What makes Uranium-235 ideal for nuclear fission? Why not use another element all together?
In: Physics
U-235 is not ideal in some respects. The biggest drawback is that it only makes up about 0.7% of naturally occurring uranium. The rest being U-238 which cannot sustain a chain reaction.
Because the two are chemically identical and only differ slightly by mass, they are next-level difficult to separate. Fuel used in nuclear power reactors always contains a majority of U-238. But this doesn’t tend to cause major problems as long as enough 235 is present.
In most cases the uranium needs to be “enriched” in U-235 to about 2-5% to be usable. This is done by selectively removing a great deal of the U-238 and discarding it. This is an extremely technically complex, and very energy-hungry process.
In order to be usable in weapons it needs to be enriched to about 50% U-235 or more.
>Why not use another element all together?
Plutonium-239 is most commonly used in nuclear weapons. It is more ideal for this because it has a lower critical mass, meaning the the cores of weapons can be smaller and lighter.
The drawback of this is Pu is not a naturally occurring element. Fortunately, Pu-239 can be made from uranium-238. This happens when U-238 absorbs a neutron in a nuclear reactor. It then undergoes two beta decays whereby neutrons transform into protons. that is, transforming into neptunium-239 then finally plutonium-239.
In fact, significant amounts of plutonium are produced in all nuclear reactors using uranium fuel, and it is present in spent nuclear fuel. Not only that, but the production and then subsequent fission of Pu may account for 40% or more of the energy output of the reactor.
In many reactors, in lieu of enriching uranium, plutonium can be separated from spent fuel and then added to fresh uranium. This creates so called MOX or “mixed oxide” fuel. This is also where Pu used in nuclear weapons comes from.
The reason that spent fuel becomes unusable is that the products of fission act to absorb neutrons, which stops the chain reaction. these need to be removed from the used fuel before it can be recycled in this way. Because many fission products are highly radioactive, this is also a difficult process.
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