Author Topic: Is a sub critical mass nuclear reactor possible?  (Read 3888 times)

Offline Patchouli

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Re: Is a sub critical mass nuclear reactor possible?
« Reply #20 on: 03/19/2019 08:45 pm »
In theory it is possible and goes by the name of the energy amplifier or ADS accelerator driven system
But they would cost more than a conventional reactor of the same output.
https://en.wikipedia.org/wiki/Energy_amplifier
« Last Edit: 03/19/2019 08:46 pm by Patchouli »

Offline RonM

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Re: Is a sub critical mass nuclear reactor possible?
« Reply #21 on: 03/20/2019 01:22 am »
What I'm looking for was described in the 2nd post of this thread. It seemed a simple enough request but apparently no one here is competent to comment on it, which is quite disappointing.

This is a spaceflight forum, not a nuclear engineering forum.

A subcritical reactor uses additional neutrons from an outside source such as a particle accelerator. You need to come up with a different name for your concept.

You have several ideas for a new technology, so you should tell us how it works. If you're really asking us if it is possible without you showing us your math, I'd have to say no.

Online Robotbeat

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Re: Is a sub critical mass nuclear reactor possible?
« Reply #22 on: 03/20/2019 01:33 am »
Can you explain in 50 words what your idea is, if it's not CANDU and is not accelerator-driven?
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline magicsound

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Offline aceshigh

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Re: Is a sub critical mass nuclear reactor possible?
« Reply #24 on: 03/20/2019 02:45 am »
there are more people with knowledge of nuclear reactors at talk-polywell forums.

Offline magicsound

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Re: Is a sub critical mass nuclear reactor possible?
« Reply #25 on: 03/20/2019 02:52 am »
Here's an expanded update on the project;

Offline Asteroza

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Re: Is a sub critical mass nuclear reactor possible?
« Reply #26 on: 03/20/2019 07:21 am »
NASA is already building it, in conjunction with a private research company:
https://www.nasa.gov/saa/domestic/24838_SAA3-1529.pdf
https://www.nasa.gov/saa/domestic/24839_SAA3-1529-1.pdf
http://coldfusioncommunity.net/wp-content/uploads/2018/05/Forsley-Lawrence-1.pdf


The 100kW thermal demo milestone is december 2018 so we should be seeing something soon?

Offline john smith 19

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Re: Is a sub critical mass nuclear reactor possible?
« Reply #27 on: 03/20/2019 07:24 am »
Can you explain in 50 words what your idea is, if it's not CANDU and is not accelerator-driven?
ADD Version
Reduce critical mass by applying compressive stress to individual pellets to raise their density at the operating temperature. Reduce proliferation by only requiring natural Uranium (no enrichment. Ever) as starter fuel and a reprocessing flow that does not allow Pu to be separated out

Short enough?
« Last Edit: 03/20/2019 07:27 am by john smith 19 »
BFS. The worlds first Methane fueled FFORSC engined CFRP stainless steel structure A380 sized aerospaceplane tail sitter capable of flying in Earth and Mars atmospheres. BFR. The worlds biggest Methane fueled FFORSC engined CFRP stainless steel structure booster for BFS. First flight to Mars by end of 2022. Forward looking statements. T&C apply. Believe no one. Run your own numbers. So, you are going to Mars to start a better life? Picture it in your mind. Now say what it is out loud.

Offline edzieba

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Re: Is a sub critical mass nuclear reactor possible?
« Reply #28 on: 03/20/2019 11:48 am »
Can you explain in 50 words what your idea is, if it's not CANDU and is not accelerator-driven?
ADD Version
Reduce critical mass by applying compressive stress to individual pellets to raise their density at the operating temperature. Reduce proliferation by only requiring natural Uranium (no enrichment. Ever) as starter fuel and a reprocessing flow that does not allow Pu to be separated out

Short enough?
So what does a sub-critical mass design offer over CANDU?
The proliferation concerns over reprocessing and enrichment are identical, being sub-critical just means you need either more than one reactor core or to stockpile fuel for that single reactor until you accumulate a critical mass. It's a huge amount of R&D for no benefit.

