Slow or fast neutrons in LFTRs (Liquid Fluoride Thorium Reactors)

  • Thread starter kiskrof
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In summary, a fast breeder reactor uses fast neutrons which have more energy than thermal neutrons. This allows them to create more neutrons per neutron absorbed, which is why thorium can work as a thermal breeder.
  • #1
kiskrof
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Some nuclear reactors are called fast breeders because:
1. they use fast neutrons, which means the neutrons have more energy than "thermal neutrons", that have the same energy as the surrounding material.
2. they are called "breeders" because they "breed" fuel. U238 is not a fissile atom, but by absorbing a neutrons and spitting out two electrons out of the nucleus (so that two neutrons can turn into protons), it can be turned into Pu239, which is fissile.
Fast breedeers are not so common. Though there is a large one in Belayarsk Russia currently working and another in Monju Japan, France stopped its own (Superphenix) some years ago.
The trouble with breeders is you need more neutrons. If your reaction is based on U235, a naturally occurring atom, you just need one neutron to make it fission and get energy. If it is based on U238, you'll need two neutrons, one to turn the U238 into Pu239, another to fission the Pu239. The problem is solved by working with fast neutrons, because high energy fissions produce more neutrons.
Thorium 232, an extremely abundant element, is fertile too. If it absorbs a neutron and spits two electrons out, it becomes U233, which is fissile. So I naively supposed that the possible reactor of the future, the utterly fascinating LFTR (Light Fluoride Thorium Reactor), would need fast neutrons too. But apparently, it uses thermal neutrons (for example, you can see this page, which looks serious though it is not written by a specialist: http://www.2112design.com/blog/lftr/) It seems to me there will never be enough neutrons. Do you have any solution to that?
 
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  • #2
Instead of looking at the number of neutrons emitted in fission, its helpful to look at the number of neutron emitted per neutron absorbed by the fissile material. The difference is that the fuel can sometimes absorb neutrons and not fission.

Lets look at this ratio for a few different fissile isotopes for both thermal and fast neutrons.


Thermal
U-235 2.08
Pu-239 2.03
U-233 2.31


Fast
U-235 2.4
Pu-239 2.9
U-233 2.5

A couple of things. First for all these isotopes a fast spectrum will produce more neutrons per neutron absorb by the fissile material. Second, for U-235 and Pu-239 the ratio is just a little above 2. It would be very hard to make a thermal breeder reactor with either of these 2 isotope. However, the ratio for U-233 is noticeably larger. While not as larger as the ratio for a fast spectrum, it is still large enough.

U-233 is the fissile material in a thorium reactor. And the fact that is has such a large ratio of neutrons produced per thermal neutron absorbed, is why thorium can work as a thermal breeder.

You also have to also consider the various neutron loss mechanisms. By looking at the ratio of neutrons produce be neutron absorbed I've accounted for one loss mechanism (neutron capture by the fissile material), but there are others. For example neutrons can be absorbed by other reactor materials (like a control rod), or they can escape out of the reactor (leakage).
 
  • #3
You can breed fuel in a thermal U-235 reactor, you just can't achieve a breeding ratio greater than 1. Meaning it would not be indefinitely self-sustaining and would require refueling with fissile material in addition to fertile material.
 
  • #4
QuantumPion said:
You can breed fuel in a thermal U-235 reactor, you just can't achieve a breeding ratio greater than 1. Meaning it would not be indefinitely self-sustaining and would require refueling with fissile material in addition to fertile material.
I think the breeding ratio for one of ORNL's designs was 1.07. They sacrificed just about every other design consideration to achieve that, however. These days, given the availability of uranium, 10 or 20% makeup fuel is not a big deal. This expands the design space nicely.
 
  • #5


I can provide some insights into the use of slow or fast neutrons in LFTRs. First of all, it is important to understand that the choice of neutron speed in a reactor is determined by the type of fuel being used and the desired reaction. In the case of LFTRs, thorium is the primary fuel and it can be converted into fissile U233 through the absorption of a neutron. This process can take place with both slow or fast neutrons, but the efficiency of the conversion is higher with fast neutrons.

The use of fast neutrons in LFTRs is not only limited to the production of fissile U233 from thorium. It also allows for the production of additional neutrons through high-energy fission reactions, thereby increasing the overall neutron population in the reactor. This is important because as you mentioned, breeding U233 from thorium requires two neutrons, one to convert thorium into U233 and another to cause fission. Therefore, having a higher neutron population is necessary for the efficient production of U233.

On the other hand, slow neutrons are used in some other types of nuclear reactors, such as pressurized water reactors, because they are better suited for the fission of U235 and other fissile materials. However, these reactors are not suitable for the production of U233 from thorium.

In terms of the future of LFTRs, there are ongoing research and development efforts to optimize the use of fast neutrons in these reactors. This includes the design of advanced fuel configurations and moderator materials that can further enhance the efficiency of U233 production.

In conclusion, the use of fast neutrons in LFTRs is essential for the efficient production of fissile U233 from thorium. While there may be challenges in maintaining a high neutron population, ongoing research and development efforts are focused on addressing these challenges and making LFTRs a viable and sustainable energy source for the future.
 

1. How do slow and fast neutrons differ in LFTRs?

Slow neutrons have a lower energy level than fast neutrons, making them more efficient at fission in LFTRs. Slow neutrons are able to better penetrate the thorium fuel and cause fission reactions, while fast neutrons tend to pass through the fuel without causing fission.

2. What advantages do slow neutrons have in LFTRs?

Slow neutrons are able to sustain a chain reaction in LFTRs without the need for an external neutron moderator, making the reactor design simpler and more efficient. They are also able to convert thorium into usable fissile material at a higher rate compared to fast neutrons.

3. Are there any disadvantages to using slow neutrons in LFTRs?

The main disadvantage of using slow neutrons in LFTRs is that they require a higher initial energy input to start the chain reaction. This can make the startup process more complex and time-consuming compared to using fast neutrons.

4. How does the use of slow neutrons affect the safety of LFTRs?

The use of slow neutrons in LFTRs increases the safety of the reactor by reducing the risk of a runaway chain reaction. Slow neutrons are less likely to cause a sudden increase in fission reactions, making it easier to control the rate of energy production.

5. Are there any other applications for slow neutrons in nuclear technology?

Yes, slow neutrons are also used in other types of nuclear reactors, such as pressurized water reactors and boiling water reactors. They are also used in neutron scattering experiments for materials research and in medical applications, such as cancer treatment.

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