How plausible is it for nuclear power to be invented but not nuclear weapons?

Question

I don’t need a super detailed answer here, so this is definitely going to be pretty opinion-based. For context, let’s say you have a world where WW2 was ended by an all-American superhero punching Hitler in the jaw (not literally but you get my point) as opposed to the atomic bombings of Hiroshima and Nagasaki. Super-soldier serum is the hottest new weapons tech anyone’s ever seen and it remains dominant throughout the 20th century and into the 21st, leading to a world where the progression of technology has certainly diverged, but only slightly. Certain developments may have been stalled and some technologies may look different or have had a different impact on the world and popular culture, but the things important to the average person are mostly the same. Someone living in 2020 would still have access to computers, the internet, movies/television and the streaming thereof, commercial airflight, modern medicine etc.

If by and large the presence of weapons-grade superheroes doesn’t impact the progression of technology towards a recognizable standard, how likely is it that nobody would have thought to pursue further research into nuclear weapons? Could superhumans supplant this particular WMD that has so thoroughly shaped our modern history or would they be developed eventually regardless?

Answer 1

You Can't Avoid the Knowledge, But You Can Avoid the Technology

The run-away chain reaction of the bomb was theorized basically as soon as the idea of splitting the atom came about. It is not plausible to say "no body thought of it."

However, there are significant technological challenges between "We have weapons grade Uranium" and "We have a nuclear bomb".

The Manhattan Project basically invented a brand new sub-field of physics describing how conventional explosions work, because they needed carefully controlled conventional explosives to create the right conditions for the nuclear reaction.

Fizzle

You create a critical mass of Uranium by compressing a sub-critical mass - it's a question of density. If you compress too slowly, the Uranium heats up, and explodes early using only a fraction of it's yield. If you compress too quickly, the required density only exists for a tiny fraction of a second before momentum separates it again - again you get a very small boom.

This is called fizzling, and is a serious problem with nuclear weapon design.

The Window of Opportunity

So it's perfectly feasible that the Manhattan Project creates a bunch of weapons grade Uranium, but the war ends before they can engineer out all the problems with fizzle. Especially if superheroes are involved.

The government, tired of war and looking to get something out of their incredibly expensive investment in the Manhattan Project, creates a civilian nuclear power program in stead of a weapons program.

A few significant reactor accidents a la Chernobyl convince everyone that contamination is a significant problem, and an Arms Treaty bans the research / use of nuclear weapons.

Governments invest in superheroes because they are cheaper and less environmentally destructive, knowing all the while that is is possible to build nuclear weapons.

EDIT:

A couple of comments have pointed out that the Gun Type bomb mitigates some of the complexity of the bomb itself. I'll be a little more concrete:

As GrumpyYoungMan pointed out in the comments, the hardest engineering challenge of the Manhattan Project was refining out the fissile Uranium. If we have some enriched Uranium, but haven't built a bomb yet, the scenario I outlined above works - the war ends early and the US government pivots to civilian power to justify the expense.

Most of the enrichment machinery came online in mid to late 1944, and the Trinity test took place July 16, 1945. If you end the war any time between those dates, then the US government has a stockpile of fissile material, and the bomb has not been proven or demonstrated.

End the war in Spring 1945, pivot to nuclear power, and have a couple of bad reactor accidents prompt an early Arms Treaty. This can probably justify a world without nuclear bombs.

Answer 2

Very implausible. Nuclear fission has to occur at a controlled and steady rate in order to be used in a reactor. The natural question that would arise in the minds of scientists and engineers is "What would happen if we let fission occur as fast as possible?" and the conclusion that nuclear bombs could be built would come immediately thereafter.

[EDIT] To clarify, nuclear power plant designers would unavoidably need to consider the potential for a runaway nuclear reaction to prevent accidental meltdowns like Three Mile Island and Chernobyl. No amount of attempts to suppress the idea would be able to hide a concept that's so blatantly obvious. Even if initial attempts succeeded, the first nuclear power plant meltdown that occurred would bring the idea of a runaway nuclear reaction to the forefront.

