# How many megatons of yield cold a sub/aircraft carrier nuclear reactor fuel be used to produce? **URL:** \ **Category:** Factual Questions\ **Created:** [July 23, 2019, 4:14pm UTC](https://boards.straightdope.com/t/how-many-megatons-of-yield-cold-a-sub-aircraft-carrier-nuclear-reactor-fuel-be-used-to-produce/837503 "2019-07-23T16:14:37Z")\ **Posts on this page:** 8\ **Page:** 3
**Author:** ![HMS\_Irruncible](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hms_irruncible/32/7394_2.png) [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)\ **Post date:** [July 25, 2019, 3:01pm UTC](https://boards.straightdope.com/t/how-many-megatons-of-yield-cold-a-sub-aircraft-carrier-nuclear-reactor-fuel-be-used-to-produce/837503/41 "2019-07-25T15:01:43Z")
> [@MichaelEmouse](#): > > After uranium, would the second limiting ingredient be plutonium? Are there nuclear reactors that at least partially rely on plutonium? No, and yes. First, you don’t really need plutonium for the bomb, U-235 also is OK. There are design differences, but both are proven. But yes on the second part, there are reactors that use plutonium. Actually if there’s any U-238 in a reactor, it will absorb neutrons and become P-239, which itself will accumulate or react. This is where plutonium comes from. > [@](#): > > If most of the yield from a thermonuclear nuke comes from fission, why bother with the fusion? Why are nukes that use fusion so much more powerful than fission-only nukes? The first small bit of fission explodes so violently that most of the fissionable fuel is dispersed before it can react. The fusion stage provides a flood of fast neutrons. By flooding the fission fuel with fast neutrons before it gets dispersed, much less of it is wasted. A bonus effect of the fast neutron flux is that U-238, which isn’t normally fissionable, undergoes fission in those conditions. So if we have a fusion stage, we can jacket the entire bomb in U-238, which ends up providing more than half the total explosive yield. Since U-238 is relatively cheap and plentiful, it gives a lot bigger bang for the buck. ---
**Author:** ![rbroome](https://avatars.discourse-cdn.com/v4/letter/r/838e76/32.png) [@rbroome](https://boards.straightdope.com/u/rbroome)\ **Post date:** [July 25, 2019, 3:48pm UTC](https://boards.straightdope.com/t/how-many-megatons-of-yield-cold-a-sub-aircraft-carrier-nuclear-reactor-fuel-be-used-to-produce/837503/42 "2019-07-25T15:48:22Z")
> [@DPRK](#): > > The “Tsar Bomba” test mentioned above proved that you can, at least in principle, build a ridiculously huge bomb without a fissionable casing in the secondary and tertiary stages. So I am wrong in believing that the Tsar Bomba relied on a damping blanket of Ur for most of its power? It was a 3 stage fission-fusion-fusion reaction without a blanket? Perhaps it was designed with a Ur blanket and that was what was replaced with lead. The bomb had been down-sized from a 100Mt to a 50Mt bomb via some mechanism. ---
**Author:** ![HMS\_Irruncible](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hms_irruncible/32/7394_2.png) [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)\ **Post date:** [July 25, 2019, 4:31pm UTC](https://boards.straightdope.com/t/how-many-megatons-of-yield-cold-a-sub-aircraft-carrier-nuclear-reactor-fuel-be-used-to-produce/837503/43 "2019-07-25T16:31:20Z")
> [@rbroome](#): > > So I am wrong in believing that the Tsar Bomba relied on a damping blanket of Ur for most of its power? It was a 3 stage fission-fusion-fusion reaction without a blanket? > > Perhaps it was designed with a Ur blanket and that was what was replaced with lead. > The bomb had been down-sized from a 100Mt to a 50Mt bomb via some mechanism. That’s what reportedly happened. Had they used a U-238 tamper it would have been 100MT in yield, but a lot of the fallout would have fallen out onto Soviet territory. So they skipped the fast-fission stage by using a lead tamper. That cut the yield in half, but it was much cleaner because it was 97% from fusion. Still: “cleaner” is relative. There was still 1.5MT worth of fission yield from it, which is a shit-megaton of dirty fission products. ---
**Author:** ![MichaelEmouse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/michaelemouse/32/101_2.png) [@MichaelEmouse](https://boards.straightdope.com/u/MichaelEmouse)\ **Post date:** [July 25, 2019, 4:41pm UTC](https://boards.straightdope.com/t/how-many-megatons-of-yield-cold-a-sub-aircraft-carrier-nuclear-reactor-fuel-be-used-to-produce/837503/44 "2019-07-25T16:41:17Z")
> [@HMS\_Irruncible](#): > > First, you don’t really need plutonium for the bomb, U-235 also is OK. There are design differences, but both are proven. But yes on the second part, there are reactors that use plutonium. Actually if there’s any U-238 in a reactor, it will absorb neutrons and become P-239, which itself will accumulate or react. This is where plutonium comes from. Thank you, that’s great information. In which use cases would you prefer to use uranium or plutonium? > [@HMS\_Irruncible](#): > > The first small bit of fission explodes so violently that most of the fissionable fuel is dispersed before it can react. The fusion stage provides a flood of fast neutrons. By flooding the fission fuel with fast neutrons before it gets dispersed, much less of it is wasted. > > A bonus effect of the fast neutron flux is that U-238, which isn’t normally fissionable, undergoes fission in those conditions. So if we have a fusion stage, we can jacket the entire bomb in U-238, which ends up providing more than half the total explosive yield. Since U-238 is relatively cheap and plentiful, it gives a lot bigger bang for the buck. A bit like making sure that a fuel is thoroughly mixed with oxidizer so that it combusts efficiently? Can both uranium or plutonium provide the fusion stage? ---
