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Source: (consider it) Thread: Nuclear Fusion
Doublethink.
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# 1984

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So, I saw an interesting feauture on channel 4 news that I don't fully understand.

They were focusing on this company's attempt to be the first to develop commercially viable nuclear fusion energy.

But so much information wasn't in the report.

I get that they need to make a charged gas / plasma - and they need to get is hotter than the sun - that they need magnets to do this, that these magnets can now be much smaller thanks to the discovery of various superconductors.

What I don't get is, for example, why so little fuel would produce so much energy *if* fusion worked, why you need magnets, what the fuel would be, what the waste products would therefore probably be and what the key safety parameters would be - (I sense a significant potential issue with making stuff many times hotter than the sun).

Also, what does nuclear mean in this context - OK to do with atomic nuclei, but is radiation an issue ? if not, why not ?

I wondered if any shippies knew more and we up for an exploratory discussion ?

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LeRoc

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The first question is simple. Such a small amount of matter can produce huge energy because E=mc².

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Doublethink.
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# 1984

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But, surely depends on conversion rate, but (dodgy) is often energy is lost in reaction voa heat - presumably not the case here.

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LeRoc

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In this case, energy is supposed to get lost as heat. It heats up steam that drives a turbine.

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alienfromzog

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Doublethink you're thinking in chemical terms here. This is not really chemical energy in the same sense. What Einstein was getting at with E=mc² is that under certain conditions it is possible to literally turn matter into energy.

Now, I'm sure you know that C is the speed of light. Or rather C (according to Einstein is the Constant of the universe) i.e. time is not constant but varies with speed. It so happens that speed seems to travel at or very close to that speed. Either way the speed of light is 3x1000000000 m/s i.e. it's a very big number. Hence if you can use this you turn a very small amount of matter into a lot of energy.

Oddly this process happens to a very small extent in common reactions but not enough to really notice. But if you can achieve a fusion reaction then the amount of energy released (or 'created' if you prefer) is absolutely massive.

Hence it's the holy grail of energy research.

AFZ

[ 03. November 2015, 19:14: Message edited by: alienfromzog ]

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Arethosemyfeet
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quote:
Originally posted by Doublethink.:
But, surely depends on conversion rate, but (dodgy) is often energy is lost in reaction voa heat - presumably not the case here.

Heat is the product you want. All big power stations (barring hydro) rely on getting something really hot and running water through it to turn into steam and drive turbines to produce electricity. That basic principle applies whether it's coal, oil, gas, peat, biomass, nuclear fission, nuclear fusion.

In the case of fusion you take advantage of the fact that fusing the nuclei of two light(er than iron, usually Helium) atoms gives you a net gain in energy. If you can contain this reaction using a small enough amount of energy to do it then your reactor can be a net producer of energy on the macro scale.

Nuclear fission, meanwhile, relies on that fact that heavy(er than iron) atoms release energy when their nuclei split. Certain unstable nuclei split very readily when bombarded with neutrons and produce neutrons in the process, causing a chain reaction.

Both processes take advantage of the fact that the mass of an atomic nucleus is not exactly the same as the sum of the masses of the component protons and neutrons. The difference in mass in each case is converted into energy according to E=mc^2. Recalling that c in this case is the speed of light (3 x 10^8 m/s) gives you some idea of the scale of the energy release even for a very small mass.

Neither fusion reactants nor products are significantly radioactive, unlike the unstable nuclei used for fission. If a fusion reaction breaks containment it just peters out very quickly, possibly with a modest explosion, but certainly no worse than you might get from a fossil fuel station with a problem. Probably dangerous to stand close to, but certainly not capable of spreading a radioactive cloud over half a continent, or even half a street.

[ 03. November 2015, 19:16: Message edited by: Arethosemyfeet ]

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Sipech
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quote:
Originally posted by Doublethink.:
So, I saw an interesting feauture on channel 4 news that I don't fully understand.

They were focusing on this company's attempt to be the first to develop commercially viable nuclear fusion energy.

But so much information wasn't in the report.

