By CKR
The NPT has been in force since 1970, and it has been signed by 187 nations. Israel, India, and Pakistan have not signed it, and North Korea has withdrawn from it.
The provisions of the NPT are simple: the five nations that had nuclear weapons at that time will continue with nuclear weapons (USA, Britain, France, Soviet Union—now Russia—and China), but they will try to decrease the numbers of those nuclear weapons. The other countries will not try to acquire nuclear weapons, but in return the nuclear weapons powers will make peaceful applications of nuclear power available to them.
In the early 1960s, President John Kennedy believed that there would be 25 nuclear powers within 20 years. The NPT has kept that number down to eight. Sweden and other countries explictly considered developing nuclear weapons and decided not to. South Africa built six and then gave them up. Brazil and Argentina had nuclear weapons programs and gave them up short of manufacture.
Technical and international developments in recent years have weakened the NPT. An Additional Protocol was developed to address some of the concerns, but not all NPT signatories have signed the Additional Protocol.
On the technical side, much of the information needed to make nuclear weapons has become more widely available than it was in 1970. Information has been leaked, stolen, declassified, and inferred. The basic principles of nuclear weapons design and a fair amount of specifics can be found on the internet. New, more concealable, technologies have been developed for isotope separation.
A gas centrifuge plant remains a large installation, but laser isotope separation potentially could be about the size of many factories. So far, no successful laser isotope separation plant has been built. The isotope separation technologies, and the production of plutonium in reactors and its chemical separation from the fuel, are difficult, expensive, and dangerous to execute. Further, large amounts of feed material and specialized equipment are necessary.
For these reasons, it is unlikely that organizations smaller than nations could build nuclear weapons from scratch. Therefore, controlling the production of nuclear weapons by nations and safeguarding nuclear materials will be the predominant method of keeping these weapons out of the hands of terrorists.
For nations, the increased availability of nuclear technology and the fact that a uranium enrichment plant can be used for reactor fuel or for weapons mean that the technical bar to nuclear weapons has been lowered slightly since 1970, but improved detection technologies mean that it is more difficult to develop a clandestine program.