
#1
Dec1004, 04:55 PM

P: 63

topic pretty much explains it. why is it that fusion releases more energy than fission?




#2
Dec1004, 05:49 PM

Sci Advisor
P: 1,137

Depends on what you mean by "releases more energy"  it may or may not. It's all the same nuclear force  whether the reaction is fission or fusion. You really can't say fusion releases more energy than fission  it depends on what specific reaction you are talking about. What really matters is what the details of the reaction are  and how much mass is turned into energy. Let's look at a couple of nuclear reactions  the fission of U235 [ as done in nuclear reactors ] and DT fusion [ which is what is being worked on for producing power]. First each fission reaction gives you about 200 MeV worth of energy. [ Of that about 10 MeV goes into neutrinos that you can't capture  but lets use 200 MeV for round numbers ] The nucleus that you are fissioning  U235 has a mass of 235 atomic mass units [ 236 if you count the incident neutron ] So fission gives you about 1 MeV / atomic mass unit of fuel. Lets do the same with DT fusion. [ Deuterium  Tritium ] DT fusion gives you 17.6 MeV of energy [ less than a fission reaction ] However, the fuel has a mass of 5 [ 2 for the D and 3 for the T ] So the energy per atomic mass unit of fuel is 17.6 MeV / 5 amu = 3.52 MeV / amu. So fission gives you more energy per reaction  but fusion gives you more energy per unit mass for these reactions. Why the energies [ 200 MeV for fission and 17.6 MeV for fusion ] are what they are is that they are the difference in the masses between the reactants and products. If you take the mass of D, add the mass of T, subtract the mass of He4 and subtract the mass of a neutron, then multiply by the square of the speed of light [ E=mc^2] you will get 17.6 MeV. Why the masses are what they are  that gets complicated. Dr. Gregory Greenman Physicist 



#3
Dec1104, 02:25 PM

P: 297

When you fuse ordinary Hydrogen into Helium, 0.7% of the mass is converted to energy. When Uranium undergoes fission only about 0.1% of the mass is converted. So in mass conversion terms, you might say that fusion is about seven times better than fission.




#4
Dec1304, 09:06 AM

Sci Advisor
P: 1,137

Why is fusion stronger than fission?You can only say that only for a particular reaction  like DT fusion. You can't say that "fusion" always releases more than "fission"  it depends on the reaction. There's nothing inherently 7X more powerful about fusion  they BOTH rely on the SAME force  the strong nuclear force. Dr. Gregory Greenman Physicist 



#5
Dec1304, 05:39 PM

P: 395

So is fusion being studied as a possible energy source because it is "cleaner" than fission even though it produces less energy per reaction? Or is it that less overall fuel is wanted for the same amount of energy?




#6
Dec1304, 06:02 PM

P: 10

Fusion has a much greater release of energy, (fission of u235 is about 10 * 8, fusion of D  T is 10 * 8.4 and Lh2/Lox is 10 * 1 per unit of mass/energy)  (all results are mathematically rounded) Fusion is much greater per unit of mass, fission releases less energy, this is converse to the binding energy of a nucleus. The energy release of fission/fusion is on the lengths of (electromagnetic waves  short at a high frequency, photons, kinetically charged atoms and particles, in the form of gamma and xrays). Fusion releases more energy than fission per unit of mass. 



#7
Dec1304, 06:52 PM

P: 395

I thought that the weak nucear force was something to the order of one billionth the strength of the strong nuclear force. What is the role of strong nuclear force in these processes?