Offline RonM

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Re: Is a sub critical mass nuclear reactor possible?
« Reply #29 on: 03/20/2019 01:22 pm »
What I'm looking for was described in the 2nd post of this thread. It seemed a simple enough request but apparently no one here is competent to comment on it, which is quite disappointing.

Can you explain in 50 words what your idea is, if it's not CANDU and is not accelerator-driven?
ADD Version
Reduce critical mass by applying compressive stress to individual pellets to raise their density at the operating temperature. Reduce proliferation by only requiring natural Uranium (no enrichment. Ever) as starter fuel and a reprocessing flow that does not allow Pu to be separated out

Short enough?

You need to turn down the attitude. You're asking people to take their time to review your idea.

3 GPa is approaching the pressure needed to make artificial diamonds. It would be extremely difficult to construct a reactor vessel to reach such pressures. Even if possible, the cost would be prohibitive.


Offline john smith 19

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Re: Is a sub critical mass nuclear reactor possible?
« Reply #30 on: 03/21/2019 06:28 am »
You need to turn down the attitude. You're asking people to take their time to review your idea.
I've read the quality of all the posts to this thread and quite a lot of them suggest the poster has spent zero time reviewing the idea before posting :(
Quote from: RonM
3 GPa is approaching the pressure needed to make artificial diamonds. It would be extremely difficult to construct a reactor vessel to reach such pressures. Even if possible, the cost would be prohibitive.
It certainly is possible. A more likely level would be the Batelle autoclave for HIP that was 0.5 dia by 2.7m long at 102 MPa in the mid 1970's.

The difficulty of autoclave construction was why I thought it was a secondary option. But it depends on what is possible.

And so far I've not seen anyone talk about what they think is possible.
BFS. The worlds first Methane fueled FFORSC engined CFRP stainless steel structure A380 sized aerospaceplane tail sitter capable of flying in Earth and Mars atmospheres. BFR. The worlds biggest Methane fueled FFORSC engined CFRP stainless steel structure booster for BFS. First flight to Mars by end of 2022. Forward looking statements. T&C apply. Believe no one. Run your own numbers. So, you are going to Mars to start a better life? Picture it in your mind. Now say what it is out loud.

Offline john smith 19

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Re: Is a sub critical mass nuclear reactor possible?
« Reply #31 on: 03/21/2019 06:31 am »
Can you explain in 50 words what your idea is, if it's not CANDU and is not accelerator-driven?
ADD Version
Reduce critical mass by applying compressive stress to individual pellets to raise their density at the operating temperature. Reduce proliferation by only requiring natural Uranium (no enrichment. Ever) as starter fuel and a reprocessing flow that does not allow Pu to be separated out

Short enough?
So what does a sub-critical mass design offer over CANDU?
The proliferation concerns over reprocessing and enrichment are identical, being sub-critical just means you need either more than one reactor core or to stockpile fuel for that single reactor until you accumulate a critical mass. It's a huge amount of R&D for no benefit.
Potential elimination of D20 as a moderator (but moderator is still a free choice at this point), potential operation of the primary cooling system at low pressure (if it's a molten metal not a gas).
Full recycling of fissionable TRU's without separating out Pu specifically. 
[EDIT And of course an operating temperature about 230c greater than CANDU's 310c, making it substantially more thermally efficient and able to use conventional steam power plant turbines at 1/10 the cost. ]
« Last Edit: 04/01/2019 09:23 pm by john smith 19 »
BFS. The worlds first Methane fueled FFORSC engined CFRP stainless steel structure A380 sized aerospaceplane tail sitter capable of flying in Earth and Mars atmospheres. BFR. The worlds biggest Methane fueled FFORSC engined CFRP stainless steel structure booster for BFS. First flight to Mars by end of 2022. Forward looking statements. T&C apply. Believe no one. Run your own numbers. So, you are going to Mars to start a better life? Picture it in your mind. Now say what it is out loud.

Offline Hobbes-22

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Re: Is a sub critical mass nuclear reactor possible?
« Reply #32 on: 03/21/2019 10:45 am »
This discussion would benefit from some numbers.

