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Inertial Confinement Fission


Bill Phil
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The quest for fusion power has been a long one. Sadly it seems that fusion is doomed to be decades away for quite some time.

However, fission reactors are a well understood technology.

Indeed, fission reactions are much easier to initiate than fusion reactions - fission requires interactions between heavy nuclei and neutrons whereas fusion requires light nuclei to overcome the electrostatic barrier between them. So fission is easier to initiate.

However current conventional fission reactors - while efficient and better suited for baseload power than other low emission concepts - are quite expensive. They also produce waste and don't burn much of their fuel sources. Not to mention they require processed fuel that requires larger concentrations of U-235 than natural uranium.

So I started thinking.

What if we took Inertial Confinement Fusion drivers and used fission fuels instead?

Assuming we also used a suitable neutron source, it's likely possible to use natural uranium or even natural thorium by employing fast neutrons. This could lead to more economical fuels than conventional fission reactors.

It should be possible to get fairly large burnup rates for the fuel as well, so less waste could be produced per unit of energy than conventional nuclear reactors and less fuel would be required.

Such reactors would also be safer than conventional nuclear reactors - meltdowns are completely impossible since a very small amount of fuel is reacting at any instant and the reactions only occur if the driver is operating. 

It could also be possible to have higher power densities than conventional reactors, so smaller facilities would be required and less shielding mass would be needed. Of course this depends on the size of the facility for the driver but the actual reactor itself could be much smaller.

Such reactors could also be more thermodynamically efficient by combining conventional heat engine technology with MHD technology. 

And compared to fusion inertial confinement fission seems to be more achievable.

So it could be more economical, safer, more efficient, lower waste production, and smaller than conventional reactors for a given power output. And compared to fusion such reactors could be possible much earlier. And it could lead to the development of more efficient drivers that could make inertial confinement fusion more practical. Or hybrid systems that use fission to initiate a fusion reaction could be possible.

Now for some applications.

The first one is obvious - baseload electricity. If such technology is more economical than current reactors it could be competitive with other energy sources as well. If developed as a small modular reactor it could be deployed quite rapidly.

The second one may be less obvious but still important, though less likely: propulsion for cargo and container ships. Ocean shipping is responsible for quite a large percentage of pollutants and a decent percentage of GHG emissions, and those emissions are expected to grow substantially over time. 

And of course the one we're all probably more interested in: space propulsion. This can be done using nuclear pulse propulsion or nuclear electric propulsion. If the power density can be made high enough then nuclear-electric systems may be capable of interplanetary missions with reasonable mass ratios. And of course nuclear pulse propulsion could do the same. Such a system would probably be similar to Mini-Mag Orion but without the Z-Pinch system and without the need for other components that limit the performance of the Mini-Mag Orion system (such as the conductive elements needed for the Z-Pinch). So fast transfers to the outer planets would be possible with manned missions.

I can't find much literature on this concept - mostly because the words "inertial confinement" are associated with fusion. But it could work with fission. And it seems that it could have some serious advantages over conventional nuclear reactors.

Thoughts?

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56 minutes ago, Bill Phil said:

The quest for fusion power has been a long one. Sadly it seems that fusion is doomed to be decades away for quite some time.

However, fission reactors are a well understood technology.

Indeed, fission reactions are much easier to initiate than fusion reactions - fission requires interactions between heavy nuclei and neutrons whereas fusion requires light nuclei to overcome the electrostatic barrier between them. So fission is easier to initiate.

However current conventional fission reactors - while efficient and better suited for baseload power than other low emission concepts - are quite expensive. They also produce waste and don't burn much of their fuel sources. Not to mention they require processed fuel that requires larger concentrations of U-235 than natural uranium.

So I started thinking.

What if we took Inertial Confinement Fusion drivers and used fission fuels instead?

Assuming we also used a suitable neutron source, it's likely possible to use natural uranium or even natural thorium by employing fast neutrons. This could lead to more economical fuels than conventional fission reactors.

