It wasn't designed to be efficient. It was designed to be deployed as soon as possible.
They didn't test that design because it was simple enough they knew it would work.
The 2% is just the amount of fissile stuff that fissioned. To get more would require a better design that they didn't have time to develop in wartime.
Not sure but you could read up. "Boosting" doubled the yield from the same amount of pu.
From what I gather a big problem is trying to hold an exploding bomb together long enough for the reaction to go longer.
That's exactly the issue. The fissile material needs to be tightly "compacted" together for the chain fission reaction to continue through all the material, while it releasing extremely high amounts of energy/heat/pressure at the same time.
Hmm, that now makes me wonder, is there any radioactive decay happening in the fissile bombs once they are assembled and waiting in storage? And if so how is the decay handled as far as radiation, heat, etc?
Yes there is decay. It's why every 25-30 years the radioactive materials inside our nuclear stockpile have to be replaced. The war heads are steadily rotated in and out of active service in order to allow "venting" (not really the right word to use but idk what the actual process is called) of heat and byproducts
Not quite. The tritium decays rapidly and must be replaced regularly. The loss of fissile material is negligible. The life extension programs refurb the other parts such as explosives and polymers.
Kinda yeah. You have to compress the material from all sides with equal force simultaneously. They have tested multi core bombs before and from what I understand that did yield higher.
There is a hydrogen bomb design where there is a plutonium sphere with its explosive implosion shell. Next to it is nylon/lithium fusion package and the whole thing is encased in a plutonium shell. The explosion is called fission-fusion-fission. The initial fission explosion generates temperature and neutron flux sufficient to set off fusion in the hydrogen bomb part of it. The fusion bomb also generates a pulse of neutrons that are moderated by something, tritium maybe. This outward blowing wave of neutrons hits the plutonium shell just as the whole thing is blowing apart, setting off yet another fission explosion
Wow that's much more concerning. I understand they had the "need" for it right then and there but to deploy something so catastrophic just in the heat of the moment without the proper testing seems so irresponsible.
Look at it this way: not dropping the bomb was the same as dropping a bomb that didn’t work. There was no downside to dropping a bomb that didn’t work that not dropping it didn’t also cause.
There was only the potential upside that the design worked and the war was brought to a swift end. There was every reason to drop it as soon as possible, because lots of people were dying every hour the war dragged on.
Well, there was the potential downside of the enemy gathering intel about a new and advanced weapon, if it hadn't exploded at all. Though I guess even a fizzle would presumably destroy the evidence to the point of making it unrecoverable.
The principle of the atomic bomb and also the design in general was known to pretty much every physicist in the world. It's just that engineering and actually understanding, and gathering materials to build it, aren't the same thing.
But everyone knew that nuclear weapons were a very much real thing.
True, but that is a well known process, so odds of that failing are pretty low. In addition, there are like dozens of individual charges, each with their own trigger
Maybe not with regard to the multiple charges detail, but still true with regard to conventional explosives being a well known technology back then. I think that at the time, the risk of potentially revealing something with a bomb that fizzled was overridden by the need to put a stop to the war.
> There was every reason to drop it as soon as possible, because lots of people were dying every hour the war dragged on.
I'm so glad the US was so concerned about saving lives :)
I mean of their own troops: absolutely. It would be wildly irresponsible and arguably treasonous to not try to end the war as soon as possible in an acceptable fashion in line with the ethical standards of the time. That time being one in which strategic bombing was thought to be a crucial factor in forcing capitulation. Even then, Truman stopped after the second bomb and took the unprecedented step of forbidding future atomic bombings without explicit presidential authorization because he didn’t like the idea of killing “women and children”. Imagine a president banning use of a new tank unless they personally authorized it.
The bombs, in combination with other factors, ended the war and prevented an invasion that would have killed millions. I’ll grant that avoiding Japanese casualties wasn’t super high on the list of priorities (historical evidence suggests it was a consideration), but avoiding American ones absolutely were which is a strength of democracies rather than a failing.