A room-temperature sphere of U-235 has a critical mass of ~50 kg. If we use compression, what can initial mass be?
A random figure I found on the internet suggests you need 1000 bar to compress a metal by 0.1%, scale that to 100 MPa and you get a 1% compression ratio, so you'd need to start with 49.5 kg of U-235. (Fermi estimate)

Unfortunately a bomb can be made with much less than 49.5 kg. An implosion design can achieve far higher densities than your reactor chamber because the pressure doesn't need to exist for more than a fraction of a second.

One interesting feature of a compressed-Uranium reactor would be that you can modify the reaction speed by changing the pressure.

Online Robotbeat

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Re: Is a sub critical mass nuclear reactor possible?
« Reply #33 on: 03/21/2019 11:24 am »
Heavy water moderator is much safer than extremely high pressure reactor core.

...and deuterium is potentially an export product for Venus and Mars. Heavy water is better, IMHO.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline john smith 19

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Re: Is a sub critical mass nuclear reactor possible?
« Reply #34 on: 03/21/2019 09:28 pm »
This discussion would benefit from some numbers.

A room-temperature sphere of U-235 has a critical mass of ~50 kg. If we use compression, what can initial mass be?
A random figure I found on the internet suggests you need 1000 bar to compress a metal by 0.1%, scale that to 100 MPa and you get a 1% compression ratio, so you'd need to start with 49.5 kg of U-235. (Fermi estimate)
Really. I found my figures in the AIP handbook.
Quote from: Hobbes-22
Unfortunately a bomb can be made with much less than 49.5 kg. An implosion design can achieve far higher densities than your reactor chamber because the pressure doesn't need to exist for more than a fraction of a second.
You do understand that I'm talking about natural Uranium, that is 0.7% U235, and the idea is to eliminate the necessity for access to enrichment technology to guarantee a supply of fuel? My apologies if this was unclear to you.

Look at it from the reverse direction. Uranium has different expansion coefficents. It can be up to 26x10^-6/K, which is higher than Aluminum. Now what happens if you wrap that pellet in Zirconium, whose TCE is more lik 6x10^-6/K ?

Quote from: Hobbes-22
One interesting feature of a compressed-Uranium reactor would be that you can modify the reaction speed by changing the pressure.
Potentially, yes.
BFS. The worlds first Methane fueled FFORSC engined CFRP stainless steel structure A380 sized aerospaceplane tail sitter capable of flying in Earth and Mars atmospheres. BFR. The worlds biggest Methane fueled FFORSC engined CFRP stainless steel structure booster for BFS. First flight to Mars by end of 2022. Forward looking statements. T&C apply. Believe no one. Run your own numbers. So, you are going to Mars to start a better life? Picture it in your mind. Now say what it is out loud.

Online Robotbeat

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Re: Is a sub critical mass nuclear reactor possible?
« Reply #35 on: 03/21/2019 10:06 pm »
3GPa for a tiny increase in density. Your reinforcement is gonna be thicker than the fuel.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline Symmetry

Re: Is a sub critical mass nuclear reactor possible?
« Reply #36 on: 03/22/2019 02:54 pm »
I'm pretty sure that if you could reach criticality with natural uranium by compressing it you could also reach critical mass with natural uranium by just using a larger mass of it.

And I'm also pretty sure that any reactor which consumes natural uranium and which involves U238 (the isotope that turns into the isotopes of plutonium used in weapon) would be much more of a proliferation threat than modern reactors because the difficulty of refining uranium is one of the major limitations on proliferation.

Offline Symmetry

Re: Is a sub critical mass nuclear reactor possible?
« Reply #37 on: 03/24/2019 12:10 pm »
To elaborate a bit on that.  Lets say you have a fission spectrum neutron in an infinity large volume of natural (99.3% U238, .7% U235).  At this energy U238 has an absorption cross section of .3 barns and U235 of 1.2 barns.  Since only a small amount of energy is transferred in an elastic collision and since the scattering cross section is ~3 for both, close enough to the absorption numbers, we can neglect the thermal spectrum shifting before absorption.  The math is easy in this case, multiplying the frequency of the isotope by the cross section and normalizing gives a 2.7% chance that any neutron reacts with an U235 atom rather than a U238.  Multiply that by 2.43 neutrons per reaction, on average, gives .066 further neutrons generated.  Well short of a sustained chain reaction.