It should be possible to get fairly large burnup rates for the fuel as well, so less waste could be produced per unit of energy than conventional nuclear reactors and less fuel would be required.

Such reactors would also be safer than conventional nuclear reactors - meltdowns are completely impossible since a very small amount of fuel is reacting at any instant and the reactions only occur if the driver is operating. 

It could also be possible to have higher power densities than conventional reactors, so smaller facilities would be required and less shielding mass would be needed. Of course this depends on the size of the facility for the driver but the actual reactor itself could be much smaller.

Such reactors could also be more thermodynamically efficient by combining conventional heat engine technology with MHD technology. 

And compared to fusion inertial confinement fission seems to be more achievable.

So it could be more economical, safer, more efficient, lower waste production, and smaller than conventional reactors for a given power output. And compared to fusion such reactors could be possible much earlier. And it could lead to the development of more efficient drivers that could make inertial confinement fusion more practical. Or hybrid systems that use fission to initiate a fusion reaction could be possible.

Now for some applications.

The first one is obvious - baseload electricity. If such technology is more economical than current reactors it could be competitive with other energy sources as well. If developed as a small modular reactor it could be deployed quite rapidly.

The second one may be less obvious but still important, though less likely: propulsion for cargo and container ships. Ocean shipping is responsible for quite a large percentage of pollutants and a decent percentage of GHG emissions, and those emissions are expected to grow substantially over time. 

And of course the one we're all probably more interested in: space propulsion. This can be done using nuclear pulse propulsion or nuclear electric propulsion. If the power density can be made high enough then nuclear-electric systems may be capable of interplanetary missions with reasonable mass ratios. And of course nuclear pulse propulsion could do the same. Such a system would probably be similar to Mini-Mag Orion but without the Z-Pinch system and without the need for other components that limit the performance of the Mini-Mag Orion system (such as the conductive elements needed for the Z-Pinch). So fast transfers to the outer planets would be possible with manned missions.

I can't find much literature on this concept - mostly because the words "inertial confinement" are associated with fusion. But it could work with fission. And it seems that it could have some serious advantages over conventional nuclear reactors.

Thoughts?

 

Sounds cool. Yet why am I thinking big heavy magnets?

Are they not needed? For the spaceship version.

 

Like all other ships it is reaching orbit the traditional way right?

Solid/chemical boosters right?

This inertial fission idea is mainly good for orbital transfers right?

Edited by Spacescifi
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1 hour ago, Spacescifi said:

 

Sounds cool. Yet why am I thinking big heavy magnets?

Are they not needed? For the spaceship version.

 

Like all other ships it is reaching orbit the traditional way right?

Solid/chemical boosters right?

This inertial fission idea is mainly good for orbital transfers right?

The use of a magnetic nozzle for a pulse vehicle is desired, though physical contact is possible.

The mass of the magnets depends on various design decisions. 

Such a vehicle would need another system to put it into orbit, or it would need to be built in orbit.

Now, another possible propulsion scheme could be a nuclear thermal rocket. A LANTR with an inertial confinement fission reactor and high enough mass flow could actually allow SSTO rockets. If the actual reactor can be made small enough then the shielding requirements could be reasonable. 

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I think the big problem is that "inertial confinement" largely uses magnetic fields to hold charged particles.  I'm not seeing how this will hold neutrons back, nor kick them back into the nuclear material.  Laser "ignition" confinement might at least hold the neutrons, but I'd be curious about kicking them back into the fission.  I might be missing something, but I don't think the fuel rods of a reactor require excess confinement (bombs are another story).

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I assume you are proposing using beams of neutrons to create burst fission in subcritical masses of fuel? There are problems with this, mainly that it's basically impossible to direct neutrons in a cohesive stream; they just go everywhere.

If you're thinking lasers, as in ICF, then I have to say, I can't see this overcoming the criticality limits on chain reactions. Perhaps mass restrictions don't apply to a non-chain-reaction?