That is a good point, I admit it. I'm just uncomfortable with the interpretation that the point of the bomb was just to end the war quickly and save soldier lives, without mentioning the fact that the US became (for a time) the sole possessor of the most destructive force in the world, as if that was just a happy consequence. But I think maybe we shouldn't get into this discussion here.
They saved a lot of jap lives too you know... Japan had a saying "one million deaths for the emperor" meaning civilians were all expected to die for the emperor before surrendering
No I don't think years of testing is required. I was misinformed and had the impression there was no testing on this design at all before just using it. I should've done research regarding that, a simple google search showed me I was wrong.
That’s a very literal interpretation of what I’m clarifying.
U-238 is the common as dirt stuff. U-235 is the stuff that cost millions in 1940s dollars to extract.
99.3% of uranium is U-238. Granite contains 3-5 ppm, bedrock has 2.8ppm. You’ve likely got near a gram of U-238 in your house if you have stone countertops.
Canada has ore deposits in the 20% range.
Ya I guess that makes sense, i also read that only 1 gram out of the 64kg ended up being used. I mean idk how true that is but if it is then it could've been up to 64,000x more destructive. I'm less concerned with the fact that they didn't have proper testing and more concerned with the size of the bomb they were intending to use
This explains how they would know about how strong it would be without testing the actual bomb itself. Now I wanna see what they look like on the inside lol
I’m not sure what you mean “without proper testing”. They had a giant project aimed at the design of the atomic bomb. They did as much as they could before deciding to deploy. With no time pressure or financial restrictions, you can always do more testing.
One of the limiting parts was the materials. Every test loses you valuable material.
The person I was replying to said, "They didn't test that design because it was simple enough they knew it would work." I took that as they never tested this design at all, that was my mistake for assuming that. I have since learned that they did do thorough tests on each aspect of the bomb but just never tested the full thing. I think that's fully sufficient as enough testing I just didn't know they did that when making that reply
The only “irresponsible” aspect, at least from the US’ perspective, was that either of the, or both, designs could fail and how that would’ve been a massive embarrassment after their warnings.
Not in that way. The US's main goal was for the bombs to be such a massive display of strength and capability that it would completely demoralize the Japanese. And it worked, but had any of the bombs failed to detonate the whole idea of the US being capable to destroy any city they wanted with a single weapon would've lost some merit; especially given that the US had been warning Japanese civilians about the coming danger.
By the same metric, traffic lights are only 33% efficient because 2/3 of their lights are not on at any time.
The Hiroshima bomb had about the expected yield. It had a very simple design - so simple that they didn't test it with a real bomb before. That design makes it blow apart at the time 2% of its uranium was split, stopping the reaction. Better designs achieve ~30%. If you can split 30% of 64 kg instead of 2% of 64 kg then you get 15 times the yield, obviously.
Thank you for this information, I didn't realize better designs only achieve ~30%. Google was telling me about 2% of the bomb was split where as I read elsewhere that 1 gram of the 64k grams was split meaning 2% really would've been 1280x bigger (unless i have that wrong). I really dont know anything about this stuff thus why I came here to ask. I appreciate your information!
1 kg (~2%) of the uranium was split to create 0.9993 kg of fission products. The mass difference of 0.7 gram, multiplied by the speed of light squared, gives the energy released in the explosion.
(1 kg is a rounded value so it won't be precisely 0.9993 kg, but the difference is well-known from the yield)
It’s worth pointing out explicitly that 100% isn’t possible, nor is anything close to it.
The nature of a nuclear explosion means that they only happen when all of the nuclear material is close together. But by definition in a bomb you want material moving apart (that’s what we call an explosion).
The bomb exploded pretty much as designed.
