Was the second - the big - explosion of the reactor core in the Chernobyl catastrophe a true nuclear explosion? There have been those who have said that it was ... but sayings to this effect do not gain much public traction. I think that explanations that invoke hydrogen production through the reaction of steam & zirconium are grossly implausible.

Any answer to this will necessarily import notions as to what constitutes a true nuclear explosion, and I am not stipulating any particuIar definition of one according to which this question 'is to be' answered. It could perhaps be said that the definition is one of the 'degrees of freedom' of it.

So the question really hinges, I think on whether the reactor was there, fatally damaged, and quite grossly overheating, with steam building up to high pressure, with free oxygen & hydrogen present, which at some point ignited explosively; or in a state in which essentially the same 'race' between escalation-of-nuclear-reaction & tendency to dissemble of the very reacting substance of the core was occuring as occurs in a nuclear bomb ... but obviously terminated very much sooner by reason of the very much lesser compaction of the core of a nuclear reactor compared to that of a nuclear bomb. Which latter compaction is in a bomb obviously contrived to be as extreme as is possible to be made with extant engineering resources.

It's quite reasonable also, I think, to consider whether the former of the two scenarios just adduced is 'worthy' of being deemed a true nuclear explosion as opposed to the latter not being. Certainly it is more similar than the latter to what occurs in a nuclear bomb, and is only really less in force than it by reason of the lesser degree of compaction of the reacting substance; whereas in the latter scenarion the nuclear reaction is indeed a copious source of heat, but the explosion is not one of the reacting substances per se.

And there is some concern amongst those who are appointed to search into these matters as to which of these two kinds of event the large explosion of the Chernobyl nuclear reactor was more nearly an instance of. I think the idea that it might have been more of a 'true nuclear explosion' according to the definition I have proposed is possibly one that has been somewhat suppressed ... not necessarily by force - I am not advancing a 'willful cover-up' theory - but more just through habits of public discussion having become settled in place.

So another way of framing the question might be this: I have seen, in the course of my 'trawling' for matter about the Chernobyl incident, matter in which the concern is with this very question; but not a very great deal & quite fragmentary; and I wonder whether anyone has seen anything really substantial about it; or what original thoughts there might be out there about it amongst folks who care about these kinds of thing.

Incidentally, I recall reading recently, in the course of the said 'trawling', that the last reading on the meter showing the heat output of the reactor core was 33GW.

Simple Mathematical Model for Exponential Growth with Two Widely Different Timescales.

This is not meant to be taken as any kind of actual engineering formula, however; it's merely a simple mathematical model for showing how it is thoroughly intrinsic to a positive feedback-system having two different timescales to flip precipitately from one to the other as a certain parameter passes from one range to another. It is expounded, & its relevance examined, more thoroughly, in the mathematics section. It shows just how imperative it is that the proportion excess of reactivity in the core be kept to substantially less than the delayed neutron fraction. It is my understanding that one of the reasons for there not being many plutonium reactors is that the delayed neutron fraction is significantly smaller for plutonium than it is for uranium, making this task much more difficult.

An expression giving the factor - say $1+\delta$ by which the activity of a reactor core increases over the mean duration of a neutron's being free in the core, according to a simple model, would be -

let $\alpha$ be the delayed neutron fraction;

let $1+\omega\alpha$ be the mean number of neutrons yielt by a neutron upon its reacting;

Let $\zeta$ be the ratio of the delay of the neutrons to the mean duration of a neutron's being free in the reactor core;

let $w()$ be $\operatorname{arcf}()$ where $\operatorname{f}(x)=x.e^x$ -

$$\delta = \frac{w(\zeta\alpha.\exp((1-\omega)\zeta\alpha))}{\zeta} - (1-\omega)\alpha .$$

The ratio $\zeta$ is a large number - a few thousand. So when $\omega$ is substantially less that 1, as it ought to be in proper operation, the argument to the lambert w function is large; and the approximation

$$w(z)≈\ln(z) - (1-\frac{1}{\ln(z)})\ln(\ln(z))$$

is valid, & the terms without $\zeta$ in the denominator vanish, leaving only (terms beyond $\ln(z)$ in the expansion of w(z))÷$\zeta$.

However, because the lambert w function tends to linearity as its argument becomes $<<1$, as soon as $\omega$ begins to exceed unity, the term without $\zeta$ in the denominator begins precipitately to preponderate.

  • $\begingroup$ I think your second paragraph sums up why this question is impossible to answer. If you don’t define what a nuclear explosion is for the purpose of answering the question, this is just a debate of semantics which it sounds like you have already found on the Internet. $\endgroup$
    – Secundus
    Commented Nov 18, 2018 at 1:58
  • $\begingroup$ It's framed somewhat differentl now. $\endgroup$ Commented Nov 20, 2018 at 5:06
  • $\begingroup$ Ok, you wonder if it went critical after meltdown? That was one of the worries at that time, if I remember correctly: nuclear reactions first raised temperatures in the engineered reactor and then went wild after that, utilizing all new geometry, viscosity, elements/compounds like water, lead , concrete etc. $\endgroup$
    – MS-SPO
    Commented Apr 16, 2023 at 7:15

2 Answers 2


We dont know. Simply we lack data beyond a certain point just before the first explosion. We therefore have to resort to mathematical modeling for deducing what happened.

Now this mathematical modeling depends heavily on what data we have, and the stability of the reactor was very bad at the moment of the accident. The modeling accuracy decreases from that point forward. Just like a weather forecast, it becomes more unreliable the further we go.