I get that they need to make a charged gas / plasma - and they need to get is hotter than the sun - that they need magnets to do this, that these magnets can now be much smaller thanks to the discovery of various superconductors.

What I don't get is, for example, why so little fuel would produce so much energy *if* fusion worked, why you need magnets, what the fuel would be, what the waste products would therefore probably be and what the key safety parameters would be - (I sense a significant potential issue with making stuff many times hotter than the sun).

Also, what does nuclear mean in this context - OK to do with atomic nuclei, but is radiation an issue ? if not, why not ?

I wondered if any shippies knew more and we up for an exploratory discussion ?

The reason for the magnets is to hold the plasma in place. Because it is so incredibly hot, if it touched the side of pretty much any container it would melt it, thus destroying the reactor.

The fuel/waste products issue is what makes fusion (smashing tiny atoms together so that they stick) so much better than fission (firing neutrons at large atoms so that they split). You don't need a radioactive material to start with and you don't end up with a radioactive by-product.

The input fuel is hydrogen, the most abundant element in the universe and something we can effectively harvest relatively easily (e.g. from sea water). The technical issue is to create a sustained fusion reaction; IIRC the longest reaction done in a lab thus far is around 20 seconds, but that never got to break even on the energy. i.e. they put more energy into making it happen than they got out of it.

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LeRoc

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quote:
Doublethink.: what the key safety parameters would be - (I sense a significant potential issue with making stuff many times hotter than the sun).
Don't let it touch anything!

(Hence the magnets, as Sipech has already said. This very hot stuff is magnetic, so you can use magnets to hold it in place).

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Doublethink.
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# 1984

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Why does matter have to be so hot to do this ? Could you fuse things chemically ? Or through pressure alone ?

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All political thinking for years past has been vitiated in the same way. People can foresee the future only when it coincides with their own wishes, and the most grossly obvious facts can be ignored when they are unwelcome. George Orwell

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LeRoc

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Matter becomes hot by fusing it. What you need to do is to slam the nuclei into each other very hard. So you turn matter into a plasma: hot, magnetic stuff. You use magnets to accelerate this and boom! Chemical processes are much to weak and slow for this: they touch the atoms' electron shells, not the nuclei.

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hatless

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I was amazed to read that a cubic metre of the sun's core produces, by atomic fusion, only about 100 Watts of energy. In other words this sphere of furiously incandescent gas that feels hot from 98,000,000 miles away, produces heat at about the same rate as a freshly made compost heap. It's just that it's much bigger than any compost heap I've ever made.

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Schroedinger's cat

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Let me try to explain fusion technology as best I can - everyone else has explained bits of it.

The essence of the physics (not chemistry) is that when you collide two hydrogen atoms (nucleus' actually) together at very high speed, they can combine into an atom of helium.

The mass of the new atom is less than the mass of the two hydrogen atoms. This excess mass is converted to energy, which is emitted from the reaction.

Whenever you convert mass into energy, because of the E=mc2 equation, the energy (E) is large compared to the original mass (m). When you achieve this across not just one atom, but a small balloon of hydrogen, the energy released is very high.

This is the essence of an H-bomb, which can produce phenomenal amounts of energy from very small sources. A bomb uses an ordinary atom bomb as the starting point, which provides the initial energy to then generate a vastly more powerful explosion.

This is the core problem - to get the initial hydrogen up to an energy level that the nuclei can collide appropriately, it needs to be very hot. Hotter means more energy, and the heat level is such that the gas is converted into a plasma. Plasmas are very difficult to control, and require a lot of energy to manage.

All of this means that there is a lot of energy you need to put into the system to start with. You need to put energy in to heat the gas, and a lot of energy to manage the plasma using strong magnets. This is a lot of energy, but, when it works, it will generate substantially more energy out of it. The calculations mean that the energy put in is significant, but is miniscule compared to the vast energy available.

The source products are hydrogen (or other safe and available materials), and the waste products are helium (or similar). They are crucially, not radioactive, and so safe to work with. The source of energy is similar to fission energy (that we use in existing power stations), but very much more efficient and - critically - the source and waste products are horrible and dangerous.

