Neutron & radioactive waste production in p11B polywells

Discuss how polywell fusion works; share theoretical questions and answers.

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ANTIcarrot
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Neutron & radioactive waste production in p11B polywells

Post by ANTIcarrot »

Irrespective of reactor design, p11B fusion produces 'less than 0.2%' of its total output as neutrons. http://en.wikipedia.org/wiki/Aneutronic ... B_reactor].

Even if we assume 0.1%, then a 1GW 'regular size' power plant will still produce a MW of neutron flux. Assuming my math is right...

This is not only hazardous to human health (unshielded at 100m it'll give you a lethal dose in about a minute) but over time it will transmute the strongest materials into weaker radioactive ones. Structural and component materials may have to be replaced. Even if not then this waste will then need to be disposed of in the conventional nuclear sense.

This is of course much better than tokamaks or fission piles, but it is still a problem. Any thoughts?
Some light reading material: Half Way To Anywhere, The Rocket Company, Space Technology, The High Fronter, Of Wolves And Men, Light On Shattered Water, The Ultimate Weapon, any Janes Guide, GURPS Bio-Tech, ALIENS Technical Manual, The God Delusion.

jlumartinez
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Post by jlumartinez »

You are expecting a miracle from Polywell. We should be quite glad even if it only may produce normal D-T fusion. I note that here people is quite enthusiastic, too much I would say. None has yet achieved fusion breakeven and we are already asking Polywell for being cheap, totally clean, aneitronic , easy to build and use, portable, and with wonder colours . Come on... Let´s be realistic and wait for future results.

Anyway, answering your question: Many fusion reactor guidelines are researching to create new materials which can withstand neutron fluxes. They are designing walls that absorb the neutrons with Lithium producing Tritium as a fission reaction to eliminate the neutron emittance . Another material is called FLIBE

Zixinus
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Post by Zixinus »

That means that aneutronic grids have to be shielded as well, I recall that Bussard wants to use boron-based material. However, an aneutronic reactor could have its operating lifetime massively increased.

EDIT: Also, I recall that neutron-producing p-b11 based reactions only occur in certain situations, something with thermalizing. This may be avoidable.
This is of course much better than tokamaks or fission piles, but it is still a problem. Any thoughts?
That there is no such thing as a "magic power machine" that is also nuclear. No matter what you do with nuclear power, you will get unwanted radiation, and neutrons. This is just how this power operates. This is also due to how engineering works, as there is never 100% efficiency with fusion machines.

MSimon
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Re: Neutron & radioactive waste production in p11B polyw

Post by MSimon »

ANTIcarrot wrote:Irrespective of reactor design, p11B fusion produces 'less than 0.2%' of its total output as neutrons. http://en.wikipedia.org/wiki/Aneutronic ... B_reactor].

Even if we assume 0.1%, then a 1GW 'regular size' power plant will still produce a MW of neutron flux. Assuming my math is right...

This is not only hazardous to human health (unshielded at 100m it'll give you a lethal dose in about a minute) but over time it will transmute the strongest materials into weaker radioactive ones. Structural and component materials may have to be replaced. Even if not then this waste will then need to be disposed of in the conventional nuclear sense.

This is of course much better than tokamaks or fission piles, but it is still a problem. Any thoughts?
Transmutation is not a serious issue.

Working fission plants are made of 404 stainless. After 10 days of cool down it is safe to work in the reactor compartment. If I recall correctly (it has been 40 years) the max time after 10 days cool down was 1 hour. Radioactive to be sure. Not extremely radioactive. BTW a significant portion of that radiation was from fission products in the core.

I remember loitering in the reactor compartment because it was so cool being near all that potential power.

BTW nuke plants are designed for 40 years operation before neutron embrittlement requires reactor vessel replacement.

Did I mention that neutrons are easily shielded with water and concrete? We actually have the technology to do this.

Sufficient neutron shielding will help take care of gammas as well.

ANTIcarrot
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Post by ANTIcarrot »

jlumartinez wrote:You are expecting a miracle from Polywell.
I'm not 'asking' polywell reactors to be anything. I'm pointing out that even if their predictions are true p11B reactors will have the same problems (and pose some of the same dangers) as any other nuclear reactor. With the added bonus that a small unshielded D-T polywell can serve as a mobile, reusable neutron bomb.

Does this sound like something you would sell to Iran or North Korea?
Some light reading material: Half Way To Anywhere, The Rocket Company, Space Technology, The High Fronter, Of Wolves And Men, Light On Shattered Water, The Ultimate Weapon, any Janes Guide, GURPS Bio-Tech, ALIENS Technical Manual, The God Delusion.