#8
Dec1404, 10:16 AM

Sci Advisor
P: 1,137

WRONG WRONG WRONG When you liberate energy via nuclear fission or fusion  it's the "strong nuclear force" that produces the energy  NOT the "weak" force [ or "electroweak", since there is a unification of the "weak" force with the Coulomb force]. The "weak" force is the one that is responsible for beta decay for example. we know what we are talking about. What is a "per unit mass/energy" The quantity mass divided by energy is not dimensionless  it has UNITS  and your statement above, in the absence of units; is meaningless. of fission /fusion is on the lengths of..." means WHAT??? You are not even dimensionally correct. Go back to your junior high science class and learn that you have to be consistent in your units. Energy release is measured in a unit of energy  MeVs are convenient for nuclear reactions, eVs are convenient for chemical reactions, and Joules are convenient for macroscopic quantities of energy. What is a "kinetically charged atom". You have to learn to separate the concepts of kinetic energy and charge. They are two different properties. Both fusion and fission derive energy the same way  they both rearrange the protons and neutrons in the nuclei of the reactants. One fuses light nuclei together  one splits heavy nuclei. The binding energy needed to hold together the products is less than that needed to hold together the reactants. This differential in binding energy is the energy that is released. Let me reiterate from my previous post. Let's look at fission on U235. The fission of U235 produces about 200 MeV of energy. Since the reactantants have a mass of 236 amu [ 235 from the uranium and 1 from the neutron ] the energy produced per unit mass is E/M = 200 MeV / 236 amu = 0.85 MeV/amu Now let's look at DT fusion. DT fusion releases 17.6 MeV of energy while the mass of the reactants is 5 amu [ 2 from D, 3 from T] Hence, E/M = 17.6 MeV/ 5 amu = 3.52 MeV/amu which is about 4.1X the analogous E/M ratio for fission above. [ So it looks like fusion is more powerful] Now lets look at another fusion reaction: D + D > He3 + n + 3.25 MeV The energy released in this reaction is 3.25 MeV and the mass of the reactants is 4 [ 2 from each of the two Deuterium nuclei]. Proceeding  E/M = 3.25 MeV / 4 amu = 0.81 MeV/amu That's LESS than the E/M ratio for U235 fission!!! Here is a fusion reaction that gives you LESS energy per mass than a fission. Or let's take the following fusion reaction: Li6 + H > He4 + He3 + 4.0 MeV The energy is 4.0 MeV, and the reactant mass is 7 amu. As before; E/M = 4.0 MeV / 7 amu = 0.57 MeV / amu That's a LOT LESS than the U235 fission reaction!!! How much energy you get, and how much energy you get per unit mass doesn't depend on whether you have fission or fusion  there's nothing inherently "more powerful" in fusion than fission. The DT fusion reaction produces more energy per mass than U235 fission while the DD and the Li6H fusion reactions above produces LESS energy per mass than U235 fission. It all depends on what reactions you are talking about!! Why does that seem to be too difficult a concept to understand? All chemical reactions rely on the Coulomb force  including all oxidation [ burning ] reactions. If you ignite some paper or wood  you will get a certain amount of energy per unit mass. If you ignite some HMX highexplosive  you are going to get a LOT more energy per unit mass. Both reactions derive their energy from the same type of reaction  oxidation  and the same Coulomb force. But how much energy you get and how much energy you get per unit mass is dependent on the particular reaction  NOT the TYPE of the reaction. Dr. Gregory Greenman Physicist 



#9
Dec1804, 07:49 PM

P: 10

A UNIT OF MASS. Don't you know what a unit of mass is?
Oh, but i thought you have already completed your doctorate. I refer to a (Kg). A Kilogram is not related to weight (because 9.81 newtons interfere  at sea level) but metric measurable mass. 1 unit of mass = 1kg. Table for energy per kJ/g  (that means Kilojoule/gram) 10*1  LH2/LOX (10 to the power of 1)  corrected/rounded 10*8  U235 (10 to the power of 8)  corrected/rounded 10*8.6  D  T (10 to the power of 8 + 6 to the power of 7) corrected Isp (lbfs/lbm) = 144.22(e{kJ/g} 1/2 Now we are talking about nuclear engineering, and I have never seen a NTR fly so excuse the referal to nuclear weapons as being the closest proven entity. 



#10
Jun2811, 08:20 PM

P: 11

It doesn't seem logical that in a fission reaction uranium235 releases less electron volts per nucleon compared to the fusion reaction TD, when uranium is around 45 times larger, obviously the size of the nucleus is not a factor. Could it be the binding energy of the atom, uranium has a higher binding energy and therefore releases less, where as the hydrogen isotopes have less binding energy, releasing more energy. Im basically asking what determines the amount of mass converted to energy, in a fission or fusion reaction? This is random but according to einstein's law e=mc2 the mass lost determines the amount of energy, and therefore it can be said mass is converted into energy, therefore wouldn't it be logical to say mass is energy, in a different maybe more concentrated form? 