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2 hours ago, wumpus said:

I think the big problem is that "inertial confinement" largely uses magnetic fields to hold charged particles.  I'm not seeing how this will hold neutrons back, nor kick them back into the nuclear material.  Laser "ignition" confinement might at least hold the neutrons, but I'd be curious about kicking them back into the fission.  I might be missing something, but I don't think the fuel rods of a reactor require excess confinement (bombs are another story).

Inertial confinement uses lasers or particle beams to highly compress a pellet of fuel. Magnetic fields can be completely ignored - except maybe for an MHD system to convert the thermal energy to electricity.

Fuel rods don't require confinement - but they do need to be kept at criticality over long periods of time.

There are a few ways to get some neutrons in to kickstart the reaction - a proton beam of sufficient energy and with enough protons could generate neutrons through spallation.

1 hour ago, SOXBLOX said:

I assume you are proposing using beams of neutrons to create burst fission in subcritical masses of fuel? There are problems with this, mainly that it's basically impossible to direct neutrons in a cohesive stream; they just go everywhere.

If you're thinking lasers, as in ICF, then I have to say, I can't see this overcoming the criticality limits on chain reactions. Perhaps mass restrictions don't apply to a non-chain-reaction?

No, I'm proposing lasers as in ICF or particle beams.

I'm not fully familiar with the physics involved.

But from my understanding the critical mass of a fissile material (like U-235) depends on its density, so if you do massively increase the density then the critical mass is massively reduced, so an ICF would work with fissile material just as is, provided the pellet mass is large enough (though if shockwaves travel through the material the density could be even larger near the center of the pellet, so current driver systems could potentially do it with minimal changes).

With non-fissile material that is still fissionable (like U-238), you have to use fast neutrons. One way to do this is to use a proton beam and generate neutrons via spallation in the pellet - of course you might need high energy proton beams. The question is whether or not that can be maintained for long enough. Perhaps we can treat it as a fusion boosted fission system, using an amount of fusion fuel in the pellet to increase the neutron flux even further. Or perhaps use even more proton particle beams at the right moment. Or alternately we can configure the system to act as a breeder reactor though such a system would be difficult.

It seems to be a more workable concept than fusion, at least in the near term.

Edited by Bill Phil
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Yep, I just realized that after I posted. Grabbing some numbers off Wikipedia, I did some math to find the density required to bring a 1 g sphere of pure U235 to criticality. Here goes...

First of all, critical mass and the square of the density are inversely proportional. We need the proportionality constant between these values. (Remember, inverse proportionality says that a × b = k where k is the constant.) 

So room temp. density x critical mass in a sphere at room temp. is

k =((19.1 g/cm^3)^2) × 52000 g 

k = 18 970 120 g^2/cm^3

Now we can set the critical mass equal to one gram and determine the square of the requisite density.

k= 18 970 120 = p^2 × 1 g      and solving for p...

the required density is ~4355 g/cm^3 .

We can then go on to calculate, using the bulk modulus, the force required to compress the U235 to this density. Through more calculations and modeling, we can find the compression generated by ablation from arbitrarily large laser banks, to find the laser energy needed to compress this metal. 

Also, this is for a sample of pure U235. I would expect civilian-grade fuel to not be weapons-grade, though. The added impurity would raise the required density even higher.

However, I somewhat arbitrarily chose the mass of one gram. Perhaps with smaller masses, other effects dominate? Also, I have been known to make rather dumb mistakes in algebra; please take my numbers with a grain of salt.:lol:

Overall, the the compressibility of Uranium looks like it would be significantly lower than that of deuterium-tritium pellets, so I'm thinking it would be more difficult to use inertial confinement fission. I am interested in whether this method would "burn" fuel with a higher efficiency, though.

***

Went and looked up the densities of fusion fuel (D-T) in ICF. Usually fuel is compressed to about 200 g/cm^3 to ignite. Remember, a whole gram of very pure U would need to be compressed to multiple kilograms per cubic cm, and smaller masses would raise this requirement.