Answer 3

You can have the weapons tech without the power tech, but the other way around, not so much. This is kind of like asking if you can invent the internal combustion engine without someone immediately figuring out Molotov cocktails. Weaponizing a fuel source is simply easier than figuring out all the mechanics of harnessing it into something controllable and useful.

That said, a world with nuclear energy and no one actually MAKING nuclear weapons may be achievable if your super humans are super enough. If your heroes are Superman tier flying around at supersonic speeds shooting lasers from their eyes, then nukes stop being able to maintain a policy of MAD. Nukes are really expensive to make. The US alone has spent around 5.8 Trillion dollars on Nukes since the technology was discovered. In our time line, those costs were justified by making sure those weapons can hit something, but once you start looking at a world where you fire 300 nukes just to see an army of supermen intercept them all, suddenly they become useless and overly expensive and go the same way as knights in shining armor did when we discovered guns, or battleships when we discovered air superiority.

Answer 4

If you are willing to alter physics maybe.

U-235 gun-type bomb is so simple, they never tested one before dropping it on Japan. (Well, that and the pain of separating weapons grade U-235). You can make U-235 not viable by adding a few more 700 million year half-lives, 4 would drop the percentage below 0.1% U-235 - bomb making seems very unlikely in that case because the initial enrichment phase would be much harder.

Pu-239 is much easier to isolate because it is a chemical separation process. But, you can't make anything better than a fizzle bomb without using the implosion bomb design.

If Pu-239 was a harder to make into a bomb, it likely would not happen, it's already a significant technical challenge. But it could still be suitable for a nuclear reactor.

Problem is Pu-240 is always mixed in with, and it starts to react before the the Pu-239 really reaches the necessary reaction density for an effective bomb - thus a fizzle. Implosion fixes the density problem. So, tweak the physics by reducing the cross sectional area (barns) and Pu-239 becomes harder to explode - a functional weapon requires a lot more Pu-239, making implosion bomb impractical.

Thorium fuel cycle presents its own set of challenges, but you could definitely make a bomb - bleed off the unwanted (neutron eating) Protactinium-233 and isolate it long enough for it to become mostly pure U-233 and you have another bomb-making isotope. This one is probably harder to hand-wave away the bomb-enabling physics, but maybe biology is the answer. Make your people much more prone to radiation damage, and the U-232 poisoning the bomb fuel becomes too hot to handle. U-232 also damages bomb electronics. The hard-gamma problem of U-232 is actually part of the decay chain of U-232, not the U-232 itself. IIRC the US exploded 1 U-233 bomb just to prove it could work.

Simpler solution:

Users won't want / need a detailed explanation, just allow the scientists decide it's just too hard to make a functional bomb. Kind of like the German scientists did during WWII. The US Manhattan Project is still an exemplar for a massive project.

From the Wikipedia article:

Heisenberg said in 1939 that the physicists at the (second) meeting said that in principle atomic bombs could be made.... it would take years.... not before five. He said I didn't report it to the Fuehrer until two weeks later and very casually because I did not want the Fuehrer to get so interested that he would order great efforts immediately to make the atomic bomb. Speer felt it was better that the whole thing should be dropped and the Fuehrer also reacted that way. He said they presented the matter in this way for their personal safety as the probability (of success) was nearly zero, but if many thousands (of) people developed nothing, that could have extremely disagreeable consequences for us.

I have read before that the reasons the Germans were convinced that it was too hard was because of a calculation error by Heisenberg, that required a critical mass significantly larger than the actual critical mass. Something related to a bad assumption re: free-neutron density being off because he a mistake re: how many free-neutrons would be available for the explosion because he flubbed the loss at the edge of the critical mass. But I can't seem to locate a reference for that. Even a true genius makes mistake.

Once you reach the mindset that something cannot be done, or is very impractical, it can remain "settled science" for a very long time.