#11
Jun2811, 09:21 PM

PF Gold
P: 11,057

padawan13, this thread is 6 1/2 years old since the last post!
Anyways, the fission of one nucleus of uranium releases more energy than the fusion of two neuclei, however the mass of the fusion nuclei are much much less than Uranium. The amount of energy released in each event is determined by the binding energy of the nuclei. The wikipedia article on Nuclear Fusion has a chart with each amount if you are intersted. Also, E=MC^2 does NOT mean that energy is converted into mass or vice versa. It means that the removal or introduction of energy is accompanied by a decrease or increase in mass. The definition of energy is the capability for a system to do work. It is a description of how two systems interact and is not something physical in itself. Mass is NOT energy. They are two completely different concepts. There are plenty of current threads on these concepts and I recommend that you visit them and not keep posting in this old thread. 



#12
Jun2811, 09:51 PM

P: 152

simply put, the binding energy curve is sharper for hydrogen (fusing) than for Uranium (fissioning)




#13
Jun2911, 08:24 PM

P: 11

haha yeah i know its old, i wasn't really expecting anyone to reply, but thanks.
So you say that mass is not being converted to energy, but a decrease in mass is accompanied by the production of energy. So where exactly does the energy come from? Is it to do with the strong nuclear force, between nucleons? Is it possible to derive the amount of mass lost for specific reactions? An explanation of the why the strong nuclear force exists would be great. 



#14
Jun2911, 09:11 PM

PF Gold
P: 11,057

Imagine it like this. The binding energy of tritium and deuterium is like a ball sitting on a table. The binding energy of helium is like that same ball sitting on the ground. Lifting the ball requires more energy if you have to lift it from the floor. Similarly dropping the ball gives you more energy if you drop it to the floor than you do to the table. Hence why fusing dt together gives you energy, it is like dropping the ball from the table to the ground. When you look at all of the elements it looks kind of like a staircase with two tops and one bottom that forms a V shape. The least massive and most massive elements occupy the top steps. Fusing the lightest ones or splitting the heaviest ones is like dropping a ball from the top step to a lower one. As you do this again and again with the elements, it is like dropping the ball from the top to a lower step, then a step lower than that, and so on until you reach Iron and Nickel. These two elements form the bottom of the V staircase. You cannot get energy out of fusing or splitting them in the same way that you cannot drop the to a lower step. There isn't a lower step! There is no explanation as to "why" any of the fundamental forces exist. They simply do. 



#15
Jun2911, 09:38 PM

P: 11

I understand what your saying, that the greater the difference in binding energy, between the fuels and the product the more energy produced. So, the elements closer to the bottom of the V stair case have smaller differences between their fuel binding energy and their products binding energy, and therefore dont produce any energy. so how do you determine the exact mass lost in a fusion or fission reaction? 



#16
Jun2911, 09:59 PM

PF Gold
P: 11,057

I mean that Helium has more binding energy than either Deuterium or Tritium, not both of them combined. (Although it might, I'm not sure. I know helium has a very high binding energy because it has exactly 2 neutrons and 2 protons)
The mass loss can be calculated, and I think it can be measured as well. 



#17
Jun2911, 11:22 PM

P: 11

if the energy needed to break the atom (nuclear binding energy) increases, then the energy holding the nucleus together (ill just call it the attractive energy) must also increase.
If the attractive energy is increasing how can energy be produced? 



#18
Jun2911, 11:33 PM

PF Gold
P: 11,057

Just like how dropping the ball further increases the amount of energy you can get out of it. Remember the staircase? If you have to pick the ball up from the bottom ALLLL the way back to the top, it takes a LOT of energy to do that. Picking up the ball from step 3 to step 1 requires less energy, but still more than it does to pick the ball up from step 2 to step 1. Dropping the ball from step 1 to step 3 gives you energy, more than if you dropped it from step 1 to step 2. Make sense?
I think you are mixing up Energy and Force. Forces provide attraction and repulsion, they are what hold the nucleus together or cause it to split. The force between the nucleons increases as you go from Hydrogen to Iron thanks to the additional nucleons providing Strong Force. After that, the additional nucleons of heavier elements start to repel each other more and more thanks to the positive charged protons overcoming the short range Strong Force, and at the heavy end of things there are NO stable elements because of this repulsion. The ENERGY required to pull the nucleons apart increases as the force does. The energy isn't holding anything together or doing anything. Energy is a description of how much work a system can accomplish on another system. (In our case the systems are the nucleons and whatever we use to pull them apart or push them together) 


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