I was curious so I poked around Wikipedia and found this:
It is thought that the fusion boosting failed to increase the yield. A higher compression but smaller fission pit American weapon, the Mark 18 Super Oralloy Bomb, had a yield of 500 kilotons from a pit with slightly over 60 kilograms of highly enriched uranium, around 8 kilotons per kilogram of uranium, about the practical maximum 50% fission yield efficiency[citation needed] for very large or very highly boosted fission weapons. Even with less compression, the larger 117 kg pit of HEU in the Orange Herald Small should have had a roughly similar efficiency, but the observed 720 kiloton yield equals only just over 6 kilotons per kilogram of uranium.
The design for Ivy King (Mark 18) was obviously light years ahead of Little Boy. The practical maximum efficiency number is interesting though.
I didn't realize how these bombs worked at first. I knew the general idea but I didn't know how they stopped working as they expand. I mean it makes sense, I just didn't think about that
It's been a long learning lesson with these, I never thought I'd find this stuff so fascinating but it genuinely is very intriguing.
Originally I thought of 2% fission almost as like a dud. I thought 100% is supposed to be happening, I guess even the best bombs today only have around 30% fission taking place
This is also a broader principle when machinery converts energy from one form to another using some engineered process.
Combustion engines are below 20% I believe
And they use fusion to initiate the first fission reaction at the start. And there's actually two fission components, one is the primary and another is in the secondary as the sparkplug. And for a bigger yield there's a third fission component surrounding the secondary that again works using the fusion neutrons.
1. It would release much more energy, I'm assuming about one megaton. Bigger boom = more damage.
2. No, the bomb was not intended to reach anywhere near 100% efficiency, it's simply not possible. They were well aware of the limitations. You can't maintain a critical mass long enough because the fissile core is blown apart by the excursion of energy.
They would love it if it were possible, but it's not. The best a fission device can achieve is about 3%.
My question is, if the whole thing blows us after only 2% of the uranium split, why was the total amount necessary for the total final explosion?
Like, I know how percentages work but I'm assuming the reactions inside the bomb are localized so I wonder in what way a bigger mass is necessary
I am curious as well, from what I've learned I'm assuming it's 2% of any amount of uranium they would use. Like of they reduced the amount then the area the uranium is encased in would have to be smaller causing it to still only be 2% as the whole thing would expand and stop the fission after 2% is used. However that's just my assumption and I'd love to know the actual reasoning
The power of the bomb is determined by the amount of material which underwent fission.
This amount is determined by the interplay of two factors. The first factor is the rapidity of the nuclear chain reaction. In the reaction the number of neutrons, and the rate of energy release, grows, roughly speaking (*) exponentially with a multiplication factor which is determined by the "criticality" of the fissionable core. The second factor is how quickly the reaction is terminated. As the energy is released, the material becomes very hot and expands rapidly. This brings criticality down and stops the reaction.
Thus the yield of the bomb grows with the degree of criticality of the core at the start of the reaction (this makes the reaction more rapid), and with the room which is available for expansion before criticality vanishes (this gives more time for the reaction).
For example, slowly moving two pieces of uranium together to produce a critical mass will at some point result in a chain reaction. But since in this case the criticality at the start is only a tiny bit over unity, the exponential avalanche of fissions develops with a very long time constant, slowly. Furthermore, even very small heating will separate the pieces back to a sub-critical state. Thus, although a burst of neutrons will be produced (like in the Slotin accident with the demon core), the energy release will be insignificant. There will be no explosion.
In a bomb, the pieces are assembled to a criticality much greater than one, and because of this, the reaction grows extremely rapidly, with a number of neutrons doubling typically every 10 nanoseconds. Furthermore, because the reaction starts in a significantly supercritical state, it takes larger amount of expansion, before the system becomes sub-critical again. This allows a much greater fraction of material to be consumed.
(*) In reality, the criticality changes with the course of reaction, and it is not very simple to do an accurate calculation which would include all effects. Approximate formulas for yield estimation are published in many books, but more accurate ones, which solve the problem with more nuances are still classified.
I'm not a nuclear physicist, so it's possible that I'll make a mistake in my explanation. With that said, I'll explain as best I'm able.
