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Reference, Questions and Help => Nuke Q&A => Topic started by: JGoedjen on Oct 24, 2017, 10:08

Title: Doppler Broadening Effect Question
Post by: JGoedjen on Oct 24, 2017, 10:08

I'm just learning about the Doppler feedback effect in U-238 as a result of fuel temperature (first year nuke student). I understand that as fuel temperature increases, the probability of capture in the non-fissile U-238 increases, therefor creating a negativite reactivity feedback effect. However, why doesn't this same effect happen in the resonance regions of fissile U-235, increasing the likelihood of capture and therefor fission? Or does this effect happen in U-235 but just to a lesser extent than U-238?

Title: Re: Doppler Broadening Effect Question
Post by: TVA on Oct 24, 2017, 12:04

It doesnt have doppler broadening to any real extent. Nor is it in the core in sufficient quantity.
U235 is not a resonant absorber

Title: Re: Doppler Broadening Effect Question
Post by: thenuttyneutron on Oct 24, 2017, 05:13

(https://upload.wikimedia.org/wikipedia/commons/thumb/e/e0/U235_Fission_cross_section.png/800px-U235_Fission_cross_section.png)
(http://images.slideplayer.com/24/6999468/slides/slide_7.jpg)
Thermal Neutrons have about 0.025 electron volts worth of kinetic energy.