While this looks simple, many of the details of the construction of the first nuclear devices are still classified. So part of the reason is that simple explanations of the design do not necessarily reflect a simple construction.
On the other hand, you get a taste of a story about how John Phillips got his Undergraduate senior thesis classified for being to informative on how to resolve some important technical details around the actual explosive design for a pit based weapon.
Plutonium is difficult to handle and accumulate in significant quantities. Here is a fun video that gives you an appreciation for the danger and rarity of the element: http://www.youtube.com/watch?v=XLufmakbiU0
*Edit: I linked the wrong youtube video. While the above is fun, this one gives you a better idea about the manufacturing difficulties of working w/ Plutonium: https://www.youtube.com/watch?v=89UNPdNtOoE
The bottom left corner of the periodic table is apparently filled with wildly interesting elements that do super bad things to your body. They get hard to come by- and in some cases they only get created in the laboratory. Maybe that's for the best.
It's not quite as easy as it looks. The implosion has to be perfectly symmetrical, or else the core will pop out the side instead of being crushed. Think trying to spear a cherry tomato with your fork. That in turn means that your chemical explosives have to be carefully arranged and they have to be detonated at the precise correct times. In fact, one theory of how the Permissive Action Links (basically, the stuff that makes it so a bomb needs a code to be detonated instead of any random person just pushing a button) work is by encrypting the timing sequence, so that without the code you don't have the information needed to detonate the bomb, rather than relying on some physical lockout. Working out the timing and building the device accordingly are both tough.
Additional difficulty comes in when you want to either increase the bomb's power or decrease its size and weight, both of which are important for making them practical to use against an enemy. There's a lot of work between Fat Man, which weighed 10,000 pounds had a yield of 21kt, and a modern weapon like the W88 which weighs 800 pounds and has a yield of 475kt.
But still, once the basic research was completed and available, producing the materials becomes the primary difficulty in building a bomb.
Making enough plutonium is pretty difficult from what I understand. In WW2 some of the biggest buildings ever built were necessary to house all the equipment to make just enough material for a few bombs (see http://en.wikipedia.org/wiki/K-25). Nowadays you need a nuclear reactor and fancy equipment like uranium centrifuges. See the Stuxnet worm and how it targeted Iran's uranium centrifuges in an attempt to disable their ability to make bomb material.
The TLDR is that you have to do a whole bunch of things that give away that you're doing it, take a lot of time and money, make you an easy target to be disrupted in your task, and failing all of that, can kill you pretty easily along every step of the way.
another complexity is that the plutonium core is not just any weapon-grade plutonium: it has to be casted in unstable delta phase (which has unusually low density; the implosion's task is to switch it into alpha-phase which is much dense: this process makes core critical and explosion happens). Not to mention toxicity of the material itself.
See this article from Der Spiegel [1]. Many methods are outlined - including getting unwitting cooperation (wink wink, it's just for energy / research, we promise [2]) from developed nations - along with the caveat that until the bomb is ready it's best to deny, deny, deny and avoid all the headache.
The article discusses Israel, South Africa, Brazil, Libya, and of course Iran.
That said, being part of the nuclear club means a huge responsibility - assuming you just want influence and don't want to kill somebody, now you've got to make sure nobody messes with your new toy or tries to take it.
Not much at all, and an interesting consequence of this is that manufacturers of high-precision CNC machines have to be very selective about which countries they're willing to sell to, and many of these machines carry GPS trackers and other sensors which will alert the manufacturer and prevent the machine from functioning if somebody tries to move it.[1]
Remember: this was done the first time in less than four years, from 9 October 1941 to 16 July 1945, using technology from the stone age.
No solid-state electronics beyond crude diodes. No electronic computation. Approximate estimates on most of the cross-sections. Hopeful guesses on many of the processes (including multiply redundant approaches to some things, including the basic design, because people weren't sure what would work or what would be possible to build.)
