# Physical Science Fusion and Fission

Discussion in 'Physical Science' started by Mizar, Nov 7, 2004.

Could it be possible to develop a reactor that would create infinite energy with a small ammount of material by first fusing it then using fission to break it then fusing it again the using fission etc.....

one of my friends said this the other day when we were talking about stars

like we make a fusion reactor for hydrogen fuse it to helium use fission break the helium to hydrogen the fuse the hydrogen again

could this work it seems too go to be true to me and i think there is a mass problem involved.

Edit: I wanted to correct all the spelling errors as this is like 3 years old and I was a bit ignorant... I want to save my current arse

2. ### Derek■֎؜♫■Staff Member

I have not the slightest clue on this, but I think would be one GF may be able to give some insight on since he works in this field. I would be interested to see his comments on this.

Now you're talking my language

Yes and no.

Yes, one could theoretically develop a process that repeatedly fissions and fuses the same material. Generally speaking, any nucleus with more than two protons can be broken into smaller nuclei through fission. Similarly, any two nuclei can be combined to form a larger nucleus through fusion. There are a number of practical issues that limit what we can do, but I won't delve into those now.

No, it wouldn't provide an infinite supply of energy - your suspicion is correct. I think a brief discussion on how we obtain energy from fission and fusion is in order.

Take an atom, let's say 12C (carbon-12 - 6 protons, 6 neutrons, 6 electrons). By definition, this atom has a mass of 12.0000... atomic mass units (amu, or just u) (12C is the benchmark for atomic mass, i.e. 1 amu = 1/12 the mass of a 12C atom). If we were to add up the masses of the component parts of this atom, we should get that same mass (within measurement error), right? Let's see what we get (I'm going to ignore electrons... they're negligible for our purpose):

6 protons * 1.007276 u = 6.043656 u
6 neutrons * 1.008665 u = 6.05199 u

Total = 12.095646 u

Wait a minute, that's almost 1% more than it should be! That difference (0.095646 u) is known as the mass defect. From that we can develop the concept of binding energy - the energy that is required to bind the nucleons together. We can calculate the binding energy using Einstein's famous equation, E=mc^2 (playing around with units, we can get c^2 = 931.5 MeV/u). In the case of 12C, the binding energy is about 89.094249 MeV (MeV is Mega electron Volts, and 1 MeV = 1.60217646e-13 Joules).

Mass defect/binding energy is not linearly related to the number of nucleons in an atom. If it were, extracting energy through fission or fusion would be impossible - the binding energy would just be distributed about the product nuclei. Here's a plot showing the binding energy per nucleon versus the mass number (number of neutrons and protons):

(image courtesy of Wikimedia Commons; used without permission, as none is needed )

That plot makes nuclear energy feasible. The peak occurs around A=60, knows as the iron peak. We gain energy through nuclear fission by splitting a heavy nucleus into smaller nuclei closer to the iron peak. We gain energy through nuclear fusion by combining two light nuclei into one nucleus that is closer to the iron peak. In both cases, the total mass of the products is less than the mass of the reactants.

If we were to split a nucleus lighter than A=60, we would have to put energy into the reaction; same if we were to combine two nuclei heavier than A=60.

As an exercise, let's calculate the energy balance for the proposed 1H + 1H + (2)n -> 4He -> 1H + 1H + (2)n reaction process:

Masses:
n = 1.0086649 u
1H = 1.0078250 u
4He = 4.0026032 u

1H + 1H + 2n:
2 * 1.0078250 + 2 * 1.0086649 = 4.0329798 u

Energy balance:

Fusion reaction: 1H + 1H + (2)n -> 4He + E
E = 4.0329798 - 4.0026032 = 0.0303766 u = 28.296 MeV

Okay, so far so good! We gained energy!

Fission reaction: 4He -> 1H + 1H + (2)n + E
E = 4.0026032 - 4.0329798 = -0.0303766 u

Not so good - we had to put in energy to make this reaction work. Not surprisingly, we had to put in the same amount we got out of the fusion reaction. And since there we can't have a perfectly efficient system, we'll have to add even more energy to make up for the losses.

So there you have it. Congratulations if you made it this far (and understood it) - that was about a week's worth of a 300 level nuclear engineering class. Thanks for bringing this up, Mizar - I'm so focused on the higher-level aspects of nuclear power that it's easy to forget the nitty-gritty that makes it all work.

Just for fun (and for the super-curious out there), I'm going to calculate the energy balance for a typical fission reaction that would occur in a commercial power plant.

