If a country wants to build nuclear weapons, why would they enrich uranium instead of turning uranium to plutonium? Firstly, I have to say that I'm bad at physics. So, please correct me if I say something wrong.
I have read that natural uranium contains 99.28% of U-238 and only 0.72% of U-235 which is the fissionable isotope of uranium. If a country wants to build nukes, they should first mine uraninite, extract uranium or UF6 and then separate the 238 isotope from the 235 isotope using a separation method like gaseous diffusion or feeding it into centrifuges. Once they have five to ten kilograms of HEU (+95%), then they can build an atomic bomb. 
Now, today I realized that you can turn U-238 into plutonium-239 by adding one neutron to U-238. I read that the decay process takes 2 days to complete. If that is true, then it seems a lot easier than building an A-bomb using HEU. So, why is uranium enrichment a big deal and a serious reason for proliferation concerns if even natural uranium can be extremely dangerous and there are other ways of building the A-bomb without uranium enrichment? 
 A: First of all, the uranium needs to be enriched to turn it into reactor grade uranium, because reactors are necessary to create plutonium. In order to build a bomb with it, the U needs to be further enriched to turn it into weapons grade uranium. Uranium bombs are easier to build, as a critical mass is achieved by using a uranium gun to bring two pieces of enriched uranium suddenly together. Making a plutonium bomb is much harder, as the critical mass is created by using HE to compress the Pu-239, which is quite a tricky thing to do, as the explosive compression has to be exactly equal all around the plutonium sphere. Another fly in the ointment is that the Pu-239 tends to be contaminated by small amounts of Pu-240. The bomb dropped on Hiroshima was a uranium bomb, while the one dropped on Nagasaki was a plutonium bomb. The Hiroshima bomb was slightly more powerful.
A: Here are a few points to which I invite the experts to add. 
Breeding plutonium out of uranium requires you to build a breeder reactor, which is a huge undertaking, and the chemical separation process for extracting the plutonium must be designed for remote operation because the radiation levels present during the process are lethal to humans. However, you can make fissionable fuel out of U238 which enhances the fissionable yield of the ore. 
A plutonium bomb is far more difficult to engineer because the gun assembly process for building a critical mass cannot be used.
Building a uranium enrichment process requires centrifuge technology which is less technically challenging than a breeder and remote separation, but the yield of fissionable uranium is fixed at the U235/U238 ratio present in the ore.
A uranium bomb is easier to engineer because the gun method (which is relatively straightforward) can be used. 