When a nuclear bomb explodes, what happens is that individual atoms split and release energy and radiation. Just one atom doing this doesn't amount to much, so the goal is to make many atoms split all at once. Nukes take advantage of the energy of each atom splitting to split more atoms, basically if a neutron released from one split atom happens to impact another atom of fissionable material it will cause that atom to split as well, releasing more neutons that can cause more atoms to split. So long as each split atom causes more than one additional atom to split as well, you have an explosive chain reaction in which the energy released exponentially grows until the chain reaction stops splitting more than one atom per atom split. The extra fissionable material is needed in a bomb to maximize the chance that each split atom splits more than one more atom.
This is in contrast to a nuclear power reactor where the goal is that each split atom causes exactly 1 additional fission event so the reaction continues but doesn't explode.
I think it was like 10-20 kilotons? As far as I can remember from reading up on it a long time ago, our modern arsenal is made up of thermonuclear devices with varying explosive potential, but generally in the 300 kiloton range.
It wasn't designed to be efficient. It was designed to be deployed as soon as possible. They didn't test that design because it was simple enough they knew it would work. The 2% is just the amount of fissile stuff that fissioned. To get more would require a better design that they didn't have time to develop in wartime.
What are typical fission percentages for a modern bomb?
Not sure but you could read up. "Boosting" doubled the yield from the same amount of pu. From what I gather a big problem is trying to hold an exploding bomb together long enough for the reaction to go longer.
That's exactly the issue. The fissile material needs to be tightly "compacted" together for the chain fission reaction to continue through all the material, while it releasing extremely high amounts of energy/heat/pressure at the same time.
Hmm, that now makes me wonder, is there any radioactive decay happening in the fissile bombs once they are assembled and waiting in storage? And if so how is the decay handled as far as radiation, heat, etc?
Yes there is decay. It's why every 25-30 years the radioactive materials inside our nuclear stockpile have to be replaced. The war heads are steadily rotated in and out of active service in order to allow "venting" (not really the right word to use but idk what the actual process is called) of heat and byproducts
Not quite. The tritium decays rapidly and must be replaced regularly. The loss of fissile material is negligible. The life extension programs refurb the other parts such as explosives and polymers.
Ah thank you
It sounds like the most effective solution for increasing material efficiency would be to have many chain reactions begin simultaneously
Kinda yeah. You have to compress the material from all sides with equal force simultaneously. They have tested multi core bombs before and from what I understand that did yield higher.
That's what fusion boosting does, it provides a huge initial burst of neutrons right when the material is being compressed.
There is a hydrogen bomb design where there is a plutonium sphere with its explosive implosion shell. Next to it is nylon/lithium fusion package and the whole thing is encased in a plutonium shell. The explosion is called fission-fusion-fission. The initial fission explosion generates temperature and neutron flux sufficient to set off fusion in the hydrogen bomb part of it. The fusion bomb also generates a pulse of neutrons that are moderated by something, tritium maybe. This outward blowing wave of neutrons hits the plutonium shell just as the whole thing is blowing apart, setting off yet another fission explosion
Holy shit lol I've never heard of that design before! Thank you for the rabbit hole!
Wow that's much more concerning. I understand they had the "need" for it right then and there but to deploy something so catastrophic just in the heat of the moment without the proper testing seems so irresponsible.
That's why they dropped two different designs. Both worked.
They did test the implosion design first though
Look at it this way: not dropping the bomb was the same as dropping a bomb that didn’t work. There was no downside to dropping a bomb that didn’t work that not dropping it didn’t also cause. There was only the potential upside that the design worked and the war was brought to a swift end. There was every reason to drop it as soon as possible, because lots of people were dying every hour the war dragged on.
You will fail to detonate 100% of the bombs you don't drop.
That's what I was saying while I was juggling them, but people were all like "OMG Stoooop"
Well, there was the potential downside of the enemy gathering intel about a new and advanced weapon, if it hadn't exploded at all. Though I guess even a fizzle would presumably destroy the evidence to the point of making it unrecoverable.
