What makes the H-bomb so efficient?

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
Richard Hester
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Re: What makes the H-bomb so efficient?

Post by Richard Hester » Sun Aug 21, 2005 8:15 pm

Who says it's efficient? A fusion bomb requires a fission bomb to trigger it, and it most likely only fuses a fraction of its fuel before it blows itself to bits. High yield, yes , but efficient? Not necessarily so. It does what it is deigned to do, namely vaporizing a city. When you look at the amount of fuel in one of those bombs and compare the explosion fusion yield against the yield if it were used in a power plant with 40% efficiency, it would most probably be a different story. How far does a 600kT explosion go towardfs fueling a city if you were to stretch out the delivery? An interesting exercise.

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Re: What makes the H-bomb so efficient?

Post by Richard Hester » Sun Aug 21, 2005 9:31 pm

In short, the H bomb has the transient high temperature and pressure generated by the X-rays from the fission explosion going for it, and nothing else. The closest one comes to this without a big explosion is the Z-pinch experiment ant Sandia, and this is many orders of magnitude away. The NIF may get there on a small scale if they can ever get the laser to work. This (the H-bomb) is the classic example of the "bigger hammer" taken to absurd extremes.

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Re: What makes the H-bomb so efficient?

Post by Goldenspark » Mon Aug 22, 2005 12:53 pm

Did a quick calculation;
New York City, in about 2000, needed about 10GW for peak demand.
1 MT equates to 4.2 x 10^15 J.
This would be enough energy to supply the city for 117 hours (4.86 days) at the peak demand.
And all that from about 50g of mass "expended" (E=MC^2).

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Re: What makes the H-bomb so efficient?

Post by Richard Hull » Mon Aug 22, 2005 6:19 pm

The H-bomb question would fall into the category of "Why don't we have nitroglycerin power cars or TNT power electric generators"? There are many small stores in nature of massive amounts of energy. Most are go-no-go reactions that can't be controlled or slowed down. They rely on their very brissant nature to do what they do.

Ya' just can't slow down an H bomb or use any part of its cycle in slow motion mode or it will not work.

Like Richard Hester said. What's efficient about an H bomb? Certainly not its mass-energy conversion ratio which is pitiable. Certainly not the fission part of the bomb either. The seed energy is in the 400 megawatt class to make the hydrogen burn during its several stages (supplied by the fission). That energy has to be applied fast and used fast for the same thing that keeps use from doing slow fusion keeps us from doing it here too. The bastards want to expand to fill the entire universe. The fission and its resultants are speedy enough to get the hydrogen stuff to burn, (fusion) before it is too expanded to do so. speed and brissance are the only reason the thing works at all.

The difference is fission can be easily controlled and scaled back. Fusion cannot, by any method thus far shown, be made to suffer ignition or produce useful energy, though there are many dreams.....Witness the dream of the ITR. or the joke of NIF.

Fission is tough to do due to materials issues, but easy to control. Fusion is easy to do but impossible to make into a usable power source. It is the way nature built things so that the univese would last a long, long time.

We outsmarted nature with fission as it was merely a matter of material concentration over and above that found in nature. Child's play, indeed. Besides, where does nature do fission other than the odd geologically ancient U rock deposit as in Africa? Nature really just doesn't do fission for power. She does it by caprice.

We have not a clue on fusion. The fuel is gulped down in small measure in every cup of coffee, every soft drink we consume and composes a small percentage of over 75 % of our body mass. Fusion fuel is never an issue. Nature has it scattered all about for the burning. Nature does fusion regularly and grossly inefficiently over the enitire universe using only gravity to drive the massive stellar mills of inefficient fusion. Fusion power is nature's engine, not man's. Nature has careful safegaurds in place to prevent small fusion fires being lit and running out of control. Nature has placed us in a position where we are attempting to light matches while submerged in gasoline, relying on an occassional air bubble to get something started. We are far too awash in fusion fuel to light it off except in self limiting flashes.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: What makes the H-bomb so efficient?

