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[Iron Man] ELI5, how Stark's chest reactor kept him alive

Where was it placed exactly? How did it keep out shrapnel? What was the nature of the shrapnel in him? What happens if no reactor was there?
gpt
Tony Stark's chest reactor does two things: powers the Iron Man suit, and replaces an existing car battery which had been used by Yinsen in order to power an implanted electromagnet. Stark has been pierced by shrapnel from a Stark Industries rocket, which was slowly moving into his heart muscle. The reactor is implanted in a metal housing which is mounted through the sternum, directly above a plate to which the electromagnet is attached. 

The nature of the fragments is unknown, but due to the fact that Stark did not develop further symptoms from the shrapnel (he developed heavy metal poisoning from the reactor, instead) it can be presumed that the fragments were not of a toxic metal such as depleted uranium. 

The fragments were burrowing into his heart muscle by the natural muscular contractions, the sharp leading edges incrementally moving towards disrupting the organ's natural function, and thus killing Stark. The way the arc reactor kept him alive was fairly simple: the electromagnet which the reactor powers offsets the motion of the shrapnel, preventing them from progressing further and thus killing him.

While Stark has been fully active since his escape from the Ten Rings and the initial creation of the arc reactor, it was necessary for preserving his life until 2013, when the shrapnel fragments were removed in a groundbreaking surgical procedure. In 2008, the arc reactor was removed by Obadiah Stane, which caused Tony's heart to fail due to the 'rebound' effect of the absence of the magnetic field, causing the fragments of metal from his earlier injury to burrow deeper into his heart and impair it from beating. Without prompt restoration of the magnetic field, Stark would have died within minutes.