Is it true that both work on the same principle of operation: the interactive fission chain reaction 235U/239Pu and the bombardment of uranium-235 by fast neutrons produce a fission chain reaction accompanied by an enormous thermal energy in addition to beta and gamma radiation?

The most probable fission products are known to be 94Sr amu and 140Xe amu plus two fast neutrons. If both reactions are a chain reaction and almost instantaneous, then why not?

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    $\begingroup$ Why Chernobyl Exploded - The Real Physics Behind The Reactor $\endgroup$
    – mmesser314
    May 26 at 14:29
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    $\begingroup$ A vague heuristic explanation: it's the same reason that if your hedge trimmer breaks down it doesn't turn into a chainsaw. That is: sure, they both burn petrol, but a chainsaw is carefully designed to direct that into sawing, and it can't really happen by accident. $\endgroup$ May 27 at 15:52

6 Answers 6


A nuclear reactor cannot explode like a nuclear weapon.

For a thermal reactor -like Chernobyl or Three Mile Island- the neutron generation lifetime is too long.

For a fast reactor (and a thermal reactor) there is no mechanism for creating and maintaining a super prompt critical assembly sufficiently long for significant release of energy from fission.

You have to really work hard to assemble the correct material to create a nuclear weapon; you need to create a system that is super prompt critical using fast neutrons and remains so sufficiently long for the chain reaction to produce enough energy before pressure causes dis-assembly into a non-critical configuration. By super prompt critical is meant super critical on the prompt neutrons alone without having to wait for the delayed neutrons to contribute. See the Los Alamos Primer by Serber, available from Amazon: the early notes on the physics of a fission weapon from Los Alamos at the beginning of the Manhattan project.

The explosions at Chernobyl were a steam explosion, and a chemical explosion caused by oxygen reacting with aerosolized graphite. (At Three Mile Island the explosion was from oxygen reacting with hydrogen released from oxidation of the over-heated zircalloy fuel cladding.)

For a nuclear reactor, the safety concern is the removal of decay heat from the radioactive decay of fission products after the fission process is terminated. Decay heat is about 5% of full power at shutdown.

A major problem with the Chernobyl design is the core was over-moderated. Specifically, the graphite was the moderator and the cooling water was not needed as a moderator and in fact acted as a neutron poison. So when the cooling water was lost the reactor power increased: positive feedback. The old N reactor in the US was a graphite moderated, water cooled reactor used to produce Pu for the weapons program, but was specifically designed to not have this problem.

Also, Chernobyl did not have a robust reactor containment system, it had a reactor confinement system.

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    $\begingroup$ Fuel composition plays into this as well, for a modern (Read Pu based) bomb you need Pu that is reasonably isotopically pure Pu239, too much Pu240 is a problem. This is where the term 'weapons grade' as opp. 'reactor grade' for plutonium comes from. Most reactor fuel had deliberate Pu240 contamination to make it unusable in a bomb. $\endgroup$
    – Dan Mills
    May 27 at 12:12
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    $\begingroup$ @Dan Mills Yes, good comment. The production reactors (e.g., at Savannah River) used a short fuel cycle time (3 months as I remember for SR) to prevent build up of too much Pu 240, and Pu-238. There is debate about using fast reactor fuel for a weapon; see Willrich and Taylor Nuclear Theft: Risks and Safeguards Taylor said it can be done (page 55). $\endgroup$
    – John Darby
    May 27 at 13:29
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    $\begingroup$ For a more detailed explanation of what "prompt" means in this context, see @Harper's answer. Delayed neutron generation was a concept I wasn't previously familiar with, but made perfect sense once pointed out. It was the the lightbulb moment for understanding what this answer was even saying. Critical to it, even. $\endgroup$ May 28 at 8:03
  • $\begingroup$ @ Peter Cordes Thanks for the reference. Glad this helped. $\endgroup$
    – John Darby
    May 29 at 23:16

In brief, the lack of a containment structure surrounding any nuclear fission chain reaction will lead to the core (nuclear material) either breaking up or quickly decaying sufficiently that the chain reaction is unsustainable.

