Difference between Thermal and Fast neutron in a reactor?

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SUMMARY

The discussion clarifies the distinction between thermal and fast neutrons in nuclear reactors, emphasizing that fast neutrons possess high energy and must be moderated to become thermal neutrons. This moderation occurs through multiple collisions with light nuclei, allowing the neutrons to reach energy levels comparable to the thermal energy of surrounding materials. The fission cross section for thermal neutrons is significantly larger, making them more effective for triggering fission in most reactors. While some reactors utilize fast neutrons, the majority operate on thermal neutrons, which require specific fuel compositions and cooling methods.

PREREQUISITES
  • Understanding of neutron types: thermal and fast neutrons
  • Knowledge of nuclear fission processes and cross sections
  • Familiarity with reactor types: thermal reactors and fast reactors
  • Basic principles of neutron moderation and collision dynamics
NEXT STEPS
  • Research the principles of neutron moderation in nuclear reactors
  • Study the fission cross section and its implications for reactor design
  • Explore the operational differences between thermal and fast reactors
  • Investigate the materials used in fast reactor fuel compositions, particularly Pu-(239,240,241)O2 and UO2
USEFUL FOR

Nuclear engineers, reactor designers, and students of nuclear physics seeking to understand neutron behavior and reactor operation principles.

satrohraj
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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?
 
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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.
 
Thanks,
Why can't we just let Fast neutrons into the nuclear reactor? Why they should be converted into Thermal neutrons
 
The fission cross section(basically 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
 
Last edited by a moderator:
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.
 

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