# How large is the primer for an H bomb **URL:** \ **Category:** Factual Questions\ **Created:** [December 13, 2012, 5:09pm UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934 "2012-12-13T17:09:03Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Buck\_Godot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/buck_godot/32/6573_2.png) [@Buck\_Godot](https://boards.straightdope.com/u/Buck_Godot)\ **Post date:** [December 13, 2012, 5:09pm UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/1 "2012-12-13T17:09:03Z")
My understanding is that the standard design of fusion weapons is that you have a fission weapon as a primer which then fuses the hydrogen for the big boom. I’m curious, if just the primer went off but no fusion reaction how big a boom would you get? ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [December 13, 2012, 5:21pm UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/2 "2012-12-13T17:21:14Z")
Its always been my impression that its a full blown fission bomb. And you can only make those so small, so as a first WAG I’m going with 10 kilotons. Though it is quite possible they NEED significantly more omppph from the fission to get the fusion going. Note that modern ones really aren’t fusion bombs (thats why they are properly called thermonuclear weapons). The initial fission causes a good bit of fusion. This fusion generates a bunch of neutrons that allows a good bit of the material that did not fission in the first go around to now fission. So, the modern weapons are mostly using the fusion to make the fission much more efficient. ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [December 13, 2012, 5:30pm UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/3 "2012-12-13T17:30:12Z")
The Russians weapons IIRC used on the SS-18 etc were three stage, fission-fusion-fission. A small fission warhead to start things off, a larger fusion explosion followed by fast fission of U-238, which provided the main explosive omph. I understand simple fission bombs can also make fast fission occur. ---
**Author:** ![Gray\_Ghost](https://avatars.discourse-cdn.com/v4/letter/g/c0e974/32.png) [@Gray\_Ghost](https://boards.straightdope.com/u/Gray_Ghost)\ **Post date:** [December 13, 2012, 6:04pm UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/4 "2012-12-13T18:04:31Z")
Per Carey Sublette’s [Nuclear Weapon Archive](http://nuclearweaponarchive.org/Nwfaq/Nfaq4-5.html) that I keep citing to again and again for questions such as these: > [@](#): > > As a rough approximation, we can say that the amount of energy required to implode a secondary is proportional to its mass, since the primary energy/secondary mass ratio defines the achievable implosion velocity. The yield of the secondary should also be roughly proportional to its mass. Thus there is a roughly proportional relationship between the primary and secondary yields, using similar design principles. > > From available data (based on known trigger tests, and fizzles where only the primary fired), it appears that this range can be from **10-200, with 30- 50 being more typical ratios.** [Emphasis added] > > If a very large yield is desired, then we must obviously have a very large primary. Large fission primaries are expensive, heavy, and potentially dangerous (due to the large amount of fissile material present). Even in very heavy weapons, the yield of the primary is limited to no more than a few hundred kilotons, limiting total yield to a maximum of 10-20 megatons. > > The high yield designs actually developed (mostly in the fifties and early sixties) seem to have used refined versions of the basic thermonuclear weapon design approach, as described above, with the addition of multiple staging to achieve even higher yields. The stated ratio by Dr. Sublette applies to fission-fusion devices. So, for your standard USA [modern SLBM warhead](http://en.wikipedia.org/wiki/W87), with a yield of 300 kT., assuming the warhead is of a fission-fusion design (this is probably a giant assumption, given the discussion of yield in the link, and its alteration by the probable inclusion of additional HEU in the secondary tamper.), the primary would be around 1/30th to 1/50th of the total yield, or ~6-10 kT, assuming those cited guessed ratios hold true. No reason it couldn’t be a lot smaller though. The [W-80](http://en.wikipedia.org/wiki/W80_(nuclear_warhead)) is stated to have a variable yield, with the lowest setting at 5 kT. The wiki states that value might be the yield of its primary alone. After that commentary on primary to secondary sizing, he then goes on to guess that the size of the Tsar Bomba primary was ~250 kt: > [@](#): > > Assuming a primary of 250 kt (to keep the fissile content relatively low for safety reasons), we might postulate secondary and tertiary stages of 3.5 Mt and 46 Mt respectively. This fusion stages would require 1700 kg of Li6D (at 50% fusion efficiency), and something like 250 kt of fission for reliable ignition. If the initial spark plug firings were 25% efficient, later fission would release another 750 kt - placing the total at 1.25 Mt (close enough to the claimed parameters to match within the limits of accuracy). The site is a blast and just interesting to read if you’ve an interest in this sort of thing. I wonder which parts of it have been blurred or elided over in the interest of security? Not like anyone who knows, can say though. ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [December 13, 2012, 6:35pm UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/5 "2012-12-13T18:35:06Z")
