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RobusEtCeleritas t1_j4pffai wrote

When uranium-235 interacts with a neutron, sometimes you get fission, and sometimes you get other processes, like radiative capture. When uranium-235 captures the neutron and de-excites via gamma emission, what's left over is uranium-236.


Rkchapman t1_j4pjo51 wrote

Yes, and the probability of each is related to the energy of the inbound neutron, right?


RobusEtCeleritas t1_j4ppbt1 wrote



iksbob t1_j4qsn4v wrote

Does thermal energy play a role? As in, is a U235 in reactor-like conditions less likely to fuse from a neutron strike of X energy, versus a U235 in the plasma cloud of a thermonuclear device?


bikedork5000 t1_j4quv9z wrote

'Temperature' is really just a statistical construct. Thermal energy is a sum of kinetic energy of all the moving massive particles in a given region. You can have a colder area that still contains fast enough neutrons to trigger fusion, but just less of them than in a 'hotter' area in a bomb, for example. But all things being equal, more neutron flux at the appropriate energy/velocity per neutron will equal more fission interactions.


iksbob t1_j4qyvst wrote

> sum of kinetic energy of all the moving massive particles in a given region

So, high thermal energy might increase the energy of one collision, but reduce that of another. Net zero?


stefab t1_j4r0rcx wrote

Well, no, high thermal energy increases the average energy of all collisions. Such is the definition of thermal energy. Yes, included in the average collision are outlying collisions where there might be excessive energy (you'll find water evaporates even at room temperature, just at a much slower rate), or a far lower collision energy, such as in the case above of U236 being produced instead of nuclear fission.


definitivelynottake2 t1_j4r2ooa wrote

So the kinetic energy of the uranium atom plays a significant role aswell?


SharkAttackOmNom t1_j4rwnau wrote

Not really. The uranium atom would be in the ground state. Higher temperature would increase the KE of the U235 atom but it would also increase the average KE of neutrons available. And as mentioned elsewhere, higher KE neutrons have less probability of being absorbed.

So for cores designed for thermal neutrons they have a “negative temperature coefficient” or if the reactor gets hotter, fission rate decreases, bringing the temp back down. This is a nice feature to keep the reactor controlled, but it wont prevent a meltdown outright.


zemsten t1_j4t7qs2 wrote

This is due to the temperature of the moderator (water) though, isn't it? At least mostly? This is important because a fast neutron has a higher probability of slowing down through water as a moderator when that water is more dense. More slowing down -> more thermal neutrons -> higher likelihood of a thermal fission. Fast fissions are negligible except for during source range reactor startups.


SharkAttackOmNom t1_j4tfd7x wrote

Yes when referencing the negative temperature coefficient, that’s the water (and in general the heat of the whole core) which lowers reactivity due to inefficient neutron-slowing. The effect is even more pronounced of the water is allowed to boil to steam. The steam bubbles making “voids” which won’t slow neutrons, basically at all.

Neat trick can be played here. Thermal output can be controlled in a BWR reactor by increasing or decreasing the coolant flow. Faster flow will drive the threshold of boiling water higher, allowing more of the fuel rod to fission. If they want to slow the reactor, slow the coolant flow rate. The water will boil lower and reaction rate slows at the top of bundles. BWR control rods insert from the bottom so it can control reaction rates from bottom up and top down.


iksbob t1_j4raeuh wrote

> high thermal energy increases the average energy of all collisions.

I was thinking that thermal-kinetic energy of a given atom could add, subtract or have no effect on the total impact energy depending on the atom and neutron's direction at the moment of impact. Within a medium, the kinetic direction of a given particle due to thermal effects would be chaotic, so effectively random.


Ok-disaster2022 t1_j4s64pi wrote

If you looked at it on an energy diagram the reaction peak gets broadened, but the area under the peak doesn't significantly change iirc. This is doppler broadening. In certain circumstands doppler broading can allow certain reactions to occur that wouldn't.