Post by HAL9000 » Fri Aug 26, 2005 6:53 pm

You are correct that H-bombs, especially modern ones, are quite efficient in regards to input/output energies. They work well that way because of the excessively high temperatures (~30 KeV) and pressures (deep in the gigabar range). Since the fusion reaction rate of any fuel at its critical temp is always faster the denser the fuel mass is (I believe that relationship is inverse cubed), the extreme core-of-the-earth type pressure is really important for the H-bomb to work. Early concepts for the H-bomb did not have a compression element in their design, and always proved infeasible on paper because of it irrespective of the temperature theorized.

Unfortunately, for those pressures at those temperatures, the only option for confinement is fast, high energy shock implosion of something already at a pretty high density (a solid or liquid) to begin with. Of course maintaining such an environment isn't really feasible for more than microseconds at best. The only way to do that seems to be to blow up the device in the reaction with ionizing radiation, see NIF (if it works at all), z-pinch, W-88, etc. Any electrostatic or magnetic system designed to continously confine such a plasma at that density and survive, even a gram of it, would be as big as the Superdome at least.

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Re: What makes the H-bomb so efficient?

Post by Donald McKinley » Fri Nov 18, 2005 7:29 am

Mike,

If published information on the H-Bomb yields is reliable (it may not be), it is definitely more energetic on a TOTAL MASS BASIS than the fission reactions. However it is not as much more energetic as you might think. It's somewhat less than 4 times as potent.

The 9 Megaton Mk-53/B-53 bomb, which apparantely is the oldest weapon in service (operational since 1962), and also the largest, weighs 4000 kg and has a ratio of 2.25 kt/kg. I believe it is fusion primarily.

This is 4.5 lbs of tnt per mg.

The 100 kt W-76 bomb is a fission only bomb which weighs 165kg has a yield/weight ratio of .061kt/kg.

This is 1.21 lbs tnt per mg.

As you can see, regarding the original question on this thread, on an equal mass basis this fuel is significantly more potent. Its true that each reaction has much less energy, but there are scads more of them. Since many of the reactions involve tritium, an energy density of 4.5 lbs per mg is probably way more potent than pure deuterium would be in a fusor.

There is also a giant fudge factor in the numbers above because the energy content is computed on the entire weight of the bombs including all the inert weight

I did a search for the bomb stats & found them at http://nuclearweaponarchive.org/Nwfaq/N ... #Nfaq4.5.3

Don M.

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Re: What makes the H-bomb so efficient?

Post by Alex Aitken » Fri Nov 18, 2005 3:46 pm

Its interesting that high temperatures are mentioned, as its my understanding this is a problem, rather than an advantage in the early stages of fuel compression.

My understanding is that if you take a high temperature plasma allready fusing and squash it, the fusion rate will rapidly increase until it balences the force you apply. Not unlike the way fusion in the sun constantly balences force due to gravity. When that force is then removed fusion rate drops rapidly and you've just fizzled, if equal force requires equal energy then you can only double the energy production in the bomb as a rough number (based on a bad estimate). If you can squash the fusile material while cold though you can achieve much higher densities as you arn't fighting fusion. Then when the plutonium 'sparkplug' also being compressed finally detonates (as a secondary fast fission bomb in its own right, aside from the primary that is doing compression) fusion starts with this much higher density and the temperature spikes much more rapidly and uses far more of the fuel. Its almost a perfect analog of the plutonium predetonation problem in pure fission weapons.

