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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.