> [@Gray\_Ghost](#): > > The site is a blast and just interesting to read if you’ve an interest in this sort of thing. I wonder which parts of it have been blurred or elided over in the interest of security? Not like anyone who knows, can say though. Yes, lots of interesting reading there. Though I must say I’d never have the balls to put that much detail about nuclear weapons out there. Particularly if I had ever been in the nuke biz and/or ever signed any security clearance paperwork. I’ve also have the suspicion that some of those “secrets” of nuclear design info is purposely misleading, if not downright wrong. Not much, but then again not much has to not work for a nuclear weapon to not work. Particularly if you are just going to build a one off, rather going into a proper R and D testing program. ---
**Author:** ![PlainJain](https://avatars.discourse-cdn.com/v4/letter/p/5daacb/32.png) [@PlainJain](https://boards.straightdope.com/u/PlainJain)\ **Post date:** [December 13, 2012, 6:39pm UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/6 "2012-12-13T18:39:15Z")
> [@gray ghost](#): > > the site is a blast … iswydt. ---
**Author:** ![Mr.Excellent](https://avatars.discourse-cdn.com/v4/letter/m/f19dbf/32.png) [@Mr.Excellent](https://boards.straightdope.com/u/Mr.Excellent)\ **Post date:** [December 13, 2012, 6:41pm UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/7 "2012-12-13T18:41:32Z")
> [@billfish678](#): > > I’ve also have the suspicion that some of those “secrets” of nuclear design info is purposely misleading, if not downright wrong. Not much, but then again not much has to not work for a nuclear weapon to not work. Particularly if you are just going to build a one off, rather going into a proper R and D testing program. Really? I mean, a uranium weapon isn’t hard to build at all, if you don’t insist upon an implosion design. Getting the uranium is tricky, but the weapon itself can be very simple - the Hiroshima bomb basically just slammed one chunk of uranium into another chunk at speed. Plenty of WW2-era conventional bombs were more mechanically complicated. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [December 13, 2012, 6:44pm UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/8 "2012-12-13T18:44:49Z")
> [@Mr.Excellent](#): > > Really? I mean, a uranium weapon isn’t hard to build at all, if you don’t insist upon an implosion design. Getting the uranium is tricky, but the weapon itself can be very simple - the Hiroshima bomb basically just slammed one chunk of uranium into another chunk at speed. Plenty of WW2-era conventional bombs were more mechanically complicated. The “spark plug” (Neutron source at the core of the bomb) is still considered a secret, even after 60 years. Simply slamming two chunks of U-235 together is unlikely to make a very reliable bomb. ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [December 14, 2012, 3:53am UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/9 "2012-12-14T03:53:34Z")
> [@Mr.Excellent](#): > > Really? I mean, a uranium weapon isn’t hard to build at all, if you don’t insist upon an implosion design. Getting the uranium is tricky, but the weapon itself can be very simple - the Hiroshima bomb basically just slammed one chunk of uranium into another chunk at speed. Plenty of WW2-era conventional bombs were more mechanically complicated. Plenty of WW2 weapons are more mechanically complex than modern nuclear weapons. That does not make it any easier. Slamming two chunks of Uranium 235 into one another is going to get you nothing but a strongly worded letter from your local environmental regulator for spreading radioactive material around. It is much more difficult getting it to become supercritical and begin a reaction before the force of the impact destroys the damn thing. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 14, 2012, 4:06am UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/10 "2012-12-14T04:06:41Z")
> [@](#): > > Quoth **billfish678** : > > Note that modern ones really aren’t fusion bombs (thats why they are properly called thermonuclear weapons). “Thermonuclear” just means that the chain reaction is sustained by high temperature, rather than by a bunch of free neutrons flying around like it is in a fission bomb. Fusion bombs are thermonuclear. ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [December 14, 2012, 4:34am UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/11 "2012-12-14T04:34:26Z")