Today, a team of reasonably intelligent engineering students with a budget of a few billion dollars could build a bomb in a year, simply because they would know vastly more than the original team.
Again: it took three years and $2 billion ($30 billion in today's dollars) to build the very first bomb from scratch knowing almost nothing about the process and having to custom build and design and invent almost every component.
To suggest that any particularly rare expertise today would be required or that the cost would be more than a tenth of the original development is implausible.
Remember this the next time you hear about Iran's nuclear program, which has been going on for twenty years, supposedly in mad pursuit of nuclear weapons. It's a ridiculous claim: if the Iranians wanted a Bomb very badly they would have one by now. If they don't have one by now the only reasonable conclusion is that they don't want one very badly.
The extreme disapprobation they get from the rest of the world on this issue may well contribute to their tepid enthusiasm, but no amount of Stuxnet and the like is going to put a dent in a strongly committed program. The Iranian's commitment just isn't that strong.
Gas centrifuge technology in particular has been a game-changer in the lowering the nuclear bar over the past couple of decades. People were warning about it in the '80's, and their warnings seem to have had some prescience. It makes practical enrichment of natural uranium to weapons grade, bypassing the need for a reactor program to breed plutonium, and all the nasty, difficult chemistry involved in plutonium extraction.
And unlike plutonium weapons, uranium weapons are ridiculously easy to detonate. Trinity was a plutonium test. Little Boy was one of the very few uranium weapons ever detonated, and it was live tested because the scientists were as certain as anything it would actually explode.
There are uraninite (pitchblende) ores that are getting on for 10% uranium, or 0.07% 235U, so a tonne of ore would yield 7 kg of 235U. It'll only get worse as seawater extraction gets better (it's already capable of producing macroscopic amounts of uranium from the relatively uranium-rich Japan Current, although post-Fukushima the primary research, which was Japanese, has likely been curtailed.)
Processing a couple of tonnes of ore is sub-industrial. It isn't that hard to hide. On the other hand, if countries want to go the plutonium route, plutonium producing reactors aren't hard to hide either: the Russians were running them underground--a few hundred metres below the surface--in the 80's and 90's.
I've always read that the primary difficulty in building a nuclear weapon isn't necessarily the difficulty in producing the bomb or detonator, but in producing the fissile material in large quantities. (U235 is something like 0.7% of bulk uranium we find in nature, and separating it from U238 is prohibitively difficult. You need 80% for "weapons-grade" according to wikipedia.)
Following your reasoning, it makes sense why North Korea was able to produce a small amount of it in a relatively short time. They were well funded thanks to the left-leaning South Korean administrations from 1998-2008 who practiced Sunshine policy. The last bit is that they certainly want it bad enough but have failed to produce a nuclear weapon with significant power (nothing in the range of WW2 nuclear weapons). So if a country that wants it so bad like North Korea have been able to produce a controlled nuclear explosion, why can't they produce anything significant enough to be recognized as a nuclear state? The only thing that would stop them seems to be either expertise or lack of key material or perhaps lack of monies.
Producing some kind of nuclear explosion is mostly only about procuring the raw materials. Getting to significant yield is the actual hard part as it involves precision machining, measurements and timing.
Machining heavy metals is complex problem in itself even without requirement of extreme precision. It is hard to procure machining equipment with sufficient precision and tooling capable of machining hard metals without raising lots of red flags.
Almost anything that is capable of switching of significant powers with small or repeatable latency is essentially non-exportable with the reasoning that it is not useful for much else than triggering nuclear weapons (one could think of lots of other applications, but the devices are too bulky and expensive for most of them).
This is off-topic, but the agreement between North Korea and the US was made in 1994 (well before there was any "left-leaning" president in South Korea), in which the US agreed to pay for two Light Water Reactors in exchange for the North Korea discontinuing its nuclear program[1].
As expected, South Korea ended up funding most of the cost, while both the US and NK violated the agreement in numerous ways until the deal completely broke down in 2003. The reactors never saw the light of the day. Bummer.
