I took freshman chemistry in college. Fifty years ago. Not my best subject. I know what an isotope is. I know how to read the periodic table. I know how to light a Bunsen burner. I have a vague idea about electron energy levels. I know where the emergency eye wash is. That’s about it. I don’t need this explained like I’m 5, but maybe more like I’m 18.
I’ve been hearing so much about enriched uranium lately I did a little lightweight research. I knew that uranium is used is bombs but I didn’t really know what enrichment is all about. The first thing I learned is that virtually all naturally occurring uranium is in the form of isotopes (I had a hard time finding out the number of neutrons directly; they generally give the atomic weight instead). The isotope needed for fissile material is rare. You have to extract that to get material rich enough to start a fission reaction. So that’s basically what enrichment is. You need about 5% for reactors to generate power but something like 90% for a bomb.
But my questions aren’t specifically about bombmaking, just basic chemistry.
How many neutrons are in non-isotopic (is that a word?) uranium?
Why does uranium occur in nature only in its isotopes? These isotopes are radioactive and I would have thought that the non-isotopic form would be most stable and therefore most abundant.
Why is U-235 fissile? What is it about a couple of missing neutrons that gives it this property, but not the other isotopes?
The first two questions are a misunderstanding of what isotopes are. There’s not generally such a thing as non-isotopic, because isotopes of an element are just the different variations of neutrons in the nucleus. Minor caveat: something like a dozen elements don’t have any naturally-occurring isotopes, but the rest do, in various numbers and quantity. Those could be considered non-isotopic.
Uranium naturally comes in U238 (vast majority), U235 (boomy stuff), and U234, which is a decay product of U238, but in tiny percentages, because it’s half-life is so much less. U238 and U235 have ridiculously high half-lifes, in the hundreds of millions of years. Fissile doesn’t mean it decays quickly on it’s own, but that it can be made so.
As for why U235 is fissile…I’ll let someone else answer, because I don’t know.
Okay, well, first off, all uranium (all elements, really) are “isotopes”. An isotope is just an atom with the same number of protons as all the others, but with different numbers of neutrons.
The atomic number is how many protons there are, which is also the number of electrons when it’s electrically neutral. U has atomic number 92, so you can figure out the number of neutrons by subtraction from the isotope, U235 vs U238.
Uranium-235 fissions with low-energy thermal neutrons because the binding energy resulting from the absorption of a neutron is greater than the threshold required for fission; therefore uranium-235 is fissile. By contrast, the binding energy released by uranium-238 absorbing a thermal neutron is less than the critical energy, so the neutron must possess additional energy for fission to be possible. Consequently, uranium-238 is fissionable but not fissile
Gotta be careful with copy-pasting those subscripts. Those should be 4.463×109 and 7.04×108.
While we’re at it, different isotopes of the same element are almost identical chemically (you can just barely detect the difference between normal hydrogen and deuterium, with sensitive chemical experiments, but it’s much harder for masses in the hundreds). Which means that enriching uranium is very, very difficult: You either have to use processes like mass spectrometry, which work on one atom at a time and are therefore very slow, or things like centrifuges, which case the bottom of your sample to have slightly more of the heavy isotope and the top to have slightly less, and repeat that process many, many times until it’s enriched enough.
An element that has two different nuclear configurations (number of neutrons) has isotopes. Uranium has six possible isotopes but of the naturally occurring ones they are almost exclusively in two isotopes: 235U and 238U, plus traces of of 233U, 234U, and 236U. 232U only exists due to synthesis in from decay of 233U. The Wikipedia article on isotopes of uranium is pretty good in explaining this.
All isotopes of uranium are radioactive (alpha emitters, 238U will sometimes undergo double beta decay) but 238U has by far the longest half-life (about 4.4 billion years). All uranium is formed from stellar synthesis processes and so all uranium on Earth (which actually comes from multiple sources but all about the same age) has the same composition in terms of ratio of 235U to 238U (0.720% to 99.3%), which has been increasing over time as 235U decays.
This is a complicated question to answer in detail, but in essence 235U has a larger neutron absorption cross-section (absorbing a neutron will cause spontaneous decay). What this means is that given a critical density 235 can sustain a fission chain reaction without any outside source of neutrons, making it a useful source of self-generating power. The need to ‘enrich’ (increase the ratio of 235 to 238) is necessary to sustain the fission chain reaction in light water and other non-moderated reactors, but a heavy water reactor like the CANDU type they can use unenriched uranium.
I’m sure you’ll have more questions following this. Please don’t hesitate to ask. More people should understand the fundamentals of nuclear technologies because there is a surfeit of misinformation about them.
Well, sorta. For one, you don’t even need to perform chemical experiments since heavy water has an easily measurable difference in density. The freezing/boiling point is also different from normal water.
As for actual chemical reactions, you can feed it to a plant or a mouse, and it’ll die once the heavy water fraction gets too high.
Alternatively, you can just taste it. Heavy water is a tad sweeter than normal water.
This is (mostly) a scientific ‘urban myth’. While the strength of hydrogen bonds is slightly affected by the presence of deuterium, the difference is very small, and you’d essentially have to replace about half of the water in your body to be biochemically impacted by heavy water.
Uranium has 92 protons. The number of neutrons is what determines its isotope. If the number of protons was different, it’d be a different element.
