Difference between Thermal and Fast neutron in a reactor?

In summary, fast neutrons released through nuclear fission can be converted into thermal neutrons by colliding with light nuclei in a moderator. The main difference between the two is their energy level, with thermal neutrons having a lower energy. This is why thermal neutrons are preferred in most nuclear reactors, as they have a higher probability of triggering fission. However, there are some reactors that can use fast neutrons as well, but they require higher enrichments and use liquid metal for cooling. This can lead to potential complications in the heat exchange process.
  • #1
satrohraj
12
0
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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  • #2
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.
 
  • #3
Thanks,
Why can't we just let Fast neutrons into the nuclear reactor? Why they should be converted into Thermal neutrons
 
  • #4
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
 
Last edited by a moderator:
  • #5
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.
 

1. What is the main difference between thermal and fast neutrons in a reactor?

The main difference between thermal and fast neutrons in a reactor is their speed. Thermal neutrons have a slower speed, while fast neutrons have a higher speed. This difference in speed affects how they interact with other particles and materials in the reactor.

2. How do thermal and fast neutrons contribute to the nuclear reaction in a reactor?

Thermal neutrons are more likely to be absorbed by heavy nuclei, causing them to split and release energy. Fast neutrons, on the other hand, can cause fission in lighter nuclei and also contribute to the production of more neutrons, which can sustain the nuclear reaction in a reactor.

3. Which type of neutron is more efficient in producing energy in a nuclear reactor?

Fast neutrons are more efficient in producing energy in a nuclear reactor. This is because they have a higher probability of inducing fission reactions and can also produce more neutrons to sustain the reaction.

4. How are thermal and fast neutrons controlled in a nuclear reactor?

Thermal neutrons are controlled by using materials, such as control rods, that absorb them and slow them down. Fast neutrons are controlled by using materials, such as moderators, that can slow them down and convert them into thermal neutrons.

5. Can thermal and fast neutrons be used interchangeably in a nuclear reactor?

No, thermal and fast neutrons cannot be used interchangeably in a nuclear reactor. The different speeds and behaviors of these neutrons require different reactor designs and fuel compositions to efficiently produce energy.

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