Which Nuclide, 235U or 239Pu, Has a Smaller Critical Mass and Why

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Homework Help Overview

The discussion revolves around comparing the critical masses of the nuclides 235U and 239Pu, specifically exploring which has a smaller critical mass and the underlying reasons for this difference.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the relationship between neutron production and critical mass, with one suggesting that 239Pu may have a smaller critical mass due to its higher neutron yield per fission. Questions arise regarding the fate of produced neutrons and the implications of neutron lifetimes on fission events. There is also consideration of the cross-sections for fission in relation to neutron energy.

Discussion Status

The discussion is active, with participants exploring various factors that influence critical mass, including neutron behavior and fission cross-sections. There is no explicit consensus yet, but productive lines of inquiry are being pursued.

Contextual Notes

Participants are considering the effects of neutron energy on fission processes and the implications of different neutron lifetimes for the two nuclides under discussion.

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Homework Statement


Which of the two nuclides 235U and 239Pu has a smaller critical mass and why


Homework Equations





The Attempt at a Solution



i'm thinking 239Pu since it produces more neutrons on average per fission. Would that be correct? Are there any other reasons I can add?
 
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What has to happen to the neutrons that are produced?
 
Hmmm cause more fissions? Ok so their lifetimes should matter too, as the longer neutrinos last the more fissions that could potentially cause. But should this be different for the two cases?

Or are you more leading me towards the respective cross-sections? As far as I know, in 235U fission can be induced even by a 0 energy neutron while high energy neutrons are needed for Pu. But then again, cross-section of fission falls for 235U for higher neutron energies. So the answer would depend on the neutron energies. :S.
 
trv said:
Or are you more leading me towards the respective cross-sections? ...so the answer would depend on the neutron energies. :S.
Yes .
 

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