What makes Uranium-235 ideal for nuclear fission? Why not use another element all together?
In: Physics
Uranium has a lot of advantages. Uranium is common. It looks like any ordinary solid metal and doesn’t require any special measures to store, transport and transform. Uranium-235 is AFAIK the only fissile isotope that occurs naturally. The fission of Uranium produces Plutonium which in turn can be used in all sorts of cool things like going back in time with a DeLorean.
Every nucleus type works differently. A few will fission if hit with neutrons. And when they do fission, even fewer will emit neutrons that can then fission other atoms (“chain reaction”).
You can use other isotopes of different elements like plutonium, however it depends how easily that isotope is available and what it leaves behind when it splits.
A uranium ore is used that is made up for different types of uranium, including u-235. If I recall something like 5% or less of the uranium in the ore is of the 235 variety.
To answer you question more specifically, every element and even every isotope of each element (say u-235 vs u-238) is stable or unstable. Some will stay as they are for all of cosmic time (maybe), others only last millionths of seconds before decaying.
Some, like uranium can be induced to decay. When you bombard the uranium ore enriched with u-235 this will cause the decay at the rates we find useful for power generation.
You could make nuclear power plants (or bombs) of many other elements and isotopes but they wont be as efficient or they may be completely useless (i.e. it takes more energy to cause the atom to decay than we get our of it).
Tl;dr – it’s the best mix of safe, economical, and viable element for what we need at this time.
A istope is fissile if when they absorb a neutron they go into fusion and not all element are fissile. Some element are fertile, meaning that if they absorb a neutron, they become fissile. Those could be used in a nuclear reactor, but you need twice the amount of neutron to sustain the reaction and so there is a big chance that the reaction will stop by itself. In Nuclear reactor the point is that the reaction produce more neutron than you need to create more fusion, so the reaction is self-sustaining. It would be like having a fire that you need to constantly restart, not very good to produce energy.
But most of element just become radiactive if they absorb a neutron, they don’t go into fusion at all. You need an element that is heavy enough.
Finally, uranium can be found in nature, meaning that you can have a good amount of the stuff at a relatively low price.
U235 is great as a fission source in nuclear reactors for many reasons: abundance, reaction cross-sections and products.
Abundance: uranium is relatively abundant. In the early 50s and 60s, uranium was thought to be more rare, so alternative fuel sources such as thorium were considered. This fell out of favor after uranium was discovered to be roughly as abundant as Thorium overall, with less processing needed to produce fuel.
However Thorium 232 much like U238 are both breeder material. That is they require capturing typically a faster neutron to eventually become u233, P239 which then fissions when hit by a neutron. U235 in particular is convenient because it has a particularly high “thermal” neutron crossection or reacts with fission more readily with slower (having the equivalent energy of a room temperature) neutrons. Remember it’s easier to slow a neutron down that it is to speed it up. The other fissionable nuclides likewise have higher lower energy cross sections. Fission neutrons are typically fast, and you can create reactors that breed their own fuel to burn as fission, however those higher energy neutrons will more readily damage your vessels and mechanics, reducing the lifetime to around 20-30 years, where some light water thermal u235 reactors will have been operating for 60+ years.
Now breeder reactors do have the benefit that the higher neutrons will essentially cause all kinds of reactions reducing the overall mass and volume of waste, however it has to be processed,refined and recycled to retrieve that other 95% of potential energy, and those processes are far more expensive.
Think of atoms as towers made of jenga blocks. You are trying to collapse them (split them up) by throwing a single block at the stack.
Some arrangements, like all the blocks laid flat on the table, can’t easily collapse. They’re as low as they can go already, so splitting them up actually takes energy.
Others might be closer to a normal stack of blocks. Sure, you could knock it over, but it’s not a sure thing and sounds like a lot of work. And since at the end of the day you need one tower to fall into the next and so on (chain reaction) this doesn’t work for you either.
But what if we could arrange a tower, balanced on top of a single block? Surely no matter how you hit it, it would fall. And the pieces of that one should hit similar arrangements nearby.
This is why we pick U-235. It’s unstable enough to fission easily when we need it to, but stable enough that it doesn’t simply decay into other elements in the time it takes to refine. It also has a long-ish decay chain, yielding lots of useable energy (the analogy doesn’t work for this part.)
Reactors have been proposed and/or implemented with other elements, notably thorium, but they are not the prevalent design.
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.
So there are some explanations here of what nuclear fission is, and some discussions of other elements that also can do it. But you might ask, why uranium-235, and not the much more common uranium-238? Why plutonium-239, and not plutonium-240, or plutonium-238? In other words, why are there only a small number of types of elements (isotopes) that work for this?
The answer gets very technical very quickly, but we can generalize it. If you are looking for an element that is _fissile_ — that can sustain a chain reaction — for various nuclear physicsy reasons they all have:
* very heavy nuclei (like the element uranium and above, which have at least 92 protons)
* they need an _odd_ number of neutrons (uranium-235 is 92 protons and 143 neutrons, whereas uranium-238 is 92 protons and 146 neutrons)
* and to be practical, it needs to be something you can produce in relatively high amounts
Why do you need these things? The heaviness requirement is because the nucleus gets sort of sloshed around. It needs to be _just_ on the edge of stability, so that a little jostling in one direction or the other makes it stretch ([fairly literally](https://www.kullabs.com/uploads/liquid_drop_model.jpg)) to the point where it is only stable as two separate pieces. Uranium is not an absolute limit for this, but it’s right on the edge of near-stability (which is why it is still around naturally after billions of years — it has a very long half-life).
As for the odd number of neutrons requirement, the issue is complicated and gets into how nuclei work. They are sort of a balance between protons and neutrons, though it is not a 1-to-1 balancing. Nuclei with odd numbers of neutrons tend to be more willing to accept a low-energy neutron, like the kinds of neutrons that fission reactions release. Whereas isotopes with even numbered nuclei, like uranium-238, only undergo fission with high energy neutrons, higher than those released by the fission process itself.
This is not very ELI5, but I’ve tried to simplify it a lot, and I have confidence you (and others) who are not literal 5-year-olds can wrap your head around it. The short answer is, there are many isotopes which can be used for fission reactions according to the above criteria, but only a few of them are practical to produce (uranium-235, uranium-233, and plutonium-239, more or less — there are a couple others, but they are much harder to produce than these three).
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