Q value - fusion vs. fission

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
User avatar
Richard Hull
Moderator
Posts: 11611
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Q value - fusion vs. fission

Post by Richard Hull » Fri Aug 31, 2012 6:24 pm

In a recent fusor.net opening page news posting, an argument was to be made at a conference about fusion versus fission to the effect of....."you get more energy out of a fusion reaction when particles join (fuse) than when they split (fission)"

This is wrong by any standard.

The Q value for our D-D fusion is, on average, (4.03 + 3.2)/2 = 3.6 mev per fusion
Q values for the famous and most studied and hoped for reaction of D-T is 17.6 mev.
These are the only two practical base line fusions being studied.
However, if fusion ever gets going in either of the above then the D-Li6 reaction is possible and puts out a whopping 22.4 mev! But then that's the result of two fusions isn't it?

A single fission (splitting of the atom) is capable of nearly 200mev leaving Fusion in the nuclear dust bin.

If one wishes to argue fusion vs. fission, I would hope that Q factors on a per reaction basis would best not be discussed as fission would win by miles on that issue.

This is why I'm a fusion doing, fission-man.

.........Of course, any form of nuclear energy is better than chemical energy... BUT.... Living in the real world all we have is one form of real nuclear energy (fission) and a who gang of useful, installed and ongoing chemical energy systems.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

User avatar
Chris Bradley
Posts: 2931
Joined: Fri May 02, 2008 11:05 am
Real name:

Re: Q value - fusion vs. fission

Post by Chris Bradley » Fri Aug 31, 2012 6:40 pm

Richard Hull wrote:
> "you get more energy out of a fusion reaction when particles join (fuse) than when they split (fission)"
> This is wrong by any standard.

even by specific mass [of reactants]?

I think the general argument/reaction proffered in this regard is to say D+T=17.6MeV (the reaction usually discussed in big-bucket fusion), which is 3.5MeV per nucleon.

User avatar
Richard Hull
Moderator
Posts: 11611
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: Q value - fusion vs. fission

Post by Richard Hull » Fri Aug 31, 2012 6:57 pm

Many things may be inferred, but the statement was rather specific "per reaction". Nothing was mentioned in the blurb about per nucleon efficiency.

Efficiency per unit nucleon involved would not be an issue to a power company as giant cars of fuel are not needed to feed either a 1 mega-watt real, extant, fission plant or, if ever successful, a dreamed about fusion plant. We are not talking fuel tonnage here, nor is nucleonic mass efficiency a real issue. However, I could see how one struggling to make a erudite point and nit-pick one's way into this as being somehow significant, relating to non-extant fusion energy systems. It would be as significant as saying I can do far less nuclear reactions in my fission plant than you can do in your silly ole fusion plant and get the same output! Silly...Silly...Silly.

If we argue, as originally noted, fusion versus fission reaction, then scientifically, the fission reaction is approximately 10 or more times more energetic than a fusion reaction.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

User avatar
Doug Coulter
Posts: 1312
Joined: Sun May 27, 2007 7:18 pm
Real name: Doug Coulter
Location: Floyd, VA, USA
Contact:

Re: Q value - fusion vs. fission

Post by Doug Coulter » Fri Aug 31, 2012 7:16 pm

I'll go with the mass in vs energy out, myself.

With one very *huge* caveat.
All you gotta do to get fission is stack up enough U with enough moderator - no energy in at all.
Zero. So when we talk about the other type of Q we use here - power in vs power out, yes, fission
wins pretty bigtime assuming the energy to get your stuff piled up is not too terribly different than extracting D or T for our fusion setups. (remember, with the right moderator, you need not even enriche your U first).

So, there's Q and there's Q - for our purposes, I like the energy in vs out definition, not the per reaction energy, or even the per nucleon efficiency, since with the fission (say, a CANDU with heavy water), the stuff that didn't react this time just sits there until it finally does, whereas with fusion the way we are trying it, the stuff that didn't react DID eat input energy to make the try...which is lost.

Make sense to anyone?
Why guess when you can know? Measure!