Uranium-238 is the most abundant on earth, it makes up 99.27% of all Uranium on earth. It has 92 protons and 146 neutrons.
Uranium-235 is the fissile material that is used in nuclear weapons. It makes up 0.72% of all Uranium on earth. It has 143 neutrons.
The other 0.01% of Uranium on earth is a mix of Uranium-232, Uranium-233, Uranium-236
Uranium-237 and Uranium-239 can occur in nuclear reactions, but they undergo radioactive decay and become other elements like Neptunium and Plutonium pretty fast.
The half lives of different Uranium isotopes is different. Uranium-238 has a half life of about 4.5 billion years. Uranium-235 has a half life of 700 million years.
Due to this half life difference, the Uranium-238 is the one that lasts while other isotopes of Uranium convert to other elements.
FWIW, about 2 billion years ago on earth there was enough Uranium-235 to cause spontaneous nuclear reactions on earth. Right now Uranium-235 is at 0.7% of Uranium, but a couple billion years ago it could be 4% which is enough to cause nuclear fission.
Certainly there’s all sorts of sensitive things going on in biology, but the experiment itself could be done by a high schooler (or a grade schooler if you don’t care about capturing the lost water).
It would take months of sustained consumption of heavy water to replace enough water in your body to cause severe impact. Plants, especially leafy plants, would be more quickly affected.
Getting back to uranium, it should be noted that it is quite toxic if absorbed, although generally not because of its radioactivity (except for possibly 232U, or if inhaled in very large doses of fine dust) but because it is a heavy metal which accumulates in the body and in many compounds in which it is used is a serious carcinogen and neurotoxin.
While we’re citing wikis, I’ll add this one to the mix:
The nature of radioactivity is such that e.g. U238 doesn’t decay to U235 to U234 to U233.
Instead …
If a nucleus is unstable, eventually it’ll spit out some hunk of neutrons and or protons. For highly radioactive substances, “eventually” may be measured in milliseconds. For highly stable substances “eventually” may be measured in hefty fractions of the lifetime of the Universe.
By the very definition of what an “element” is, if a nucleus of whatever element spits out one or more protons, it becomes a different element. And now the decay properties of this new nucleus are those of the element (and isotope thereof) it became, not the one it was before.
Over time each nucleus does a slow march down the chain of possible decays and eventually settles at something highly stable.
In the cited wiki you can scroll down to the Uranium section for a decent graphic of how that all works. There’s a multi-branching chain, but eventually all the U238 turns itself into lead via one route or another.
It’s misleading to say that decays happen by “spitting out a hunk of neutrons and/or protons”. For one thing, by far the most common “hunk of neutrons and/or protons” is an alpha particle, which is exactly two of each (same as an ordinary helium nucleus). This results in atomic number decreasing by 2, and atomic mass decreasing by 4. While I’m sure that there are a few isotopes that sometimes spit out some other hunk, they’re certainly rare enough that I can’t name any offhand.
For another, there is also another common decay mode, that doesn’t involve spitting out any neutrons or protons at all. That’s beta decay, where one of the neutrons in a nucleus turns into a proton, and spits out an electron (plus an antineutrino, but those almost never matter). This leaves the mass number unchanged, and actually increases the atomic number.
Because both of these processes change the mass number by a multiple of 4, and because all other decay processes are so rare, decay chains fall into four families, based on the mass number mod 4.
Not relevent to Uranium, but for completeness, there is yet another mode which is sort of the reverse of the above: electron capture. A proton absorbs one of the atom’s electrons and becomes a neutron. Thus reducing the atomic number by 1.
Usually occurs in unstable isotopes of lighter elements with a high ratio of protons to neutrons.
The simplistic explanation I’ve heard for the reactivity of U235 vs U238, is that the number of protons and neutrons forms a less fragile nucleus so it takes a stronger impact from, say, a neutron to split the nucleus. Something about filling a symettrical number of spots in the arragement of the nucleons.
And conveniently, when U235 splits it gives off 3 neutrons with high enough energy that if they hit another U235 nucleus before departng the uranium mass, they each could split that atom. So the “critical mass” is the amount and density of U235 in a volume -does more than 1 neutron have the odds of hitting a vulnerable nucleus? If so, then that produces 3 more, which could produce 9 more, which produce 27 more, …etc. (Plus, if the uranium is too rich with U238, those neutrons statistically would mostly hit U238 and not have the same effect)
Hence the process of exploding an atomic bomb - compress as much of the right amount of U235 together so that before, it was safely not enough U235 within a given volume, and for a monent after, enough. And the process of running a nuclear reactor is the opposite - keeping the reaction going with enough uranium within the critical mass, but no so much that the process runs away and melts down.
One thing that helps a lot in a nuclear reactor is that not all of the neutrons are “prompt neutrons”, emitted immediately as soon as they’re triggered. If they were, then it’d be very difficult to balance the reaction at that just-barely-critical level without it running away. Rather, a fair fraction of the neutron emissions are delayed by as much as a few seconds (and then a fair fraction of the ones they trigger by a few more seconds, etc.), which means that a runaway would be much slower. So you just have to keep the reactor in a state where the prompt neutrons aren’t enough to make it critical, but all of the neutrons together are enough, and then it’ll be manageable.
How do centrifuges actually work? I mean, it’s not like you drop a lump of U238 in a tube and spin it around, right? What goes into the tubes? U238 powder? U238 suspension?