North Korea is believed to have used plutonium-based bombs in its first and second nuclear tests in 2006 and 2009.
Although not confirmed, experts believe their third test last year used a uranium-based bomb, since the detonation was two times greater than in previous tests.
The difference between the materials is significant.
Plutonium is made by reprocessing fuel used in nuclear reactors.
It is a long process and requires a large reprocessing facility.
Uranium, on the other hand, requires only an enrichment process.
With just a centrifuge, a nuclear bomb can be produced almost anywhere, even in a two-story building.
North Korea is said to have the ability to greatly expand its nuclear bomb reserves because of plentiful uranium deposits, which also have a long half-life.
"There is a high chance that plutonium will not explode, because the explosive device is elaborate. However, uranium involves a relatively simple technique. If the two are separated and put together, it will explode 100 percent of the time."
However, there are other limitations.
Miniaturizing a uranium bomb and mounting it on an intercontinental ballistic missile is impossible.
So how does this all relate to the North's recent threat to carry out a new form of test?
Experts say it could mean the regime is looking at trying out a reinforced type of nuclear weapon.
If tritium and heavy hydrogen are placed in the core of a bomb and surrounded by plutonium and uranium, it can increase the explosive power by up to ten times.
It is more powerful than a regular nuclear bomb, but less so than a hydrogen bomb.
Such a bomb can be easily miniaturized and also has the advantage of being light.
Kim Min-ji, Arirang News.