack has increased even as the danger of apocalyptic nuclear warfare has receded. Recent revelations that the Islamist terrorist group ISIS may have been targeting a nuclear facility in Belgium make this all too clear.

The initial focus has been on HEU, because of simple physics. Whereas plutonium must be compressed with explosives to produce a nuclear explosion — a feat that is probably beyond the technical capability of terrorist groups — the process is simpler for weapons-grade HEU, which is also used in many reactors. The United States and Russia, which have supplied the world with the bulk of HEU, have stepped up efforts to secure, remove or blend these materials into low-enriched uranium (LEU), which has 20% or less of the key isotope uranium-235. Security has been upgraded at 32 facilities, and 12 countries have been declared HEU-free since 2010.

Many of these materials are located at civilian research reactors. The risks were recognized long ago; in 1978, for example, the United States began eliminating HEU fuel in these reactors. In 1992, the US Congress enacted a law requiring countries that receive its HEU to commit to converting reactors to LEU fuel. To maintain reactor performance, however, scientists needed to develop a new generation of high-density LEU fuels, which are now available for most research facilities.

This is good news, but challenges remain. Existing high-density LEU fuels cannot be used without degrading performance in 11 specialized US and European research reactors. Certifying new fuels and converting these reactors could take nearly two decades. In January, the US National Academies of Science, Engineering, and Medicine recommended that specialized US reactors adopt an interim solution and convert to less-enriched fuel sources containing 30–45% uranium-235. This could — and should — be accomplished over several years, without impeding efforts to complete the shift to safer LEU fuel as soon as possible.

Researchers also need a comprehensive strategy to maintain research reactors. The European Commission is sponsoring a research consortium called HERACLES to do just that. The White House Office of Science and Technology Policy should convene agencies and research facilities to develop a path forward, and engage internationally. Many of these specialized research reactors are getting old; in some cases, given delays with new LEU fuels, it may make sense to start anew.

But research reactors are just one part of the puzzle, and the question now is how to carry the broader nonproliferation agenda forward once Obama leaves office. His four nuclear summits have boosted political attention and accelerated progress, but the world is awash with nuclear materials. Nuclear safety and security falls to a problematic patchwork of international institutions, including the International Atomic Energy Agency (IAEA), Interpol and the United Nations, and the latest summit produced a variety of initiatives to bolster these institutions.

That is a start. Ultimately, the world needs a new convention that sets specific standards for nuclear security and allows inspections and enforcement by the IAEA. In the meantime, governments must work through existing institutions to share and implement best practices. Regardless of cost, research facilities must ensure that their nuclear materials are safe and secure.