User avatar
Chris Bradley
Posts: 2931
Joined: Fri May 02, 2008 11:05 am
Real name:

Re: Q value - fusion vs. fission

Post by Chris Bradley » Fri Aug 31, 2012 7:28 pm

Right! It's just a sales pitch for fusion.

Just like where the UK government has said in their publicity information that JET has 'broken even' on power, when actually they've put 20MW into the plasma and got 18MW out, for 2 seconds. Close enough they feel a burning need to make the claim, I guess.

Of course, they'll've conveniently forgotten about the 1GJ of energy they have just expended to create the magnetic field.

Incidentally, did you know that when (if?) ITER gets to operate, its magnetic field will have 41GJ of stored energy. Imagine if that all goes wrong in a hurry - that'll be 10 tons of TNT equivalent of magnetic energy looking for somewhere to go.

And not forgetting (or maybe it will) that this stored energy is equivalent to the 500MW output of this device for 80 seconds - or more likely around 250 seconds after energy is converted to primary electrical power. So it'd needs to operate for over 250 second at 500MW just to power its own magnetic field, let alone the power its needs to drive the plasma during that time.

Yet I am sure that we'll hear some claim of 'proven break even' power very soon in its operation.

So these sound-bites of pseudo-information are just a sales pitch to the masses. But let 'em have their $30 billion to build ITER. It's all engineering and science (and is a triviality of money compared with the amount of money the governments have stuffed into the hands of the bankers recently). One day, from all of this we might even collectively learn enough to understand what the real useful next steps will be ....

User avatar
Jim Kovalchick
Posts: 344
Joined: Thu Apr 14, 2011 12:00 am
Real name:

Re: Q value - fusion vs. fission

Post by Jim Kovalchick » Fri Aug 31, 2012 7:29 pm

Well said Richard.

Not to confuse the issue, but fire itself involves individual reactions that only amount to an an eV or so. A chemical reaction like that is still useful as an energy source because the quantity of the reactions and immediacy of the reaction transfer to thermal energy. The comparision between fission and fusion involve similar, but very complex dynamics. The point of useful collection of the energy and potential impact on subsequent fusion and fissions are part of that complexity.

Dan Tibbets
Posts: 578
Joined: Thu Apr 17, 2008 5:29 am
Real name:

Re: Q value - fusion vs. fission

Post by Dan Tibbets » Sat Sep 01, 2012 8:35 pm

It depends on how you look at it. A mole of uranium fission is way ahead of a mole of hydrogen fusion.

But a mole of uranium weighs ~ 135 grams.
A mole of D-T fusion fuel weighs ~ 5 grams.
This where weight is the dominate consideration for a deliverable warhead, fusion wins.
135 g/ mole / 5 g/ mole (sum of tritium and deuterium which combine)= 27
27 *yield per unit weight brings the hydrogen fusion up to ~ 460 MeV in the weight of fusion fuel equal to the weight of the fission fuel.
Then you have to consider the burn up efficiency. In a good fission bomb this may be ~ 50-80%. Fusion fuel burn up in a hydrogen bomb may approach 100 % (?). Another gain for the fusion Q per unit weight (or mass)

In a fission nuclear plant, how much of the uranium 235 is burned up? 20%- 80% ?
I have heard that the decay products add ~ 10 %more heat to the original fission reaction, so this consideration is modest, and more of a problem for managing the reactor rather than useful heat generation.

I speculate that because of the tremendous challenges with controlled fusion reactor confinement issues, a high burn up of the fuel will be necessary, perhaps several fold higher than the burn up in fission reactors. So a fusion reactor, by necessity will need to use the fuel more efficiently, and this will push the final net Q higher towards (though not matching) fission reactors .

And, of course the question is not which system can generate more useful energy per mole or unit of weight, but which is more sustainable (amount of fuel available) and which is less problematic when environmental issues are included. Fission is limited to a few hundred years or perhaps a few 10s of thousand of years if thorium can be used. There is enough deuterium , boron, lithium etc. to last billions of years. The bottleneck here may be the availability and cost of lithium. Pure deuterium, or aneutronic fusion systems avoid this issue (no need for generated tritium. In fact with D-D fusion, tritium waste product will need a use. The easiest solution may be to burn it in the same reactor for additional power.

