Fast reactors: How can they work without a moderator?

In summary, fast-neutron reactors are controlled using delayed neutrons and neutron-absorbing control rods or blades, rather than relying on changes to moderators like thermal reactors do. Doppler broadening from the molecular motion and thermal expansion of the fuel provide negative feedback in fast reactors. However, the amount of fissile material present is the biggest issue in fast reactors. They must be kept in the regime where delayed neutrons are the difference between critical and sub-critical in order to be controlled effectively. A "fast reactor" refers to the speed of the neutrons, not the overall speed of the reactor.
  • #1
artis
1,481
976
Pardon if this is easy to find but I tried google with little success,

so how can a fast reactor work without a moderator? Now I understand that a moderator is commonly used within thermal reactor which is why they are thermal in the first place as the neutrons get slowed down and their energy decreases, I guess what I am asking is how can a fast reactor be controlled without a runaway?

This just came to me while reading about fast reactors. The short passage in an older book says that the fuel is also more dense in a fast reactor I wonder what would that mean as I thought the fuel is just a more highly enriched uranium which contains U235 say 30% and the rest being U 238, apart from enrichment what else differs?

thanks.
 
Last edited by a moderator:
Engineering news on Phys.org
  • #2
The answer is at your fingertips with a simple search.

https://en.wikipedia.org/wiki/Fast-neutron_reactor#Control

Like thermal reactors, fast-neutron reactors are controlled by keeping the criticality of the reactor reliant on delayed neutrons, with gross control from neutron-absorbing control rods or blades.

They cannot, however, rely on changes to their moderators because there is no moderator. So Doppler broadening in the moderator, which affects thermal neutrons, does not work, nor does a negative void coefficient of the moderator. Both techniques are common in ordinary light-water reactors.

Doppler broadening from the molecular motion of the fuel, from its heat, can provide rapid negative feedback. The molecular movement of the fissionables themselves can tune the fuel's relative speed away from the optimal neutron speed. Thermal expansion of the fuel can provide negative feedback. Small reactors as in submarines may use Doppler broadening or thermal expansion of neutron reflectors.
 
  • Like
Likes artis, DEvens and Klystron
  • #3
ok, thanks I glossed somehow over it.

although isn't the text a bit misleading from the aspect where it talks about Doppler broadening? first it says "So Doppler broadening in the moderator, which affects thermal neutrons "
then a bit lower it says "Doppler broadening from the molecular motion of the fuel, from its heat "

from what I have read myself I have always understood that the Doppler broadening in nuclear fuel is related to the temperature of the fuel itself while the moderator plays no role in it. In other words U 238 and Pu240 within the fuel element heat up and then absorb more neutrons compared to when the temp. is lower.I guess the next thing would be to understand how those moderators are used in real reactors, I am familiar with thermal reactor designs to certain extent but much less with those of fast ones.
 
  • #4
The biggest issue is the amount of fissile material present. A reactor with an efficient moderator can have much less fissile material relative to a reactor with little or no moderation.

Very broadly speaking, the reactor will be critical if, on average, each neutron in a generation results in one neutron in the next generation. That requires that enough, on average, find a nucleus and cause it to fission.

The moderator slows the neutrons down. That means that you get a larger cross section for fission. (See the graph.) So, if the neutrons are thermal, they will see that larger cross section. So they will have an easier time finding some U235.

But the cross section is not zero at higher energies. So, by working harder at the design and by putting more U235 in, you can make it critical with less, or no, moderation.

Controlling the reactor is then the challenge. You must, as anorlunda said, keep it in the regime where delayed neutrons are still the difference between critical and sub-critical.

A prompt neutron is one that is released, as the name suggests, immediately on fission. A delayed neutron is released from a fission fragment through radioactive decay. These can be emitted from seconds to minutes after a fission happens. They then follow fission power up and down, but with a delay.

If you stay in the region of delayed neutrons, then you can control the reactor quite easily. The characteristic time will be many seconds. If there is a small uptick in power, you have enough time to control it using fairly ordinary mechanical systems. If you get into the region where the reactor is critical without delayed neutrons, called the prompt region, then the characteristic time is milliseconds. Or possibly even a fraction of a millisecond in some designs. Such situations can't be controlled by normal mechanical systems, and you get huge extremely bad problems. Explosive in some designs.

So a "fast reactor" means that the neutrons average faster, not that the reactor is doing anything unusually fast otherwise. Broadly speaking, things are quite similar except for:
- The spectrum tends much more strongly towards the higher energy parts.
- The fuel tends to have much higher concentrations of U235. Or other fissile material if it is another fuel type.

https://en.wikipedia.org/wiki/Neutron_cross_section#/media/File:U235_Fission_cross_section.png

U235_Fission_cross_section.png
 
  • Informative
Likes anorlunda

Related to Fast reactors: How can they work without a moderator?

1. How do fast reactors work without a moderator?

Fast reactors use a different type of fuel and a process called "fast fission" to sustain a nuclear chain reaction without the need for a moderator. In this process, the neutrons released during fission are not slowed down by a moderator, allowing them to continue splitting atoms at a faster rate.

2. What is the benefit of using fast reactors without a moderator?

By eliminating the need for a moderator, fast reactors can use a wider range of fuel types, including depleted uranium and plutonium, which are byproducts of traditional nuclear reactors. This allows for more efficient use of nuclear fuel and reduces the amount of radioactive waste produced.

3. Are fast reactors more dangerous without a moderator?

No, fast reactors are not more dangerous without a moderator. In fact, they have built-in safety features that make them less prone to accidents and meltdowns compared to traditional reactors. These include the use of liquid metal coolant and the ability to shut down the reactor automatically in case of an emergency.

4. How do fast reactors compare to traditional reactors in terms of energy production?

Fast reactors have the potential to produce more energy compared to traditional reactors. This is because they are able to use a wider range of fuel types and are more efficient in their use of nuclear fuel. Additionally, fast reactors can also be used to "burn" nuclear waste, further increasing their energy output.

5. Are fast reactors a viable alternative to traditional nuclear reactors?

While fast reactors have many potential benefits, they are still in the early stages of development and have not yet been widely adopted. There are also some safety and technical challenges that need to be addressed before they can be considered a viable alternative to traditional reactors. However, many scientists and experts believe that fast reactors have the potential to play a significant role in the future of nuclear energy.

Similar threads

  • Nuclear Engineering
Replies
2
Views
1K
Replies
3
Views
1K
  • Nuclear Engineering
Replies
9
Views
2K
  • Nuclear Engineering
Replies
2
Views
1K
  • Nuclear Engineering
Replies
11
Views
2K
Replies
7
Views
2K
  • Nuclear Engineering
Replies
1
Views
1K
Replies
13
Views
3K
  • Nuclear Engineering
Replies
4
Views
2K
  • Nuclear Engineering
Replies
1
Views
954
Back
Top