Edited by SOXBLOX
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57 minutes ago, SOXBLOX said:

Yep, I just realized that after I posted. Grabbing some numbers off Wikipedia, I did some math to find the density required to bring a 1 g sphere of pure U235 to criticality. Here goes...

First of all, critical mass and the square of the density are inversely proportional. We need the proportionality constant between these values. (Remember, inverse proportionality says that a × b = k where k is the constant.) 

So room temp. density x critical mass in a sphere at room temp. is

k =((19.1 g/cm^3)^2) × 52000 g 

k = 18 970 120 g^2/cm^3

Now we can set the critical mass equal to one gram and determine the square of the requisite density.

k= 18 970 120 = p^2 × 1 g      and solving for p...

the required density is ~4355 g/cm^3 .

We can then go on to calculate, using the bulk modulus, the force required to compress the U235 to this density. Through more calculations and modeling, we can find the compression generated by ablation from arbitrarily large laser banks, to find the laser energy needed to compress this metal. 

Also, this is for a sample of pure U235. I would expect civilian-grade fuel to not be weapons-grade, though. The added impurity would raise the required density even higher.

However, I somewhat arbitrarily chose the mass of one gram. Perhaps with smaller masses, other effects dominate? Also, I have been known to make rather dumb mistakes in algebra; please take my numbers with a grain of salt.:lol:

Overall, the the compressibility of Uranium looks like it would be significantly lower than that of deuterium -lithium pellets, so I'm thinking it would be more difficult to use inertial confinement fission. I am interested in whether this method would "burn" fuel with a higher efficiency, though.

 

There are other mechanisms that can be used besides direct compression.

According to this:

https://physics.aps.org/story/v5/st3

Fission was accomplished with lasers in 2000, 20 years ago.

"Before each high-power laser shot, the Livermore team hit their solid gold target, mounted on a copper sample holder containing uranium, with a lower-energy pulse to briefly create a plasma of electrons at the target’s surface. They then used the world’s first petawatt (10^15 W) laser to blast the gold with a 0.5 ps,260 J pulse of infrared light–packing more than [10^20] W/cm2–which accelerated plasma electrons to energies of tens of MeV. The “quivering” of these electrons created gamma rays known as Bremsstrahlung radiation that liberated high-energy neutrons from gold and copper nuclei. These neutrons split uranium-238 nuclei and caused other nuclear reactions. With two strategically placed detectors, Cowan and his colleagues monitored the energies of electrons escaping the plasma during the laser pulse."

Another team used tantalum instead of gold. 

And the laser pulse seemed to be very low energy at that - though I don't know how much energy the fission actually released. If the fission yield could be made considerable then it could be doable.

And another option is to use uranium-238 as a kind of tamper-pusher. Essentially we put a shell of U-238 around the fusion fuel. Then the tamper compresses the fusion fuel after its bombarded by lasers and the fusion releases fast neutrons which then cause fission in the U-238. So a thermonuclear micro-explosion reactor. Might be better to use cylindrical pellets.

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Confinement plays a very different role in fusion and fission. In fusion, you have to have high density and high temperature to get the process going. In fission, increase in temperature actually slows down the reaction, and the only purpose of increasing density is preventing neutrons from flying out of the fuel without contributing to reaction. There isn't actually a need for proximity between nuclei. Because of that, there are limits to how much you can decrease critical mass by compressing the fuel. And that's basically where all of this breaks down. You can take a fuel "pellet" that's significant fraction of critical mass of uranium and get it to split. Congratulations, you just invented a nuke. You cant go much smaller, because there is no amount of pressure that will get it to critical. Alternatively, you can try and look for fuels that will make explosions small enough that you can harness this energy inside a power plant. So something with critical mass in milligram ranges or less. Now we're talking about isotopes that have a very short half life and that you can't practically store, because even at decent spacing, placing enough of them to power a plant in one place is likely going to make the whole store critical. If you somehow manage to avoid that, the background radiation is still going to be quite significant. And all these isotopes will have to be manufactured by some other process.

There have been proposals for making a big cave, dropping in a nuke every once in a while, and using heat of the walls to drive turbines. That is the only practical way I'm aware of of making a nuclear fission reactor that uses inertial confinement.