ANTIcarrot
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Re: Neutron & radioactive waste production in p11B polyw

Post by ANTIcarrot »

MSimon wrote: Working fission plants are made of 404 stainless. <snip> Did I mention that neutrons are easily shielded with water and concrete? We actually have the technology to do this.
Ah! Good good. A couple of issues still though.

For obvious reasons a polywell can't have water or other kinds of shielding inside the core itself, like a PWR does. It may also have relatively exotic materials needed for the magnetic coils. It is also my understanding that at the end of 40 years the reactor core is left standing, because dismantling it is not safe. I was under the impression that this was due to long term contamination, which lasts longer than ten days. Would that make any difference?
Some light reading material: Half Way To Anywhere, The Rocket Company, Space Technology, The High Fronter, Of Wolves And Men, Light On Shattered Water, The Ultimate Weapon, any Janes Guide, GURPS Bio-Tech, ALIENS Technical Manual, The God Delusion.

MSimon
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Re: Neutron & radioactive waste production in p11B polyw

Post by MSimon »

ANTIcarrot wrote:
MSimon wrote: Working fission plants are made of 404 stainless. <snip> Did I mention that neutrons are easily shielded with water and concrete? We actually have the technology to do this.
Ah! Good good. A couple of issues still though.

For obvious reasons a polywell can't have water or other kinds of shielding inside the core itself, like a PWR does. It may also have relatively exotic materials needed for the magnetic coils. It is also my understanding that at the end of 40 years the reactor core is left standing, because dismantling it is not safe. I was under the impression that this was due to long term contamination, which lasts longer than ten days. Would that make any difference?
No long term contamination with fusion.

Only neutron activation.

rexxam62
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Post by rexxam62 »

I bought a telescope made just for looking at the Sun. What a wonderful Fusion reactor the Sun is. And it works, right now!

ANTIcarrot
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Re: Neutron & radioactive waste production in p11B polyw

Post by ANTIcarrot »

MSimon wrote:No long term contamination with fusion. Only neutron activation.
Contamination can mean more than just 'can't clean away the last of the uranium'.

Mr Bussard assumes the use of super conductors in his machines. Two of the better ones which is beginning to be used in mains systems (albeit experimentally in limited quantities) are BSCCO and YBCO. It may not be possible to shield these coils, so these materials may also have to withstand neutron activation. If any of them decay into other materials afterwards, then that is also an issue of contamination; one that coudl inhibit the super conducting effect.

Of bismuth, calcium, copper, strontium, oxygen, ytterium and barium, there are a few potential activation problems:

40Ca to 41Ca HL 1e5 years and decays to potassium
88Sr to 89Sr HL 50 days and decays into rubidium
89Y to 90Y - HL 2 days and decays to zirconium
Some light reading material: Half Way To Anywhere, The Rocket Company, Space Technology, The High Fronter, Of Wolves And Men, Light On Shattered Water, The Ultimate Weapon, any Janes Guide, GURPS Bio-Tech, ALIENS Technical Manual, The God Delusion.

MSimon
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Re: Neutron & radioactive waste production in p11B polyw

Post by MSimon »

ANTIcarrot wrote:
MSimon wrote:No long term contamination with fusion. Only neutron activation.
Contamination can mean more than just 'can't clean away the last of the uranium'.

Mr Bussard assumes the use of super conductors in his machines. Two of the better ones which is beginning to be used in mains systems (albeit experimentally in limited quantities) are BSCCO and YBCO. It may not be possible to shield these coils, so these materials may also have to withstand neutron activation. If any of them decay into other materials afterwards, then that is also an issue of contamination; one that coudl inhibit the super conducting effect.

Of bismuth, calcium, copper, strontium, oxygen, ytterium and barium, there are a few potential activation problems:

40Ca to 41Ca HL 1e5 years and decays to potassium
88Sr to 89Sr HL 50 days and decays into rubidium
89Y to 90Y - HL 2 days and decays to zirconium
41Ca is not going to be very radioactive.
90Y will be gone in 10 days - 20 max
89Sr if it is significantly activated is going to be a little problem. 500 days to 1/1,000th activity. 1,000 days to get to 1E-6 of initial activity.

Activation rates will not be really high because the neutrons coming out of the reactor are going to be at 2 Mev or greater where the capture cross sections are low.

If pB11 is used as fuel the neutron output will be 1/50th of an equivalent nuke plant. Plus there will be essentially zero thermal neutrons in the reactor.

