Water vs. Liquid Sodium: The Optimal Coolant for Fast Reactors?

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In summary: Higher heat capacity and high boiling point mean you don't need as much of it in the core to provide coolingWater has a really high specific heat capacity; something like liquid sodium or some other metal is actually inferior to water in terms of its heat capacity. But you need something fairly heavy as the coolant so it's not moderating the neutrons, and the liquid metals meet that requirement whilst also providing reasonably good specific heat capacity, even though the specific heat capacity would strictly speaking be inferior to water.If you take those into account - liquid sodium is a MUCH, MUCH BETTER coolant than water.
  • #1
mohsenman
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Whats the problem with using water in a fast reactor. I mean moderating neutrons in a fast reactor just increases the fission chance and that is not bad is it?
 
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  • #2
Moderating neutrons increases the fission chance of Pu-239 and U-235 but U-238 has effectively no chance to absorb thermal neutrons. So, it might work, depending on the fuel type and enrichment, e.g. if the reactor is high-enriched uranium or plutonium then I think it would work. But if it is, say 20% plutonium and 80% U-238, the U-238 can't do anything with thermal neutrons and so you will just burn up your feed plutonium without breeding any more.
 
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  • #3
mohsenman said:
Whats the problem with using water in a fast reactor. I mean moderating neutrons in a fast reactor just increases the fission chance and that is not bad is it?
If one moderates a fast reactor then it's not really a fast reactor.

The idea of a liquid metal is that it has high thermal conductivity and won't necessarily boil.

Water may boil, as well as moderate. The problem is that to minimize moderation, one would minimize the water, but there needs to be sufficient water so that it doesn't boil or change to steam in significant volume.

PWRs and BWRs already produce a fair amount of Pu-239/240, but the breeding ratio is about 50%, so they are considered a converters, not breeders. In BWRs, it's easier to do spectral shift because the upper 2/3's of the core has boiling with void fractions from 10% to 80%.
 
  • #4
You must also consider resonance capture. Since in the resonance bands capture is generally favored over absorption, you don't want neutrons in the fuel at resonance energies or else there is a good chance the neutron will be lost to absorption.

If the reactor is high in U238 or thorium this isn't as bad because the resulting nuclei are fissile. If the resonance capture occurs in something like U235 or Pu-239 the results isotopes might never undergo fission (or might require one or two more neutrons before it will).

In thermal reactors, moderators are used to slow the neutrons down to thermal energies away from the fuel such that they have better resonance escape probabilities. The worst case (neutron economy) would be a situation where you have enough moderation to slow neutrons to the resonance bands, but not enough to slow them to a region where fission is favoured.

Liquid metals are favored for fast reactors for two reasons:
-Heavier nuclei means less effective moderator
-Higher heat capacity and high boiling point mean you don't need as much of it in the core to provide cooling
 
  • #5
mohsenman said:
Whats the problem with using water in a fast reactor. I mean moderating neutrons in a fast reactor just increases the fission chance and that is not bad is it?

As Astronuc states - if you put water in a fast reactor - you don't have a fast reactor
anymore.

You may WANT to have a fast neutron spectrum Many reactions like U-238 fission or
certain reactions you want to burn-up nuclear waste - are threshold reactions. They
don't happen with slow neutrons - only fast neutrons.

Additionally the power densities in fast reactors can be large - so you need a more efficient
coolant than water - something like a liquid metal.

Dr. Gregory Greenman
Physicist
 
  • #6
Hologram0110 said:
-Higher heat capacity and high boiling point mean you don't need as much of it in the core to provide cooling

Well, water has a really high specific heat capacity; something like liquid sodium or some other metal is actually inferior to water in terms of its heat capacity. But you need something fairly heavy as the coolant so it's not moderating the neutrons, and the liquid metals meet that requirement whilst also providing reasonably good specific heat capacity, even though the specific heat capacity would strictly speaking be inferior to water.
 
  • #7
minerva said:
Well, water has a really high specific heat capacity; something like liquid sodium or some other metal is actually inferior to water in terms of its heat capacity. But you need something fairly heavy as the coolant so it's not moderating the neutrons, and the liquid metals meet that requirement whilst also providing reasonably good specific heat capacity, even though the specific heat capacity would strictly speaking be inferior to water.

miverva,

It's not just specific heat - but density and heat transfer properties.

If you take those into account - liquid sodium is a MUCH, MUCH BETTER coolant than water.

Dr. Gregory Greenman
Physicist
 

What is a fast reactor?

A fast reactor is a type of nuclear reactor that uses fast neutrons instead of thermal neutrons, allowing for the fission of heavier elements such as plutonium. This allows for more efficient use of fuel and the ability to use plutonium as a fuel source.

Why is moderation important in fast reactors?

Moderation is important in fast reactors because it slows down the fast neutrons to a speed that makes them more efficient at causing fission. This allows for a self-sustaining chain reaction and produces more energy. Without moderation, the fast neutrons would pass through the fuel without causing fission.

What materials are commonly used for moderation in fast reactors?

Common materials used for moderation in fast reactors include heavy water (deuterium oxide), light water (ordinary water), graphite, and beryllium. Each material has different properties that affect the speed of the neutrons, allowing for control of the chain reaction.

What are the benefits of using fast reactors?

Fast reactors have several benefits, including more efficient use of fuel, the ability to use plutonium as a fuel source, and reduced nuclear waste. They also have the potential to be used as a source of energy for thousands of years, as they can use both uranium and plutonium as fuel.

What are some challenges of using fast reactors?

Some challenges of using fast reactors include the need for specialized materials for moderation, the potential for nuclear proliferation due to the use of plutonium, and the potential for accidents and safety concerns. There are also economic challenges, as fast reactors are currently more expensive to build and operate compared to traditional nuclear reactors.

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