Nuclear bombs use a strong container and (usually) a lot of high explosive to force the core to stay dense until the chain reaction reaches extremely high fission rates. At that point the explosion still blows the core apart but the total energy released is "big" .

If I recall correctly, the Hiroshima and Nagasaki bombs' cores only fissioned about 5% of the nuclear matter.

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    $\begingroup$ Do you have a citation for nuclear bombs being dependent on physically strong containment? $\endgroup$
    – ikrase
    May 28 at 23:13

There are three major issues at play.

Fast neutrons vs. thermal neutrons

When fission occurs, between 2 and 3 neutrons are emitted. These are fast neutrons, that is they have a very high neutron energy, or kinetic energy. In a reactor, they are slowed down by a moderator to become a "thermal neutron".

In a reactor, the name of the game is sustainable fission - that is, each fission causes 1.0 additional fissions. (or maybe 1.02 if you're trying to increase power.) At 1.0 this is called "criticality". Exceeding 1.0 is "supercritical" (above critical). But reactors aren't designed to exceed criticality by a whole lot, why give it the ability to something it should never, never do?

For the fuel typically used in a reactor, fast neutrons perform poorly. They are much less likely to cause another fission than a thermal neutron. Even a small reduction in performance breaks the ability to maintain criticality. If fast neutrons were allowed to run amok in a reactor, it would just shut down.

And moderation takes time. That will be important.

Prompt criticality vs delayed criticality

Look closely at the periodic table. Notice that as atoms get bigger, they have proportionately more neutrons per proton.

  • Oxygen-16 has 8 neutrons 8 protons, ratio 1.0.
  • Iron-56 has 30 neutrons 26 protons, ratio 1.15.
  • Iodine-127 has 74 neutrons and 53 protons, ratio 1.39.
  • Tungsten-184 has 110 neutrons and 74 protons, ratio 1.48.
  • Uranium-235 has 143 neutrons and 92 protons, ratio 1.55.

When Uranium-235 fissions, it expels 0-3 neutrons and splits into two new atoms with (between them) 140-143 neutrons and 92 protons. That's still a ratio of 1.52-1.55 -- much too high a ratio for the smaller atoms they become.

For instance a common fission product is Iodine-135. 82 neutrons 53 protons (ratio 1.54, as expected) and that's way off for iodine. 8 neutrons too many!

This will cause the new atoms to "decay" via various decay modes. One decay mode is "neutron emission" - plain and simple they "throw" an excess neutron or two. The time it takes is called the "half life". (here we are back at "time" again).

These delayed neutrons are part of the neutron economy of the reactor, and the reactor is designed to rely on them. This slows down the reactivity of the reactor to within human reaction times. For a reactor to sustain criticality entirely on "prompt (immediately generated) neutrons" would be bad, and make the reactor extremely uncontrollable.

I mentioned Iodine-135. That decays (via beta; a neutron becomes a proton) into Xenon-135, which loves to "eat" neutrons. (turning into harmless Xenon-136). Normally this is just accounted for in the reactor's "neutron economy", but if you make a power reduction and hold it for 8-16 hours, the I-135 previously made by the full-power operation will become Xe-135 while the reactor is at reduced power. This disproportionate amount of Xe-135 will "poison the reactor" in what's called a "Xenon pit" or "Iodine pit".

Xenon decays on its own after 24 hours or so (into harmful Cs-135). But if you can't wait, some reactors include extra control rods that can be pulled out to increase reactivity enough to get critical and use neutrons to burn up the Xe-135. Obviously, this is risky. It's a great deal more risky if your reactor has a bizarre control rod design that increases reactivity when control rods start to be inserted, and then, you insert all of them at once. (And that is how an RBMK reactor explodes).

Holding the package together, though.

Prompt criticality is easy enough - just push enough fissile material together and it will just happen. It will generate an exponentially increasing amount of power until... that energy pushes the material apart, and you no longer have a critical mass. This brings an end to the criticality.

And that is what happened at Chornobyl - the power excursion caused a steam explosion that blew the reactor to pieces, making it unable to be critical anymore.