If we slow the neutrons down to the room temperature with some heavy water (but ignore  things are very different.  At 0.0253-eV U235 is 585 barns across and U238 is .000017.  That means that U235 reactions are essentially all of the reactions, you get nearly 2.43 neutrons per neutron lost, and you're prompt critical.  The heavy water will capture a bit of the neutrons, of course, but it doesn't like to do so as much as light water so it doesn't ruin things even across all the collisions needed to get the energies down this far.

In any real assembly you're going to be losing neutrons to them exiting it.  To a first approximation you can take all the atoms in the assembly, multiply them by their cross sections, divide them by the distance from neutron emission to them, and that's the chance that the escaping neutron will interact with them first.  Of course compressing the mass of uranium will tend to decrease all those distances, and increase the chance of some interaction before it escapes.  But of course as we showed in the first paragraph, even if the chance of escape went down to literally zero it won't give you a nuclear chain reaction with natural uranium unless you introduce a moderator.  And the different propensities of H1 and H2 to absorb neutrons are why you need H2 for a natural uranium reactor while you can use the H1 in light water for low enriched uranium.  Compression doesn't change the relationship between the sort of moderation and the sort of enrichment you need for a chain reaction, it just changes the mass of uranium you need.  Achieving a chain reaction in natural uranium simply by compressing it with no moderation won't work.

For a natural uranium reactor you need a heavy water (or graphite) moderator and heavy water reactors are well known to be the most dangerous sort of reactor from a proliferation standpoint.

Offline Symmetry

Re: Is a sub critical mass nuclear reactor possible?
« Reply #38 on: 03/24/2019 02:31 pm »
And an interesting deep historical note related to all that.  The current natural isotope ratio of uranium hasn't been constant across Earth's history.  Uranium 438 has experience one half life but uranium 435 has experience 6.  That means that there was once much more U235, proportionally, in the past and that it was once possible to run a natural uranium reactor with light water.  And sure enough we've been able to find evidence of this at Oklo in Gabon, a deposit of uranium where the U235 is depleted well below natural levels in a geological formation where a natural reactor seems feasible.  There were likely many others on the floor of the Earth's oceans but that is geologically recycled very rapidly compared to the continents so it's unlikely we'll ever find evidence.

When we travel to other solar systems with different ages and different natural isotope ratios we might find that the natural uranium we find there might easy fuel for light water reactors without processing.  Or it might be so depleted that it will require breeding with fissiles brought from Earth for nuclear reactors to be possible.

Offline john smith 19

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Re: Is a sub critical mass nuclear reactor possible?
« Reply #39 on: 03/24/2019 09:51 pm »
Achieving a chain reaction in natural uranium simply by compressing it with no moderation won't work.
Which no one (except you) is disputing.

Had you read the 2nd post at the start of this thread you'd have noticed the part about "pellets" of fuel.

This is by synonym with a UO2 fuel pellet, the smallest individual element of fuel in a common LWR (PWR or BWR).

I don't expect a single large lump of Uranium fuel to work. I'm looking at this at the fuel pellet level. The pellets are then mounted between whatever moderating structure the designer feels is necessary.

Personally I'm quite intrigued by a "dual moderator" structure like TRIGA, with the pellets (say 1mm in dia) either of UH or with a coating of H2 (or D2, although getting pure D2 molecules is quite difficult) on the surface before coating with the restraining shell, which I think Zirconium is the prime candidate for. Then placing it into a moderating structure of graphite.

I don't like Uranium Oxide as a fuel (unless I wanted to run a nuclear submarine) as the O2 has too big a cross section to be viable for natural uranium.

Perhaps you would like to read the 2nd post on this thread and see how it changes your thinking?
BFS. The worlds first Methane fueled FFORSC engined CFRP stainless steel structure A380 sized aerospaceplane tail sitter capable of flying in Earth and Mars atmospheres. BFR. The worlds biggest Methane fueled FFORSC engined CFRP stainless steel structure booster for BFS. First flight to Mars by end of 2022. Forward looking statements. T&C apply. Believe no one. Run your own numbers. So, you are going to Mars to start a better life? Picture it in your mind. Now say what it is out loud.

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