As a physics graduate I've been following the discussion on the North Korean explosion over at ArmsControlWonk.com with some interest.First, here's what the North Korean state media had to say:
KCNA Report on One More Successful Underground Nuclear Test
Pyongyang, May 25 (KCNA) -- The Korean Central News Agency released the following report on Monday in connection with one more successful underground nuclear test in the DPRK.
The Democratic People's Republic of Korea successfully conducted one more underground nuclear test on May 25 as part of the measures to bolster up its nuclear deterrent for self-defence in every way as requested by its scientists and technicians.
The current nuclear test was safely conducted on a new higher level in terms of its explosive power and technology of its control and the results of the test helped satisfactorily settle the scientific and technological problems arising in further increasing the power of nuclear weapons and steadily developing nuclear technology.
The successful nuclear test is greatly inspiring the army and people of the DPRK all out in the 150-day campaign, intensifying the drive for effecting a new revolutionary surge to open the gate to a thriving nation.
The test will contribute to defending the sovereignty of the country and the nation and socialism and ensuring peace and security on the Korean Peninsula and the region around it with the might of Songun.
Here's two important points - the first from commenter JF:
the equation [for yield Y] is
M = a + b log Y
where a and b are empirically determined constants
for Novaya Zemlya [the Russian testing ground], the eqn would be
M = 4.45 + 0.75 log Y
this gives a yield Y of 2.2 kt
for Nevada Test Site, it would be
M = 3.92 + 0.81 log Y
this gives a Y of 9.2 kt
Nobody has calibrated the North Korean test site and so the empirical constants can only be guessed (or chosen to give the yield you wish to claim)
Which brings us to our second point from Über-wonk Geoffrey Forden - the importance of calculating the yield:
If they had gone with the “fail safe” WWII design [i.e., one with a 20 Kiloton yield], it would probably mean it was too heavy to mount on a missile. They would be making a political bomb that would undoubtedly use a lot of high explosive to ensure it got a good compression of the plutonium pit. The 4 KT bomb, however, might very well fit on a DPRK missile. If they have stayed with this design, it probably indicates that weaponising it is even more important than ensuring a successful test.
Using data from the 1998 India-Pakistan tests a yield of ~4 kilotons has been calculated, which would indicate that this is not a political 'for show' bomb, but another step in developing a bomb capable of being mounted in a long-range missile. Indeed, this is exactly what the North Koreans said they would do after the United Nations Security Council (UNSC) issued its resolution after the failed 'satellite' launch earlier this year:
The UNSC should promptly make an apology for having infringed the sovereignty of the DPRK and withdraw all its unreasonable and discriminative "resolutions" and decisions adopted against the DPRK.
This is the only way for it to regain confidence of the UN member nations and fulfill its responsibility for maintaining international peace and security, not serving as a tool for the U.S. highhanded and arbitrary practices any longer.
In case the UNSC does not make an immediate apology, such actions will be taken as:
Firstly, the DPRK will be compelled to take additional self-defensive measures in order to defend its supreme interests.
The measures will include nuclear tests and test-firings of intercontinental ballistic missiles.
Secondly, the DPRK will make a decision to build a light water reactor power plant and start the technological development for ensuring self-production of nuclear fuel as its first process without delay.
As Totalwonkerr Joshua Pollack put it: "We are now at one nuclear test and counting."