Neutron cross section in fission

[cragar]

I was reading about control rods in a nuclear reactor. The carbon rods slow down the neutrons so they can interact better and induce fission. Is the reason that they don't interact when raveling fast is this because the have a smaller de broglie wavelength?

 

[ZapperZ]

I was reading about control rods in a nuclear reactor. The carbon rods slow down the neutrons so they can interact better and induce fission. Is the reason that they don't interact when raveling fast is this because the have a smaller de broglie wavelength?

Again, do the "If this is true, then..."

If having a "smaller de broglie wavelength" is all there is affecting the interaction cross section, then why do we have "fast breeder" reactor that does not thermalize the neutrons?

Zz.

 

[anorlunda]

The relationship between fission cross section and neutron energy is highly complex and nonlinear. No simple explanation is sufficient.

http://www.world-nuclear.org/uploadedImages/org/info/Nuclear_Fuel_Cycle/crosssection.gif

 

[SteamKing]

I was reading about control rods in a nuclear reactor. The carbon rods slow down the neutrons so they can interact better and induce fission. Is the reason that they don't interact when raveling fast is this because the have a smaller de broglie wavelength?

You seem to be mixing different things here.

Control rods are made of certain materials which absorb neutrons:

http://en.wikipedia.org/wiki/Control_rod

Control rods act to increase or decrease the activity inside the reactor by their relative placement inside the fuel elements where the nuclear fission takes place. They are also a safety device which, when fully inserted into the reactor core, will absorb so many neutrons, the fissioning of the fuel stops altogether.

In early reactor designs, the so-called atomic piles, nuclear reactors were crude piles of uranium pellets encased in graphite blocks:

http://en.wikipedia.org/wiki/Chicago_Pile-1

The uranium naturally gave off neutrons as it decayed, which neutrons would then strike other uranium nuclei, causing them to split. In order to increase the likelihood of these free neutrons causing uranium nuclei to fission, the carbon in the graphite served as a moderator to slow the neutrons down.

Other reactors were subsequently built which used heavy water (D₂O) to serve as the moderator substance in place of graphite blocks. The heavy water also served to cool the nuclear fuel when the reactor was active and to transmit the heat from the nuclear reaction to other machinery.