That's the rub with making a bomb - you want the bomb to do more than just crack the critical mass in two and quit, like it does in criticality accidents. OK, so you build the bomb to be highly prompt supercritical, so all the fuel fissions in a microsecond (before the mass can physically disassemble itself). The time unit they use is called the "shake". (of a lamb's tail).

And even if you figure out all that, there's another problem. To get to "highly prompt supercritical", you must pass through "barely critical". And you're moving "at the speed of stuff", so it takes a whole lot of shakes. And probably a burger and fries. If fission starts before full assembly, the nuclear energy developed will oppose the assembly energy and win, causing Rapid Unscheduled Disassembly before assembly can finish. So nuclear yield will be extremely poor.

enter image description here

A fizzle of something intended to be a bomb. It's hard to do better by accident.

All that to say, making a bomb work is super, super hard even when you're really trying. Not something that is going to happen by accident, especially not on a machine that is engineered to be unable to do that.

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    $\begingroup$ It might be worth referencing the Kiwi-TNT test, where the reactor of a nuclear thermal rocket was deliberately blown up to test a beyond-worst-case scenario. The control elements normally rotated at 45 degrees/second, which meant the reactor would have just melted apart while they were opening. They were modified to rotate at 4000 degrees/second to get the reactor power high enough for an explosion before it damaged itself too badly. The explosion was compared to 90-140 kg of black powder...not just small, but slower and less violent than TNT. $\endgroup$ May 28 at 23:26

To expand upon John Darby's excellent answer with regards to Chernobyl specifically, where actually, to answer the original question, there is a school of thought that it might have done!

John writes:

The explosions at Chernobyl were a steam explosion, and a chemical explosion caused by oxygen reacting with aerosolized graphite.

It is my understanding that there is some degree of scientific debate still ongoing about this, which is relevant to your original question. I am going to put forward another hypothesis on the origin of the first explosion mentioned here. It is not mine – and nor am I wedded to it – and there is a chance that it has subsequently been disproved. That being said…

These two explosions were witnessed by several eyewitnesses -- both at the plant and fishermen elsewhere. These explosions were both large, and the second, and largest, occurred a couple of seconds after the first. The first one is widely believed to have been a steam or vapor explosion where the energy in the hot cooling water together with the energy generated across the reactor core pressurized the steam pretty much instantaneously and to such a degree that it catastrophically failed while under load. Some authors estimate that the power of the RBMK reactor went up a factor at least 100 over its design value of 3.2 GW (thermal) for a few seconds. The next explosion is often described as a hydrogen explosion, where water present is hydrolysed by the extreme temperatures and pressures present by the zirconium fuel rod cladding, producing hydrogen and oxygen (which then subsequently exploded, and set the graphite moderator on fire).

However, some authors have posited a "nuclear jet" hypothesis that corresponds to this initial explosion being a prompt criticality event proper, and the second explosion being a chemical one. The main evidence for this hypothesis, first posited in 2009, comes from the geographical location of two radioisotopes of xenon, $^{133}$Xe and $^{133m}$Xe, which were experimentally detected at Cherepovets (a Russian city 370 km north of Moscow and 1000 km north-northeast of Chernobyl) about four days after the explosion. These xenon isotopes have a half life of 5.24 days and 2.19 days respectively; in "ordinary" reactor fission their ratio can be estimated through Monte-Carlo simulations, in which the fission process is modeled computationally. It's worth noting that these types of simulations are very well regarded; in my professional work (as a medical physicist) there are two packages that are largely used, MCNP4 (which requires an export license from the US government, claims to have a million man-hours of programming behind it, and, I believe, handles fission excellently) and Geant4 (which, being written by CERN and released to the world as open-source software, doesn't).

The three authors of this paper, from the Swedish Defence Research Agency, Meteorological and Hydrological Institute and Stockholm University, have done this accurately for the RMBK reactor type used in Chernobyl and have estimated a branching ratio for these two isotopes under differing power levels and thus what might be expected to be present within the reactor. The authors furthermore claim that the code used to produce this estimate is experimentally validated. Below are the expected branching ratios for thermal neutron fission in the reactor:

Decay chain leading to isotopes of Xe-133

The authors then have the ability to try different scenarios in the distribution of the reactor core, and look at the production of $^{133}$Xe (or $^{133m}$Xe). One of the scenarios they examined is that one reactor rod had a prompt criticality incident and produced a nuclear explosion with an energy release equivalent to 75 tons of TNT. The reactors is destroyed in this process, and the ratios of the two isotopes then decay back towards equilibrium:

Xe-133 production over time.