Sorry, forgot about this one

Alter physics in a different way. Make the nuclear weapon ignite the atmosphere. During the Manhattan Project, the issue came up re: whether setting off a nuclear bomb would trigger a chain reaction in the atmosphere. Calculations were done and they accurately concluded that the nitrogen would not start a global chain reaction. Although they had done the calculation they were not 100% certain because they knew that there was at least a possibility that they had overlooked something in their calculations, specifically the possibility of a different mechanism of nitrogen fusion.

So could come to a different conclusion, either due to altered physics or by just being more risk adverse by nature.

I like this because it could be an interesting sub-story, and certainly not one that most people ever thought about. Changing physics is not actually needed in this case.

Altering physics is hard and not easy to weave into a good story unless the story includes changing physics as an element.

Answer 5

Nuclear bombs were deemed irrelevant after an underfunded project produced a series of unimpressive fizzles.

In real life, the Manhattan Project was absurdly well funded - to the point they used an 80 lbs solid gold sphere as a doorstop - because this was the technology that would win the war.

Perhaps in your world the Manhattan Project didn't start until superheroes had already won the war - and it has 90% less money. With far fewer resources and much less motivation, they weren't able to perfect the design and they could only ever demonstrate a partial detonation.

The partial detonation gave a bigger blast than you'd get with conventional explosives - but not much bigger, and you've got all this inconvenient fallout to deal with, and it's very expensive. So the military decided to stop throwing good money after bad and shut down the project.

Who needs a big explosion, if you've got an army of bulletproof teleporting psychic time-travelling superhumans on your side?

These days, nuclear bombs are like cold fusion - never convincingly demonstrated, with something of an embarrassing history - never entirely disproven or defunded, but never funded to the degree needed to be a success.

Answer 6

This would be a good way to go if your book wants to highlight the injustice of racism.

"Jewish physics"

Change something in WWII relating to the treatment of the Jews, which worsens antisemitism worldwide. As such, nuclear bomb potential is rejected by everyone (instead of just Germany) as "Jewish physics".

Antisemitism never dies

Remember: being a Jew in 1940 was not a bed of roses. Blatant antisemitism existed throughout the world and was just a normal part of life. And believe you me - America was completely capable of that level of racism at that time; they were already doing it to African Americans. But in our world, Germany took antisemitism to its final conclusion. The horrors of what this actually looked like shocked people out of it.

Here this doesn't happen for some reason. Say Hitler actually "crunches the numbers" and realizes the burden to the economy of a) operating massive concentration camps, and b) removing many highly effective workers, scientists and businessmen from the economy, will break the German economy and lose the war.

Now, Germany does NOT commit unspeakable horrors against Jews, they just use media manipulation to notch up antisemitism all over the world. The Allies arrive in Germany and find no Shoah, just baked-in German antisemitism that looks a lot like the antisemitism they have back home, just done a little more thoroughly and efficiently, but while still keeping the Jews in the economy. The antisemitism they find makes sense to them and many countries actually copy the techniques.

The upshot is that at war's finish, antisemitism is alive and stronger than ever.

In real-world Germany, nuclear research was rejected for years as "Jewish physics". That set them so far back that they never got near the Bomb. This continues worldwide: nobody can get funding to research it; merely broaching the subject raises questions about your parentage.

Getting bomb material is hard...

Let's review how one obtains bomb material. There are two branches, both starting with natural uranium. Uranium ore has a bunch of other crud in it; that's easy enough to separate to get pure uranium. But uranium itself has two isotopes: 0.7% is U-235 and 99.3% is U-238. Now there are two branches to a bomb, both staggeringly expensive, Manhattan Project level, budget-busting, large-state commitments:

The Oak Ridge path: separate the isotopes of Uranium to get the directly fissile U-235. So you are taking 25,000 pounds of U to make 141 pounds of 80% U-235. With only 3 atomic weights between them, that's biblically difficult. A lab could do a few atoms at a time - but you need 16 million billion billion atoms. The problem is scaling up; you just need a whole lotta machines. This is why we get excited about Iran getting centrifuges.

