Iran's nuclear prowess is growing, fuelling a range of global tensions. Are scientists' abilities to sniff out covert nuclear weapons tests keeping up? Two new techniques could at least make it harder for countries to develop nuclear weapons on the sly.
Last week, the Iranian government flaunted its growing nuclear might, unveiling a suite of more efficient centrifuges for enriching uranium. Though it insists this is only for peaceful purposes like generating electricity, the US and other western nations have suggested Iran may be secretly developing nuclear weapons.
If Iran is, at some point it will have to test them. Existing sensors rely on monitoring for abnormal seismic activity and spikes in radioactive gases in the atmosphere. These are not foolproof.
Exploding a nuclear weapon in a large cavity underground, for example, can reduce the intensity of seismic waves and keep radioactive gases contained.
Such techniques could render the smallest nuclear explosions undetectable, and for larger blasts could be a way to conceal the total energy released, or yield – a sign of the extent of a nation's nuclear capability. The two new tools could help flag tests that would otherwise go unnoticed and reveal more accurate information about yield.
One looks for disturbances in the ionosphere, the region of space just above the atmosphere. The force of a subterranean nuclear blast causes the earth above to instantly flex upwards, disturbing the air. The resulting waves ripple through the atmosphere all the way up to the ionosphere. As it contains lots of electrically charged particles, radio signals from GPS satellites get distorted by these waves.
Now Yu-Ming Yang of Purdue University in West Lafayette, Indiana, and colleagues have found ionospheric disturbances coinciding with two underground nuclear tests carried out by North Korea in 2006 and 2009, in data collected by a network of 1235 GPS receivers in Japan.
Both tests were also big enough to have been detected previously by their seismic waves, but the ionospheric disturbances allow for a more detailed picture of the blast, says Yang's colleague James Garrison.
Further research on the ionospheric imprints might allow researchers to calculate the yield from them too, he says. Solar flares can also disturb the ionosphere so further work is needed to home in on nuclear disturbances.
For a much more subtle, long-term signature of nuclear tests, Paul Vincent at Oregon State University in Corvallis and colleagues turned to satellite-based radar measurements of the Lop Nor underground nuclear test site in China.
These showed that the ground swelled in an area about 2 kilometres wide over several years in the 1990s. The team attributes this to tests carried out then – one in 1992 and two in 1995. The swelling was greatest at the centre of this zone, where the ground rose by 2.7 centimetres. The team suggests that colossal amounts of heat from the nuclear tests gradually warmed the rock above, causing it to swell.
Such swelling takes years to become noticeable, unlike the seismic and ionospheric signatures. Like the ionospheric technique though, in combination with other methods, swelling could help to calculate a device's yield.
The US and other nations have tightened sanctions on Iran in recent weeks. Today, the UN's International Atomic Energy Agency announced its team was leaving Tehran after two days of talks with Iranian officials that failed to clarify issues relating to a possible military dimension to Iran's nuclear programme.
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