Nuclear mattersHow safe are the world's nuclear fuel stockpiles?

Published 22 April 2010

There is a lot of weapon-grade nuclear material in the world — 1,600 tons of HEU and 500 tons of separated plutonium; keeping these stockpiles safe will take more than barbed wire; one method is a seal for HEU fuel rods with a pattern of flaws visible on ultrasound scans that cannot be removed without leaving telltale signs; the seals were installed last year in Romania and Pakistan; scientists work on other detection and safety methods

Could a terrorist build a nuclear bomb? Opinion is divided — but there people out there certainly eager to buy the ingredients. In March police in the republic of Georgia stopped a gang trying to sell weapons-grade, highly enriched uranium (HEU) on the black market. It was the eighth such interception in Georgia since 2000.

Debora MacKenzie writes in New Scientist that such tales helped persuade 47 world leaders meeting in Washington, D.C. last week to pledge to lock up the world’s weapons-grade material by 2014. By current estimates, this is 1,600 tons of HEU and 500 tons of separated plutonium.

How do we go about doing that? MacKenzie writes that one thing is clear: barbed wire and armed guards will not be enough. Safer reactor fuels and smarter detection technology will be needed, says Benn Tannenbaum of the American Association for the Advancement of Science.

The effort is overdue. It used to be assumed that only governments could use fissile materials to make a bomb, so international safeguards focus on what national nuclear agencies do with their stockpiles. “Keeping civilian stockpiles safe from criminals has been a poor cousin,” says Wyn Bowen, director of the Centre for Science and Security Studies at King’s College London.

One urgent task is to find cost-effective ways to convert HEU-burning reactors to a safer fuel, especially the world’s 130 often poorly guarded research reactors. In the meantime, technologies are available that could reveal tampering with fuels. The European Commission’s Joint Research Center in Ispra, Italy, has developed a seal for HEU fuel rods with a pattern of flaws visible on ultrasound scans that cannot be removed without leaving telltale signs. The seals were installed last year in Romania and Pakistan.

If thieves steal spent fuel, they will need to get the plutonium out of it. Current methods for detecting illicit extraction of plutonium require samples to be taken nearby, which can be politically impossible. Extraction also releases the radioactive gas krypton-85, which can be carried away on the wind and picked up by distant sensors.

The iGSE project led by Simon Hebel of the University of Hamburg, Germany, has developed ways to detect krypton-85 and track its source from several hundred kilometers away. It is testing this for the International Atomic Energy Agency (IAEA).

If all else fails, stolen material could be detected en route to its target. The United States had planned to install 1,400 detectors in ports worldwide to pick up neutrons and gamma rays emitted by HEU and plutonium, but because the neutron detectors require helium-3, which is in short supply (see “Nuclear security push bleeding cryogenic science dry”), their number has been cut.

MacKenzie writes that gamma rays are more difficult than neutrons to detect reliably. Existing germanium-crystal detectors have a high false positive rate, and a clued-up smuggler would not find it hard to block the gamma rays altogether.

Muon detectors have been developed as a back-up, but they work very slowly. Tannenbaum says the liquid argon detectors now used by physicists to spot neutrinos could, in principle, be used to detect both gamma rays and neutrons, but there are no plans to implement this idea.

Funding is a big stumbling block. No money was pledged at last week’s summit, beyond what countries already contribute to the IAEA’s small research program, Bowen says. The iGSE group’s funding runs out next month.