The Hanford path: Inside a reactor, neutrons will hit the U-238 and turn it into Pu-239, which is also fissile. However, neutrons will hit the Pu-239 and turn it into Pu-240. Pu-239 is great for bombs but Pu-240 makes it useless for bombs. And with only 1 atomic weight between them, forget about it - ain't gonna happen. So to get Pu-239 and not Pu-240, the exposed fuel must be exposed only for a short time, then removed, then Pu chemically separated from U. Which is easy enough; the problem is your yield. You're leaving the fuel in the reactor for such a short time that you have to process a staggering quantity of fuel to get a little bit of Pu239. But this chemical separation scales much easier than the Oak Ridge path.

Neither of these giga-projects is going to happen given that you can't even talk about the science without being blacklisted.

... But nuclear power is easy.

All you need is ordinary mined Uranium (0.7% fissile U-235). This is obtained via normal mining techniques; nothing special to it. This is probably already happening, since uranium is a rather useful metal for things like aircraft ballasts and anti-tank bullets. So the only thing that changes is that somebody assembles a big pile of it, inside a pressure vessel, on purpose.

In fact, two reactor types are already sized to be able to accept natural uranium fuel loads: CANDU I and RBMK. And they're operating right now in the two large arctic countries. BWR and PWR/VVER can't, but that's only a sizing issue.

This works perfectly well with natural uranium. In fact, the abundant U-238 will capture neutrons and turn into Pu-239, 240 and 241. All of them are useful reactor fuel. Further, where there's uranium, there's thorium; Thorium-232 can do the same trick; it picks up neutrons and becomes fissile U-233. The reactor can be tuned to make more fuel via neutron capture than it consumes in U-235, this is called breeding. So the concepts of natural uranium reactors + breeding fuel can take civil nuclear reactors rather far indeed, without delving too deeply into the impolitic "Jewish Physics" and the brobdingnagian problems of refining bomb fuel.

And the world is satiated with this approach to nuclear power.

Even if the bomb is discovered in theory, it is taboo

If the possibility of a bomb is discovered, the antisemitism bleeds over to the very notion of developing and using such a bomb.

Governments are quick to sign pacts that such weapons will never be explored, just as they have already agreed regarding chemical and biological weapons. Given the Manhattan Project difficulty of doing so and the large footprint that would make on both land and economy, this becomes easy to enforce.

Perhaps, somewhere, a civil reactor has already blown up and contaminated a large region, and so they know how horrible that is. (mind you, bombs are orders of magnitude less toxic than reactor accidents, downtown Hiroshima is a bustling city... but people couldn't possibly know that at this point).

Answer 7

They are made not to think of it

One of your supers a telepath who makes Chuckles X look like a cold reading fraud. The Mind of Minerva is literally in the head of every person on Earth, whether she wants to be or not. For the most part, she does nothing but observe and learn. She doesn't interfere, partly because she feels it isn't her place (since no-one can keep her out) and partly because being in contact with everyone has skewed her morals to the extreme neutral. But when it comes to nukes, she make an exception. Whenever someone start to think of weaponizing fission or fusion, she nudges them in a different direction. Distracts them, or makes them think of and accept a reason nuclear weapons would never work.

Nukes will make things worse

Your supers are nuclear powered. They absorb various types of radiation and grow stronger in the process. Heck, one of the ways to treat injured heroes is to bring them into the core of an active reactor.

So nuclear weapons are not a good idea. You drop a nuke on your enemies, you might kills a lot of the normals. But what you are left with is supers who are ten times more powerful (approximately) and are pissed and have little left to lose.

Answer 8

I'm afraid that I disagree with the majority here and am impenitent: it is entirely plausible to have a steady-state nuclear reactor generating a useful amount of energy, without developing the technology to produce nuclear weapons.

