Is Direct Energy Conversion Possible in Nuclear Fusion Reactors?

[DaemonStudent]

Is there any possible way to extract energy that isn't in terms of heat in a nuclear reaction? It seems like a very passive way to collect energy. There are many medium transports from the reactor to the actual steam turbine (e.g. supercritical water, molten sodium, etc). Is there any proposed way to use the movement of particles ejected by the reaction (anything other than PIDEC)? Can anyone point me to some reading about this topic? I really like thinking about it! I'm a freshman in college studying chemistry and I need other "sciency" and conceptual things to think about besides remedial lab reports and homework. Thanks!

 

[bcrowell]

There's this sort of thing: http://en.wikipedia.org/wiki/Betavoltaic

I don't think it can be done with a fission chain-reaction, however, because in order to get a critical mass, you need the material to be very thick, and that means the alphas are going to stop before they escape.

 

[fatra2]

Hi there,

Is there any possible way to extract energy that isn't in terms of heat in a nuclear reaction? It seems like a very passive way to collect energy.

Why?

Heat is a very "good" form of energy. Plus, you have neutrons decaying at energies of ~2MeV. Try to imagine their initial speed. To be able to sustain chain reaction, in a sensible way, it is much easier to catch slower moving neutrons. Therefore, the slowing down process creates heat. Let's use this heat in good terms, and make electricity out of it.

Cheers

 

[DaemonStudent]

Heat is a very "good" form of energy.

I just wish that we could use the ejected particles and EM waves from radiation. Maybe we could created some kind of electromagnet to make the direction of high energy particles more coherent. Is that even possible? I just feel that there must be a better way to extract energy other than boiling water... Does everyone get my reference to the proposed reactor called PIDEC? Or should I post an article?

 

[fatra2]

Hi there,

But the problem with your electromagnet is that we are talking essentially about gamma radiation and neutrons with do not react or react very little to magnetic fields.

Your idea is not that bad, and could be extrapolated to nuclear waste in pools. The only problem would be the extra investment needed, for such a little amount of usable energy.

Cheers

 

[DaemonStudent]

Would there be anyway to harness the neutron? Its mass is relatively great and it move at very fast speeds when ejected. Is there some way to turn that kinetic energy into mechanical? Some kind of neutron turbine... I hope this isn't so theoretical that its annoying.

 

[vemvare]

Perhaps combining a gaseous fission reactor with a MHD-generator? Neutrons could be absorbed by boron vapor or something.

 

[Astronuc]

Collecting energy directly from neutrons is not feasible since they are by nature 'neutral'. One possibility would be to put a hydrogenous material around the neutrons and allow the neutrons to knock out the protons, and then try charge collection. This is problematic because the neutron flux diminishes over some distance which makes a thin collection system difficult if one wants to place it outside the neutron rad field. Otherwise, the structural materials will become activated through neutron capture.

Related to the OP, in-core thermionics have been considered for some small compact fast reactor systems. That technology is rather challenging.

The problem with fission products is that they travel about 3-6 microns in solid material, so one needs a thin fission source. However, one of the objectives of nuclear fuel design is to retain the fission products close to where they are produced. The fuel is essentially the first barrier between fission products and the environment.

 

[fatra2]

Hi there,

However, one of the objectives of nuclear fuel design is to retain the fission products close to where they are produced. The fuel is essentially the first barrier between fission products and the environment.

And the key word here is "FIRST" barrier.

For the harnessing of neutrons, this is what happens in a light water reactor. The neutron is slowed down, the energy released by this neutron is used to heat the water. The hot water can then be converted into electricity.

I still find this system pretty convenient, and quite efficient.

Cheers

 

[Astronuc]

Hi there,


And the key word here is "FIRST" barrier.

For the harnessing of neutrons, this is what happens in a light water reactor. The neutron is slowed down, the energy released by this neutron is used to heat the water. The hot water can then be converted into electricity.

I still find this system pretty convenient, and quite efficient.

Cheers

Um - no! The energy released from the 'fission' process is what heats the water, in addition to the gamma and beta radiation, which accounts for about 0.026 of the thermal energy.

Neutrons are slowed down from the low MeV range to 0.025 eV (thermal range), but they contribute little to thermal energy.

The fission of U-235 or Pu-239 liberates about 200 MeV per fission, of which about 170 MeV is in the form of kinetic energy of the fission products. The rest of the energy is in the form of beta and gamma radiation, that is deposited in the fuel or the coolant or core structural material.

 

[QuantumPion]

Well if you weren't worried about the radiation, you could make a direct-cycle nuclear gas turbine perhaps. This is still based on thermodynamic principles of heating/expanding a fluid though.

 

[RocketSci5KN]

As noted, the basic answer to your question is 'no'. The vast majority of the energy involved in fission comes from the two daughter products of the fission. They travel only very small distances in any liquid or solid material as they are highly ionized. The other fact of life is that these products cause the reactor core to become very radioactive during its burn cycle (typically 3 yrs or so of burn-up), and you really do want to keep them immobilized in the fuel pellets themselves (typically UO2, a ceramic) for everyone's safety.
Some fusion concepts may be able to do direct conversion, such as the current research into DPF (dense plasma focus), http://www.lawrencevilleplasmaphysics.com/.

 

[Astronuc]

Here are some direct energy conversion ideas for fusion reactors. They have been envision for tandem mirror reactors.
http://www.askmar.com/Direct_Energy.html

Electrostatic direct energy converters are linear accelerators run backwards, e.g. fast ions enter the “exit” of the accelerator, are decelerated by retarding electric fields, and collected on high-voltage electrodes forming the positive terminal of the direct energy converter power source.

 

[mheslep]

Here are some direct energy conversion ideas for fusion reactors. They have been envision for tandem mirror reactors.
http://www.askmar.com/Direct_Energy.html

As I recall there's a seminal paper around, decades old, that surveyed the fusion product direct conversion concepts and found them very difficult to implement. Nothing there saying direct conversion was scientifically impossible, just a difficult engineering challenge. For instance, IIRC, it is not possible with current materials to convert MeV products in a single electrostatic stage; multiple stages are required which adds additional difficulty, etc, etc.