Now, the interesting thing about this is that the ratio experimentally detected in Cherepovets happens to line up almost exactly with this scenario, and it is claimed that it is not possible to easily explain it any other way, given the known time delay between explosion and detection, and the unambiguous determination of the isotopes (via gamma-ray spectroscopy):

Branching ratios

The authors of this work also claim that another significant piece of experimental evidence comes from the curious fact that Cherepovets is far to the north/northeast of Chernobyl, yet the majority of the fallout and dispersed isotopes was observed went to the west and northwest, causing (famously) "radioactive sheep" in Wales, amongst other issues en route over Scandinavia. We have got a lot better at weather forecasting since the late 1980s, and again the authors of this work use historical observational data and modern models to simulate what may have occurred. They claim that a these simulations show, as a function height, that a significant fraction of the large amount of liberated radioactive fission products at low altitudes would head northwest toward Scandinavia but, at higher altitudes made a sharp turn back east around the Gulfs of Riga and Finland. This would permit them to explain the geographical distribution of $^{133}$Xe/$^{133m}$Xe that matches with the measurement made in Cherepovets, by having a jet of nuclear material reaching about a 3 km altitude:

Distribution of Xe-133

Finally, the authors make the (to me, far less proved) claims that this hypothesis explains the observations made by eye of the two explosions:

[...] a witness that was out fishing on the cooling pond some 500 m away from Block 4 when the accident happened. He heard a large clap followed by an explosion. Then, in a couple of seconds he saw a bright blue flash that was followed by an enormous explosion. It is well known that criticality accidents emit a blue flash, or rather glow, which derives from fluorescence of excited oxygen and nitrogen atoms in the air [and perhaps Cerenkov radiation].


[After the second explosion ] with the fuel fully exposed, the air was irradiated, and the typical blue glow was lit. An employee of the power plant, Alexander Yuvchenko, has described how he and a colleague “ran out of the building and saw half of the building gone and the reactor emitting a blue glow of ionized air.” But, the flash observed by the fishing man was a bright blue flash before he heard the second explosion [and before reactor products were directly exposed to the air].

The authors make the argument that this blue flash has to be nuclear, unlike a blue glow, which may be thermal:

[As a steam] explosion would not create a flash, an explanation could be that the surface of the jet peeled off some hot material to the air and/or that the jet with a temperature of several tens of thousand degrees heated a column of air around its track. Within a few seconds that hot material would cool down through the temperature interval around 7000°K, where for a short time before it cooled down further, it would radiate blue light by blackbody radiation—a blue flash, not a glow

So, there you have it. One (admittedly small) potential for an (admittedly small) nuclear explosion in the worst nuclear disaster in human history. Maybe.


To get the exponential nuclear chain reaction to occur such as that in a nuclear bomb, you have to reach critical mass in order to have the chain reaction occur. However, the "explosion" part wants to disperse the mass which reduces the mass below critical and ends the chain reaction.

So in order for an atomic weapon to work it needs to either initiate the chain reaction fast enough so the mass can't disperse in time or it needs to keep the mass together long enough for enough of the chain reaction to occur, and neither happens at random.

Otherwise the self-sustaining reaction just gets really hot and stays that way which is what happens in a meltdown.


A fission warhead uses implosion compression equally on all sides to achieve criticality. Whereas a reactor fissions in a controlled continuation. A bomb fissions virtually everything simultaneously. An analogy is comparing a car's engine running to a fuel tank explosion.

So no. You'd have to squish all the fuel into a ball... surround it with explosives and have it all detonate simultaneously.

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    $\begingroup$ You may wish to lookup gun-type fission warheads. They are inefficient, but exist. $\endgroup$
    – TLW
    May 28 at 22:52

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