Polyknikes t1_j4se6xz wrote

This is an interesting concept that, despite being fairly well educated, I hadnt really internalized before. Thanks!


Pedromac t1_j4t4005 wrote

I feel like what you just said was so painfully obvious but it just never occurred to me the temperature would fluctuate so much at the atomic level but reading what you said makes perfect sense. Thank you for that


Rawkapotamus t1_j4pul40 wrote

Yeah, the cross section (probability) of any given reaction is based on the relative velocity (energy) of the particle. Cross section graphs are fun to look at.


AmphoraExplorer t1_j4puzs7 wrote

Is there another factor that influences the probabilities? I want to get 3 dimensional


maddumpies t1_j4q3cmd wrote

While cross sections are heavily dependent on neutron speed, there are other factors that affect the cross section and other factors that influence the reaction rate.

The temperature of the medium also matters and you can have effects like doppler broadening that will affect the cross sections (an important part of reactor safety). Number density of course plays heavily into reaction rates and going beyond that, material geometry and type of course heavily influences a reactor design (reflectors, shielding, absorbers, etc...).


AmphoraExplorer t1_j4q3nds wrote

Ok now which of these should I use to make the trippiest visuals for a post-rock music video?


NeverPlayF6 t1_j4q8mhj wrote

Doppler broadening- the higher the temperature of the fuel, the faster the nuclei are vibrating. Nuclei can only absorb neutrons of a certain energy. If the nuclei were at rest, they would only be able to absorb a narrow range of neutrons based on the neutron's velocity/energy. Since the nuclei are in motion, the relative velocities/energies between the neutrons and the nuclei are spread out. The higher the temperature, the wider the spread. The wider the spread, the more likely that a neutron is to encounter a nuclei with the correct relative energy to absorb it.

Imagine that it is only possible to catch a baseball that is moving between 15 and 20 mph relative to the person trying to catch it. If you have 1,000 people standing still, then any baseball slower than 15 mph or higher than 20 mph cannot be caught. Now imagine that those 1,000 people are all walking around in random directions at 3 mph. It is now possible for a baseball thrown at 12 mph or 23 mph to be caught. If they're moving faster (the same as increasing the temperature of the fuel)- say 19 mph, it is possible for a baseball thrown between 1 mph and 39 mph to be caught.


jobblejosh t1_j4r38l9 wrote

Is this what causes positive temperature coefficients?


Crashastern t1_j4ru8l9 wrote

No, that has more to do with the material choices in the construction of the reactor. The moderator (the medium used to slow the neutrons to the desirable range for continued absorption but the fuel for additional fission) in different reactor designs isn’t always water. As I understand it, it’s the moderator which carries the temperature coefficient attribute. Water is negative, graphite (like in Chernobyl’s RBMK style reactor) is a positive temperature coefficient.

With a water-moderated reactor: temperature goes up -> total fission goes down -> power goes down (all else being kept equal). Which makes temperature come down. Which makes power go back up. This results in a sort of sine-wave oscillation of the reactor’s power level for a short time until other elements of operation come into play.

Graphite moderated reactor: temperature goes up -> total fission goes up -> power goes up. Which makes temperature go up. And the cycle repeats. This was a key oversight in design for what happened in Chernobyl, and why the choice of a water moderator helps to create a reactor design which is inherently stable.

Edit: Doppler broadening is more about why it’s preferable to operate with the fuel at a higher temperature from an efficiency standpoint in terms of using the available neutron flux to create sustained chain reactions.


NaomiNekomimi t1_j4q9b5p wrote

Is it related inversely or proportionally? Does a higher energy neutron make capture and fission more likely? Or is capture more likely with a low energy neutron?


Dravous t1_j4pynd2 wrote

can that U236 then react with a neutron and get you fission?


Themarshal2 t1_j4q0fg2 wrote

It's way less likely to do so, it might however absorb more neutrons and turn into another fissile nuclei down the line


Dravous t1_j4q0pv9 wrote

could it become U239?


