3 ms·

In the case we are considering, it doesn’t, but it could with other materials. Consider that the wavelength of the neutron is a function of its energy, and tha

by dumah 2y ago

In the case we are considering, it doesn’t, but it could with other materials. Consider that the wavelength of the neutron is a function of its energy, and that the cross sections for interaction between nuclei and neutrons are strong and complex functions of energy. If the cross section for the interaction of interest gets smaller with decreasing energy, then it would be the case that the neutrons mean free path length would increase as energy decreased.

pfannkuchen 2y ago
> In the case we are considering, it doesn’t, but it could with other materials. Sorry, I said something subtle and easy to miss and also made a confusing typo, writing too fast. "average distance a [nucleus-hitting neutron]" As in, as more material is added, the percent of neutrons that successfully collide and don't just fly out increases. But, for the class of nucleus-hitting neutrons, the average distance prior to collision increases. If the neutron loses energy as it travels, then as the average distance increases I suppose the probability of splitting the collidee nucleus decreases. So as the class increases in size, its rate of nucleus splitting may fall below the threshold, which bounds the useful size increase. Perhaps this doesn't occur until the object has grown in size way past the point of basically guaranteed criticality, I haven't done the math, just curious since GP's statement sounded as if neutrons do not lose energy across any distance and the object could therefore could be increased to an arbitrary size while maintaining the same qualitative per-iteration behavior, and I find that surprising.
- Tuna-Fish 2y ago
  > If the neutron loses energy as it travels Excluding collisions, it does not. As far as the neutron is considered, it's traveling through empty space, just as if it was in vacuum. > I suppose the probability of splitting the collidee nucleus decreases. In this regime, probability of splitting a nucleus goes up as energy decreases.
Tuna-Fish 2y ago
In the regime that's interesting for pure fission devices, the opposite is true. The cross section increases as energy decreases. This is why moderators are a thing in nuclear reactors.

by dumah 2y ago

pfannkuchen 2y ago

> In the case we are considering, it doesn’t, but it could with other materials. Sorry, I said something subtle and easy to miss and also made a confusing typo, writing too fast. "average distance a [nucleus-hitting neutron]" As in, as more material is added, the percent of neutrons that successfully collide and don't just fly out increases. But, for the class of nucleus-hitting neutrons, the average distance prior to collision increases. If the neutron loses energy as it travels, then as the average distance increases I suppose the probability of splitting the collidee nucleus decreases. So as the class increases in size, its rate of nucleus splitting may fall below the threshold, which bounds the useful size increase. Perhaps this doesn't occur until the object has grown in size way past the point of basically guaranteed criticality, I haven't done the math, just curious since GP's statement sounded as if neutrons do not lose energy across any distance and the object could therefore could be increased to an arbitrary size while maintaining the same qualitative per-iteration behavior, and I find that surprising.

Tuna-Fish 2y ago
> If the neutron loses energy as it travels Excluding collisions, it does not. As far as the neutron is considered, it's traveling through empty space, just as if it was in vacuum. > I suppose the probability of splitting the collidee nucleus decreases. In this regime, probability of splitting a nucleus goes up as energy decreases.

Tuna-Fish 2y ago

In the regime that's interesting for pure fission devices, the opposite is true. The cross section increases as energy decreases. This is why moderators are a thing in nuclear reactors.