37
$\begingroup$

This seems like a pretty simple question, but I can't seem to come up with a satisfactory answer. When a nuclear bomb is detonated a large fireball forms. What is the fuel that drives this fireball? Or is it not a fire in the traditional sense (i.e. requiring fuel, oxygen, and a spark)?

$\endgroup$
2
  • 2
    $\begingroup$ It seems like a better comparison is not to combustion*/fire, but to the *flame you see with a fire. There are other Q&A here that address what a flame is, like physics.stackexchange.com/questions/23469/is-fire-plasma (even though the title asks about fire). $\endgroup$ Aug 7, 2023 at 3:18
  • 1
    $\begingroup$ Friendly reminder: please use comments to clarify and improve the question. To post a brief answer, post an answer. $\endgroup$
    – rob
    Aug 7, 2023 at 12:05

5 Answers 5

47
$\begingroup$

A fireball marks the radius at which the plasma - ionised atoms and free electrons from the air, the ground (if detonated near the ground) the bomb casing and the nuclear explosive - become transparent to visible radiation. It is a "fireball" because visible radiation is produced by hot plasma (a few thousand Kelvin and upwards) via a number of processes involving electrons interacting with ions or recombining with ions. The visible radiation escapes to us from the outer part of the fireball - a bit like the photosphere of the Sun.

The plasma is made hot by material absorbing energy in the form of radiation (predominantly gamma and x-rays and the kinetic energy of reaction products) released in the initial fission or fusion explosion and then kept hot from absorbing its own radiation thereafter. Ultimately, the energy arises from the potential energy of the strong nuclear force that binds neutrons and protons together, which is millions of times greater than the atomic chemical potential energy associated with normal "burning".

Unlike the solar photosphere the fireball from an explosion evolves rapidly - expanding and cooling as it does so - because there is nothing like the gravity of the Sun to constrain the hot plasma.

$\endgroup$
6
  • $\begingroup$ amazon.com/… disagrees, saying the 100,000’s kelvin fireball is shielded by the opaque heated to luminescence air at the shock front. It also describes the first milliseconds as X ray diffusion, propagating much faster then any shockwave. $\endgroup$
    – JEB
    Aug 5, 2023 at 21:49
  • 13
    $\begingroup$ @JEB how is that disagreement? Who's talking about milliseconds? A visible fireball is at 1000s of K - as I say in my answer, precisely because of opacity. $\endgroup$
    – ProfRob
    Aug 5, 2023 at 21:55
  • $\begingroup$ @JEB That's actually the second effect that leads to a fireball. Both, the radiative and the shock wave fireballs exist, and they swap places after a while. As an observer, you only ever see the outer fireball which is radiative as ProfRob describes at first, but then turns into the shock front fireball as the shock wave emerges from the surface of the radiative fireball. I've added this as a separate answer, because it's indeed missing from the other answers. $\endgroup$ Aug 6, 2023 at 11:05
  • 6
    $\begingroup$ Note that this makes the fireball something that only shows up for an atmospheric detonation. Set off a bomb in space, and the only material available is the bomb itself, for a "fireball" that's a brief flash a few tens of meters across. $\endgroup$
    – Mark
    Aug 7, 2023 at 1:31
  • $\begingroup$ My answer to the 'directionality' question of the nuclear explosions says ~4000 K as the temperature of the reactions, but seeing how it was nearly a decade ago that I wrote it, I have no recollection of the source of that information. $\endgroup$
    – Kyle Kanos
    Aug 7, 2023 at 14:00
21
$\begingroup$

The fireball is not itself burning in the sense of combustion, but is instead the superheated remnants of the atmosphere, ground, water etc. near the point of detonation, which has been turned into a plasma by the enormous temperatures generated by the bomb.

Wikipedia describes a nuclear mushroom cloud as follows:

Initially, the fireball contains a highly ionized plasma consisting only of atoms of the weapon, its fission products, and atmospheric gases of adjacent air. As the plasma cools, the atoms react, forming fine droplets and then solid particles of oxides.

$\endgroup$
13
$\begingroup$

Like ProfRob says, the surface of the fireball is where the cold translucent air turns into hot radiating plasma. However, there are two distinct ways in which the fireball is formed:

  1. Air is heated by hard radiation (gamma rays and particles). At first from the bomb material, but later also from the growing fireball.

