Nuclear Direct Electricity Conversion

Why are nuclear plants operate in a secondary steam engine for electrical conversion from nuclear energy, wherein thermal efficiency is less 30% in comparison to direct conversion which should be higher than30%? Is it not possible that nuclear reaction can produce direct electricity?

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Simon Bridge
Homework Helper
There is no way to convert one form of energy "directly" into another one.

ZapperZ
Staff Emeritus
Why are nuclear plants operate in a secondary steam engine for electrical conversion from nuclear energy, wherein thermal efficiency is less 30% in comparison to direct conversion which should be higher than30%? Is it not possible that nuclear reaction can produce direct electricity?
Would you like to offer an explicit "direct" mechanism from nuclear fission to current in a wire, with a back-of-the-envelope efficiency calculation?

Zz.

I am not sure what you mean, but I am wondering why it is uncommon and unexplored.

ZapperZ
Staff Emeritus
I am not sure what you mean
In science, and especially in physics, as my signature says, you can't just say "what goes up must come down". You must also say when and where it comes down. In other words, "God is in the DETAILS"!

You need to clearly describe the detailed mechanism of the conversion from "fission" to "current in wire". In other words, how do you take the energy out of the fission process, and convert that into electricity in a wire. You claim that there is a direct process and that this can be done. I'd like to know what it is exactly. You simply can't just make a flippant statement that it can and should be done and leave it at that! And here, the "when and where it comes down" means a quantitative analysis that it can be done and it is more efficient. You can't just say it is better, you have to prove it!

, but I am wondering why it is uncommon and unexplored.
Because as of now, there is nothing that we know of that can convert one into the other without going through all the intermediary process? That was why I asked you if you know of such a direct conversion.

Zz.

Ronie Bayron
got it and if I knew, a forum should not be the right place for it, right? I'll take note of that. Thanks.

I don't know why you got such hostile replies.

In this context, "direct conversion" means "a way to convert kinetic energy of fission fragments to electricity, without intermediate step of turning it into heat".

Direct conversion of *fusion* energy to electricity is being looked at. In particular, polywell proponents have schemes to do so.

For fission, it is not feasible because fission fragments are very easily stopped by matter - even better than alphas. Even if you would have a reactor where fuel is in gas phase, you still will have very hard time preventing most of kinetic energy of reaction products from turning to thermal energy at once.

ZapperZ
Staff Emeritus
I don't know why you got such hostile replies.
Er.. it may appear "hostile" to you, but that wasn't my intention at all. Rather, it is to ask the OP to clearly dissect the question and figure out what he/she meant by "direct".

In this context, "direct conversion" means "a way to convert kinetic energy of fission fragments to electricity, without intermediate step of turning it into heat".
You don't know that. This is your assumption of what the OP meant as "direct conversion". After all, there are other intermediary steps without just turning it into heat. Maybe this isn't direct enough for the OP either.

Direct conversion of *fusion* energy to electricity is being looked at. In particular, polywell proponents have schemes to do so.
This isn't the topic that was asked, unless you think we already have "nuclear plants" running on fusion, which was what the OP asked about. I didn't bring this up because it is a distraction.

For fission, it is not feasible because fission fragments are very easily stopped by matter - even better than alphas. Even if you would have a reactor where fuel is in gas phase, you still will have very hard time preventing most of kinetic energy of reaction products from turning to thermal energy at once.
But this assumes that the fissile material are in its usual surrounding of water, etc. There's nothing to say that we can't have it suspended in vacuum and use other methods at harnessing the fission by products. That is exactly the type of mechanism that I was asking about, and why I asked the OP to think about such mechanism.

Zz.

mfb
Mentor
For fission, it is not feasible because fission fragments are very easily stopped by matter - even better than alphas.
To make it worse, you want to contain and ideally slow down neutrons to get a stable chain reaction, and you want some self-regulation from the neutron moderator. Everything that contains and slows down neutrons slows down fission fragments as well and converts their energy to heat. It's not as simple as suspending fissile material in a vacuum, because that would either give a bomb (if dense enough) or no fission chain reaction (if too scattered).

Astronuc
Staff Emeritus
Why are nuclear plants operate in a secondary steam engine for electrical conversion from nuclear energy, wherein thermal efficiency is less 30% in comparison to direct conversion which should be higher than30%? Is it not possible that nuclear reaction can produce direct electricity?
Boiling water reactors (BWRs) boil water in the core, and the steam is passed through large pipes (steam lines) directly to a high pressure turbine, so it is a direct cycle. Nuclear plants achieve between 32 to nearly 38% thermal efficiency.

Direct conversion, which has been envisioned for some fusion systems, requires charge separation in which the electrons are collected to provide a current to the load, and then sent to a recombiner where they neutralize the positive ions which are collected.

In a nuclear (fission) reactor, charge separation would be difficult, if it wasn't impossible, to achieve. Conventional nuclear systems use fuel assemblies, a fraction of which are replaced periodically as fission products accumulate over time. In the fission process, the electrons are rapidly collected by the ionized fission product atoms, so there is little opportunity to collect free electrons.

I have seen concepts for thermionic based electrical systems involving emitter/collector systems in core (of a compact fast reactor). It was a problematic system. One of the challenges is placing the conductors and insulators in the core where they are subject to neutron irradiation. It is an expensive (uneconomic) concept for commercial electrical generation on a large scale.

Ronie Bayron
mheslep
Gold Member
Queries on nuclear direct conversion have been answer on PF earlier. No its not done on any practical scale, yes some research has been done especially by Mark Prelass at MU, and not just via charge collection.
https://en.wikipedia.org/wiki/Photon-intermediate_direct_energy_conversion
http://phys.org/news/2009-04-hot-air-nuclear-power-scientists.html
http://munews.missouri.edu/news-releases/2009/0413-prelas-energy-summit.php