I hope this helps to explain why this is such an important area of work.

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Doublethink.
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Dumb question, why does it have to be a specifc fuel, could you use methane from rubbish ?

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Leorning Cniht
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quote:
Originally posted by Doublethink.:
why you need magnets,

You need magnets to confine the plasma.

A quick nuclear physics primer (Dr. Cresswell will probably show up soon...):

Nucleons (protons and neutrons) like being bound together in a nucleus (up to a point). Iron and Nickel are the most tightly-bound nuclei: you can break up nuclei that are much heavier than these in order to release some binding energy (this is nuclear fission - the thing that happens to Uranium in existing nuclear reactors), or you can join together lighter nuclei (nuclear fusion).

Nuclei are all positively charged (they have charged protons and uncharged neutrons), so tend to repel each other electrostatically. To overcome this, they have to be moving fast, which means they need to be hot.

That's why you need a very hot gas to make fusion happen.

Next problem: what do you put your hot gas in? It will destroy any physical container on contact, so you need some kind of non-contact confinement. So you confine the charged plasma with magnetic fields, in order to keep it away from the walls of the reactor chamber.

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LeRoc

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quote:
Doublethink.: Dumb question, why does it have to be a specifc fuel, could you use methane from rubbish ?
You're thinking chemically [Smile]

Methane is a molecule, consisting of one carbon atom and four hydrogen atoms, bound by electrical forces.

For fusion, you need to go down to the atomic level and even further, to the nucleus. Electrical bonds are irrelevant, you need pure hydrogen.

In theory, it is possible to fuse carbon. Some stars do. But you'd need much more pressure than with hydrogen.

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Leorning Cniht
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quote:
Originally posted by Doublethink.:
Dumb question, why does it have to be a specifc fuel, could you use methane from rubbish ?

In principle, you can fuse anything lighter than iron. This is what stars do - they start with hydrogen, and gradually turn themselves into iron (you only get the heavy stuff from neutron absorbtion in supernovae).

However, not all the possible reactions are equally-good. Helium-4 is an especially good target nucleus: its symmetry makes is exceptionally stable for its size. It's easier to fuse lighter elements because you have less electrostatic repulsion to overcome (if you want to fuse carbon, it needs to be much hotter).

So Deuterium + Tritium -> Helium-4 + neutron usually works out to be the best candidate, although there are other possibilities.

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hatless

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What happens to two hydrogen atoms when they fuse? Where do they find two neutrons from to become helium?

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Leorning Cniht
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quote:
Originally posted by Leorning Cniht:

So Deuterium + Tritium -> Helium-4 + neutron usually works out to be the best candidate, although there are other possibilities.

I should mention that deuterium and tritium are both isotopes of hydrogen.

A normal hydrogen nucleus is a single proton.

A deuterium nucleus is a proton and a neutron. Combine deuterium and oxygen, and you get heavy water. An amusing, but expensive, party trick is to make ice from heavy water - it will sink in your drink. You can't replace all the water in your body with heavy water and live, but it's fine in small quantities.

A tritium nucleus is a proton and two neutrons. Tritium is radioactive (it decays to helium-3 with a half-life of about 12 years.) Tritiated water contaminating the water supply is on the list of things that people who worry about radiation safety like to worry about.

(hatless - this is where the neutrons come from. It's D+T -> He-4 + n.

In the sun, the primary reaction is proton-proton fusion, which fuses 2 protons to make He-2, which then decays by positron emission to deuterium. This is a rare process: usually, the He-2 diproton just splits up into a pair of protons. The fact that this is rare (and slow) is the reason that the stars still shine, and didn't all consume themselves in fiery ecstasy billions of years ago.

[ 03. November 2015, 20:42: Message edited by: Leorning Cniht ]

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Jay-Emm
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I wonder if one way [if all that is required is to see that there would be a difference and guess the direction] to picture the difference between Chemical and Nuclear energy, would be to ignore E=mcc [qm, etc] and just consider the bonds in a very very naive fashion.