**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:** [July 25, 2019, 4:48pm UTC](https://boards.straightdope.com/t/how-many-megatons-of-yield-cold-a-sub-aircraft-carrier-nuclear-reactor-fuel-be-used-to-produce/837503/45 "2019-07-25T16:48:17Z")
> [@MichaelEmouse](#): > > Can both uranium or plutonium provide the fusion stage? Neither. U-235 or PU-239 is used as the “primary” - a fission bomb. The energy from that explosion (mostly in the form of x-rays) is used to “ignite” the fusion stage. The Neutrons from the fusion reaction are used to induce fission in the tamper and casing of the bomb. In “dial-a-yield” weapons the amount of fusion can be selected, resulting in a huge range of yields. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [July 25, 2019, 5:02pm UTC](https://boards.straightdope.com/t/how-many-megatons-of-yield-cold-a-sub-aircraft-carrier-nuclear-reactor-fuel-be-used-to-produce/837503/46 "2019-07-25T17:02:35Z")
> [@MichaelEmouse](#): > > Thank you, that’s great information. > > In which use cases would you prefer to use uranium or plutonium? You mean in a reactor? You would prefer to use natural uranium, because that’s what comes out of the mine. OTOH in a situation where MOX fuel is cheaper you could design your core to run on that. > [@](#): > > A bit like making sure that a fuel is thoroughly mixed with oxidizer so that it combusts efficiently? > > Can both uranium or plutonium provide the fusion stage? U-238 is not fissile like U-235, so it cannot sustain a chain reaction. For the fusion boosting you may use deuterium and tritium gas. ---
**Author:** ![HMS\_Irruncible](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hms_irruncible/32/7394_2.png) [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)\ **Post date:** [July 25, 2019, 5:11pm UTC](https://boards.straightdope.com/t/how-many-megatons-of-yield-cold-a-sub-aircraft-carrier-nuclear-reactor-fuel-be-used-to-produce/837503/47 "2019-07-25T17:11:43Z")
> [@MichaelEmouse](#): > > In which use cases would you prefer to use uranium or plutonium? There’s a lot to that question. In general uranium is less radioactive, so easier to work with, but the necessary critical mass is like 4-5x what you’d need for plutonium. > [@](#): > > A bit like making sure that a fuel is thoroughly mixed with oxidizer so that it combusts efficiently? Sort of, I guess, but that’s a totally different process. I wouldn’t want to use an analogy there. > [@](#): > > Can both uranium or plutonium provide the fusion stage? The primary can be uranium and plutonium. These are always fission reactions. Either one could provide enough energy to initiate a fusion stage, but I don’t know if you can just swap a uranium pit for a plutonium pit and expect the same behavior without changing the bomb design. ---
**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:** [July 25, 2019, 6:54pm UTC](https://boards.straightdope.com/t/how-many-megatons-of-yield-cold-a-sub-aircraft-carrier-nuclear-reactor-fuel-be-used-to-produce/837503/48 "2019-07-25T18:54:03Z")
Uranium occurs naturally, but most natural uranium is the relatively stable U-238. To do anything interesting with uranium, you need U-235, which is also present in the ore, but only at a few percent concentration. You can separate out the two isotopes, but that’s very difficult, because they’re chemically the same, and only differ in their mass, which is about 1% different. Plutonium basically isn’t found in nature (or rather, in trace amounts far too small to be useful), but it’s possible to turn uranium into plutonium in a nuclear reactor. For this, you can start with the abundant U-238, and you can even get useful power out of the reactor while you’re doing it. A lot of nations that are pursuing “peaceful” nuclear power technology just happen to choose this kind of reactor, even though most of the rest of the world would really prefer that they use other sorts of reactors that can’t be used to manufacture plutonium. During the Manhattan Project, it wasn’t known whether it’d be easier to use uranium or plutonium, and so they pursued both. The Trinity test device and Fat Man were both plutonium-based, and Little Boy was uranium-based. I think it turns out that, once you’ve got the technology established, plutonium is easier, so most US weapons use plutonium, but you still want to keep an eye on anyone developing the uranium-based technology. [Previous page](https://boards.straightdope.com/t/how-many-megatons-of-yield-cold-a-sub-aircraft-carrier-nuclear-reactor-fuel-be-used-to-produce/837503.md?page=2)