UN delegates meeting in Vienna are expected to adopt new measures to protect nuclear material from terrorism, sabotage and smuggling.
9/11 raised fears about the possibility of nuclear terrorism
The review of the Convention on the Physical Protection of Nuclear Material was prompted by the 9/11 attacks.
Tougher security measures are envisaged for nuclear sites such as storage plants for spent fuel.
Proposals also provide for more co-operation between nations to locate and recover stolen nuclear material.
Delegates from 80 countries are considering the proposals at the UN's International Atomic Energy Agency (IAEA) headquarters in the Austrian capital.
The 1980 convention was designed mainly to protect nuclear material in international transport and includes some provisions on domestic storage and use.
But following the 11 September 2001 attacks, a decision was taken to strengthen the convention amid concerns about the possibility of nuclear terrorism.
"What is happening now is the realisation that virtually any nuclear reactor could be the target of a highly co-ordinated terrorist attack," said Daryl Kimball, of the Washington-based Arms Control Association.
"If damaged, [a reactor] could become a weapon by scattering nuclear material over a broad area."
The five-day conference is expected to adopt a series of amendments expanding the scope of the convention, to include the protection of nuclear material within countries.
Mohammed ElBaradei, head of the UN's atomic watchdog, said the amendments will be another milestone in international efforts to protect nuclear facilities.