Recall that the hard part of launching a rocket is the first part. The nearer to the centre you start the harder it is, and going from 10000 miles to 20000 miles away requires more energy than 20000 to 40000 (and you get the same energy back if you reverse the motion). With opposite charges it is the same situation.

In chemical bonds you have a negative electron then a positive proton, then a negative electron then a positive proton*. All about 0.1 nanometer apart. So to burn hydrogen, we have to pull one of the p-e pairs, do the same for another molecule of hydrogen and oxygen (3 splittings). The we gain the energy when they come back together H-O-H H-O-H (4 pairings) so you have a profit of the bringing a positive-negative charge 0.1 nanometers together (which is enough to make a small pop).

For nuclear bonds the distances are about 1000th of the size, but the charges are the same. So the energy is much higher. If you imagine the nucleus as a tiny mix of +ve and -ve's you can see how net profit is potentially vastly greater.

Now if you think about it electrostactic energies all go in the opposite way, (and hence also reinforce each other, but lets ignore that for the moment). And there's another force (the Strong one) that does the attracting. But you can kind of see the profit/loss in a change remains similarly high.


*except, in this naive model the electrons are spinning so it alternates between e-p-e-p and p-e-p-e.


quote:
Originally posted by hatless:
I was amazed to read that a cubic metre of the sun's core produces, by atomic fusion, only about 100 Watts of energy.

It's a bit like electricity being fast, slow, fast isn't it.

[ 03. November 2015, 20:50: Message edited by: Jay-Emm ]

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Doublethink.
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(Doublethink now has acquired physics theory eclectical confusion disorder - not helped by random googleage.)

Why are researchers so much more sold on hot fusion than muon-catalysed fusion ?

--------------------
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Penny S
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When I was twenty I read all this up while I was at college. The Soviets were using tokomaks (sp) which were bottle shaped magnetic containers, and the west was using toroidal containers with the plasma as a ring. Wasn't liquid lithium being used as a coolant, or to absorb stray particles?
Over 45 years pass. Where have we got to? Not very far.

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alienfromzog

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Try this... it's fun and educational! [Biased]

AFZ

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Leorning Cniht
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quote:
Originally posted by Doublethink.:

Why are researchers so much more sold on hot fusion than muon-catalysed fusion ?

Muon beams are pretty expensive. To make muons, you have to accelerate protons, collide the protons with some stuff, focus the pions that are produced, let them decay into muons, and then use them quick, because the muon lifetime is 2 microseconds. It's challenging to make this energetically favourable.
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Adeodatus
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quote:
Originally posted by Doublethink.:
(Doublethink now has acquired physics theory eclectical confusion disorder - not helped by random googleage.)

Why are researchers so much more sold on hot fusion than muon-catalysed fusion ?

The problem with all fusion experiments at the moment is that, generally speaking, they use up more energy than they produce. (In the past few years some have produced "flashes" of net energy output, but as far as I know, none have maintained it yet.)

The problem with muon-catalysed fusion is that it takes huge amounts of energy to create enough muons to get the reaction going. The problem with hot fusion is it takes a huge amount of energy to heat the plasma and power the magnets.

There is another reason why hot fusion is still the most popular line of research, and that's simply that it's had so much time and energy invested in it. Early on, scientists thought it would be pretty easy to do, and a lot of government money went into hot fusion. Then in the late 50s J.D.Lawson showed that it was going to be a lot more difficult than they thought. (I've heard someone who was there tell the story that he literally worked out the maths on the back of a cigarette packet!) But by that time, the hot fusion bandwagon was well and truly rolling.

For the past 60 years, scientists have been saying that a working fusion reactor is "less than 20 years in the future".

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Doublethink.
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I get the impression that hot fusion also requires significant up-tech, presumably one could focus on increasing muon production etc

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All political thinking for years past has been vitiated in the same way. People can foresee the future only when it coincides with their own wishes, and the most grossly obvious facts can be ignored when they are unwelcome. George Orwell

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Martin60
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If it could be done, we'd have done it, like all the other bunk in Sci-Fi.