> [@billfish678](#): > > Its always been my impression that its a full blown fission bomb. The [Davy Crockett](http://en.wikipedia.org/wiki/Davy_Crockett_%28nuclear_device%29) was a tripod launched weapon of 10 to 20 ton explosive capacity. ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [December 14, 2012, 2:25pm UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/12 "2012-12-14T14:25:17Z")
> [@Chronos](#): > > “Thermonuclear” just means that the chain reaction is sustained by high temperature, rather than by a bunch of free neutrons flying around like it is in a fission bomb. Fusion bombs are thermonuclear. And what pray tell is the point of the these high temperatures? To keep the other elements warm and toasty? No, to cause fusion. Which nicely produces neutrons that can be used to increase yield even further. And I doubt the Davy Crocket is powerful enough to do much fusing BTW. ---
**Author:** ![Chefguy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chefguy/32/138_2.png) [@Chefguy](https://boards.straightdope.com/u/Chefguy)\ **Post date:** [December 14, 2012, 4:33pm UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/13 "2012-12-14T16:33:24Z")
An H-bomb is just a large black ball with a fuse sticking out of it. It’s usually lit by some cartoonish character like a coyote, who then sticks his fingers in his ears. ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [December 14, 2012, 4:54pm UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/14 "2012-12-14T16:54:05Z")
It’s pretty conclusive that the fusion primary can be as small as 0.3 kilotons in modern “dial-a-yield” weapons such as the B61. (yields of 0.3, 5, 10 and 80 kilotons) It stands to reason that the lowest yield would be the unboosted fission primary, and the subsequent higher yields would be various methods of boosting, with the 80 kt yield coming from the full-house thermonuclear bomb with everything detonating. I don’t know however if tritium boosting or external neutron sources are required to set the secondary off though. ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [December 14, 2012, 5:01pm UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/15 "2012-12-14T17:01:54Z")
Based on what I know, a 0.3 kiloton primary should be plenty though. And it isn’t the temp that does the fusing directly in a fusion bomb. It works a lot like an inside-out rocket in concept. Basically when the primary goes off, it radiates a colossal amount of energy into the bomb casing, which heats up the outer casing of the secondary very, very rapidly, causing its outer layers to instantly expand into gas and ablate off. This ablation exerts force on the rest of the secondary, causing it to compress, and as it compresses, the internal temperature heats up. There’s also a plutonium or enriched uranium “spark plug” in the center of the secondary, that as the secondary compresses, it fissions, throwing out a lot more neutrons to jumpstart and/or accelerate the fusion reactions. ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [December 15, 2012, 12:19am UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/16 "2012-12-15T00:19:19Z")
> [@bump](#): > > It’s pretty conclusive that the fusion primary can be as small as 0.3 kilotons in modern “dial-a-yield” weapons such as the B61. (yields of 0.3, 5, 10 and 80 kilotons) > > It stands to reason that the lowest yield would be the unboosted fission primary, and the subsequent higher yields would be various methods of boosting, with the 80 kt yield coming from the full-house thermonuclear bomb with everything detonating.. I’d go with 0.3 being a controlled fission only “fizzle”, 5 being about the minimum for a full blown nuke, and 10 and 80 being additional hydrogen fusion initiated by the full blown 5 kiloton fission. Or 10 being a fission bomb working a bit better and the 80 being the 10 setting off the 80 fusion. Or something in between. ---
**Author:** ![DesertDog](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/desertdog/32/11503_2.png) [@DesertDog](https://boards.straightdope.com/u/DesertDog)\ **Post date:** [December 16, 2012, 3:39am UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/17 "2012-12-16T03:39:49Z")
> [@AK84](#): > > Plenty of WW2 weapons are more mechanically complex than modern nuclear weapons. That does not make it any easier. Slamming two chunks of Uranium 235 into one another is going to get you nothing but a strongly worded letter from your local environmental regulator for spreading radioactive material around. It is much more difficult getting it to become supercritical and begin a reaction before the force of the impact destroys the damn thing. [Little Boy](http://en.wikipedia.org/wiki/Little_Boy) was the first and only uranium bomb made and the makers were confident enough that it would work, it was dropped on Hiroshima without testing the design first. With the implosion plutonium bomb [Fat Man](http://en.wikipedia.org/wiki/Fat_Man), on the other hand, there were enough doubts (as well as enough plutonium) that they proofed the concept with the Trinity test. I wouldn’t suggest a bunch of high school shop students could design a gun barrel uranium bomb, but a large enough bunch of post-doc and grad students conceivably could. That’s who did it before and half of doing something is knowing it’s possible. ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [December 16, 2012, 4:27am UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/18 "2012-12-16T04:27:40Z")