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This sound more like an advanced orion pulse drive engine, where you compress uranium or plutonium without an bomb benefit is that you only carry the mass of the plutonium and not the bomb. 

Now an small nuclear reactor like the one used on ships is fail safe. If something goes wrong the moderator rods fall down and stop the reactor, even if no pumps circulate cooling water the reactor will not overheat catastrophic. 

An large power generating reactor don't have this luxury as its much larger so it has more thermal mass and also more rest radioactivity after shutdown so you need to actively cool it. 

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 I think the target didn't go critical, since it's *hard* to circumvent the critical mass requirement. So the laser caused a reaction, it just wasn't a chain reaction. This makes sense, as it was driven by Bremsstralung from scattered electrons, which lose energy and can't be recycled. Of course, it didn't go critical (I don't think) so it didn't release anywhere near enough energy (I don't think) to "break even".

It seems you have to use either compression to lower the critical mass or make up for lost neutrons by bombarding the substance with extra neutrons, bringing us back to the neutron beam problem. Another method would be to put neutron reflectors, like beryllium or tungsten carbide, around it. However, this only works down to a point; after that, it can't bring the fuel critical.

Of course, if we're thinking about using compression to 4 kg/cm^3, then it might be time to worry about uranium fusion instead of fission.

Edited by SOXBLOX
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On 6/3/2020 at 4:42 PM, SOXBLOX said:

 I think the target didn't go critical, since it's *hard* to circumvent the critical mass requirement. So the laser caused a reaction, it just wasn't a chain reaction. This makes sense, as it was driven by Bremsstralung from scattered electrons, which lose energy and can't be recycled. Of course, it didn't go critical (I don't think) so it didn't release anywhere near enough energy (I don't think) to "break even".

It seems you have to use either compression to lower the critical mass or make up for lost neutrons by bombarding the substance with extra neutrons, bringing us back to the neutron beam problem. Another method would be to put neutron reflectors, like beryllium or tungsten carbide, around it. However, this only works down to a point; after that, it can't bring the fuel critical.

Of course, if we're thinking about using compression to 4 kg/cm^3, then it might be time to worry about uranium fusion instead of fission.

Now  uranium fusion get me curious as it stealth google and might be an island of stability pathway. 
curious cat is curious vKadA01.gif

Edited by magnemoe
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20 hours ago, Dragon01 said:

This is similar to something called "mini-mag Orion". Basically, a way to avoid some of the Orion's problems by not using full nukes as pulse units, which enables them to be much smaller, meaning a higher pulse rate and lower peak accelerations:
http://www.projectrho.com/public_html/rocket/enginelist3.php#id--Pulse--Zeta-Pinch--Zeta-Pinch_Fission--Mini-Mag_Orion

Extremely interesting design, it does not suffer from the fusion problem for current design in that you do not need to power this externally, but use some of the plasma to create power for the next blast. 
And yes you could always slam two blocks of U 235 into each others having far smaller pulses makes this much easier to build. 
Yes think it was boron who could be used to fusion in fusion style setting but this is sounds more promising. 
Why the low cycle time however, yes you have thermal issues but this looks like an very nice design. That unless Curium 245 is hard to make, an 8500 year half life so its stable and around 10 time more radioactive than plutonium. 
 

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On 6/3/2020 at 5:06 PM, Dragon01 said:

This is similar to something called "mini-mag Orion". Basically, a way to avoid some of the Orion's problems by not using full nukes as pulse units, which enables them to be much smaller, meaning a higher pulse rate and lower peak accelerations:
http://www.projectrho.com/public_html/rocket/enginelist3.php#id--Pulse--Zeta-Pinch--Zeta-Pinch_Fission--Mini-Mag_Orion

There are similarities to Mini-Mag Orion (which I am familiar with), but that uses Z-Pinch. Inertial confinement is a different concept.