I'm leaning to MgB2 superconductors.
Superconductors

What are the problems with those?

ANTIcarrot
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Re: Neutron & radioactive waste production in p11B polyw

Post by ANTIcarrot »

MSimon wrote:41Ca is not going to be very radioactive.
As I just said, radioactivity is not an issue when it comes to superconductor failure.
Activation rates will not be really high because the neutrons coming out of the reactor are going to be at 2 Mev or greater where the capture cross sections are low.
Why do you believe this?
If pB11 is used as fuel the neutron output will be 1/50th of an equivalent nuke plant. Plus there will be essentially zero thermal neutrons in the reactor.
Yes, but here's the important thing: In a fission reactor, all the sensative equipment is on the outside of the shielding. In a polywell, significant delicate equipment (superconductos) may have to be on the inside of the shielding. So even though neutron production is down, neutron exposure may be up.
I'm leaning to MgB2 superconductors. What are the problems with those?
*MgB2 only works at 26K and requires liquid hydrogen for cooling.
*High temperature superconductors (like YBCO) use liquid nitrogen which will use less power to run the compressors.
*Containing the hydrogen in turn requires more energy and more exotic materials than YBCO; which can also lead to additional neutron-activation related failure modes. Hydrogen also has a bad (if undeserved) PR reputation.
*YBCO and BSCCO are commercial materials in the beginning stages of mass production. MgB2 is just another lab experiment.
*11% of Mg is 26Mg - which is not stable after activation and may decay into another element
*80% of boron is 11B - which is not stable after activation and may decay into another element.
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Post by MSimon »

anti asks,
Quote:
Activation rates will not be really high because the neutrons coming out of the reactor are going to be at 2 Mev or greater where the capture cross sections are low.
Why do you believe this?
Because I'm a nuclear engineer. How about you?

Do you know the difference in absorption cross sections between 2 Mev neutrons and thermal neutrons? Is it typically a 10% difference or more like 2X to 10X difference?

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Post by MSimon »

Any superconductors used in a Polywell will have to be cooled in the 20K range (or lower depending on the material) because of the magnetic field requirements.

There are no 77K high mag field superconductors. You knew that didn't you?

Hydrogen in multi-ton quantities confined in gas bags (how much hydrogen do you contain?) has a bad rep. Hydrogen used in the Space Shuttle has a pretty good rep. So for Polywell I propose no H2 storage in gas bags. Such a simple solution.

B10/B11 has a low high energy neutron cross section. You knew that didn't you? It is why thermalization is required if you want to use Boron as a neutron regulator. When was the last time you studied fission reactor technology? You seem very ignorant on the subject.

If you have neutrons you are going to have activation problems. These can be minimized by the fuel selection, operating conditions, and proper design. The problems are much smaller with fusion because there are no thermal neutrons in the reactor.

You ask why I believe capture cross sections are lower for 2 Mev neutrons than thermal neutrons. It is simple really. I looked it up.

Stefan
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Post by Stefan »

High temperature superconductors can't be used for strong electromagnets.
ITER uses niobium-tin and niobium-titanium alloy.

Even if YBCO or BSCCO could be used, it is likely that cooling further down would increase the critical current density enough to warrant the effort.

I see no reason to use hydrogen for cooling. Unless I am very mistaken usually liquid helium is used (boiling point ~4K, critical temperature for NbTi ~10K).

chazmataz
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Post by chazmataz »

Stefan wrote:High temperature superconductors can't be used for strong electromagnets.
ITER uses niobium-tin and niobium-titanium alloy.

Even if YBCO or BSCCO could be used, it is likely that cooling further down would increase the critical current density enough to warrant the effort.

I see no reason to use hydrogen for cooling. Unless I am very mistaken usually liquid helium is used (boiling point ~4K, critical temperature for NbTi ~10K).
The major disadvantage of using liquid He for cooling is obviously COST. Liquid He costs dozens of times as much as liquid hydrogen. When you use liquid helium, you also need to be very effective in your recycling. Yes, the lower temperature of helium will let you get a higher magnetic field, even with the MgB2 superconductors, but overall the hassles and expenses of working with liquid helium are big enough that basically everyone who uses it would happily switch to LH2 if they could.

Traditional low-temperature superconductors generally don't work at the 20K boiling point of hydrogen. MgB2 doped with carbon is superconducting up to about 40K, so it is capable of generating high magnetic fields at 20K, and even at the 33K critical temperature of LH2.

The only rationale to go with traditional NbTi superconductors and liquid helium would be if your research project was extremely well funded and/or if you could get off-the-shelf coils that fit your design perfectly.

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