Hypothetical properties of a Fission Stack?

[GrahamF]

What properties would a material need to be able to efficiently convert energy from a fission reaction into electricity?

Let's say we had a device that used thin round plates of fissile material. The fissile material is well below critical mass, but you can stack them to achieve critical mass. Now imagine we have something called... idunno, a "Magischeibe" which is a flat disk that's stacked alternating with the fissile material like a voltaic pile. The Magischeibe can allow enough energy to pass through for the critical mass reaction, but is able to absorb a portion of the energy to convert to electricity.

there's a couple potential ways to control the power output:

• The stack can be spread open like an accordion to control the distance between the fissile plates in a neutralizing liquid medium
• The Magischeibe can be slid in and out of the pile as part of longer plate where the other half is a reaction neutralizing material
• The Magischeibe can change it's properties to change how much energy it's converting to a useable form and how much it's passing through

What material properties would be needed to make such a device, and what level of efficiency could be possible?

 

thanks

[swansont]

39 minutes ago, GrahamF said:

but is able to absorb a portion of the energy to convert to electricity

...

The Magischeibe can change it's properties to change how much energy it's converting to a useable form and how much it's passing through

The devil's in the details here. How does it do this?

[Bufofrog]

13 hours ago, GrahamF said:

What properties would a material need to be able to efficiently convert energy from a fission reaction into electricity?

I don't know of any material that can directly turn the KE of fission products into electricity.  The heat produced by fission or radioactive decay can directly turned into electricity.  This is a method used to power many unmanned space craft.  Look up 'thermopile' for more information on this.

[swansont]

42 minutes ago, Bufofrog said:

I don't know of any material that can directly turn the KE of fission products into electricity

There are for beta decay, but I suspect alpha decay would damage the semiconductor, and the higher temperatures might not be well-tolerated.

https://en.m.wikipedia.org/wiki/Betavoltaic_device

[npts2020]

On 4/14/2023 at 9:27 PM, GrahamF said:

Let's say we had a device that used thin round plates of fissile material. The fissile material is well below critical mass, but you can stack them to achieve critical mass. Now imagine we have something called... idunno, a "Magischeibe" which is a flat disk that's stacked alternating with the fissile material like a voltaic pile. The Magischeibe can allow enough energy to pass through for the critical mass reaction, but is able to absorb a portion of the energy to convert to electricity.

 

Take the stack and put a hole through the middle for a rod of neutron absorbing (but not fissile) material like boron or hafnium to control the reaction and you have essentially the same kind of configuration as reactors commonly in use for power production today.

[swansont]

9 hours ago, npts2020 said:

Take the stack and put a hole through the middle for a rod of neutron absorbing (but not fissile) material like boron or hafnium to control the reaction and you have essentially the same kind of configuration as reactors commonly in use for power production today

Are there any reactors that directly generate electricity, rather than heating water to run a steam turbine?

[mistermack]

2 hours ago, swansont said:

Are there any reactors that directly generate electricity, rather than heating water to run a steam turbine?

I don't know the answer to that, but I seem to remember reading that there is the potential for some fusion systems to do that, producing charged particles that can directly release their charge to an electrical circuit. Obviously, fusion is nowhere near doing anything of the sort at the moment, but maybe a long time in the future, that might be possible. 

[swansont]

25 minutes ago, mistermack said:

I don't know the answer to that, but I seem to remember reading that there is the potential for some fusion systems to do that, producing charged particles that can directly release their charge to an electrical circuit. Obviously, fusion is nowhere near doing anything of the sort at the moment, but maybe a long time in the future, that might be possible. 

The difficulty will be in stepping down the energy of the individual particles - you want more charge carriers at lower energy. e.g. you want 10^20 charge carriers at 1 keV, not 10^17 at 1 MeV

(and for the fusion case you want this in a conductor, not a plasma)

[mistermack]

16 minutes ago, swansont said:

The difficulty will be in stepping down the energy of the individual particles - you want more charge carriers at lower energy. e.g. you want 10^20 charge carriers at 1 keV, not 10^17 at 1 MeV

There is work being done around that situation. Wikipedia has a very interesting page on various direct energy conversion research including high enerergy protons, and in the fission field as well as fusion. 

"In 1992, a Japan–U.S. joint-team proposed a novel direct energy conversion system for 14.7 MeV protons produced by D-³He fusion reactions, whose energy is too high for electrostatic converters." ^[18]    

https://en.wikipedia.org/wiki/Direct_energy_conversion   

[mistermack]

I ought to add that if direct energy conversion were ever to become a success, that wouldn't mean that the heat produced would just be allowed to dissipate. You would surely have steam turbine generation alongside it. It would just help to make the overall process more productive. 

[npts2020]

16 hours ago, swansont said:

Are there any reactors that directly generate electricity, rather than heating water to run a steam turbine?

I wasn't aware the OP was for direct conversion from fission to electricity, rather than asking about a novel configuration to do essentially what we now do. My apologies for not understanding that to be the case.

[GrahamF]

18 hours ago, npts2020 said:

I wasn't aware the OP was for direct conversion from fission to electricity, rather than asking about a novel configuration to do essentially what we now do. My apologies for not understanding that to be the case.