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Crœsos
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quote:
Originally posted by Adeodatus:
There is another reason why hot fusion is still the most popular line of research, and that's simply that it's had so much time and energy invested in it. Early on, scientists thought it would be pretty easy to do, and a lot of government money went into hot fusion.

To be fair, hot fusion was fairly easy to do. Controlled hot fusion, on the other hand . . .

There are a couple competitors to the folks mentioned in the OP. The first is the ITER reactor, currently under construction and behind schedule. Lockheed-Martin claims to be close to developing a compact fusion reactor, but they're being squirrelly about details and schedules (though some of this might simply be because they're a for-profit company trying to protect its intellectual property).

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Adeodatus
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quote:
Originally posted by Doublethink.:
I get the impression that hot fusion also requires significant up-tech, presumably one could focus on increasing muon production etc

I'm not that well up on the physics (my old professor was a hot fusion man), but I gather there's a certain minimum energy needed to create and isolate the pions that decay into muons, which you can then fire at your deuterium/tritium fuel. Combined with certain problems once the muons are in there (decaying in about 2 microseconds, and another problem where some of the muons essentially clog up their own reactions), the best energy ratio that's been achieved is currently 40%. "Break-even" is 100%, and some of the hot fusion experiments are a lot closer.

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Leorning Cniht
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quote:
Originally posted by Doublethink.:
I get the impression that hot fusion also requires significant up-tech, presumably one could focus on increasing muon production etc

A more promising route for accelerator-driven reactors is subcritical accelerator-driven fission of thorium. We know the math works for that - it's "just" a case of the necessary technical advances. Nobody knows how to make the math work for muon-driven fusion.
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LeRoc

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On the other hand, we could just reverse the polarity of the neutron beam.

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I know why God made the rhinoceros, it's because He couldn't see the rhinoceros, so He made the rhinoceros to be able to see it. (Clarice Lispector)

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Ricardus
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# 8757

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quote:
Originally posted by Leorning Cniht:

So Deuterium + Tritium -> Helium-4 + neutron usually works out to be the best candidate, although there are other possibilities.

Someone Told Me that for fusion to work on an industrial scale, you would need a lot more tritium and deuterium than is currently readily available, and the only reliable way of generating these products is by nuclear fission ...

(As an arts graduate I have no idea if that is true.)

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Then the dog ran before, and coming as if he had brought the news, shewed his joy by his fawning and wagging his tail. -- Tobit 11:9 (Douai-Rheims)

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Adeodatus
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# 4992

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quote:
Originally posted by Crœsos:
To be fair, hot fusion was fairly easy to do. Controlled hot fusion, on the other hand . . .

Well ... yes ... [Hot and Hormonal]

ITER looks interesting. Mainly because I like twisty twirly things. It's messy, which probably means scientists hate it and engineers love it. Tokamaks are more elegant and symmetrical, which means scientists will like them and engineers think they're boring. Muon fusion is the most elegant of all, since it essentially persuades the atoms to fuse, instead of hitting them with a large hot hammer till they do.

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"What is broken, repair with gold."

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Adeodatus
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# 4992

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(It's 11pm and y'all have got me looking out my old physics notes! I could never manage the really clever particle physics, but I was good enough at it to be pretty good at astrophysics. I liked astrophysics. Any subject where the lecturer can say "Ignore anything that's happening below 6 million Kelvin..." is my kind of subject!)

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"What is broken, repair with gold."

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Hedgehog

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# 14125

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quote:
Originally posted by LeRoc:
On the other hand, we could just reverse the polarity of the neutron beam.

We are not reversing the polarity. We are confusing the polarity!

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"We must regain the conviction that we need one another, that we have a shared responsibility for others and the world, and that being good and decent are worth it."--Pope Francis, Laudato Si'

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Alan Cresswell

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# 31

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quote:
Originally posted by Adeodatus:
It's 11pm and y'all have got me looking out my old physics notes!

In my case, it's just gone 8am and I don't have any of my notes to reference. I feel late to the party, much has been said and all I've got is a lecture I can deliver, but don't know if there's anyone wanting to hear it.

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Don't cling to a mistake just because you spent a lot of time making it.