Example 1: https://en.wikipedia.org/wiki/Chicago_Pile-1 (ran for about 4.5 minutes, was shut down in good order).

Example 2: https://en.wikipedia.org/wiki/Oklo_Mine (ran for in excess of 100,000 years, shut down when fuel was depleted).

A nuclear reactor can be made comparatively easily and does not require advanced control equipment, provided that the fuel and moderator are chosen appropriately. The popular view of such things tends to be skewed by the unfortunate fact that dominant reactor technologies (pressurised or boiling water) are derived from military practice where the overwhelming requirement is compactness, these are inherently unstable hence dangerous due to being operated near the phase transition between water and steam.

With the exception of "dirty bombs" (conventional explosive scattering radioactive material) a nuclear weapon can not be made easily, since it relies either on isotopes of high (and carefully-monitored) purity, or on a precisely fabricated layout of explosive lenses and a robust tamper, or in most cases both. For just one example of the difficulties, see https://en.wikipedia.org/wiki/Thin_Man_(nuclear_bomb)

To move from the Chicago pile to the "Trinity" test took two and a half years of the most aggressive R&D known to mankind, and producing the enriched or synthesised fuel for the early weapons took industrial plants covering thousands of acres and staffed by tens of thousands of men and women.

It would have been far easier to have perfected reactors for power generation, as has been done by a number of non-aggressive countries such as Canada. And having a source of energy "too cheap to meter" might have curbed the voracity of the Superpowers hence avoided the Cold War and "proxy wars" on every continent (with the arguable exception of Antarctica).

Answer 9

It's quite plausible.

There were a number of false leads in various nuclear-weapons projects during World War II. For example, the Germans calculated that the minimum critical mass for a uranium bomb was high enough that a bomb could only be delivered by train, not by airplane. The Americans calculated that a gun-type plutonium bomb was feasible, and spent considerable effort pursuing the non-viable "Thin Man" design.

These false leads don't cause problems for reactor design, though. A train-sized atom bomb isn't reasonable, but a factory-sized reactor is. A bomb that pre-detonates every time you try to set it off doesn't work, but a reactor that starts up faster than expected just needs different control settings.

If World War II ends early (say, in late 1942), there's no driving force to sort out these issues with nuclear weapons: conventional forces and superheroes work just fine at winning wars, while nuclear weapons design is expensive and error-prone. But there's plenty of demand for the electricity from nuclear reactors, so research on those continues.

Answer 10

If potential nuclear weapons are seen as a "silver bullet" against superheroes, then yes.

In a world ruled by super powered individuals, technology and military development in particular would go differently. Why build tanks and battleships if they can easily be sabotaged or even destroyed by infiltrating or just flying straight at it enemy agents. Sure, a country can have its own "supers" protecting its military assets, but this makes those assets a vulnerable liability.

In this world, any weapon capable of stopping and killing supers, would be extremely valuable. Accordingly, supers (from all sides!) would be happy to see such weapon never existed. Thus, there might be a good chance that Manhattan Project may end up being sabotaged by America's own superheroes (and I'm not even talking about supervillains).

There is, of course, an opposite chance of some evil genius developing nuclear weapon on his own, but let's just say this is just a chance.

Answer 11

It would require an arms treaty

Today, we have the ability to create all kinds of horribly devastating chemical weapons and biological weapons. We have the ability to create a weaponized viruses that are far, far more deadly than our current pandemic.

Why aren't countries threatening to destroy each other with these kinds of weapons of mass destruction? Because of binding international treaties/laws/conventions.

This never happened with nukes (for various reasons). But if it had, we likely wouldn't have had the cold war.

To be clear, the knowledge of how to make nuclear weapons would be known, and maybe a few would even have been created in secret, but... they would be mostly useless, because no country could use them or even threaten to use them without facing serious consequences from all other countries; possibly including facing all of the world's countries in war, solo.

Answer 12

Too risky to Build...

