# How does nuclear fusion work?

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1. Nov 5, 2012

### Edgewood11

I need a detailed explanation as to how nuclear fusion works. How it produces energy and why? Be very specific as I have a general understanding.

2. Nov 5, 2012

### Staff: Mentor

You'll probably get better answers if you start with wikipedia and other web sources, try to build on the general understanding that you already have, come back here with a specific question when you hit something that you don't understand and need help with.

3. Nov 5, 2012

### FeynmanIsCool

(Basically)
In a Fusion Reactor (if we are using Tritium and Deuterium) energy comes from heat this heat comes from rogue neutrons that are expelled by the fusion process.
When we fuse Tritium (Hydrogen 3) and Deuterium (Hydrogen 2) we get Helium 4 (two protons/two neutrons).
In a reactor (in this case a Tokamak) magnetic confinement is used to keep the 150M degree plasma away from the reactor walls. The neutrons expelled by the fusion of DT releases a neutron with no influences from the magnetic field. These neutrons will pass through the walls creating heat. This heat is used to create steam power.
If you want to know WHY how D and T fuse, you may want to read up on wiki:
http://en.wikipedia.org/wiki/Nuclear_fusion

Last edited: Nov 5, 2012
4. Nov 5, 2012

### FeynmanIsCool

In very simplistic terms:
When two nuclei get very close together, the nuclear forces between them will grow more powerful that the electrostatic forces that repel them (+and+). If the two nuclei can get close enough, they will fuse. The problem is getting them close enough....which is where the 150 million C comes in.

5. Nov 7, 2012

### Edgewood11

Why does nuclear fusion produce energy?

I understand that strong forces overcome electrostatic forces and a neutron is released. But why does this happen? Where does e=mc2 come? What matter is converted to energy?

6. Nov 8, 2012

### K^2

Re: Why does nuclear fusion produce energy?

"Matter" is mostly just energy of interaction fields in particles. Only about 10% of the mass of the proton is in the valence quarks. The other 90% are in various particles that pop in and out from vacuum fluctuation and interaction fields. The fraction of "matter" that gets converted to energy is from these interactions. Effectively, two protons and two neutrons that make up a helium nucleus are lighter than each particle isolated by itself.

7. Nov 8, 2012

### Staff: Mentor

Re: Why does nuclear fusion produce energy?

About 1%, or 10 MeV out of ~940 MeV.

That is true for special fusion processes only.

Quantum mechanics.

Where does the universe with its physics come from?

8. Nov 8, 2012

### Edgewood11

How is mass lost in fusion and how is this mass turned to energy?

I need a very detailed explanation as to how mass is lost in deuterium/tritium fusion and how energy is produced (e=mc2).

9. Nov 8, 2012

### Staff: Mentor

Re: How is mass lost in fusion and how is this mass turned to energy?

Is this for schoolwork?

10. Nov 8, 2012

### K^2

Re: Why does nuclear fusion produce energy?

You are thinking of current mass vs dynamic mass. But even if we look at dynamic mass, only about 10% is due to the valence quarks. The other 90% is dynamic mass of the sea.

11. Nov 8, 2012

### Edgewood11

Why isn't helium's binding energy less than deuterium and tritium combined?

Helium= 28 MeV
Deuterium= 2 MeV
Tritium= 8 MeV

I thought fusion created energy because helium's binding energy is less than d and t?

12. Nov 8, 2012

### Staff: Mentor

Re: Why isn't helium's binding energy less than deuterium and tritium combined?

When binding energies are given with a positive sign, they're describing the energy that must be added to tear the nucleus apart, and therefore that is released when the nucleus is assembled. So your numbers are saying that if we were to break a deuterium and tritium nucleus down into two protons and three neutrons, then reassemble them into one He-4 nucleus and a free neutron, we'd have to put 10 MeV in but would get 28 MeV out.

The sign is just a convention, so sometimes you'll see the binding energy given as negative number. For example, -28 MeV for the He-4 nucleus is saying that the He-4 nucleus has 28 MeV less energy than the two free neutrons and two free protons from which it formed; to make the books balance this 28 MeV must have been released when the nucleus formed.

It all comes out the same way as long as you're consistent about which convention you use.