PlutoniumChemist t1_j4q3tia wrote

This is a great question, and yes. 239U has a half life of less than 30 minutes and will radioactively decay into 239Np. This happens because there are too many neutrons in the 239U nucleus, so one of the excess neutrons will "decay" into a proton & an electron (a form of radioactive decay called beta minus decay). When that happens, the nucleus goes from 92 protons to 93 protons, which makes it Np instead of U. The 239Np will then decay into 239Pu using the exact same method with a half life of a couple minutes.

That's how we create 239Pu for nuclear weapons.

The 239Pu has a half life of 24,000 years, so only a small amount of it will radioactively decay during irradiation. Some of it will fission in the reactor, but the rest of it has a chance to "capture" more neutrons to form heavier isotopes of Pu, like 240Pu, 241Pu, & 242Pu. If this is allowed to go on long enough, then the Pu is no longer suitable for a nuclear weapon. This is why commercial power reactors don't produce weapons grade Pu - the fuel sits in the reactor too long and produces these heavy isotopes of Pu

241Pu can decay into 241Am, which can capture neutrons and decay into heavier elements, which can then capture neutrons and decay into heavier elements again, etc etc. This process can repeat all the way up to the element Fm inside a nuclear reactor.


TerjiD t1_j4q996f wrote

This is a really interesting explanation. Made me smarter.

I gotta ask though - is "decay" the correct term, when an element goes up in protons?

Only curious, just trying to get smarter.


PlutoniumChemist t1_j4qdvq8 wrote

There are multiple types of radioactive decay, some make the atomic number go up, some make the atomic number go down, but it's correct to refer to them all as radioactive decay.


cited t1_j4qfpjm wrote

Remember it also goes down a neutron when it does it. Everything wants to get to the happy line on the chart of the nuclides.


biggyofmt t1_j4qn9ek wrote

This type of decay is known as Beta minus decay. A beta minus particle (aka an electron) and a photon are emitted from the nucleus. This has the effect of raising the atomic number by one, as a neutron was converted to a proton. It is still decay because particles were emitted from the nucleus


Wiz_Kalita t1_j4s134f wrote

It's decay because the atom is going from a higher energy state to a lower energy state. It's very much like rolling down a hill except instead of picking up speed it converts potential energy into radiation.


kira01070 t1_j4ran62 wrote

Graat explanation!! Just curious, why stop at Fm?


PlutoniumChemist t1_j4rcme7 wrote

Beta minus decay is the only mode of radioactive decay that increases atomic number.

These heavy radionuclides become more and more likely to decay by alpha decay or by spontaneous fission as their atomic numbers increase and less likely to decay by beta minus.

The half lives of these radionuclides also decrease as they get heavier, meaning they don't "survive" long enough to reliably capture neutrons before they decay into something else

By the time you get to Fm, there are no accessible beta minus decaying isotopes of Fm that can be created through neutron capture before the nuclide decays by some other mode.

This logic applies to nuclear reactors. Nuclear weapons have... a much higher neutron flux. This means the Fm isotopes could potentially capture a very large number of neutrons in a very short period of time in order to create an exotic beta minus decaying isotope of Fm before it decays by some other decay mode. Not sure if this was ever observed during the various nuclear weapons testing phases across the world


UpperCardiologist523 t1_j4q24oq wrote

Not a scientist so forgive me if i mess this up.

I often hear that When you hit a U235 atom with a neutron, it splits and results in 2 OR 3 new neutrons. I've always wondered about this OR part and stuggle to grasp at this seemingly random result.

Would the chance of an U235 atom ABSORBING the neutron and becoming U236, be in the about same ballpark chance? I understand these two different actions to not be correlated/connected, but i want to understand more, struggle with reading theory on my own and just wonder.


echawkes t1_j4s3z3l wrote

U-235 has 143 neutrons. When you strike it with one more neutron, the total is 144.
Fission usually splits one atom into two atoms, with some left over neutrons. There are a number of different pairs of atoms that are possible outcomes. Sometimes, the pairs of atoms produced add up to 141 neutrons, with three free neutrons left over, and sometimes they add up to 142 neutrons, with only two free neutrons left over.