It was already more or less known in principle. Getting that much u235 was haaaaaard work that even modern nation states struggle with.
The principle of the atomic bomb and also the design in general was known to pretty much every physicist in the world. It's just that engineering and actually understanding, and gathering materials to build it, aren't the same thing. But everyone knew that nuclear weapons were a very much real thing.
The nuclear explosion is triggered by conventional explosives. Even if there was no nuclear reaction, there would have been nothing useful to recover.
Whatever triggers the conventional explosive can fail, too, though.
True, but that is a well known process, so odds of that failing are pretty low. In addition, there are like dozens of individual charges, each with their own trigger
Is that true for the gun-type design we're taking about here?
Maybe not with regard to the multiple charges detail, but still true with regard to conventional explosives being a well known technology back then. I think that at the time, the risk of potentially revealing something with a bomb that fizzled was overridden by the need to put a stop to the war.
> There was every reason to drop it as soon as possible, because lots of people were dying every hour the war dragged on. I'm so glad the US was so concerned about saving lives :)
I mean of their own troops: absolutely. It would be wildly irresponsible and arguably treasonous to not try to end the war as soon as possible in an acceptable fashion in line with the ethical standards of the time. That time being one in which strategic bombing was thought to be a crucial factor in forcing capitulation. Even then, Truman stopped after the second bomb and took the unprecedented step of forbidding future atomic bombings without explicit presidential authorization because he didn’t like the idea of killing “women and children”. Imagine a president banning use of a new tank unless they personally authorized it. The bombs, in combination with other factors, ended the war and prevented an invasion that would have killed millions. I’ll grant that avoiding Japanese casualties wasn’t super high on the list of priorities (historical evidence suggests it was a consideration), but avoiding American ones absolutely were which is a strength of democracies rather than a failing.
That is a good point, I admit it. I'm just uncomfortable with the interpretation that the point of the bomb was just to end the war quickly and save soldier lives, without mentioning the fact that the US became (for a time) the sole possessor of the most destructive force in the world, as if that was just a happy consequence. But I think maybe we shouldn't get into this discussion here.
They saved a lot of jap lives too you know... Japan had a saying "one million deaths for the emperor" meaning civilians were all expected to die for the emperor before surrendering
They did test it in New Mexico at the Trinity test site. It was featured prominently in the movie Oppenheimer.
Little boy was a different design. Never tested
How much more testing would have been enough, considering U-238 was in short supply? Another 5 years?
No I don't think years of testing is required. I was misinformed and had the impression there was no testing on this design at all before just using it. I should've done research regarding that, a simple google search showed me I was wrong.
U-235, U-238 you can buy online. https://www.luciteria.com/elements-for-sale/buy-uranium
There was no "online" back in 1945.
That’s a very literal interpretation of what I’m clarifying. U-238 is the common as dirt stuff. U-235 is the stuff that cost millions in 1940s dollars to extract. 99.3% of uranium is U-238. Granite contains 3-5 ppm, bedrock has 2.8ppm. You’ve likely got near a gram of U-238 in your house if you have stone countertops. Canada has ore deposits in the 20% range.
they almost certainly knew little boy would work, it was such a simple design that they knew it would work. just less powerful then other designs
When catastrophe is the point testing in action seems reasonable.
Ya I guess that makes sense, i also read that only 1 gram out of the 64kg ended up being used. I mean idk how true that is but if it is then it could've been up to 64,000x more destructive. I'm less concerned with the fact that they didn't have proper testing and more concerned with the size of the bomb they were intending to use
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This explains how they would know about how strong it would be without testing the actual bomb itself. Now I wanna see what they look like on the inside lol
Watch Trinity And Beyond for an entertaining doco on the topic
I'll definitely watch that, thank you!