With regard to TNT or nitroglycerine powered cars the reason is actually different. With a H Bomb there is no question that it achieves more energy than available by other means as well as short term power. With nitroglycerine/TNT this is simply not true. The tradeoff for making a molecule that releases its energy in such a small amount of time (less than a microsecond, most of the time in a half a millisecond ish detonation is just wating to be hit by the det wave) is that the total energy released is less per unit mass than most fuels. A kilogram of TNT or Niroglycerine when detonated actually release Much less energy than a kilogram of gasoline. The two key reasons for this are that detonating explosives have to carry the oxidiser as well as the fuel in the mass (cf difference between a rocket and a jet engine), and that intramolecular redox reactions of chemicals stable enough to be handled produce less energy than intermolecular redox reactions (cf the difference between single molecule explosives like TNT and versions with aluminium or silicon powder and oxidisers or even just straight low order propellents).

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Re: What makes the H-bomb so efficient? A: (with no math)

Post by Retric » Tue Nov 22, 2005 8:08 pm

Sorry it took this long but I am trying to make this a simple as
possible.

When you look at overall fusion efficiency there are 4 aspects:

1: Energy per reaction.
2: Average energy input per fuel molecule.
3: Percentage of fuel undergoing reaction.
4: What percentage of output energy is useful.

Now H-Bombs maximize #1 by using D-T reactions AND using the
neutrons as a fission “catalyst”. (These are both vary “dirty” methods
in that they produce a lot of high-energy neutrons, but hey it’s a
bomb.)

Now #2 uses a cool trick in that they use a bomb to get into the
30+kev range but as these things take place at such a high pressure
the mean free path is vary low thus they get to use the “free” energy
from the early fusion reactions to increase the plasma temperature for
the later reactions. Also they get to compress things to extreme
levels before hitting the insane temperature levels which would rapidly
produce too much pressure to allow for compression.

Now #3 is a function of both containment time and plasma
temperature AND plasma density. Containment time sucks for an H-
Bomb but plasma temperature gets into the Mev range and pressure
goes though the roof which causes the density to quickly drop as
containment fails but the mean free path is so low that you get a lot of
reactions even as the thing is detonating. Basically if you have a 6cm
ball at 30kev that becomes a 12cm ball at 300kev the larger ball is still
going to undergo some fusion. You also get a little fusion as the high-
energy fusion materials encounter atmospheric hydrogen.

Finally H-Bomb’s don’t waste any of the input energy so even a 60%
efficient H-Bomb is still useful. Basically a 1MT bomb that becomes
a 1.6MT bomb is much better where a power plant needs to be
~300% power input to break even. It’s my understanding that early
H-Bomb’s got little of there energy from fusion but focused on fission
+ some fusion + fusion catalyzed fission = bigger bomb.

Continuing the basic idea. If you wanted to build an insanely big H-
Bomb you could surround a super cooled high (extreme density) ball
with normal H-Bomb’s to drive up the density and hit 30+ Kev igniting
a huge ball for a (relatively) long period of time. Even if the central
ball only produced 50% of the energy of the containment H-Bomb’s
your new (mega) H-Bomb is now as powerful as say 6 H-Bomb’s
while being slightly larger than 4 normal H-Bombs. You would have
serious issues trying to time the 4+ H-Bomb’s to get this to work but
it’s still doable. (I don’t know why you would need a bomb that size
but it gives you an idea of the basic premise behind H-bombs.)

PS: Or on a more basic level when building an H-bomb you get to
work with insane energy levels and don’t have to worry about
containment losses so it’s easy to set thing on fire.

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Re: What makes the H-bomb so efficient? A: (with no math)

Post by Donald McKinley » Wed Nov 23, 2005 8:35 am

JKirby,
greetings

Could you amplify a little teeny tiny bit on the 30--300Kev issues, as well as the 3rd to last paragraph regarding the

"power plant needs to be ~300% power input to break even".

I know its been sort of covered, but I'm still on the uptick here. Don't quite have the whole process under control.

I'm interested, if possible, on any known sweet spots in the process where voltages to pressures () may be ideal. So far I've gleaned perhaps wrongly that there is a large factor of "more is better".

I'm slowly working my way through the "Fundamental Plasma Limitations etc." Phd Thesis of Tod Rider recommended by Brian M. It'll be awhile.