Dan Tibbets

User avatar
Frank Sanns
Site Admin
Posts: 1622
Joined: Fri Jun 14, 2002 6:26 pm
Real name: Frank Sanns
Location: Pittsburgh, PA USA

Re: Q value - fusion vs. fission

Post by Frank Sanns » Sun Sep 02, 2012 2:29 am

Dan,

FYI. A mole of U-235 is 235g. (92 protons and 143 neutrons)

A mole of deuterium is 2g (1 proton and 1 neutron)
A mole of tritium is 3g (1 proton and 2 neutrons)

A 50:50 isotopic mix would be 2.5 g/mole average.

Frank Sanns

User avatar
Frank Sanns
Site Admin
Posts: 1622
Joined: Fri Jun 14, 2002 6:26 pm
Real name: Frank Sanns
Location: Pittsburgh, PA USA

Re: Q value - fusion vs. fission

Post by Frank Sanns » Sun Sep 02, 2012 2:40 am

Have I missed something here or is nuclear binding energy being thrown out of the window? The delta energy for a low Z fusion reaction is much greater than for a heavy Z fission. For fusion, there is more mass change (energy release) than for a heavy fission. It is the same result reaction for reaction, mole for mole, or mass for mass.

I am lazy tonight and do not feel like going through the calculations. Here is a hyperphyics photo with some hard numbers.

Frank Sanns
Attachments
fifun.gif
fifun.gif (18.37 KiB) Viewed 4939 times

Dan Tibbets
Posts: 578
Joined: Thu Apr 17, 2008 5:29 am
Real name:

Re: Q value - fusion vs. fission

Post by Dan Tibbets » Sun Sep 02, 2012 4:42 pm

Oops... A rather glaring error . Mass comparison would be 235 g / 5 g intead of 135g / 5g. The fusion yield per gram of fuel would be even greater by that ratio, so the greater yield of fusion would be ~6 times as great.

As for using 5 g for the fusion fuel. This is the sum of the mass for one deuterium atom and one tritium atom that is consumed in this fusion reaction which yields ~ 17 MeV.

For the energy yield for light element fusion vs- heavy element fission, looking at the nuclear binding energy per nucleon graphs the energy differences is apparent. Just measure the change in the X- axis (some adjustment is needed when doing fusion as there are two starting reactants, and in fission where there are two products).

Keep in mind that there is a much smaller change in the chosen fusion reaction A (atomic weight) than in the fission reaction. In this case it would be ~2.5 average A to 4 A (here the average weight between the reactants would apply) just as the average weight between the fission products would apply. * The difference between the fission reactant and products is much greater. The fusion binding energy change per A is greater, but there is a greater A change with Fission. The fusion D-T to He4 travels only a short distance on the curve (Y axis), while the fission travel distance on the curve is much greater. The greater slope on the fusion side before you approach Ni62 too closely, where the slope levels out, more than makes up for the distance traveled on the graph, especially for the Helium 4 product.

http://hyperphysics.phy-astr.gsu.edu/hb ... ucbin.html

These energies are calculated per individual reaction or atoms. This extrapolates to energy per mole or energy per gram. It is all dependent on your point of view. One mole of uranium 235 releases more energy than one mole of D-T, but one gram of D-T releases more energy than one gram of Uranium235. Another way of saying this is that there are much fewer atoms of uranium in one gram compared to hydrogen isotopes and this accounts for the different perspectives.

*Note that we are talking about the nuclear binding energy per nucleon, or the potential energy of the isotope. In this case , while the neutron may be carrying a lot of kinetic energy, it is insignificant when talking about the binding energy because the neutron by itself carries almost no binding energy, or potential energy within this nuclear binding system in isolation (just like the proton or even the beta particle, the alpha particle is a more interesting situation).

Dan Tibbets

Post Reply