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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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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 4 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 4 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 4 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. –  Richard Apr 4 at 20:24
    
@Richard: Thanks, I think it was a translation error... –  arc_lupus Apr 4 at 20:25

2 Answers 2

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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.

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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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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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 4 at 19:32
    
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 4 at 19:38
    
I didn't state the size. I quoted Scientific American. I'll check my copy and novelization this weekend, though. –  phantom42 Apr 4 at 19:43
    
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 4 at 21:41

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