Thanks
Don

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Re: What makes the H-bomb so efficient? A: (with some math)

Post by Retric » Wed Nov 23, 2005 5:44 pm

However, this is an outline of what’s going on so I am not going to be
overly technical but you can’t really compare things without a little
math.

"power plant needs to be ~300% power input to break even".

Let’s start with http://www.sciencemag.org/cgi/content/summary/278/
5335/29a where “JET produced 50% of the energy supplied to the
reactor--close to twice the previous record.” Ok well energy can’t be
created or destroyed so what are they talking about? Well for a few
seconds JET can liberate some atomic energy as heat, but it takes a
constant supply of electrical energy to do so. So if you started with
100kw/s of electricity you get 150kw/s of heat which better than a
space heater. However, if you want to turn that heat back to
electricity you lose about 60% of the energy. So that 150kw of heat is
only 60kw/s of electricity which is less than we started with. If you
want to get twice as much electricity out as you put in you need a
power plant that adds 4kw of heat per 1kw of electricity you add.
(4kw+1kw)(Energy) 40%(efficiency) = 2kw of electricity.

As to the 300% isue: (2kw + 1kw) * .4 = 1.2kw out per 1kw in which
is probably not really useful, it could start to be useful at that level, but
I pick 300% output (from fusion) to input as a true breakeven. After
that more becomes more useful but that’s about where the threshold
starts.

However, let’s take a 1MT A-Bomb that produces 1MT worth of heat
energy. Now if you take all that heat(not really possible) and use it to
drive a 50% efficient reactor you get 1.5MT of heat but as your talking
about a bomb you get to use all that heat to blow things up. (Not all of
that energy starts out as heat you get things like x-ray’s ect but it all
quickly becomes heat). Now in the real world you might only get 5%
of the A-Bomb’s heat to drive fusion but the fusion is going to be way
more than 50% efficient.

Now onto the 30>300KV issues:

If they took the basic jet design and rebuilt the same device they
would be stuck with the same performance. However, they are
scaling up the same basic design. ITER is not really going to be much
hotter or have much higher dencity than JET but it’s going to be a lot
larger. http://www.jet.efda.org/pages/content/tokamak-
description.html Vs. http://www.iter.org/plasma.htm (from http://
www.iter.org/index.htm)

Now in fusion each of the ion’s is going about 1000+ miles per
second (3,600,000mph) but the ITER plasma is only ~6m wide which
mean they need to use a strong magnetic field to contain the plasma.
Which is all well and good but it has limits after fusion you get a
neutron with 14,100kev of energy that ignores the magnetic field and
a 3,500kev Helium nucleus, which is going way to fast to be stopped
by that field. Now as the size or density goes up you increase the
chances of those particles hitting some deuterium or tritium which
would increase the overall plasma temperature but you have the
same problem, the magnetic filed is not strong enough to contain a
higher temperature particle so you hit a limit as to how hot things can
get.

But let’s take another approach instead of having a huge ring of low
density plasma let’s have a small ball 1cm wide with 1,000 times that
much plasma. First off the chances of helium or neutron hitting some
deuterium or tritium are now much higher. So after things start to fuse
the plasma temperature is going to rise. 60kev in gives ~17,600 + 60
kev out, but now the 1cm wide ball is going to start blowing up at a
good chunk of the speed of light so things need to happen fast.
Luckily as the temperature rise the probability of fusion goes up
dramatically and the density is way higher than ITER so in that tiny
fraction of a second before things are blown up you get an insane
amount of fusion.

More temperature, and or more volume, and or more density yield
more fusion. There is no sweet spot it’s just a question of how large,
how hot, and how well contained plasma you can create. In a bomb
you basically give up on containment time and try to maximize the
temperature, density, and volume.

PS: Density is more valuable than volume if you can pack the same
number of ions into a smaller space you get much more fusion.

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