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Doublethink.
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# 1984

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I'd be interested.

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All political thinking for years past has been vitiated in the same way. People can foresee the future only when it coincides with their own wishes, and the most grossly obvious facts can be ignored when they are unwelcome. George Orwell

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Adeodatus
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# 4992

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Is this where we all shout "We want to read the lecture, Alan!!"? [Big Grin]

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"What is broken, repair with gold."

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Leorning Cniht
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# 17564

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quote:
Originally posted by Ricardus:
Someone Told Me that for fusion to work on an industrial scale, you would need a lot more tritium and deuterium than is currently readily available, and the only reliable way of generating these products is by nuclear fission ...

Deuterium is naturally occuring; heavy water is produced by separating it from normal water.

Tritium is made (in useful concentrations) by neutron activation of Lithium-6, and the most convenient source of neutrons is in a nuclear reactor. We haven't made much.

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Alan Cresswell

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# 31

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Actually, we've made a lot of tritium. Most of it has been squandered in obscene quantities (ie one or more) of bombs.

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Don't cling to a mistake just because you spent a lot of time making it.

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Leorning Cniht
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# 17564

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quote:
Originally posted by Alan Cresswell:
Actually, we've made a lot of tritium. Most of it has been squandered in obscene quantities (ie one or more) of bombs.

I thought the total was something less than a tonne. (Mostly produced for weapons programmes, and mostly decayed in storage or in stored warheads.)

A tonne of tritium might generate something like 10 GW-years of electricity, which is 900 TWh, or about 5% of the world's current annual electricity consumption.

If we're going to generate a significant chunk of our power with D-T fusion, those fusion reactors will also need to breed tens of tonnes of tritium per year.

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Alan Cresswell

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# 31

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OK. Nuclear fusion 101 (much of it already covered).

I'm going to start with a nuclear/particle physicists energy unit, the electron volt (eV) which is the energy gained by an electron accelerated across a potential of 1 volt. If you remember cathode-ray TVs (wot we 'ad afore these new fangled flat screens) they accelerated electrons across a potential of 25,000V or so - ie: the electrons had an energy of 25keV.

Second, nuclear binding energy (well, any sort of binding energy really). When things are bound together they have less energy than when they were unbound (otherwise they'd be unstable, seeking the lower energy unbound state). A single proton (H) or neutron there is, naturally, zero binding energy. If you fuse them then there is an increase of 2MeV in binding energy (D has a binding energy of 1MeV per nucleon - and, two nucleons), that is there is 2MeV less in the D than in the unbound p+n - this reaction also produces a positron and neutrino, which carry away most of that energy. 4He is especially stable, it has significantly greater binding energy than nuclei to either side of it (3H, 3He, 6Li, 7Li). So, fusion producing 4He is particularly good at producing energy.

For fusion to occur you need to overcome the Coulomb repulsion between two positively charged ions until they're close enough for the attractive, but very short range, nuclear force to take over. The nuclear force is stronger for heavier nuclei. All fusion with hydrogen isotopes have the same Coulomb force (there is one proton in each nucleus), but for heavier isotopes you can get fusion with the nuclei not needing to get quite as close. So, you can fuse D and T at lower temperatures than D and D, and that at lower temperatures than D and H. Which is why D-T fusion is the preferred process for fusion power generators.

OK, so the specifics of D-T reactions. The products of the reaction are 4He and a neutron, and 17.6MeV of energy - of which 3.5MeV is carried by the neutron which will not be contained, and hence that energy is lost into heating the physical containment (and, is likely to result in production of radioactive activation products within the walls surrounding the structure). Before you ask, to surround the reactor with 6Li to use those neutrons to produce tritium would be an excellent idea, but prohibitively expensive and impractical. If water was used as a coolant to drive turbines some of that water would become tritiated - but it would not produce enough tritium to fuel the reactor.

In comparison, nuclear fission produces about 200MeV per reaction. So, for the same energy output you need a fusion rate at least 10x greater. Chemical reactions (burning coal and similar) only produce a few eV per reaction.

