The vast majority of uranium produced today is destined for use in civilian power plants. In 2015, nuclear energy generated around 11% of the world’s electricity.
After yellowcake is safely transported and converted, it is ready to be used as a source for nuclear power via fuel fabrication or enrichment. For nuclear weapons, additional processing or enrichment is required to make the material weapons-grade. Whether the path is ‘ore to bomb’ or ‘ore to electricity’, the process is long, challenging and requires sophisticated technologies.
Types of Enrichment
The majority of nuclear power reactors use uranium which is enriched to 3 to 5 percent of U235, the only isotope existing in nature that can sustain a chain reaction, or is “fissile.” Enriched uranium containing  less than 20 percent of U235 is considered “low enriched uranium” (LEU) while highly enriched uranium (HEU) has a concentration of U235 of 20 percent or greater. Fissile uranium used in nuclear weapons is usually enriched to 90 percent or more U235, known as weapons-grade.
Plutonium also has fissile isotopes. Plutonium is almost entirely human-made as a byproduct in nuclear reactors where some of the neutrons released by the fission process convert U238 into plutonium. Weapons-grade plutonium contains around 90-95 percent plutonium-239 (Pu239) while the range of 50-60 percent plutonium-239 is known as reactor-grade plutonium. Any plutonium that does not fission before the fuel is removed from the reactor remains in spent fuel, which some countries reprocess to extract plutonium and uranium to make new fuel. 
Plutonium does occur naturally in very small amounts from uranium ore called “pitchblende,” or uraninite, which is one of the primary mineral ores of uranium. While pitchblende can contain 50-80 percent of uranium, it only contains a few parts per trillion of natural plutonium.

The nuclear weapon dropped on Hiroshima, Japan on 6 August 1945, code-named “Little Boy,” was made from HEU. The “Fat Man” bomb dropped on Nagasaki three days later was a plutonium fission bomb. At the time, the United States was the only country to have developed nuclear weapons. 

The World’s Uranium and Plutonium Stockpile
According to the International Panel on Fissile Materials, the global stockpile of HEU is estimated to be around 1370 tonnes as of January 2015, with the stockpile of separated plutonium about 500 tonnes, of which approximately 270 tonnes are in civilian custody.

China, France, Russia, UK and US, the five recognized nuclear weapons states (NWS) under the Treaty on the Non-proliferation of Nuclear Weapons (NPT), had been exercising a voluntary moratorium on the production of fissile material for use in nuclear weapons since the 1990s. In 2012, however, Russia resumed HEU production for icebreakers and domestic and overseas research reactors. Of the three states that never signed on to the NPT (India, Israel and Pakistan), India and Pakistan continue to produce fissile material for use in nuclear weapons. Israel still operates its Dimona reactor, but it is believed to be producing tritium rather than plutonium. North Korea is also producing weapons-grade plutonium and HEU and is the only country to have withdrawn from the NPT.

Between 1945 and 1996 when the Comprehensive Nuclear-Test Ban Treaty (CTBT) opened for signature, more than 2,000 nuclear tests were carried out all over the world. Since 1996, France, Russia and the UK have ratified the CTBT while China and the US have signed but not yet ratified. India conducted five tests in 1998 as did Pakistan – neither has signed the CTBT. Of the 191 states that have joined the NPT, North Korea is the only one to have acceded (in 1985) and then announced its withdrawal (in 2003). North Korea conducted its first nuclear test on 9 October 2006, followed by a second one on 25 May 2009, a third on 12 February 2013, fourth on 6 January 2016, fifth on 9 September 2016, and a sixth on 3 September 2017.
The International Atomic Energy Agency (IAEA), which promotes peaceful uses of nuclear energy, verifies NPT safeguards compliance among member states.