John Carlson is a Visiting Fellow at the Lowy Institute and the former Director-General of the Australian Safeguards and Non-proliferation Office.
From seismic analysis, it appears North Korea's latest nuclear test had a yield of 6 to 7 kilotons (thousand tons of TNT). This is larger than the previous two tests (the 2006 test is believed to have had a yield of 400 tons, the 2009 test is believed to have been around 4 kilotons), but is still small by nuclear weapon standards – the bomb dropped on Hiroshima was around 15 kilotons, and modern weapons are an order of magnitude larger.
The implications of this relatively small yield are not clear. A full scale explosion is not necessary to prove a warhead design. Were there technical problems, so the test failed to meet its objectives, or was the small yield intended? It may take some time for the answer to this to emerge.
There is a world of difference between detonating a nuclear explosion in a tunnel and being able to deploy a reliable warhead on a missile. If North Korea is determined to build and deploy a nuclear arsenal, a series of tests will be required to develop a warhead that is relatively compact and reliable, including being able to withstand the stresses of missile flight.
North Korea has claimed this test was a successful demonstration of a 'miniaturised' warhead design. At this stage we don't know enough to assess this, but it is unlikely three tests would be sufficient to achieve a deployable weapon. Unless North Korea can be dissuaded from taking its nuclear weapon program further, in all likelihood it will conduct further tests.
A key question is whether this test used plutonium or highly enriched uranium (HEU), or maybe both.
North Korea has a limited supply of plutonium, maybe sufficient for 6 to 8 weapons, but reduced by the 2006 and 2009 tests (and also by the latest test, if it used plutonium). Following the shutdown of the Yongbyon reactor in 2007, North Korea is not able to produce additional plutonium (it still has a smaller research reactor, but the plutonium production capabilities of this reactor are limited).
However, if North Korea has established significant uranium enrichment capability, it would be able to expand the fissile material available for nuclear weapons by producing HEU. North Korea has shown it has an enrichment plant at Yongbyon, claimed to be for producing low enriched fuel for a power reactor it is building. It is generally assumed there must be at least one other enrichment plant at a secret location, used for HEU production.
It will not be possible to tell if the latest test used HEU unless there are gas leaks from the test, which can be detected and analysed. This could take several days or even weeks.
Where will this test lead? As already mentioned, if North Korea is determined to deploy nuclear weapons it is likely to need further tests. North Korea is likely to be several years away from having deployable weapons, but as it continues down this path tensions in North Asia will increase. Trying to dissuade North Korea from this path is complicated because it appears internal politics may be more influential on the regime's behaviour than its claimed concerns about external threats.
If nothing changes, the region, and beyond, will face ever-increasing risks: proliferation pressures on other countries in the region, notably South Korea and Japan, and corresponding pressures on the US to counter this through increased military involvement in the region; North Korea's role as a proliferation exporter, including its ties to Iran; questions about North Korea's ability to secure nuclear weapons from unauthorised launch and to secure fissile materials from theft; and the danger of weapons and materials falling into other hands when the regime does eventually collapse.
As Rory Medcalf has pointed out, China has more leverage than any other country to halt this slide toward disaster, and in many ways China has most to lose by inadequate effort. Changing the behaviour of North Korea is a challenge for all the P5 and regional countries, but it is especially a test of China's ability to meet the responsibilities of being a world power.
Photo by Flickr user oracle monkey.