> [@DesertDog](#): > > [Little Boy](http://en.wikipedia.org/wiki/Little_Boy) was the first and only uranium bomb made and the makers were confident enough that it would work, it was dropped on Hiroshima without testing the design first. With the implosion plutonium bomb [Fat Man](http://en.wikipedia.org/wiki/Fat_Man), on the other hand, there were enough doubts (as well as enough plutonium) that they proofed the concept with the Trinity test. > > I wouldn’t suggest a bunch of high school shop students could design a gun barrel uranium bomb, but a large enough bunch of post-doc and grad students conceivably could. That’s who did it before and half of doing something is knowing it’s possible. Little Boy was made by a team compromising some of the greatest minds who ever lived who were funded and supported by the infrastructure of the worlds richest nation. So your comparison is completely of base. Furthermore it was overengineered and not because it was easy to do so, but because they did not have enough uranium to so test. Even then, I believe the bomb was very in efficient utilizing about 1% of it’s fissile material mass. Furthermore if what you said was true, then we would have dozens of nuclear powers and that would be the choice of every aspiring power. As it is, most programmes failed despite having access to technicL skills and resources your post doc and grad students could only dream of having ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [December 16, 2012, 5:12am UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/19 "2012-12-16T05:12:27Z")
The issue isn’t in the design and engineering of the nuclear weapon; that’s not trivial, but now, almost 70 years later, it’s not a huge deal. The issue now (and really, then as well) is getting the right fissile materials of sufficient purity to actually make a functioning bomb. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [December 16, 2012, 11:39am UTC](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934/20 "2012-12-16T11:39:16Z")
> [@DesertDog](#): > > I wouldn’t suggest a bunch of high school shop students could design a gun barrel uranium bomb, but a large enough bunch of post-doc and grad students conceivably could. That’s who did it before and half of doing something is knowing it’s possible. There is a great deal we don’t know about the physics of a real weapon that is known to those nations that have built successful ones. It isn’t just a matter of understanding the basics - the idea of a gun assembly device, and the basics of the fission reaction. As witnessed by the damp squib bomb of the North Koreans, ensuring that the bomb doesn’t just spray a lot of molten uranium about the neighbourhood has eluded at least one country that deeply desires a working weapon. We do know that the Manhattan project invested a huge amount of effort in measuring the physical properties of materials and understanding the manner in which they behaved in extreme circumstances. A lot of this knowledge was both hard won, and remains classified. A team of post doc and grad students may well be able to design a viable weapon if presented with all the parameters they need. But I suspect a great deal of the nitty gritty information they need is not available to them, and is not derivable ab-initio with any ease. The advent of very cheap compute may mean that some can be worked out well enough without actually performing the measurements, but there is always devil in the details. As mentioned earlier, the neutron source spark-plug is critical to get right, and one would bet that the original designs of these were extensively tested and characterised. Whilst Little Boy might have been an untested end to end system, the components, and especially the neutron source, were probably tested many many times. Again, whilst the basic principle of a neutron source is easy to understand, your ability to make one that will deliver a reliable pulse of neutrons to microsecond precision whilst the weapon is in the midst of explosive disassembly is not something that is going to be worked out on a blackboard and a few computer programs. [Next page](https://boards.straightdope.com/t/how-large-is-the-primer-for-an-h-bomb/643934.md?page=2)