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In Pacific Rim their plan is to throw a 1.2 megatons nuclear bomb into the breach.

Although this is reasonably sizeable for a nuke (Hiroshima was only 12.5 kilotons, about a thousand times less powerful) I would have thought that they would use something like the 58-Megaton Tsar-bomba.

Is there something I did not consider, or is there a mistake in the movie?

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arc_lupus
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    I presume the 1.2 ton figure is the actual mass of the bomb. The 12.5 kiloton figure for Little Boy, the Hiroshima bomb, refers to its yield, equivalent to that mass of TNT; the bomb itself massed 4,400 kg. According to phantom42's answer, the 1.2 ton bomb had a yield of 1.2 megatons, nearly 100 times as powerful as Little Boy. (Disclaimer: I haven't seen the movie.) – Keith Thompson Apr 04 '14 at 19:57
  • In the german version, they tell us that the bomb weighs 1.2 tons which is equivalent to 1.2 tons of tnt (and that is the reason for my question). – arc_lupus Apr 04 '14 at 20:02
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    Then it's probably just a mistranslation. It makes no sense for a thermonuclear bomb weighing 1.2 tons to have a yield just 1.2 tons of TNT. You might as well just use 1.2 tons of TNT. – Keith Thompson Apr 04 '14 at 20:10
  • @KeithThompson - yes, he's got the calculation way out. The bomb in Pacific Rim is 1.2 megatons, not 1.2 kilotons. He's out by a factor of a thousand. – Valorum Apr 04 '14 at 20:24
  • @Richard: Thanks, I think it was a translation error... – arc_lupus Apr 04 '14 at 20:25
  • @arc_lupus - No worries. It's very easily done. I've answered the question of why they don't just build a bloody huge bomb in my answer. – Valorum Apr 04 '14 at 20:26
  • @Richard: The question originally said the bomb was 1.2 tons, not 1.2 kilotons. Since the OP was referring to a source that actually said 1.2 tons, I suggest your edit is inappropriate; it removes the information that was being asked about. – Keith Thompson Apr 04 '14 at 20:28
  • @KeithThompson - I'll let OP decide but the error seemed obvious and easy to fix. I've even tidied up the language error that the revision caused and added a nice vid of a bloody huge bomb – Valorum Apr 04 '14 at 20:30
  • @Richard: Fixed – arc_lupus Apr 04 '14 at 20:35
  • @KeithThompson directly from the novel: "We'll strap a thermonuclear warhead on Strike's back. Twenty-four hundred pounds, with a detonation yield of 1.2 million tons of TNT." – phantom42 Apr 04 '14 at 21:46
  • Aren’t you a little short for a stormtrooper? – Paul D. Waite Jul 31 '14 at 09:34
  • The Tsar Bomba weighed ~27 metric tons (roughly 30 tons U.S.), according to http://en.wikipedia.org/wiki/Tsar_Bomba – Joe L. Sep 01 '14 at 13:47
  • @JoeL. yah, which makes delivery rather problematic. They had to specially modify a bomber to deliver it, cutting out a large section of its belly. And even then it was a tight squeeze, and the bomber had serious performance problems, barely made it out in time. The rocket planned to deliver the production version was the N1, quite a bit larger than the Saturn V that put the Apollo missions on the moon. – jwenting Sep 02 '14 at 07:43

3 Answers3

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They couldn't build a bigger bomb because they didn't have the material.

In an earlier version of the script we learn that they'd tried (repeatedly) blowing up the Kaiju and the rift with nuclear bombs and had since depleted the world's supply of weaponisable Uranium.

LETHBRIDGE : It doesn't matter. There can't be a bomb. We're past peak uranium...

JIAO : He's right. The mines have all but dried up. We have no single source that could provide enough material.

[Raleigh speaks up from the back. In an earlier version of the script we learn that they'd tried (repeatedly) blowing up the Kaiju and the rift with nuclear bombs and had since depleted the world's supply of weaponisable Uranium.]

RALEIGH : Yes we do. The Mark-1's. That's what we did with the last of the uranium. We used it to power the first Jaegers. Like Gipsy Danger..

On top of that, the film transcript makes it quite clear that the aim is to "collapse" the rift rather than simply blowing the enemy into smithereens. Their modelling may well indicate an optimum yield size rather than a mentality of simply hitting it with "the biggest bomb ever built" in the hope that brute force will work.

"And this is what we call "The Throat," the passage between the Breach and us. We know that it's atomic in nature.

I predict that the increased traffic will force the Breach to stabilize and remain open long enough to get the device through and collapse its structure."