Fusion technology falls into two categories:
"Hot" reactions where the Coulomb repulsion is overcome just by pure kinetic energy
"Cold" reactions where some form of "catalyst" allows the nuclei to get closer together by shielding the electrostatic repulsion in some manner.

"Hot" technologies are mainly magnetic confinement of a plasma (the differences between the different designs are just various claims as to which shape is more efficient at containing the plasma) which is very hot, beam driven reactions where the energy is supplied by an accelerator (which doesn't need to be LHC size, you could probably fit an accelerator for that in a garage), or implosion devices where pellets of fuel are compressed by powerful lasers. All these technologies work, they fuse the nuclei ... but currently use more energy than they produce - JET did produce a net output for short periods, but not enough to overcome inefficiencies in recovering that energy.

"Cold" technologies only have one viable candidate at the moment (Fleischman and Pons may disagree with me), muon catalysed fusion. The biggest barrier to fusion is getting the nuclei close enough. Neutral atoms (ie: a D or T nucleus + an electron) can, of course, approach without the repulsive force. But, the electron is (on average) a considerable distance from the nucleus, and as the electron orbits overlap the nuclei start to repel. To overcome this you need more energy - unfortunately so much that you ionise the gas and no longer have atoms. Muons are like very heavy electrons, and you can form muonic atoms with the muon replacing the electron but it's much more closely bound. Muonic atoms can approach to the point where the nuclear force takes over, at room temperature. But, as mentioned muons have very short half lives, require an accelerator to generate them, and are also happy to form atoms with He and any other nuclei that happen to be around. Also, heating your gas speeds the muonic atoms to the point where they pass each other too quickly to fuse ... and, the whole point of the process is to generate heat.

Other proposed "cold" technologies are some form of solid state system, where two hydrogen atoms are forced into spaces between larger atoms in some form of crystal where fusion can occur. Despite claims made be some people, I'm not convinced this has ever occurred, and certainly not a rates to be useful potential power sources.

"Cold" technologies are the Holy Grail of fusion research, but "hot" technologies will be with us first.

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Don't cling to a mistake just because you spent a lot of time making it.

Posts: 32413 | From: East Kilbride (Scotland) or 福島 | Registered: May 2001  |  IP: Logged
Alan Cresswell

Mad Scientist 先生
# 31

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quote:
Originally posted by Leorning Cniht:
quote:
Originally posted by Alan Cresswell:
Actually, we've made a lot of tritium. Most of it has been squandered in obscene quantities (ie one or more) of bombs.

I thought the total was something less than a tonne. (Mostly produced for weapons programmes, and mostly decayed in storage or in stored warheads.)

I think the US admits to having produced about 250kg of tritium for their weapons programme. Presumably the USSR (as was) produced similar quantities. So, less than a tonne is about right. It's still been enough to produce and maintain an obscene number of thermonuclear weapons (but, then again IMO one such weapon is an obscenity).

If we did develop a viable fusion reactor technology then tritium supply would be the biggest problem. Current heavy water reactors generate tritium at a rate of several kg per year. Irradiation of 6Li in commercial reactors would be possible, but at the cost of reduced power output and difficulties of handling highly radioactive components removed from the reactors. Production of tritium, and extraction of deuterium from sea water, would have an energy cost that needs to be included in the "does this produce a net amount of energy?" question.

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Don't cling to a mistake just because you spent a lot of time making it.

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Karl: Liberal Backslider
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# 76

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quote:
Originally posted by hatless:
What happens to two hydrogen atoms when they fuse? Where do they find two neutrons from to become helium?

There are two ways.

Firstly you can start with Deuterium - heavy hydrogen. The deuterium nucleus has a proton and a neutron. I believe this is the easiest way, for certain values of "easy".

If you're starting with hydrogen, then in a plasma there will be collisions between protons (this is why you need a plasma, so that the protons are "naked"; normally they'd not get near enough to each other). Two colliding protons can result in one decaying to a neutron and a positron. Two protons would never bind because of electronic repulsion, but a proton and a neutron can because of one of the nuclear forces; I've forgotten which one.

That gives you the deuterium nuclei, bind two of those together and you have your helium nucleus.