Did a nuclear explosion happen? It is possible. Atleast that possibility is coroborated by a analysis done by Yuri V. Dubasov in a 2009, this was further examined by Lars-Erik De Geer in 2017. While both suggest that probability of nuclear fizzle is there. Their conclusion is a bit different as to wether the first or second event was nuclear though.

See this is still very much a reasearch publication worthy material, and you need serious scientific chops to deal with this adequately. Yes its your conclusion has been suggested, and is somewhat probable. But gist of the matter is we dont know. You can not expect this forum to solve this for you.

  • $\begingroup$ Thanks for that - I feel a bit less in the dark now. Sometimes I almost begin to imagine that I have seen these things ... and it doesn't help with that when I try to find the item again ... & it turns out that I can't! But venturing somewhat despite the 'serious scientific chops' (I like that!) requirement: I have seen much emphasising how very very imperative it is that the reactivity-excess be kept well within the delayed neutron fraction ... (& that little bit of differential equation stuff is a figure for that) ... & at that occurence the rods were jammed right out! $\endgroup$ Commented Nov 20, 2018 at 6:05
  • $\begingroup$ @AmbretteOrrisey They were testing a safety system that was known to not work. So they had disabled the other safety system, only they werent supposed to do that the day shift was supposed to to. Anyway you can read the actual papers the 2009 paper is here and 2017 here $\endgroup$
    – joojaa
    Commented Nov 20, 2018 at 9:44
  • $\begingroup$ oh yes! These are good! I was also thinking in the meantime ... one day now, what with this great arch containing it remote-controlled crab $\endgroup$ Commented Nov 20, 2018 at 14:31
  • $\begingroup$ cranes to dismantle it, there probably will be enough evidence to decide. We mustn't be too impatient though: I read somewhere that the scheduled completion date is 2080! Gi $\endgroup$ Commented Nov 20, 2018 at 14:33
  • $\begingroup$ Throwing in a bit of latitude for delay, that'll make it 100 years to fix that accident - a full century! ¶ By the way, one of the buttons on my virtual keypad acts as a post button - & I seriously wish it wouldn't, coz I keep accidentally hitting it. $\endgroup$ Commented Nov 20, 2018 at 14:36

We can define a nuclear explosion as one caused by prompt criticism calory, which is what is used in nuclear eespone. It's not implausible in the least that the second explosion was a nuclear explosion in this sense.

You see, the reactor had a positive void effect of 5 times the delayed neutron fraction. This meant that all other reactivities being zero, just 20% of the water boiling away was enough to cause a prompt criticality.

At 01:23:43, power began to double every second, exceeding 520MWth. Given that this initial surge was confined to the lower 3 meters of the core, it means that at 01:23:44, the rods had already overheated.

The next development came at 01:23:47. Pressure rise rate suddenly skyrocketed, and water flow rate dropped sharply. By this time the rods were at a staggering four times their design capacity.

A. S. Dyatlov, the deputy chief engineer, whi was in the control room at that time, later stated that this was the moment of a local prompt criticality which comprised the first explosion.¹ ²

At 01:23:48, the pressure rise rate sharply reduced, and flow rates were restored.

At 01:23:49, alarm indicating a burst fuel channel sounded.

The Steinberg Commission report, which is Annex I of the IAEA INSAG-7, noting that the power surge was highly uneven even across the of the width of the reactor, concluded that a localized power surge in the bottom of the core burst a handful of fuel channels, which released steam into the gaps between fuel channels, which lifted the reactor lid, tearing the remaining fuel channels apart.

While this is a steam explosion, and while the Commission does not tie this event to any specific time or witness observation, we may conclude from what I presented earlier that this was the consequence of a prompt criticality.

N. V. Karpan, Dyatlov's successor, read explanatory notes and seismic records, and concluded that there were three explosions - two "lower" ones in rapid succession forming a 3 second rumble, followed by one "upper" one.³ ⁴

So it seems that the first two explosions, collectively referred to as the first explosion proceeded like this:

01:23:47: local prompt criticality caused handful of fuel rods to explode.

01:23:48: handful of fuel channels burst.

01:23:49: steam pressure in voids between fuel channels lifts the reactor lid, destroying all the remaining fuel channels.

Now we move on to the third, bigger explosion, commonly referred to as the second one.

Consider the state of the reactor now. The reactor lid is sitting ajar, and steam is hemorrhaging from it. Remember that just 20% boiloff is enough to trigger prompt criticality.

Witnesses reported a flash/flame at the moment of the second explosion. Thus is in contrast to the first explosion, regarding which I have come across no visual observations at all.5 6

This suggests that the second explosion was much hotter than the first one, suggesting it was a hydrogen or prompt criticality explosion, not a steam explosion.

This is one of my major criticisms of the Lars-Erik de Geer paper mentioned above.

Of these two possibilities, the most 'natural" one would seem to be the prompt criticality. If hydrogen failed to blow up the reactor, a global prompt criticality was inevitable.

This explosion, not the first one, was likely the one which finally threw the reactor lid into the air.

  1. Dyatlov, How It Was: An Operators Perspective, Nuclear Engineering International 1991

  2. Dyatlov, Chernobyl: How It Was chapter 1

  3. Karpan, Analysis of the version 'earthquake is the cause of the Chernobyl accident'

  4. http://accidont.ru/evid02.html

  5. See aforementioned de Geer report

  6. Higginbotham, Midnight at Chernobyl


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