On 6/3/2020 at 9:42 AM, SOXBLOX said:

 I think the target didn't go critical, since it's *hard* to circumvent the critical mass requirement. So the laser caused a reaction, it just wasn't a chain reaction. This makes sense, as it was driven by Bremsstralung from scattered electrons, which lose energy and can't be recycled. Of course, it didn't go critical (I don't think) so it didn't release anywhere near enough energy (I don't think) to "break even".

It seems you have to use either compression to lower the critical mass or make up for lost neutrons by bombarding the substance with extra neutrons, bringing us back to the neutron beam problem. Another method would be to put neutron reflectors, like beryllium or tungsten carbide, around it. However, this only works down to a point; after that, it can't bring the fuel critical.

Of course, if we're thinking about using compression to 4 kg/cm^3, then it might be time to worry about uranium fusion instead of fission.

The target didn't go critical, but I can't find anything on the actual yield of the target. Perhaps similar technology could be made to break even...

Another concept would be to make the fuel pellets similar to the secondary in a thermonuclear bomb - essentially surrounding the fuel pellet with a tamper of fissionable material. From my understanding the fissionable material (uranium or thorium) would also act as a pusher. I might be entirely wrong about this, but such a setup could bring about more efficient compression of the fusion fuel and perhaps better confinement as well. And fusion releases neutrons which can cause fission in U-238. The question then becomes how many fission events can we get per pulse. A high enough fusion burn rate may allow for very large fission burn rates. 

Basically, it may be possible to use the fusion of D-T fuel as a source of fast neutrons for the fast fission of U-238 or Th-232 fuel. It is well known that D-T fusion releases a large amount of neutrons which themselves have large amounts of energy. Could work.

We need a quantitative analysis though.

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Carlo Rubbia at CERN has done decades of research using proton beam spallation to drive subcritical reactors.  This technology is spectacular all it needs is inexpensive high energy proton beams.   Or a seat in the Van Allen belts. 

There is an economical device to accelerate electrons using a so called laser wakefield.  If proton acceleration could be achieved on a similar scale, then beam driven subcritical reactors would be a reality.  

 

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On 6/3/2020 at 1:49 PM, K^2 said:

Confinement plays a very different role in fusion and fission. In fusion, you have to have high density and high temperature to get the process going. In fission, increase in temperature actually slows down the reaction, and the only purpose of increasing density is preventing neutrons from flying out of the fuel without contributing to reaction. There isn't actually a need for proximity between nuclei. Because of that, there are limits to how much you can decrease critical mass by compressing the fuel. And that's basically where all of this breaks down. You can take a fuel "pellet" that's significant fraction of critical mass of uranium and get it to split. Congratulations, you just invented a nuke. You cant go much smaller, because there is no amount of pressure that will get it to critical. Alternatively, you can try and look for fuels that will make explosions small enough that you can harness this energy inside a power plant. So something with critical mass in milligram ranges or less. Now we're talking about isotopes that have a very short half life and that you can't practically store, because even at decent spacing, placing enough of them to power a plant in one place is likely going to make the whole store critical. If you somehow manage to avoid that, the background radiation is still going to be quite significant. And all these isotopes will have to be manufactured by some other process.

There have been proposals for making a big cave, dropping in a nuke every once in a while, and using heat of the walls to drive turbines. That is the only practical way I'm aware of of making a nuclear fission reactor that uses inertial confinement.

As I understand in modern nuclear bombs the plutonium need to be compressed more than an metallic sphere at one bar to go supercritical, you need less plutonium and more important safety is vastly improved as unless both detonators go off in microseconds of each other you just have an dirty bomb. 

Instead of an bomb in an cavern have an shaft and an orion style pusher plate connected to an crankshaft drop the nuke and a some water into it, you now have an rather OP steam engine, probably best to make it triple expanding :) 
More realistic you could generate plasma and tap it. 

Again the 
Curium 245 idea sounds nice, might even work for power as its smaller blasts but again cost. 

