what was the feared runaway nuclear reaction from Oppenheimer?

462 viewsOtherPhysics

Can someone explain the feared runaway nuclear reaction that Oppenheimer presented Einstein in the film? The one where detonating a nuke would’ve exploded the whole world?

Wouldn’t that scenario require many orders of magnitude more energy than the output of the what the first (or current) nuclear weapons were capable of?

In: Physics

15 Answers

Anonymous 0 Comments

So just to make one thing clear — in real life, Oppenheimer didn’t go to Einstein for anything, much less on this topic. That was done for dramatic/narrative effect. Einstein was not actually part of the actual work on the bomb in any way.

The actual concern about the “runaway” reaction was that the intense heat of the atomic bomb might cause elements in the Earth’s atmosphere or crust to undergo nuclear fusion. The fear was that these fusion reactions would release more heat, and that this would continue the fusion reaction.

Upon further study, the most “dangerous” reaction — in the sense that it was the most likely to occur out of the all of the elements that might be available in quantity when an atomic bomb was exploding — was a nitrogen-nitrogen fusion reaction. (Air is 78% nitrogen.) They did not know the exact energy output of the atomic bomb, and they did not know the exact “cross-section” of the nitrogen-nitrogen reaction. The “cross-section” is basically the probability that the reaction will happen at a given amount of input energy and density of material.

Even with these unknowns, they made reasonable guesses as to what the answers might be, and then made even more “pessimistic” versions than were reasonable, and found that the amount of energy required to start and sustain the nitrogen-nitrogen reaction in quantity was likely to be several orders of magnitude more than any atomic bomb developed during World War II and probably any nuclear weapon developed ever.

It should be emphasized that they still did not know much about terrestrial nuclear fusion reactions at this stage. They later found out (while trying to invent the hydrogen bomb) that it is even harder to produce a nuclear fusion reaction, even with an atomic bomb as the source of “input” energy, than they thought.

I would also emphasize that there were many “known unknowns” and many “unknown unknowns” involved in deciding that this was not possible. The scientists were aware that they were only just learning about these things. But they concluded that the chance of igniting the atmosphere seemed small-enough to not be a source of worry, even though they did not have the ability to definitively rule it out as a possibility.

Later computer simulations by weapons scientists showed that the only way you could ignite a planet-destroying nuclear fusion reaction on Earth would require greatly increasing the amount of deuterium in the oceans (by a factor of 20X), and then having a bomb that was on the order of 200 teratons of TNT. Which is a very technical way to say, “it’s not actually possible.”

You are viewing 1 out of 15 answers, click here to view all answers.