I would argue that, based on superhero scenarios, governments might be afraid to build nuclear weapons specifically BECAUSE of superheroes. Not that they would complain, but that they would be stolen by villains and used. A proliferation of powers means there are too many ways to get a hold of a bomb and too few ways to keep them safe. Invisible/mind controlling/shapeshifting Soviet spies could walk in and steal them. Terrorist superheroes could do the same. Why build the means for supervillains to kill you?

Further, I remember Schoolhouse Rock from when I was a kid, with a superhero cranking a generator to run a power grid. With all those amazing powers drawing from (extradimensional?) sources of power, major technologies might come to be dominated by well-paid supers manipulating reality. With alternate physics powering supers, perhaps names like Einstein would get buried in physics textbooks, and no one even cares about dangerous, radioactive nuclear power. Governments and physicists might even deliberately bury the idea. Many physicists were deeply conflicted about the development of nuclear weapons, and they could see it as an unnecessary and deadly idea in a world where the next super-powered dictator could use them to cement his rule over the city-state he founds. New York City with Dr. Doom and a neutron bomb, anyone?

Maybe a side-effect of the supers is a hero that suppresses radioactive fusion and fission. Once someone figures out how it works, governments build the suppressors so nuclear weapons don't work. Who builds big, expensive bombs that can be shut down by a simple coil of adamantium radiating super-neutronic stabilizer energy?

Finally, there are some wild physics that need to be possible in order to have superheroes. Even if the supers aren't able to make a nuclear bomb, that doesn't mean a better weapon isn't available based on what powers heroes. A dark-force generator that cloaks regions in perpetual darkness would slowly choke out an area with no broken windows. I have a set of Marvel Comics books detailing every super power of every Marvel hero before 1990, and it's a HUGE list. Any one of these taken to a logical extreme could make a weapon equal to a nuclear weapon.

Answer 13

When does a dangerous physics experiment stop and nuclear bomb development begin?

"What happens when we spit to atom?" Was the question on some scientists minds in the late 30s. There was interest, and interest leads to experimentation.

Because we were looking for a weapon, we found one. Of course, a way of turning a square km of desert into glass will obviously have potential for a weapon, but so do many dangerous and exciting things done in physics labs.

We learnt much about high energy physics and such from early nuclear testing. They were clearly weapons because wed used them as weapons previously, but had we not dropped 2 on Japan, these would be dangerous physics experiments done in remote locations, not "nuclear weapon testing".

In this new reality, we've basically got a CERN project that's setting off nukes underground / remote islands in the 50s. It's dangerous physics that needs to be done far away from people, not bomb creation.

In an alternative past, we may of done more research in nuclear physics before getting returns in the form of military bombs or commercial fission reactors. If theres an army of superheroes keeping the peace, it's just in the real of plausible that we could've made it to Thorium reactors rather than uranium and plutonium. The fuel is much more abundant, it's harder to melt down, and it's much harder to weaponized.

Answer 14

Nuclear reactors WERE invented first. The Bomb was spearheaded by wartime necessity, if not for that pesky war; commercial power would have been doable. The first nuclear reactor was the "Chicago Pile-1" in 1942 the x-10 reactor later. The experimental breeder reactor-I was the first reactor to produce electricity in 1951.

Answer 15

I'd say that by the very way nuclear power works, you always pass by nuclear weapons before you reach nuclear power. Nuclear power IS a nuclear bomb, just slowed down so it takes months to release the energy instead of nano-seconds.

Answer 16

Have a look at https://en.wikipedia.org/wiki/Nuclear_weapon_design . Modern "implosion" type weapons require extremely precisely timed explosive charges arranged around the core such that the shock wave converges all at once and squeezes the core to the point of fission.

We have modern computers, so the technology for precise timing is there - to prevent this from working our only other choice is to interfere with the development of explosives such that fast-detonating plastic explosives were never invented to the required level to make nuclear implosion type weapons feasible. Perhaps there is little call to develop these explosives when your super soldiers are capable of punching holes through a bunker door barehanded (or just ripping it out of the surrounding concrete) - you would not have much use for breaching charges in that case, and given the economic burden of the war you would not want to waste money developing a redundant technology.

