In 2009, US President Barack Obama described the possibility of nuclear weapons falling into the hands of "terrorists" as the most immediate and extreme threat to global security.
Since then, the US has been leading global efforts to secure, consolidate and dispose of nuclear material, culminating in this week's United Nations Nuclear Security Summit in Washington.
Diplomacy has been a large part of these efforts, but they have also been accompanied by efforts to increase surveillance and monitoring at the world's ports, including the port of Rotterdam, Europe's largest.
More than 440 million tonnes of cargo pass through Rotterdam each year, making it a frontline in the effort to stop the transportation of potentially dangerous nuclear materials.
"These are sources coming from all over the world," Rene de Goede, from Dutch Customs, told Al Jazeera.
"They are coming from abandoned hospitals, things like contaminated steel, for instance. I know [of] one case where small pipes were contaminated with enriched uranium."
The port requires radiation sensors to be installed in the claws of the cranes used to unload scrap metal. Shipping containers are also checked as they are driven through the port.
Both systems send out an alert if the material is found to have suspicious or unusual levels of radiation.
Nuclear security summit to focus on dirty bomb scenario
Each year, about 150 shipments from this port are isolated and passed over to government scientists.
Suspicious nuclear material is then taken to a government laboratory in the Dutch city of Bilthoven, where samples are taken and analysed to try to understand what it is and whether it poses a threat.
"We investigate contaminated scrap metal to enable us to go back to the facility where it has most likely gone loose," Klaus Mayer, a leading nuclear scientist with the European Commission, told Al Jazeera.
"We help them improve their handling procedures and make sure they take better control of contaminated conventional waste."
Recent finds have included depleted uranium cylinders from Pakistan, nuclear testing material from Russia and even an unused nuclear fuel pellet from Germany.
Although all these pose a threat to health - and could potentially be used in a so-called dirty bomb - most of the nuclear material found in Rotterdam shows only low levels of radioactivity.
Frequent importers are unaware that their cargo is contaminated and blame suppliers for not checking the safety of their materials.
Mayer says detection efforts are catching only a fraction of the nuclear material being transported illegally, and some of it could be much more dangerous.
"The material which is discovered out of regulatory control and intercepted is most likely not all the material that is out there," he said. "What we see is the tip of the iceberg, but we don't know how big the iceberg is."
Securing high-grade nuclear materials
While low-level nuclear materials pose only a small threat to public health, of much greater concern are those which are more radioactive, such as nuclear fuel or weapons-grade material.
"Terrorist organisations are also acting upon opportunities that they have," said Brecht Volders, a nuclear weapons researcher at the University of Antwerp.
"That's why the nuclear security summit is so important. You secure the fissile material so they don’t have an easy opportunity to obtain the material."
Source: Al Jazeera