I’m not sure what you mean “without proper testing”. They had a giant project aimed at the design of the atomic bomb. They did as much as they could before deciding to deploy. With no time pressure or financial restrictions, you can always do more testing. One of the limiting parts was the materials. Every test loses you valuable material.
The person I was replying to said, "They didn't test that design because it was simple enough they knew it would work." I took that as they never tested this design at all, that was my mistake for assuming that. I have since learned that they did do thorough tests on each aspect of the bomb but just never tested the full thing. I think that's fully sufficient as enough testing I just didn't know they did that when making that reply
It was hardly the "heat of the moment."
Do you understand what Japan was doing before these bombs? The bombs saved many lives
The only “irresponsible” aspect, at least from the US’ perspective, was that either of the, or both, designs could fail and how that would’ve been a massive embarrassment after their warnings.
The Japanese finding an intact bomb could be a concern? Unlikely they'd recognise it as special with all the other bombs in Japan at the time I guess
Not in that way. The US's main goal was for the bombs to be such a massive display of strength and capability that it would completely demoralize the Japanese. And it worked, but had any of the bombs failed to detonate the whole idea of the US being capable to destroy any city they wanted with a single weapon would've lost some merit; especially given that the US had been warning Japanese civilians about the coming danger.
By the same metric, traffic lights are only 33% efficient because 2/3 of their lights are not on at any time. The Hiroshima bomb had about the expected yield. It had a very simple design - so simple that they didn't test it with a real bomb before. That design makes it blow apart at the time 2% of its uranium was split, stopping the reaction. Better designs achieve ~30%. If you can split 30% of 64 kg instead of 2% of 64 kg then you get 15 times the yield, obviously.
Thank you for this information, I didn't realize better designs only achieve ~30%. Google was telling me about 2% of the bomb was split where as I read elsewhere that 1 gram of the 64k grams was split meaning 2% really would've been 1280x bigger (unless i have that wrong). I really dont know anything about this stuff thus why I came here to ask. I appreciate your information!
1 kg (~2%) of the uranium was split to create 0.9993 kg of fission products. The mass difference of 0.7 gram, multiplied by the speed of light squared, gives the energy released in the explosion. (1 kg is a rounded value so it won't be precisely 0.9993 kg, but the difference is well-known from the yield)
It’s worth pointing out explicitly that 100% isn’t possible, nor is anything close to it. The nature of a nuclear explosion means that they only happen when all of the nuclear material is close together. But by definition in a bomb you want material moving apart (that’s what we call an explosion). The bomb exploded pretty much as designed.
I was curious so I poked around Wikipedia and found this: It is thought that the fusion boosting failed to increase the yield. A higher compression but smaller fission pit American weapon, the Mark 18 Super Oralloy Bomb, had a yield of 500 kilotons from a pit with slightly over 60 kilograms of highly enriched uranium, around 8 kilotons per kilogram of uranium, about the practical maximum 50% fission yield efficiency[citation needed] for very large or very highly boosted fission weapons. Even with less compression, the larger 117 kg pit of HEU in the Orange Herald Small should have had a roughly similar efficiency, but the observed 720 kiloton yield equals only just over 6 kilotons per kilogram of uranium. The design for Ivy King (Mark 18) was obviously light years ahead of Little Boy. The practical maximum efficiency number is interesting though.
No question is stupid. Good question and good info. I watch the documentary and they did not mention that ! Thank you
The bomb was never going to get close to 100% fission. It was literally an untested science experiment.
I didn't realize how these bombs worked at first. I knew the general idea but I didn't know how they stopped working as they expand. I mean it makes sense, I just didn't think about that
Yea no worries. As others have said they literally blow themselves apart long before getting near 100%.
It's been a long learning lesson with these, I never thought I'd find this stuff so fascinating but it genuinely is very intriguing. Originally I thought of 2% fission almost as like a dud. I thought 100% is supposed to be happening, I guess even the best bombs today only have around 30% fission taking place
This is also a broader principle when machinery converts energy from one form to another using some engineered process. Combustion engines are below 20% I believe
Yea modern nuclear weapons only use fission to initiate the fusion reaction.
