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Rkchapman t1_j4pjo51 wrote


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?