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50 minutes ago, farmerben said:

Carlo Rubbia at CERN has done decades of research using proton beam spallation to drive subcritical reactors.  This technology is spectacular all it needs is inexpensive high energy proton beams.   Or a seat in the Van Allen belts. 

There is an economical device to accelerate electrons using a so called laser wakefield.  If proton acceleration could be achieved on a similar scale, then beam driven subcritical reactors would be a reality.  

 

I'm aware of accelerator driven reactors. Cool stuff. I think it's worth developing.

27 minutes ago, magnemoe said:

Again the Curium 245 idea sounds nice, might even work for power as its smaller blasts but again cost. 

For Z-Pinch Curium-245 isn't necessary - that was proposed for Mini-Mag Orion for reasons that seem to be unrelated to physics. Any fissile isotope should do.

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4 hours ago, farmerben said:

This technology is spectacular all it needs is inexpensive high energy proton beams.

I mean, granted, cheap proton beams are comparatively simple, but this reminds me of, "We can build room-temperature controlled fusion right now. All it needs is inexpensive muon beams." Or my personal favorite, "We can have cheap nuclear isotope batteries with million times the capacity of LiPo. All it needs is inexpensive hard gamma beams to charge." Unfortunately, if you want inexpensive, safe(ish), and high energy and current, the only ones we've managed are electron beams. And even there, to get the kind of current you need for applications fancier than dental X-Ray, it's actually quite a challenge. I can build a proton accelerator with sufficient energy in my living room, unless my wife stops me, but I won't be able to generate a current that lets me go beyond detectable outputs without a budget of a modest particle accelerator and hiring people who know what they're doing to handle the build. Most accelerator designs just don't scale in terms of getting necessary currents. That's why magnetic confinement fusion still isn't a thing beyond experimentation.

Still beats using neutrons. So maybe this approach has legs.

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I think muons are more likely to initiate fusion if you get one to replace an electron in the ground state of H22  There are fast muons raining down on us all the time, we just need more slow ones.  Maybe a giant airship full of deuterium gas could get some.  

The Van Allen belts have plenty of high energy protons.  We just need to concentrate the current.  A giant magnetic field like a torus could pinch the protons into higher concentration.  Or we could beam electrons at a target giving it a negative charge and allow it to pull in protons that way.  

If we are talking about compressing Uranium or Thorium into supercritical densities, I don't know if that is possible.  Plutonium has several different density states, and the transition from one to the other can go from subcritical to supercritical. I believe that transition could be achieved by hitting plutonium with a hammer.  Other metals like iron can change structure by hitting them with hammers, but they barely change density at all.  Is that what we are talking about?  Using lasers or magnets to hammer solid metals into super densities?  I'd like to see more evidence that this phenomenon is a thing.  

 

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19 hours ago, K^2 said:

. I can build a proton accelerator with sufficient energy in my living room, unless my wife stops me, but I won't be able to generate a current

True that.  A 2-3 diameter cyclotron has sufficient energy.  Maybe if you had dozens of them surrounding a reactor then you would have something.  Nobody to my knowledge has tried to mass produce proton accelerators for the price of TVs.

Anyhow alpha-beryllium neutron sources are an easier way to get neutrons.  And still to this day the most inexpensive fission reactor is the RBMK 1000.  

 

If we position a reactor in the right spot, we could harvest high energy protons.  

Ap8-omni-400.0MeV.png

Imagine a reactor target with 1000 cathode ray tubes surrounding it.  The cathode ray tubes would deposit electrons on our target while becoming positively charged themselves.  

Edited by farmerben
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22 hours ago, Bill Phil said:

I'm aware of accelerator driven reactors. Cool stuff. I think it's worth developing.

For Z-Pinch Curium-245 isn't necessary - that was proposed for Mini-Mag Orion for reasons that seem to be unrelated to physics. Any fissile isotope should do.

One idea is to use it on nuclear waste to break it down fast, benefit is that this does not need to make cheap power as it also break down nuclear waste fast. 

However the charges as in 40 gram and 60 gram beryllium the pulse is also small, could you go so small with plutonium or u235? 
Its an benefit to keep the explosion small as long as this don't make the engine too heavy. 