For low-energy neutrons striking U-235, the chance of fission is about 6 times as high as absorbing the neutron and becoming U-236.


UpperCardiologist523 t1_j4sgdet wrote

Does the level of energy the neutron hit with, decide what new atoms and therefore how many neutrons are left over? If not, what does? Or is it random?

Oh, and in your last paragraph. Does this mean that when they enrich uranium/plutonium, reactors are run on lower energy?

Sorry if I'm way off here. I'm a TV repair man, but curious about this.


echawkes t1_j4snn8g wrote

>Does the level of energy the neutron hit with, decide what new atoms and therefore how many neutrons are left over?

The kinetic energy of the incoming neutron can affect which pairs of atoms are produced. Higher energies generally result in more neutrons being released, which means that different fission products must result.

>Does this mean that when they enrich uranium/plutonium, reactors are run on lower energy?

Enrichment isn't done in a nuclear reactor. Typically, something like a centrifuge is used to separate the isotopes of uranium. U-235 is a little lighter than U-238, so the centrifuge can be used to get two output streams in which one has more U-235 than the input (higher enrichment) and the other has less U-235 (lower enrichment).

I think you might be asking about something like a breeder reactor, which can produce Pu-239 from U-238, or U-233 from Th-232. There have been very few of these, because some of the neutrons are used up in transmuting one element into another (by absorption without fission), which is a technical challenge. The usual technique is to use a fast reactor (high energy neutrons), so that there are more neutrons produced per fission.

There haven't been many breeder reactors because they are more complicated and expensive to build and operate than normal reactors. Uranium is pretty common and not that expensive, so we usually just mine it and use that. The uranium in power reactors is usually enriched a little. Natural uranium is 0.7% of the uranium you would find in the ground, and it is usually (but not always) enriched as high as 5% in nuclear power plants.


UpperCardiologist523 t1_j4sx3h2 wrote

Wow, thanks for a great answer. I've always thought (probably because i misunderstood or remember wrong) while watching videos about Thorium-Salt reactors, how they were better than the breeder reactors we've currently on, and that current reactors were breeders, because of how inexspensive they were to build. I better go back and watch those videos one more time.

I knew about Hanford, which is a breeder.

Anyways. Thanks a lot for answers.


zowie54 t1_j4t26ae wrote

Think about it like breaking a cookie. It will break slightly differently each time you break it, and produces two major pieces (not necessarily equally sized), and some crumbs. While it is easy to break a cookie, exactly how it will break will be determined by lots of variables, so many, that measuring the statistical frequency is usually how outcomes are predicted. Some reactions require certain minimum energy thresholds to be overcome, and the energy of an incoming particle can determine how likely certain types of decay are. That being said, a neutron is actually absorbed by the 235, which becomes unstable and breaks apart. U-235 fission produces an average of 2.41 neutrons per fission, the neutrons being analogous with a seed or something in a cookie that cannot break apart easily, and so either is in one half or the other, or in neither as a crumb.


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mfb- t1_j4posh4 wrote

The ratio of the two processes doesn't depend strongly on the energy. Here is a plot (from here), the upper red curve is fission, the lower green curve is capture and U-236 production. It's a logarithmic plot, so a constant offset between the curves means a constant ratio (of ~3).


tdgros t1_j4qbwij wrote

Are the shape of the curves (clean, a few spikes, lots of spikes, super clean from ~0.002MeV onward) related to some physical processes we know? is it just due to the scale of the plot?


mfb- t1_j4qiu6w wrote

The mess in the middle is the resonance region. If you look at zoomed in plots then you see the cross section spiking for individual resonances. Here are examples, see e.g. figure 2, that's for uranium-238 but the idea is the same for U-235.

The leftmost spikes look nicer because it's a log plot so the same energy difference gets more space in the plot.


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