235U + n -> 92Kr + 141Ba + (3)n + E

Masses:
235U = 235.0439231 u
n = 1.0086649 u
92Kr = 91.9261528 u
141Ba = 140.9144064

235U + n:
235.0439231 + 1.0086649 = 236.052588 u

92Kr + 141Ba + (3)n:
91.9261528 + 140.9144064 + 3 * 1.0086649 = 235.8665539 u

E = 236.052588 - 235.8665539 = 0.1860341 u = 173.29 MeV

This specific reaction is actually kinda low - we generally use 200 MeV/fission as a rough estimate. Some of this energy goes to the freed neutrons and fission product nuclei as kinetic energy (about 1 MeV each), while the remainder is released as photons. Most of this energy becomes heat in the fuel pellets, cladding, or coolant; this heat can then be turned into mechanical energy in the steam turbines, which can be converted to electrical energy in the generator, which can then be delivered to you to be converted into light or heat. Yay physics!

4. ### Derek■֎؜♫■Staff Member

Great post GF! Very informative, and I actually grasped most of what you posted.

Even though this does not seem like a very efficient way to produce electricity, using the same amount of energy input to create a fusion reaction, is the reason for continued use because the impact on the environment is less than say, burning fossil fuels?

My pleasure, bud. I spent a lot of money gaining that knowledge - best I share the wealth.

See, I always knew I wasn't much of a teacher. I'm not very good at explaining these things, and for that I apologize.

I guess what was confusing was my example where all the energy released had to be put back in. That was in response to Mizar's question regarding a combined fission/fusion process, where the products of a fusion reaction are subsequently used in a fission reaction. That has a net gain of zero, and due to inefficiencies requires energy input. Using nuclei that already have a high mass number (A>60, e.g. uranium) in a fission reaction, or that have a low mass number (A<60, e.g. hydrogen) in a fusion reaction, does result in a net gain of energy - luckily for us, the universe has produced an abundance of both.

As of yet, we haven't been able to develop a fusion reactor that generates more energy than is input. The physics do allow for considerably more energy to be released than is input, however. The ITER project should be the first to achieve this.

All commercial nuclear power plants use nuclear fission for energy. This is certainly able to generate more power than is input, not only in theory but in practice. Using some numbers off the top of my head, a plant that can generate 1000 MW (electric) only needs maybe 50 MW input (mainly for the coolant pumps). This does not include energy required to make the fuel or other capital energy costs, but I can't imagine that those are all that much.

As for the impact on the environment... The greenhouse gas emissions are certainly much less (virtually nil) than fossil fuels, but the implications of digging the uranium out of the ground, and the storage of the spent fuel, is more a political question than a technical one. There is a lot of potential energy left in that "waste" fuel, but the politicians have decided that we can't undertake the steps necessary to utilize it. I'll leave it at that.

6. ### Derek■֎؜♫■Staff Member

Naw, it has nothing to do with you being a bad teacher, it has to do with me being a tad on the short side of understanding processes like what you mentioned! I have to re-read many things more than twice to gain what I perceive as a full understanding.

Thanks for clearing that up for me.

I thank you GoneFission for your response. Although this is an old post, I am glad to have it finally responded to and answered. There is a slight hint of nostalgia in reading that as I no long deluge into scientific matters too often. My interest in astronomy has been peaking again however.

-Mizar

8. ### bodeblissThe Zoc-La of Kromm-BPremium Member

The rhyme of late about alternate fuels is insane.

The world will need a projected double amount of power production by 2050, a quadruple amount of power production by 2100, and a 8 fold power production increase by 2150.

I hope something like fusion comes along soon.

[Edited on 3/26/2007 by bodebliss]

i actually found that quite easily read aswell GF, thanks for that, it's not often someone explains these things so accessibly, usually i find i have to go look up half the words.

i may be pushing it a bit but would you mind explaining a little about the fusion process and why it is so difficult for scientists to master it. it seems on the surface of it to be a simpler process, so i've obviously missed the point.

Yes no doubt. I'm in the oil business and I can say we have enough reserves to last for 20 years, after that I'm seeing that the trendline will begin to fall below demand and probably never recover. One of two things WILL happen.

A) We will develop a new source of energy.

B) We will hit a technologic brick wall, a plateau, peak, stagnation, take your pick. Fuel will become an asset of the elite while lower societal levels will turn to renewables. Solar and wind will become more common in towns and local clusters, co-ops will power very small sections and there will be no mass power grids as there are now.

From the length of those answers I think you can see which way I think we will go.