You should be able to build an orion engine with two cannons shooting two u235 projectiles at each other after all, no nuclear bombs here, yes you get some problems like that you probably hit at an angle as you want to hide your cannons and their reload and recoil systems in the pusher plate and you still need to handle an kiloton explosion. 

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The researchers behind Mini-Mag Orion proposed Z-Pinch of around 40 grams of fissile material. Curium-245 was chosen but other fissile isotopes were possible. 

This implies that it is possible to compress masses of tens of grams to critical mass density.

If 40 grams of Plutonium-239 is the target, then it would have a volume of 0.002 cubic centimeters (assuming a density of 19.86 kg/cm^3, though different allotropes have different densities, and a hollow space would be required for the DT - which acts as a neutron source and potentially a fusion boost). Less compression would be required to reach criticality than would be required for DT targets to reach fusion conditions. Larger target masses would require even less compression. There is likely an optimal mass that would be possible with current drivers, considering that the fissile target will be much less compressible than the fusion target. However new facilities would likely be required, but no advances in driver power may be required.

This setup, if possible, would still have advantages over IC fusion systems:

Since the target masses are much larger more energy will be released per shot, and fairly large burnup rates can be possible if boosting is used. The yield per shot would likely be in the hundreds of gigajoules - the reactor must be able to survive large numbers of consecutive shots. The rate at which the reactor operates will determine the power output - at 100 GJ per shot a 1 GWth reactor would only need a shot every 100 seconds, assuming all or the vast majority of the energy ends up as thermal energy. However a shot every 100 seconds is likely easier to accomplish than 10 shots per second, as IC fusion might require - assuming 100 MJ per fusion shot. Of course larger fusion targets are possible as well but those would require much more powerful drivers, as I understand it. An IC fission reactor may not need drivers that are much more powerful than already existing drivers. I'm not sure about the numbers though.

It would also have advantages over conventional fission reactors, which I outlined in the OP.

One disadvantage would be that using nearly pure fissile material as the fuel could make nuclear proliferation a much larger issue. However, surrounding the fissile material with other dense materials which could act as a "pusher", or just a shell of decent mass, can reduce this risk. This shell can be a dense material with a higher melting point than the Plutonium, so that melting fuel pellets - if some party were to obtain them - would give said party an impure mixture of molten material. So the fuel pellets can be engineered so that they are not easy sources of fissile material.

There are technical challenges, but they appear surmountable.

From what I can find Plutonium is more compressible than Aluminum, and much more compressible than steel. Obviously that varies with its conditions and allotropes, but I think it's compressible enough that a 40 gram target could be made to undergo fission, though I'm not sure about the amount of energy necessary.

40 grams of Plutonium-239 would actually release around 800 GJ at a 25% burnup rate. So to get 1 GWth a reactor would only need to fire a shot every 800 seconds. Only 108 in a day. That's around 4.32 kilograms. To keep a reactor fueled for a period of six months each reactor would need 777.6 kg of fissile material. However this would be stored in discrete pellets with other elements in their makeup. They could also be stored in extremely secure storage containers, each one containing perhaps around a day's worth of fuel. Storage in a highly secure area with automated delivery to the reactor so that as few people as possible are involved in fuel handling would be preferred.

So I believe it is possible, and perhaps more achievable than IC fusion.

Or perhaps a Z-Pinch could be used to compress the fuel? As in Mini-Mag Orion.

It may be possible to capture the waste and separate out the fission products - some percentage of the remaining mass could be usable fuel. Perhaps as much as half or even more. This could be directly sorted on-site using a mass spectrometer, where the actual waste could then be dealt with. Though if such a system is too energy intensive or too difficult to create on the desired scale a dedicated facility could be preferred. Breeding Pu-239 from natural Uranium may be possible (and perhaps more effectively than in a breeder reactor) using neutron spallation from particle accelerators - if the neutron economy in the IC fission concept can't be made to reliably reproduce its fuel supply.

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