Moderation in Fast Reactors

mohsenman

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?

QuantumPion

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.

Astronuc

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%.

Hologram0110

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

Morbius

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

minerva

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.

Morbius

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