Perhaps the explosives would be developed later after the war by the civilian sector (e.g. for mining or demolitions), but if a chernobyl-like event had occurred by the time they became feasible for implosion weapons then getting approval for nuclear weapons testing might prove politically not worth the hassle, at least until the next war.

They could still build a "gun" type nuclear weapon, but given the safety issues involved (if the "bullet" becomes loose in the tube - e.g. by the plane carrying it having to make evasive manoeuvres to avoid AA fire - and contacted the target you could have a sub-critical accident) and the fact that they have super soldiers available meant that line of technology was never developed to a usable state, or it was but was deemed impractical for wartime use.

Answer 17

Yes, and it's easy.

Many nuclear power plants operate on controlled, sustained fission of uranium or plutonium.

But that's not the only way to get power out of nuclear reactions. Radioactive materials decay and produce heat, and you can always turn a heat gradient into power (example that is commonly used: https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator).

You could also use uncontrolled reactions. There may be some slight downsides, but meltdowns produce a huge burst of heat and heat equals power. I have a hard time imagining specifically how you could do that safely, but it's not hard to imagine an alternate Manhattan Project going that direction instead. In fact, if Groves was hit by a bus in 1938, the project likely would have stayed in Chicago, and likely would not have produced any functional weapons. Instead, it might well have produced better-controlled piles, which could easily be the basis of very large scale generation.

You could also imagine more exotic solutions, roughly analogous to solar panels for radiation. They likely need to be built from handwavium, but there's no obvious reason something analogous to solar cells couldn't have been developed and refined since the '40s (except that fission already exists and is efficient).

Answer 18

Delayed Neutrons

If you're willing to modify the laws of physics, or some elements and isotopes specifically, this is an option.

Criticality

The difference between Bombs and Fast reactors on the one hand, and regular 'Slow' reactors on the other, is whether criticality (the number of neutrons being produced, on average, by one neutron, reaches one) is achieved by prompt neutrons (the neutrons being released directly by the fission event) alone, or if you need delayed neutrons (neutrons released by the decay of fission products) also.

Slow Reactors

When it comes to regular 'slow' uranium reactors, on the order of 1% of the neutrons is delayed. If you tune your reactor such that each neutron produces slightly less than 1 prompt neutron, but slightly more than 1 prompt+delayed neutrons (that is, you're prompt-subcritical, but delayed-supercritical), your reaction rate will slowly increase, on a human timescale. A large part of reactor design is about ensuring your prompt neutron economy approaches one, but never exceeds it.

Bombs

If you want to build a bomb, human timescales are not what you want. You want the reaction to be over and done with before you can blink. That means you cannot use delayed neutrons and have to rely on prompt neutrons alone and need prompt-supercriticality.

Your physics modification

You change the properties of all the fissile isotopes (or you invent a new element/isotope and keep your planet depleted of our regular fissiles) as follows:

• When bombarded with neutrons, the element will fission, but release less than 1 neutron
• The fission products will release enough further neutrons on a timescale of milliseconds to minutes to make a chain-reaction viable

Bombs may still be theorized (if only these properties were a little different, we could have this reaction happen so much quicker!), and people may invent all sorts of very clever ideas to work around this and a workable solution may even eventually be found, but it becomes much easier to build a reactor and much harder to build a bomb.

Possible societal effects

Building a reactor becomes quite a bit easier because you don't have to worry about prompt criticality, but this may also result in quite lax standards in the many reactors all across the world. After you finally have the equivalent of Chernobyl (that was a steam explosion mostly due to poor control of the 'slow' reaction, which is in no way ruled out in this world), the very idea of nuclear weapons may become very taboo. (you want to create a worse Chernobyl, ON PURPOSE?)