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Valorum
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    I think it was a good idea to cut those lines from the script. World uranium reserves are estimated at 16 million metric tons. If all of this was converted into nuclear weapons and detonated, I suspect radioactive fallout would make the Earth uninhabitable long before the Kaiju killed us off. – Royal Canadian Bandit Oct 29 '14 at 09:39
  • This is rather silly anyway. Modern nukes don't use uranium, and in any case thermonuclear devices require relatively small amounts of fissile material. – Fhnuzoag Feb 20 '15 at 14:07
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    @Fhnuzoag - Don't shoot the messenger. – Valorum Feb 20 '15 at 14:44
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    "We know that it's atomic in nature." Ugh, this pseudo-science. The only thing this really says is that it's not quark-gluon plasma, which I didn't assume anyway. – Fabian Röling Nov 15 '17 at 11:00
  • @Fabian Well, it rules out energy, right? Or am I missing something? – Feathercrown Nov 15 '17 at 13:35
  • I guess? But I don't know where this starts becoming the claim that it's just not the scifi-weirdness of "this object consists of energy". There's too little information on this portal or whatever it is to say for sure. – Fabian Röling Nov 15 '17 at 13:43
  • This answer says that it's a fold in spacetime (a wormhole). So it makes even less sense to call it "atomic". – Fabian Röling Nov 15 '17 at 14:46
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    Quick comment on that: this is a huge goof because Uranium is ubiquitous. Especially granite can be used to extract uranium. Sure, it is not economically profitable with today's market values of uranium, but in an emergency like the one we are talking about here... – MichaelK Nov 22 '17 at 08:34
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    @MichaelKarnerfors - Hush now. Your well-argued points and sensible grasp of science have no place on this question about giant robots fighting giant monsters. – Valorum Nov 22 '17 at 08:56
  • Oh... so sorry @Valorum. *looks down in shame and kicks some dirt* – MichaelK Nov 22 '17 at 08:59
  • @MichaelK Developing new extraction technologies to deal with "peak" resources sounds like a great way to get more material in say 3-6 months, but not a great way to get material tomorrow. I assume they were fairly pressed for time. – Cadence May 16 '18 at 22:28
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Yes. There are general issues with the stated size of the bomb.

Scientific American even wrote up an article about it.

Early in Pacific Rim the plan to stop the kaiju invasion is revealed: Bomb the “breach” and destroy the inter-dimensional portal. To do this, the jaeger Striker Eureka is outfitted with a 2,400-pound thermonuclear device—equivalent to 1.2 million tons of TNT. That sounds impressive, but it is quite small in the annals of blowing up huge bombs. The largest nuclear device ever detonated—Tsar Bomba—was 50 times larger. Even so, the jaeger bomb isn’t anything to mess with. The energy it carried was about how much a large hurricane expends in any one second.

Size isn’t everything; depth matters too. Nuclear explosions have very different behavior depending if they are near the surface or deep in the sea. In fact, if you place a nuclear bomb deep enough, there will hardly be anything happening at the surface after it explodes. Maybe an upward rush of water, but no mushroom cloud. Conversely, if the bomb is only a few hundred feet under the water the resulting plume is absolutely incredible. So to find out what we’re dealing with in Pacific Rim we have to estimate the depth.

They go into the math, science and logistics and estimate that...

putting everything we could glean from Pacific Rim together, the jaeger bomb would create a bubble as wide as the Hindenburg—about 245 meters.

But the article is really about the plausibility of what is shown, with the Jaegers surviving the blasts and such.

The real question, however, is how much of a bomb was needed to destroy the Breach. Since the breach was really an unknown, it would have made more sense to go with as big of a bomb as possible.

However, since the Jaeger program was officially canceled months earlier, it is likely that they simply did not have access or funding for anything larger.

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phantom42
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    And in the end they ended up using Gipsy Danger's reactor. I would think that this had even less power, since it's not a weapon and thus not designed to deliver maximum energy in practically a single spike. – Sebastian_H Apr 04 '14 at 19:32
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    In the german translation they even say that they are using a 1.2 ton-bomb, and not (as you stated) a 1.2 Mton-bomb (which would be a much better version than the 1.2 ton-bomb... – arc_lupus Apr 04 '14 at 19:38
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    I didn't state the size. I quoted Scientific American. I'll check my copy and novelization this weekend, though. – phantom42 Apr 04 '14 at 19:43
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    Confirmed: the novelization says "We'll strap a thermonuclear warhead on Strike's back. Twenty-four hundred pounds, with a detonation yield of 1.2 million tons of TNT." – phantom42 Apr 04 '14 at 21:41
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    @arc_lupus: That's probably a direct translation of the 2400 weird weight units named pounds. – Joey Oct 29 '14 at 14:45
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1.2 megatons is a perfectly good sized bomb. There are still 30 kiloton yield nuclear weapons in the US arsenal. Anything in the megaton range is huge.

Note: Even if the world exausted its fissionable uranium, it's still fully possible to produce fissionable plutonium from the typical abundant Uranium-238

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    This seems to be more of a comment on the answers above. Can you flesh it out a bit into a proper answer? – Valorum Feb 20 '15 at 10:23