Nuclear Fusion

So many of our books couldn't get over the idea that Fusion! is the Energy! of the Future!  I found their reasoning pretty simplistic and I'm not so convinced (nor am I sure the massive expenditures will pay off), but, regardless, it sure is interesting to learn about! I found what I think is the best activity EVER to demonstrate fusion. It's described here, but basically, when you apply heat and pressure to mini-marshmallows, they will "fuse" together. It's like magic! We loved it.

It's also helpful to show a chart of electromagnetic energy when you talk about the energy fusion releases; this is from here

We had our periodic tables handy, and we would choose two elements and get marshmallows to represent the number of protons in each one. For the simplest example, you take two atoms of hydrogen (one proton each; so you take one marshmallow in each hand)---add heat and pressure (roll them together between your palms very fast)---and they fuse to form a new element, Helium (2 protons)! At the same time they release lots of energy (represented by the lentils).

This happens at very high temperatures, in a plasma in fact, so the electrons are free and we don't even have to worry about them. :) If you really rub your hands together hard, the marshmallows fuse so completely, they look like one marshmallow again! Which is cool, but it's more helpful to see the individual "protons," so we backed off and didn't rub so hard. Still: magic!

I let the children create as many elements through fusion as they wished, as long as they told me which elements they started with, and their atomic numbers, and then figured out which new elements they were creating (also based on atomic number). They had so much fun, and the fused nuclei were truly amazing. They gained a lot of familiarity with the periodic table as well.

As we learned more about fusion we became curious about the transuranic elements: that is, the elements PAST Uranium on the Periodic Table that are man-made. How did they make them? None of our books seemed to mention this; they just said they were man-made and didn't occur naturally.

So, we asked Karl (my brother the physicist). Here's what we learned.

(our questions in black; Karl's answers in blue):

Now we have another question: how do they create the transuranic elements, and why is it not fusion when they do so? We read that they can be made in particle accelerators, but we thought that's what what's-her-name--Lise Meitner---was TRYING to do when she discovered fission. And she discovered that bombarding the nuclei DIDN'T add particles, but split the nucleus instead. I know some things are more fissionable (?) than others, like U-235, but they were using U-238 in those original experiments and that still split instead of adding protons.

Fission happens because very few configurations of protons/neutrons are stable.  When you send two nuclei together, they will most likely create an unstable nucleus which will then split into two more-stable elements, not necessarily the two that collided in the first place.  You might get lucky and find that one of the resulting elements is more massive than either target.  

And, even if they do have a way of making the protons "stick" and add themselves to the nucleus, why doesn't that create tons of energy like fusion? And if they can hardly even make Hydrogens fuse, how on earth could they make heavier elements fuse---which seems like it would be harder? 

To get excess energy from fusion, you need to have the mass of the combined nucleus be less that the mass of the separate nuclei by more than the amount of energy it takes to shove them together.  Larger nuclei are harder to shove together, and the mass difference is smaller, so no excess energy.  Yes, they are very hard to get to fuse, it takes huge accelerators.  Think of trying to push the N poles of two magnets together vs. trying to push 100 magnets together. 

The problem with H fusion (Deuterium, actually), is not getting them to fuse (they do it all the time).  The problem is getting it to be self-sustaining and not energy-consuming.  I just read an article by a high school science teacher claiming he had calculated that even D fusion could never be a net energy producer.  I was skeptical given that thousands of scientists clearly think it can be.  (But you might say they have a conflict of interest because they want funding.)  Hmmm. 

I bet it will take 100 years of a fusion generator to make back all the energy spent on fusion so far. 

Karl.

Interesting, eh?