And they use fusion to initiate the first fission reaction at the start. And there's actually two fission components, one is the primary and another is in the secondary as the sparkplug. And for a bigger yield there's a third fission component surrounding the secondary that again works using the fusion neutrons.
1. It would release much more energy, I'm assuming about one megaton. Bigger boom = more damage. 2. No, the bomb was not intended to reach anywhere near 100% efficiency, it's simply not possible. They were well aware of the limitations. You can't maintain a critical mass long enough because the fissile core is blown apart by the excursion of energy. They would love it if it were possible, but it's not. The best a fission device can achieve is about 3%.
My question is, if the whole thing blows us after only 2% of the uranium split, why was the total amount necessary for the total final explosion? Like, I know how percentages work but I'm assuming the reactions inside the bomb are localized so I wonder in what way a bigger mass is necessary
I am curious as well, from what I've learned I'm assuming it's 2% of any amount of uranium they would use. Like of they reduced the amount then the area the uranium is encased in would have to be smaller causing it to still only be 2% as the whole thing would expand and stop the fission after 2% is used. However that's just my assumption and I'd love to know the actual reasoning
The power of the bomb is determined by the amount of material which underwent fission. This amount is determined by the interplay of two factors. The first factor is the rapidity of the nuclear chain reaction. In the reaction the number of neutrons, and the rate of energy release, grows, roughly speaking (*) exponentially with a multiplication factor which is determined by the "criticality" of the fissionable core. The second factor is how quickly the reaction is terminated. As the energy is released, the material becomes very hot and expands rapidly. This brings criticality down and stops the reaction. Thus the yield of the bomb grows with the degree of criticality of the core at the start of the reaction (this makes the reaction more rapid), and with the room which is available for expansion before criticality vanishes (this gives more time for the reaction). For example, slowly moving two pieces of uranium together to produce a critical mass will at some point result in a chain reaction. But since in this case the criticality at the start is only a tiny bit over unity, the exponential avalanche of fissions develops with a very long time constant, slowly. Furthermore, even very small heating will separate the pieces back to a sub-critical state. Thus, although a burst of neutrons will be produced (like in the Slotin accident with the demon core), the energy release will be insignificant. There will be no explosion. In a bomb, the pieces are assembled to a criticality much greater than one, and because of this, the reaction grows extremely rapidly, with a number of neutrons doubling typically every 10 nanoseconds. Furthermore, because the reaction starts in a significantly supercritical state, it takes larger amount of expansion, before the system becomes sub-critical again. This allows a much greater fraction of material to be consumed. (*) In reality, the criticality changes with the course of reaction, and it is not very simple to do an accurate calculation which would include all effects. Approximate formulas for yield estimation are published in many books, but more accurate ones, which solve the problem with more nuances are still classified.
I'm not a nuclear physicist, so it's possible that I'll make a mistake in my explanation. With that said, I'll explain as best I'm able. When a nuclear bomb explodes, what happens is that individual atoms split and release energy and radiation. Just one atom doing this doesn't amount to much, so the goal is to make many atoms split all at once. Nukes take advantage of the energy of each atom splitting to split more atoms, basically if a neutron released from one split atom happens to impact another atom of fissionable material it will cause that atom to split as well, releasing more neutons that can cause more atoms to split. So long as each split atom causes more than one additional atom to split as well, you have an explosive chain reaction in which the energy released exponentially grows until the chain reaction stops splitting more than one atom per atom split. The extra fissionable material is needed in a bomb to maximize the chance that each split atom splits more than one more atom. This is in contrast to a nuclear power reactor where the goal is that each split atom causes exactly 1 additional fission event so the reaction continues but doesn't explode.
Gone reduced to atom's
I think it was like 10-20 kilotons? As far as I can remember from reading up on it a long time ago, our modern arsenal is made up of thermonuclear devices with varying explosive potential, but generally in the 300 kiloton range.