There's a story (not sure how much of it is true) that, shortly before the deal, the Clinton administration went within hours of air strike on North Korean nuclear facilities, while Seoul was completely left in the dark. The realization that South Korea was essentially reduced to a spectator (and a deep pocket) in NK-related matters doubtlessly affected the two following administrations' policy toward North Korea, trying to have at least some of our own leverage. ("The US will take care of us" is not a strategy.)
But we get to blame our "leftist" presidents for their nukes. Niiice...
> Terms of the pact and consequent agreements included ... In exchange two light water reactors would be constructed in North Korea by 2003 at a cost of $4 billion, primarily supplied by South Korea.
In the case of North Korea it is likely the raw material they lack. They are a small and insular country, and may not have any significant indigenous reserves of uranium ore.
This is a very good thing, and unfortunately one that sea-water extraction technology--which is otherwise a very good thing because it makes natural uranium a renewable resource--may eventually have an effect on.
Yes, in fact the website in the OP is for a book which advocates for a disarmament approach based on eliminating weapons-grade fissile material stockpiles:
Main thing that stops any country is availability of refined fission material. First you need to have access to Uranium mines which US and Russia pretty much has covered all over the world. Second, you need to dig tons of ore and go through very tedious and complicated refinement process that is very expensive and requires very large scale operation. To avoid these, countries like Iran would just use the material generated from reactor fuel that can be obtained more easily.
What stops so many other countries from building nuclear weapons?
That's an interesting question, perhaps the best answer is simply affluence, the perception of geopolitical safety, and the perception of unwanted negative repercussions of obtaining nuclear weapons. Any developed country with a significant industrial base can build nuclear weapons. Trivially so for countries with significant local nuclear power industrial infrastructure such as fuel reprocessing or Uranium enrichment. Brazil, Mexico, Canada, Japan, South Korea, Germany, all these countries, and more, could build nuclear weapons quite rapidly and in great quantity just by deciding to do so.
For everyone else, well, one hopes that it is sufficiently difficult and costly that it's just not worth it for most countries, and difficult to hide as well, though mostly that's a lie we tell ourselves to make us feel safe. The hardest part is acquiring the fissile materials, which turns out to be slightly tricky. Plutonium requires production in reactors, but more so it requires a rather inefficient process to produce weapons grade Plutonium in reactors. Reactors are hard to hide, and if a reactor is under IAEA observation it's non-trivial to use the reactor to produce Plutonium. I say non-trivial but it is certainly far from impossible, as it's been done, but it makes it difficult to produce in large quantity while under the IAEA's eyes.
Uranium enrichment is a bit of a different story, and not a particularly heartening one either. It does require very sophisticated equipment and significant industrial resources to enrich Uranium, but it turns out that this equipment can be hidden fairly well and it actually takes much less of an investment to create enrichment facilities for making weapons versus for making reactor fuel (the enrichment level needs to be much higher, but the quantity needs to be vastly smaller). For example, to process natural Uranium into 5% enrichment takes about 75% of the work as to process it into 90% enrichment (weapons grade), and you only need perhaps 16kg or so of highly enriched Uranium to make a bomb. Getting back to observability, these facilities can be easily hidden and operate without notice since they blend in well with other industrial facilities externally. The world learned of Saddam's vast Calutron facilities only due to the first Gulf War. And the world learned of North Korea's centrifuge cascades only when that regime announced their existence.
In the last 50 years half a dozen countries have acquired nuclear weapons without sanction from existing nuclear powers and in most cases without the world being certain of the existence of those nuclear programs until they were announced (such as through weapons tests). That gives some perspective on the true difficulty of preventing nuclear weapons proliferation.
As a side note, if the entire worldwide civilian Uranium enrichment capacity was diverted from making reactor fuel to making nuclear bombs it could produce around 400 nuclear bomb cores per week.
So what stops so many other countries from creating such seemingly simple device? Is it the trouble in acquiring the materials?