So: (P+ = Proton; N = Neutron; p+ = Positron; e- = Electron)

2P+ + 2e- --> P + N + p+ 2e-

The positrons go off and find electrons to mutually annihilate themselves with (producing energy)

then:

(P+ + N) + (P+ + N) --> 2P+N (Helium nucleus) + 2e- + More Energy

As this all happens in plasma, it's nuclei we're interested in.

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Posts: 17938 | From: Chesterfield | Registered: May 2001  |  IP: Logged
Alan Cresswell

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# 31

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quote:
Originally posted by Karl: Liberal Backslider:
Two protons would never bind because of electronic repulsion, but a proton and a neutron can because of one of the nuclear forces; I've forgotten which one.

The strong nuclear force allows them to bind, but 2He is incredibly unstable (ie: basically never exists) because the strong force between two protons isn't strong enough. The weak nuclear force allows one of the protons to transmute to a neutron (producing a positron and neutrino), giving 2H (deuterium), which is stable because there's no Coulomb repulsion.

Fusion involving 2H adds the strong nuclear force from the neutron to the mix, making resulting heavier nuclei stable (relatively anyway, in astrophysics anything with a half life more than a few milliseconds exists long enough to fuse sometimes). Still more 1H around than 2H, so that's the most likely reaction.

That is the main reaction set in the Sun (and similar stars).
1H + 1H -> 2H + p+ + n
2H + 1H -> 3He + gamma
3He + 3He -> 4He + 2x1H

It is, however very slow and only works in stars because of the high pressure under gravitational attraction. With a smaller mass of plasma (ie: something we can actually have on Earth) the reaction rate would be too slow to be useful unless we have it very hot. Which is why the 3H+2H reaction is preferred, it can run at lower temperatures.

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Don't cling to a mistake just because you spent a lot of time making it.

Posts: 32413 | From: East Kilbride (Scotland) or 福島 | Registered: May 2001  |  IP: Logged
Alan Cresswell

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# 31

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quote:
Originally posted by Karl: Liberal Backslider:
So: (P+ = Proton; N = Neutron; p+ = Positron; e- = Electron)

2P+ + 2e- --> P + N + p+ 2e-

Where do those electrons come from?

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Don't cling to a mistake just because you spent a lot of time making it.

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Karl: Liberal Backslider
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# 76

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quote:
Originally posted by Alan Cresswell:
quote:
Originally posted by Karl: Liberal Backslider:
So: (P+ = Proton; N = Neutron; p+ = Positron; e- = Electron)

2P+ + 2e- --> P + N + p+ 2e-

Where do those electrons come from?
They're from the hydrogen atoms; it'd have been simpler without them.

I'll give myself a D-, could try harder, and plead thirty years since A level physics in mitigation.

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Might as well ask the bloody cat.

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Alan Cresswell

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# 31

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A plasma is a phase of matter where the atoms have been ionised, they have had their electrons stripped away. So, no electrons associated with the hydrogen nuclei.

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Don't cling to a mistake just because you spent a lot of time making it.

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Dave W.
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# 8765

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quote:
Originally posted by Alan Cresswell:
A plasma is a phase of matter where the atoms have been ionised, they have had their electrons stripped away. So, no electrons associated with the hydrogen nuclei.

Presumably the electrons are still in the plasma, though no longer bound to individual nuclei, right?
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LeRoc

Famous Dutch pirate
# 3216

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quote:
Dave W.: Presumably the electrons are still in the plasma, though no longer bound to individual nuclei, right?
No they're gone. We just have a stream of protons (and possibly neutrons) which is positively charged. That's why we can manipulate it with magnets.

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I know why God made the rhinoceros, it's because He couldn't see the rhinoceros, so He made the rhinoceros to be able to see it. (Clarice Lispector)

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Alan Cresswell

Mad Scientist 先生
# 31

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Yes, a plasma is neutral - the positive and negative charges cancel. But, the electrons will be travelling independently of the protons and so will not be involved in any reactions.

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Don't cling to a mistake just because you spent a lot of time making it.

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