North Korea’s powerful National Defense Commission announced its intention this week to carry out “a nuclear test of higher level.” In a Q&A, James M. Acton explains the technological reasons North Korea would want to test another nuclear device and what to watch for if it does.

When is the test likely to take place?

From a purely technical perspective, a test could almost certainly take place within weeks—possibly within just a few days. Over the last year, numerous satellite images have shown that North Korea has worked hard, first to prepare its test site at Punggye-ri and then to ensure a high state of readiness. This included strenuous efforts in the fall to repair extensive damage from summer rains.

The exact amount of time required to prepare for a test, however, is uncertain. If the device is not stored in the tunnel, it must be placed there. The device (or conceivably devices) as well as diagnostic equipment to measure and transmit data useful to the weapon designers must be prepared. And the tunnel must be sealed or “stemmed” to try and prevent the leakage of radioactive material.

How far advanced these preparations are is unclear. Interestingly, a satellite image from January 23 shows that a pile of dirt on the test site is smaller than it was at the end of December. One possible interpretation of this is that North Korea is currently in the process of sealing the tunnel or has already done so. If so, it may be close to completing its preparations.

Beyond the time required to make final preparations, are there any other timing considerations?

Possibly. North Korea has sometimes chosen to conduct its missile tests on significant dates.

The 2009 test of an intercontinental ballistic missile took place on July 4, U.S. Independence Day. The first of two intercontinental ballistic missile tests in 2012—both of which were in the guise of satellite launches—took place on April 13, two days before the centenary of the birth of Kim Il Sung, North Korea’s founder.

February 16 is the birthday of Kim Jong Il, the son and successor of Kim Il Sung and the father of North Korea’s current leader, Kim Jong Un. It is possible (but by no means certain) that North Korea will conduct a test on or around that date.

Is there anything distinctive about North Korea’s testing arrangement?

Actually, yes. North Korea appears to be testing in a horizontal tunnel dug into the side of a mountain. In this arrangement the tunnel is probably curved and divided into a number of segments, each sealed by blast doors and possibly backfilled with earth.

Although the United States sometimes used a similar setup when studying the effects of nuclear weapons, most U.S. tests—and indeed most tests by most other nations—were conducted in vertical shafts. The North Korean arrangement, however, is reportedly similar to that used by Pakistan.

If North Korea does conduct another nuclear test, what should we look for?

Analysts will be looking at two things in particular: the size of the explosion—its yield—and the fissile material (highly enriched uranium or plutonium) from which the device is made.

Is it true that North Korea’s previous nuclear tests had quite low yields?

North Korea conducted two previous nuclear tests, on October 9, 2006, and May 25, 2009. Most studies suggests that the first explosion (which created an earthquake of magnitude 4.3) had a yield of about 900 tons (the amount of TNT required to make an equivalently large conventional explosion). There is much more uncertainty about the yield of the second test, which led to an earthquake of magnitude 4.7. Most estimates range from about 2,000 tons up to 7,000 tons. By way of comparison, the first U.S. test, called Trinity and conducted in New Mexico on July 16, 1945, had a yield of about 21,000 tons.

Given the small size of both North Korean tests, most analysts have concluded that they performed significantly less well than intended.

That said, a minority of experts have argued that North Korea may have intended to produce lower-yield devices (indeed, before the 2006 test Pyongyang is reported to have told Beijing that it was aiming for a yield of 4,000 tons). Surprisingly perhaps, such smaller devices are more difficult to manufacture than the simple design used in the Trinity test but could be lighter and hence easier to carry by missile. If this speculation is correct, then North Korea’s nuclear program may be more advanced than generally believed.

Is a third test likely to be a higher yield?

That certainly seems like Pyongyang’s plan given the National Defense Commission’s mention of a “nuclear test of higher level.” Exactly what this statement means, however, is open to interpretation.

If the first two tests were indeed much smaller than intended, North Korea may simply try for a successful test of a relatively simple design with a yield that measures in tens of thousands of tons.

However, it is also conceivable that North Korea could test a much more ambitious design. According to media reports, the Japanese government has concluded, based on data about imports, that Pyongyang is “likely” to test a “boosted fission” weapon in which the efficiency of the device is improved by using nuclear fusion to ensure that more of the plutonium or uranium is split and releases energy. Finally, if North Korea has received some form of outside assistance, the possibility that it could test a true thermonuclear weapon—an H-bomb—cannot be excluded.

Apart from possibly increasing the yield of its devices and trying out highly enriched uranium, are there any other technical reasons North Korea would want to conduct a third nuclear test?

Yes. Given North Korea’s aggressive ballistic missile development program, it is likely that Pyongyang’s goal is to develop a warhead that is small and light enough to be mounted on a missile since this would enhance its ability to threaten South Korea, Japan, and eventually the United States. Indeed, its successful satellite launch in December 2012 using intercontinental ballistic missile technology was a significant stride toward this goal.

Testing would be helpful to North Korea in miniaturizing its warheads if it has not done so already (and many analysts believe it has not). If radioactive emissions are present, they may give clues about the design—and hence the size—of the device. However, it is unlikely that the U.S. intelligence community, which will certainly try to detect such emissions, will comment on them beyond confirming their existence, as it did in 2006.

What is the significance of the fissile material from which the device is made?

Radioactive emissions from North Korea’s first nuclear test in 2006 indicated the device was made from plutonium. North Korea’s plutonium stockpile is only sufficient to produce a handful of weapons, and, given the dilapidated state of its plutonium-production infrastructure, producing more would be slow going and very noticeable. Consequently there was reason to hope that North Korea’s arsenal would remain very small.

In November 2010, however, North Korea revealed the existence of a sophisticated uranium-enrichment effort that appears to be capable of producing highly enriched uranium in significant quantities and, as a result, of significantly expanding North Korea’s arsenal. Many analysts, therefore, anticipate that a third test would be a highly enriched uranium device.

There is no guarantee, however, that the radioactive emissions from a third North Korean test would be detectable. Without such emissions it is impossible to identify the material from which the device is made. Indeed, North Korea’s second test in 2009 did not produce measurable radioactive emissions, and it is not known whether it was made of plutonium or highly enriched uranium.

If a third nuclear test takes place without producing radioactive emissions, how will we know it has occurred?

Even if there are no radioactive emissions, it will still be possible to detect the shock waves that propagate through the earth after a large explosion. Seismic monitoring is a reliable means of detecting nuclear tests and estimating their yields.

The earth is covered in seismic sensors, including those that are part of the International Monitoring System for the Comprehensive Nuclear-Test-Ban Treaty (an agreement to ban all nuclear tests that has yet to enter into force because a number of countries, including the United States and North Korea, have not ratified it).

This sensor array, along with many others, detected both previous North Korean nuclear tests and provided sufficient data to allow analysts to estimate the yields with reasonable accuracy.

This Q&A has been updated to note the uncertainty in the yield of North Korea’s second nuclear test.