Doppler effect neutron absorption

[girts]

Just a quick question I got in my mind while reading up about fission, the so called Doppler effect which manifests itself in fertile material like U238, I read it helps control reactor stability , hence the majority of fuel consists of U238, as the fuel heats up at some point, U238 starts to absorb more neutrons and hence less neutrons are available for fission which decreases the further rise of reactivity,

now firstly I assume this is a natural physical phenomenon that just so happens to serve in our favor in a reactor core?
secondly tell me please what exactly is the "magic" trick here, is it that while the temperature is lower in the fuel the nucleus of U 238 has lower motion and hence a smaller cross section so only neutrons of specific energy get absorbed while others are not and when the temp rises the target nucleus kind of "moves" more and hence it somehow can accept a wider range of neutrons energies and absorb them?

 

[Astronuc]

. . . . while reading up about fission, the so called Doppler effect which manifests itself in fertile material like U238, I read it helps control reactor stability, hence the majority of fuel consists of U238, . . . .

Not hence. Natural uranium is predominantly U-238 (~0.992) with some U-235 (~0.007), with traces of U-234. U-235 is fissile, so commercial LWR fuel is enriched (up to 4.95%) in order to operate for a given energy generation over a given time (cycle). U-238 and many heavy isotopes (not usually found in a core) absorb neutrons in the so-called 'resonance' region of the neutron spectrum, that is in the high eV to ~ 1 MeV. Pu-240 is another isotope in LWR fuel that provides resonance absorption, but it is formed from transmutation of Pu-239, which is formed from U-238 through neutron absorption and subsequent beta decay.

a natural physical phenomenon that just so happens to serve in our favor in a reactor core?
secondly tell me please what exactly is the "magic" trick here, is it that while the temperature is lower in the fuel the nucleus of U 238 has lower motion and hence a smaller cross section so only neutrons of specific energy get absorbed while others are not and when the temp rises the target nucleus kind of "moves" more and hence it somehow can accept a wider range of neutrons energies and absorb them?

In resonance absorption, a high-Z nucleus, which preferentially absorbs neutrons of specific energy (resonance), helps control reactivity in the core, and it helps prevent over-power events (e.g., reactivity insertion accident (RIA)), where a control rod is inadvertently ejected (PWR) or dropped (BWR) from a core, which leads to a power excursion. When the power increases, the fuel temperature increases, and as the fuel temperature increases the atomic vibrations increase. The vibrational increase in U-238 and Pu-240 means that the resonances broaden allowing more neutrons to be capture, which prevents the power from increasing.

Decreases in fuel and moderator density also contribute to a reduction in core/fuel reactivity.