# Whats the advantage of an H-bomb over an A-bomb
**URL:** https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800
**Category:** Factual Questions
**Created:** [December 14, 2002, 10:59pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800 "2002-12-14T22:59:04Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@PolskiKing](https://boards.straightdope.com/u/PolskiKing)
#### Post date: [December 14, 2002, 10:59pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/1 "2002-12-14T22:59:04Z")
Why is the H-bomb better than the A-bomb?
\*\*Better i mean bigger, more effective
-pk
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### Author:  [@pravnik](https://boards.straightdope.com/u/pravnik)
#### Post date: [December 14, 2002, 11:08pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/2 "2002-12-14T23:08:48Z")
A hydrogen bomb is about one thousand times more powerful than an atomic bomb. It uses an uncontolled fusion reaction instead of an uncontrolled fission reaction.
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### Author:  [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)
#### Post date: [December 14, 2002, 11:10pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/3 "2002-12-14T23:10:40Z")
Because it relies on a nuclear fission reaction (an A-bomb) to trigger a nuclear fusion reaction, which is far more powerful than the fission reaction. Fusion releases far more energy than fission (at least with our current bomb technology. I misremember my physics, I think fusion always yields more energy than fission, but I’m not 100% sure).
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### Author:  [@Podkayne](https://boards.straightdope.com/u/Podkayne)
#### Post date: [December 14, 2002, 11:13pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/4 "2002-12-14T23:13:55Z")
Depends on which part of the periodic table you’re in, **Qadgop.** Causing elements heavier than iron to fuse requires more energy than is released in the reaction.
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### Author:  [@pravnik](https://boards.straightdope.com/u/pravnik)
#### Post date: [December 14, 2002, 11:15pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/5 "2002-12-14T23:15:50Z")
Whoops. Misread your question. You were asking _why_. Okay.
An A-bomb’s fission reaction stops when pieces of uranium or plutonium fly far enough apart during the early stages of the explosion. The H-bomb, on the other hand uses a fusion reaction of light atomic nuclei of hydrogen that has no theoretical limit. Larger bombs can be made just by adding more hydrogen.
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### Author:  [@Revtim](https://boards.straightdope.com/u/Revtim)
#### Post date: [December 14, 2002, 11:33pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/6 "2002-12-14T23:33:10Z")
I think it was “Dark Sun” by Richard Rhodes where he describes that a practical limit exists where you end up blasting a Pennsylvania-sized chunk of the Earth’s atmosphere into space. You can make the bomb more powerful, but all you accomplish is sending that same-sized chunk into space faster.
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### Author:  [@Coriolanus](https://boards.straightdope.com/u/Coriolanus)
#### Post date: [December 14, 2002, 11:36pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/7 "2002-12-14T23:36:59Z")
Does an A-bomb require a TNT explosion to start the chain-reaction? (furthermore…) Does an H-bomb require an input of energy that an A-bomb provides to smash (fuse) Deuterium and/or Tritium molecules into eachother?
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### Author:  [@Hail\_Ants](https://boards.straightdope.com/u/Hail_Ants)
#### Post date: [December 14, 2002, 11:42pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/8 "2002-12-14T23:42:28Z")
> [@](#):
>
> It uses an uncontolled fusion reaction instead of an uncontrolled fission reaction.
Actually H bombs use a fission reaction to start a fusion reaction to boost another fission reaction! Or the first fission reactions provides the intense heat (ten million degrees!) required to start a fusion reaction and the fusion reaction then provides a huge blast of neutrons to create another much larger fission reaction.
If you want a good, very readable description of it read the chapter ‘Three Shakes’ in Tom Clancy’s book Sum of All Fears (forget the movie, it absolutely stunk).
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### Author:  [@Revtim](https://boards.straightdope.com/u/Revtim)
#### Post date: [December 14, 2002, 11:45pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/9 "2002-12-14T23:45:18Z")
IIRC, a standard (non-nuclear chemical) implosion compresses material in an A bomb to start the fission, and the heat from the fission of an A Bomb starts the fusion in the H bomb.
So an H bomb is actually three bombs, sort of.
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### Author:  [@Coriolanus](https://boards.straightdope.com/u/Coriolanus)
#### Post date: [December 14, 2002, 11:48pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/10 "2002-12-14T23:48:16Z")
Urr… I see I missed **Qadgop’s** answer to my question before I asked it (read twice - post once!).
Fusion reactions - even uncontrolled reactions such as a bomb - would tend to spread less radioactive material (other than the stuff from the triggering A-bomb). H-bombs have the potential, vis-a-vis the relative availability of their ingredients, to be far more immense explosion-wise (yet less massive) than A-bombs which require lots of processed Uranium or even more-processed Plutonium.
Radioactive material is of course nasty - yet the vast amount of debris a H-bomb kicks up might very well wipe every living dinosaur off our planet.
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### Author:  [@pravnik](https://boards.straightdope.com/u/pravnik)
#### Post date: [December 14, 2002, 11:51pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/11 "2002-12-14T23:51:29Z")
The A bomb that exploded over hiroshima used two pieces of weapons grade U-235 (each not big enough to start a chain reaction alone) that came together in a gun barrel to form a supercritical mass that exploded instantaneously. I don’t know exactly what the propellant in the gun barrel was.