I was mostly trying to determine would would be needed to devise a more efficient method absorbing and using the energy from the nuclear reactions. Like, say, a molecular level lattice system that has a complicated configuration of various elements that when struck with high energy particles directly absorbs them and releases electricity 

[swansont]

1 hour ago, GrahamF said:

I was mostly trying to determine would would be needed to devise a more efficient method absorbing and using the energy from the nuclear reactions. Like, say, a molecular level lattice system that has a complicated configuration of various elements that when struck with high energy particles directly absorbs them and releases electricity 

A high energy particle like a fission fragment striking (or passing near) an atom will tend to ionize the atom, but those electrons will recombine with some other nucleus that has been ionized. Accelerated charges (as they scatter) produces bremsstrahlung. So there are some lossy mechanisms with regard to the energy of a current. These charges aren’t flowing in a particular direction, so there isn’t a current, as such.

Then there’s the question of whether you can make this in a conductive material and have it hold up to the fission damage.

[npts2020]

On 4/18/2023 at 6:37 PM, GrahamF said:

I was mostly trying to determine would would be needed to devise a more efficient method absorbing and using the energy from the nuclear reactions. Like, say, a molecular level lattice system that has a complicated configuration of various elements that when struck with high energy particles directly absorbs them and releases electricity 

One of the problems is that fission occurs much better (in uranium, anyway) with thermal (slow) neutrons but those fissions make fast (high momentum) neutrons. The hydrogen in water is very efficient at slowing those neutrons down by absorbing some of their energy causing the water to heat up enough to make steam for running a turbine to turn a generator for making electricity. If one or more of those steps can be eliminated, you may have something worthwhile. The sticking point comes in channeling exploded atoms with virtually no net charge into an electrical current and finding materials that will hold up to nuclear bombardment, like swansont points out above. It's all about turning atomic kinetic energy, which almost always exhibits itself as heat, into a usable form of energy.

 

[GrahamF]

10 hours ago, npts2020 said:

One of the problems is that fission occurs much better (in uranium, anyway) with thermal (slow) neutrons but those fissions make fast (high momentum) neutrons. The hydrogen in water is very efficient at slowing those neutrons down by absorbing some of their energy causing the water to heat up enough to make steam for running a turbine to turn a generator for making electricity. If one or more of those steps can be eliminated, you may have something worthwhile. The sticking point comes in channeling exploded atoms with virtually no net charge into an electrical current and finding materials that will hold up to nuclear bombardment, like swansont points out above. It's all about turning atomic kinetic energy, which almost always exhibits itself as heat, into a usable form of energy.

 

...what about an electromagnet? Could there be a way to set up a superconductor that can absorb the energy of the neutrons as they pass through, slowing them down and generating electricity that way?

[swansont]

11 hours ago, npts2020 said:

One of the problems is that fission occurs much better (in uranium, anyway) with thermal (slow) neutrons but those fissions make fast (high momentum) neutrons. The hydrogen in water is very efficient at slowing those neutrons down by absorbing some of their energy causing the water to heat up enough to make steam for running a turbine to turn a generator for making electricity. If one or more of those steps can be eliminated, you may have something worthwhile. The sticking point comes in channeling exploded atoms with virtually no net charge into an electrical current and finding materials that will hold up to nuclear bombardment, like swansont points out above. It's all about turning atomic kinetic energy, which almost always exhibits itself as heat, into a usable form of energy.

What atoms with virtually no net charge? The fission fragments would be highly ionized. This isn’t the sticking point. Because they are charged they deposit their energy in a short distance, as they collide with other nuclei and ionize atoms along their path. 

 

[GrahamF]

Would it be possible to set up a very thin electromagnet capable of forcing neutrons to slow down dramatically and converting the energy to electricity, or using a series of thermocouples to directly convert the heat to energy?

[Genady]

1 hour ago, GrahamF said:

electromagnet capable of forcing neutrons to slow down

Neutrons don't have electric charge. They are not affected by electromagnet.

[swansont]

23 minutes ago, Genady said:

Neutrons don't have electric charge. They are not affected by electromagnet.

They have a magnetic moment, though, so they can be affected by a magnetic field and have been confined in a magnetic trap. But these neutrons were slowed by other means.

[npts2020]

On 4/20/2023 at 11:57 AM, swansont said:

What atoms with virtually no net charge? The fission fragments would be highly ionized. This isn’t the sticking point. Because they are charged they deposit their energy in a short distance, as they collide with other nuclei and ionize atoms along their path. 

Of course they are ionized temporarily. What is the net charge of all the fragments?

[swansont]

13 hours ago, npts2020 said:

Of course they are ionized temporarily. What is the net charge of all the fragments?

It depends on how highly ionized they are

[npts2020]

8 hours ago, swansont said:

It depends on how highly ionized they are

Why is that? Wouldn't the highly ionized negative particles have just as highly ionized counterparts with positive charge?

[swansont]

8 hours ago, npts2020 said:

Why is that? Wouldn't the highly ionized negative particles have just as highly ionized counterparts with positive charge?

The negatively charged particles aren’t atoms, they are electrons. The fission fragments are the positively charged atoms.

[npts2020]

On 4/26/2023 at 8:22 AM, swansont said:

The negatively charged particles aren’t atoms, they are electrons. The fission fragments are the positively charged atoms.

Sure. Perhaps I am mistaken, but I had always thought the difference in fragments was in size of mass not charge and that in fission it was mass lost not charge. That is why you don't get a useful electric current directly from fission. I could see a magnetic field, atomic filter or some similar scheme possibly causing an electric current to be produced, as the OP seems be asking, but the catch (as usual) will be in getting more energy out than you put into the process.