Nuclear Waste + the Sun


by SHISHKABOB
Tags: nuclear, waste
chasw
chasw is offline
#37
Feb4-13, 08:30 AM
P: 4
Stars are fusion engines, converting hydrogen into helium into lithium, etc. and releasing the resulting energy. I read in "The Grand Design" that the instant fusion progresses to the point where manganese is converted into iron, a supernova is triggered.

Would introducing heavy elements (as in nuclear waste) thereby trigger the sun's destruction? Probably not, since the main fusion engine is at the core of the sun, not at its surface. I'm certain nickel-iron meteors impact the surface of the sun all the time, without any measurable effect, therefore injection of our nuclear waste would likewise be safe.

Unfortunately, the energy cost of lifting the waste out of Earth's gravity well is prohibitive at this point in time. Its cheaper and more practical to dilute it, encase it in protective shielding and bury it underground somewhere in Nevada. - CW
D H
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#38
Feb4-13, 09:32 AM
Mentor
P: 14,468
Quote Quote by chasw View Post
Stars are fusion engines, converting hydrogen into helium into lithium, etc. and releasing the resulting energy. I read in "The Grand Design" that the instant fusion progresses to the point where manganese is converted into iron, a supernova is triggered.
That's not quite right. More below.

Would introducing heavy elements (as in nuclear waste) thereby trigger the sun's destruction?
No.


The very presence of iron (or heavier elements) in the core of a star is not what triggers a supernova. Except perhaps for some red dwarfs that were formed amongst the very first generation of stars, every star we now see has some heavier elements in its core. What triggers a type II supernova is when a large star has converted all of the lighter elements in the core to silicon. First look at what happens in a red dwarf. Fusion in a red dwarf converts hydrogen into helium. Because red dwarfs are fully convective, the helium that is produced disperses throughout the star. A red dwarf is able to burn a large portion of its initial hydrogen into helium before dying. The temperature never gets high enough to go beyond hydrogen burning in a red dwarf.

That's not what happens in a very large star. The waste products stay in the core. Once all the hydrogen has been converted into helium, the core collapses and increases in temperature until pressure and temperature become high enough so that helium burning can commence. This stops the collapse for about a million years until all of the helium has been consumed. The core collapses again until pressure and temperature become high enough to enable the carbon burning process. Each stage of the collapse is temporarily held at bay when pressure and temperature build up to the point where some other form of fusion can take place.

This burnout / collapse / restart cycle ends when the core becomes completely iron and nickel. (Note: It's *not* when the star starts producing nickel and iron. That's about three days before the end.) The problem with iron and nickel is that they can't fuse into some more massive element and yield energy. Fusion of iron and nickel are endothermic rather than exothermic processes. There's no new energy source to halt the collapse once a star's core becomes pure iron/nickel.

As far as our Sun is concerned, it's not massive enough to get past the helium burning stage, and that won't happen for another five billion years. The additional presence of a tiny, tiny bit of heavier elements won't do anything. Our Sun already contains far more heavy elements thanks to its third generation origins than we could ever hope to add. For example, the Sun contains almost 500 Earth masses worth of iron.
chasw
chasw is offline
#39
Feb4-13, 01:23 PM
P: 4
DH: Thanks for the explanation, very interesting. - CW


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