Difference between Thermal and Fast neutron in a reactor?

[satrohraj]

I read a sentence in my book saying,
The fast neutrons released through the first nuclear fission are passed into the moderator, thus producing (becoming) thermal neutrons.

What is a Thermal neutron?
What is the difference between Thermal and a Fast Neutron?

 

[Azael]

Its a difference in energy.

A fast neutron has a high energy, you lower its energy by making it collide a lot of times with light nuclei(in the moderator). After a number of collisions the neutron will have been lowered to roughly the same energy as the surrounding materials thermal energy.

 

[satrohraj]

Thanks,
Why can't we just let Fast neutrons into the nuclear reactor? Why they should be converted into Thermal neutrons

 

[Azael]

The fission cross section(basicly the probability that a neutron will trigger fission)is much larger for thermal neutrons.

There are rectors that can use fast neutrons aswell. But the majority of reactors use thermal neutrons. Here is some info about fast reactors
http://www.uic.com.au/nip98.htm

 

[Astronuc]

Fast reactors require higher enrichments and preferably fuel with about 20% Pu-(239,240,241)O₂ dispersed in UO₂, which is about 3 times the maximum enrichment used in MOX fuel for thermal reactors.

Fast reactors use liquid metal for cooling and to be useful, heat must usually be passed to a Rankine cycle. This introduces the complication of the potential for liquid metal - water interaction if there is a leak in the heat exchanger.