There are three chief  fissile materials that are used   in  nuclear reactions: Uranium-233 (233-U),  Uranium-235 (235-U)  and  Plutonium-239 (239-PU). In addition, Plutonium-240 (240-PU) and  Plutonium-241 (241-PU) are  produced and  consumed in  Nuclear power  production but neither can  be used  for Nuclear Weapons. Uranium-233 is produced by neutron capture of Thorium-232 in much the  same way  as  Plutonium-239  is  pro duced. But Uranium-235 is currently the most common  fuel  in  nuclear reactors. Natural Uranium must be enriched to contain about 3-5 per cent 235-U before  it can be used  in most conventional reactors. To create a weapon, Uranium must be enriched above  80 per  cent. Highly enriched Uranium, more  than 20  per cent  enrichment is also used for reactors in  naval vessels and  for research  reactors. Various techniques can be used  to enrich Uranium. Plutonium- 239 is the  preferred isotope for nuclear weapon design as it has a lower critical mass and  is easier to produce in large quantities than 235-U. In the context  239-PU and  240-PU are produced in  nearly all nuclear reactors by neutron capture on naturally occurring 238- U, and  can be easily separated from the uranium.
Working of a reactor/ fast reactor
A nuclear reactor is one where a controlled self-sustaining nuclear reaction takes place  in which  no uranium nuclei fission or break up releasing energy, manifesting as heat. In it a chain reaction is sustained and  controlled in order to produce nuclear energy, radioisotopes, or  new  nuclides. The  fuel  available for use  in  a  fission reactor are Uranium-235, Uranium-233 and Plutonium-239; only the  first occurs  in nature, the others have to be produced artificially. When  a uranium-235 nucleus is made to undergo fission  by the  impact of neutron it breaks into  two roughly equal fragments, which  release either two or three very  high  energy neutrons. These fast  neutrons need  to be  slowed down  to  increase the probability that they will  cause further fissions of 235-U  nuclei and  thus sustain the chain reaction. The  slowing down  process occurs  naturally to a certain extent when the neutrons collide  with other nuclei; unfortunately, however, the predominant uranium isotope, 238-U, absorbs fast neutrons to such  an  extent that in natural uranium the  fission reaction is not self-sustaining. In  order  to create a controlled self-sustaining chain reaction it is necessary either to slow down the neutrons to greatly reduce the  number absorbed by 238-U, or reduce the predominance of 238-U  in  natural  uranium by enriching it with more  235-U  than it normally contains.
While in fast reactor, the fission  process takes place with  high-energy neutrons, not requiring a moderator. But it is necessary to use  concentrated fissile  materials such  as  highly enriched uranium or plutonium. In these reactors, large amount of heat are produced from a small volume thus requiring special materials  for taking  away   the heat. Removal of heat from thermal reactor is  done  with  coolants such  as  carbon dioxide gas or light water or heavy water. In fast  reactors, it is necessary to employ  a coolant such  as  molten sodium. Even  in thermal reactors, there are  two basic  types, those that can  use  natural uranium as  fuel  and  those that require enriched uranium as fuel.  Naturally, occurring uranium has  two components 235-U, present to the  extent of one part in one hundred and  forty parts, which  is  fissionable and  238-U,  which  is  not fissionable. But 238-U  gets converted to artificially created fissionable material plutonium 239, after irradiation in a reactor. Similarly thorium 232  is not fissionable but  gets  converted to fissile  233-U  after irradiation in a reactor.
Status in big nations
T h e  U S ,  U S S R ,  B r i t a i n  a n d  C h i n a  b u i l t enrichment plants as part of their weapons programmes. France and  later India used  reactor-produced plutonium for the initial nuclear  explosions. The  US,  USSR,  Britain and  China also produced reactor-made plutonium for their  weapons. The  US and  USSR  took  up development of nuclear propulsion reactors for submarines  and  these reactors used  enriched uranium as fuel  and  light water as  moderator and  coolant. These reactor designs were scaled up  to provide designs for production of electricity. Such reactors are called Light Water Reactors (LWR) in  the  West and  VVER  in  the  Soviet  Union. Typically, these reactors use  uranium enriched to between three and  five  per c e n t ,  w h i l e  s u b m a r i n e  r e a c t o r s  u s e  a  h i g h e r l e v e l  o f enrichment.  Again  the Light Water Reactors as  developed in the US  have   two  variants; those that  produce steam in  the reactor vessel are called  Boiling  Water Reactors (BWRs)  and those where the hot  water from  the reactor produces steam in external steam generators are called  the Pressurised Water Reactors (PWRs).  On the  other hand, Britain and  France which initially did not have  large uranium enrichment capability developed a  graphite moderated carbondioxide cooled  reactor that could use natural uranium as fuel. In line,  Canada worked on  another reactor design that could  use  natural uranium as fuel with  heavy water as moderator and coolant. The Canadians call it CANDU reactor and the international nuclear community calls it the Pressurised Heavy Water Reactor (PHWR).
Nuclear devastations
The nuclear technology developed in the context of World War-II has produced an unimaginable horror and destructive potential which demonstrated at Hiroshima and Nagasaki. The  Cold  War  encouraged nuclear innovation. The development and  innovation related to nuclear energy were closely bound to the military context. Even  today the  very word “nuclear”  carries with it an  association of fear.  In  comparison to other sources of nuclear energy, the  fission  weapon in which a mass of plutonium or uranium in excess of critical is assembled very  quickly, with  a flood of neutrons from  a device  known as an initiator. The release of energy is extremely rapid and results in  a  massive explosion. The  concepts  of nuclear reactor and nuclear plant are different in context. In a nuclear power reactor, the reaction is  far slower and   more controlled  –  the heat produced can be harnessed to boil water to spin  turbines for the generation of electricity and this has been in practice for decades. The use  of nuclear reactors for power  generation began on 27th June 1954,  at the Obninsk power  plant in  the former Soviet Union and  has  continued in different countries to this day. Likewise a nuclear weapon is different from a nuclear plant, as in  the former there is  no  need  to control  or slow  down  the reactions that lead  to a catastrophic energy release in a short time interval which  is the  essence of bomb.  But a nuclear plant needs moderation of the reaction to sustain  a  steady but controlled release of energy. Gradually, it was  co-opted  and moved  to the  civilian sector for the production of energy for the development of global  peoples’  need, comfort and  aspiration.