H bombs do apparently require a good input of energy to start the reaction. The"super bomb" version used a small atomic bomb to start the hydrogen reaction, the “booster bomb” uses a relatively large fission reaction to ignite a smaller amount of hydrogen, which then releases neutrons that “boost” the fission reaction. The Soviets apparently combined these two into a Fission-Fusion-Fission reaction: a fission explosion causes a fusion reaction which causes a fission reaction with the bomb’s uranium casing.
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### Author:  [@Squink](https://boards.straightdope.com/u/Squink)
#### Post date: [December 14, 2002, 11:52pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/12 "2002-12-14T23:52:04Z")
> [@](#):
>
> A fusion reaction produces about five times more energy than a fission reaction, but fusion bombs can be made to yield thousands of times the destructive force of the largest fission bomb; in fact, the former have no clear upward yield limitations. The fission bomb dropped on Nagasaki in 1945 had a destructive force equivalent to 20,000 tons of TNT (20 kilotons or kt), while fusion warheads have been tested with yields equivalent to 60 million tons of TNT (60 megatons or mt). A kind of intermediate type is the “boosted-fission” device, using a small amount of deuterium and/or tritium inside the core of a fission device to fission thesurrounding material more completely, thus maintaining the same yield while greatly reducing the overall size and weight (important for warheads designed to be carried by missiles, for example).
[http://www.csis-scrs.gc.ca/eng/miscdocs/200110\_e.html](http://www.csis-scrs.gc.ca/eng/miscdocs/200110_e.html)
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### Author:  [@Hail\_Ants](https://boards.straightdope.com/u/Hail_Ants)
#### Post date: [December 15, 2002, 1:39am UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/13 "2002-12-15T01:39:52Z")
> [@](#):
>
> The"super bomb" version used a small atomic bomb to start the hydrogen reaction
All Hydrogen bombs require a fission reaction (i.e. an A bomb) to start the fusion reaction. That’s the reason a fusion reactor has been impossible so far. You need 10 million+ degrees to get it going and the only place that temperature exists (other than the center of the sun) is in a nuclear explosion.
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### Author:  [@Revtim](https://boards.straightdope.com/u/Revtim)
#### Post date: [December 15, 2002, 2:00am UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/14 "2002-12-15T02:00:42Z")
> [@](#):
>
> \*Originally posted by Hail Ants \*
> \*\*That’s the reason a fusion reactor has been impossible so far. You need 10 million+ degrees to get it going and the only place that temperature exists (other than the center of the sun) is in a nuclear explosion. \*\*
Actually, terrestrial fusion reactors have to reach 100 million degrees (because of the lesser pressure), and it’s been more than reached:
[http://news.bbc.co.uk/1/hi/sci/tech/1573450.stm](http://news.bbc.co.uk/1/hi/sci/tech/1573450.stm)
"The world’s largest tokamak is called Jet, the Joint European Torus. It is also at Culham.
Using the Jet, scientists have heated plasma to 300 million degrees - more than is needed to achieve fusion ignition."
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### Author:  [@pravnik](https://boards.straightdope.com/u/pravnik)
#### Post date: [December 15, 2002, 2:02am UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/15 "2002-12-15T02:02:11Z")
Yes, they all need a fisson reaction to get started. The simplest version of the bomb relies on the fission reaction to start the fusion reaction in the hydrogen and nothing else, but you need large amounts of hydrogen for a really big boom.
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### Author:  [@pravnik](https://boards.straightdope.com/u/pravnik)
#### Post date: [December 15, 2002, 2:04am UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/16 "2002-12-15T02:04:45Z")
Yowza!! I hadn’t heard that, **Revtim**! That is potentially great news!
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### Author:  [@Revtim](https://boards.straightdope.com/u/Revtim)
#### Post date: [December 15, 2002, 2:14am UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/17 "2002-12-15T02:14:12Z")
I guess it’s progress, but as usual the article says commercial fusion power is “decades away”, like it’s been since the 50’s.
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### Author:  [@Revtim](https://boards.straightdope.com/u/Revtim)
#### Post date: [December 15, 2002, 4:10am UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/18 "2002-12-15T04:10:34Z")
FYI, there’s info on the Joint European Torus at [http://www.jet.efda.org/](http://www.jet.efda.org/)
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### Author:  [@Revtim](https://boards.straightdope.com/u/Revtim)
#### Post date: [December 15, 2002, 4:29am UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/19 "2002-12-15T04:29:14Z")
I apologize for this continuing JET fusion reactor hijack, but I just noticed that the BBC article I quoted earlier conflicts with the FAQ on the JET site.
The BBC article states that JET heated plasma to 300 million degrees, but the FAQ says they only reached around 100 million degrees (but low level fusion was still achieved). I suppose the author of the BBC article might have got confused by the fact that an eventual commercial fusion power plant will run at 200 to 300 million degrees.
The FAQ is dated after the BBC article, so I presume it’s more accurate describing what has and what has not been achieved by JET.
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### Author:  [@rowrrbazzle](https://boards.straightdope.com/u/rowrrbazzle)
#### Post date: [December 15, 2002, 7:52am UTC](https://boards.straightdope.com/t/whats-the-advantage-of-an-h-bomb-over-an-a-bomb/142800/20 "2002-12-15T07:52:19Z")
There’s lots more about the design of A- and H-bombs here: [http://www.milnet.com/milnet/nukeweap/Nfaq2.html](http://www.milnet.com/milnet/nukeweap/Nfaq2.html)
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