Those pesky North Koreans and by extension, the Iranians. Making a bomb is too easy to think that a country can't make one. Virtually every country can do it. The only hard part is the Diffusion part.

Pure uranium is dried and filtered into coarse powder called yellowcake. The substance is exposed to fluoride gas and heated to 133 degrees F. This creates a gas called uranium hexafluoride. The gas consists of two isotopes—U-238, which is too stable for atomic detonation, and U-235, which is lighter and highly fissionable. So they have to be separated. (U-238 IS used in a truly difficult to make plutonium bomb, the one tested at Los Alamos and dropped on Nagasaki).

To separate the isotopes, the gas (uranium hexafluoride) is pumped through a succession of fine, porous barriers—centrifuges. U-235 isotopes are lighter and propel faster through the barriers and concentrate. After passing through several thousand barriers, called centrifuges, the gas contains about 2 percent of U-235—only enough for a nuclear reactor.

But an atom bomb requires nearly 95 percent purity. So we move to refinement—the sophisticated, expensive, and time-consuming part. The slightly enriched uranium undergoes magnetic separation before being fed into another centrifuge. After passing through more than 1,500 barriers, the gas is about 20 percent pure. The process is repeated for nearly a year until the purity reaches nearly 90 percent. At this point, the uranium gas is considered bomb grade. It is then converted into metal powder.

So we have to assume that North Korea has done the hard part, if they haven't they have bupkis. Same for Iran. Setting the stuff off goes like this, The metal powder is molded into a ball weighing between 50 and 100 pounds. Size matters less than the purity of the uranium. It is packed into a warhead containing a detonator (the United States used artillery shells in its early versions) and fuse, which can be remotely set.

The even easier way to explode it is called "the gun" type. In this type you just take two clumps of U-235 (not large enough to sustain a chain reaction) and bring them together rapidly inside a gun barrel so that a supercritical mass is formed, and boom.

So it's possible that the NKs had a couple of ten or twenty pound clumps and detonated that, thus producing a weaker bomb.

It seems impossible that our intel is so bad that we can't tell if NK has the 50,000 or so centrifuges necessary to refine the U-235, but then it has already been demonstrated the limits of our intel. And remember we are talking a U-235 bomb, not the much more powerful plutonium bomb which needs advanced technology.