  2. Air is heated by compression when the shock front reaches it.

At first, the radiative heat transport is faster. However, as the surface temperature of the fireball drops, the surrounding air takes more and more time to reach fireball temperatures. That is why this radiative fireball is eventually overtaken by the shock front which is itself strong enough to form a fireball. Obviously, the shock front gets weaker the further it travels, and at some point it stops being strong enough to turn the air into a radiating plasma. At that point the fireball stops growing quickly while the shock front continues to move outwards to do its destructive work.

$\endgroup$
3
  • 1
    $\begingroup$ +1 for the important mention of the growth as a shock wave. However, do you have some citation for these two fireballs? The sources I can quickly find, albeit old, talk about radiative growth gradually transitioning into shock expansion. E.g., Brode (1968) doi.org/10.1146/annurev.ns.18.120168.001101 or Brode (1964) Fireball Phenomenology rand.org/content/dam/rand/pubs/papers/2006/P3026.pdf It should also be noted that after some time the shock wave separates from the fireball and the fireball will cease to expand while the shock wave travels to very large distances. $\endgroup$ Aug 7, 2023 at 9:27
  • $\begingroup$ @VladimirFГероямслава My source is a German book by Wilhelm Schraml, also quite old (1969). But good science does not get old, does it? $\endgroup$ Aug 7, 2023 at 11:54
  • 1
    $\begingroup$ @VladimirFГероямслава I've also added two sentences concerning the detachment between shock front and fireball. Thanks for pointing out that this was still missing from my answer. $\endgroup$ Aug 7, 2023 at 12:00
9
$\begingroup$

The main channel of energy transport from the nuclear reaction to the surrounding environment is x-ray radiation. This is intense enough to ionize any matter in the vicinity. The fireball is the resulting plasma.

$\endgroup$
2
$\begingroup$

When you heat material up enough it becomes Plasma. Plasma is quite opaque to light, because it has a large number of free electrons - and free electrons interact with a broad spectrum of light. Plasma is also hot.

Hot things emit radiation as black bodies.

So when you dump enough energy into a bunch of matter, you get a ball of plasma. This ball of plasma acts a bit like an ideal gas -- PV=nRT -- the high temperature causes the pressure to skyrocket, which in turn causes the volume to expand and the temperature to drop. The plasma also radiates light - above the ground, into the transparent air. This transparent air heats up, which can in turn turn it into plasma.

Higher energy particles -- be they photons or not -- also penetrate the plasma, and collide with the air. This is the initial source of energy to heat matter up to a plasma, and at least for some period will outpace the plasma radiation itself.

So you have 3 "waves" of energy:

  1. The direct high-energy products of the explosion (photons, neutrons, alpha and beta particles, high-energy nucleons produced by fission). You could easily describe this as more than 1 wave, as some of it move far closer to c than other parts.

  2. The hot plasma itself radiating black body radiation.

  3. The hot plasma expanding and colliding with the surrounding material.

None of this is "fire". Fire that you know of is rapid oxidation -- it is a chemical reaction. Fire can make things hot enough that they become a partial plasma and hot gas (this is what flames usually are), so that is why the ball of plasma looks like a "fire"ball.

The glow of the black body radiation from the plasma is what you are seeing. And the edge of the fireball is the point at which the air (and other material) hasn't heated up enough to turn to plasma, so is mostly transparent to the light. Once it heats up enough it in turn glows and you can no longer see the plasma behind it it -- the "fireball" grows.

The surface of the sun is also a plasma. It glows like a nuclear bomb does. Unlike a nuclear bomb, most of it is being held down by gravity (lots of particles escape, but little compared to what remains). The sun's surface is kept hot by heat radiating from deep inside the sun, where nuclear fusion is providing the energy to keep the sun glowing.

But, if you where to take a chunk of stellar surface matter and put it on the surface of the Earth, it would behave a lot like a nuclear bomb. Without gravity holding the plasma down, the plasma expands, and the black body radiation of the plasma also blasts in front of it.

You'd lack the "type 1" energy deposit in my list above, but that is just what gets the fireball started.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.