It depends on how you make the comparison. Typically, fission (of U235 or Pu239 for example) releases on the order of 100's of Mev per event, while fusion of light elements releases on the order of 10's of Mev per event. On the other hand, by weight fusion is more energetic. If you are thinking nuclear weapons, then you have to keep in mind there is a fundamental size limit on fission that dosn't apply for fusion. In fission, you must start out with the fuel in a sub critical mass, so making a very large fission only bomb is a challenge.
And are the neutrons from fission in preceding generation, in fission chain reaction (the 2-3 neutrons which are released) accelerated? In that case they won't be able to make another fission in the next generation.
http://en.wikipedia.org/wiki/Image:Binding_energy_curve_-_common_isotopes.svg This image explains it, basically when fusing the change in binding energy per nucleon is larger than in fission.
You have what is called a moderator (no that is really true) is can be graphite, water or a number of materials which slow the neutron, it slows fast neutrons to the thermal range where the barns cross-section is appropriate for capture and subsequenct fission. You also have what are called reflectors materials that surround a reactor that keep neutrons from just escaping the reactor core. http://en.wikipedia.org/wiki/Neutron_moderator http://en.wikipedia.org/wiki/Neutron_reflector
Are you comparing the same elements or different elements? I mean, if I fuse two hydrogen nuclei and a couple of neutrons to make a helium nucleus, then it will release exactly the same amount of energy as it would require to split my helium nucleus into its component particles. To fuse, say, two protons, you first need to get them close enough together for the nuclear force to start acting. And that means you have to overcome their electric repulsion. That's why you need them to be moving fast. For which element? When Uranium fissions, for example, the neutrons produced are generally fast neutrons. If that's what you're asking. There's less chance, sure. That's why nuclear reactors contain moderators to slow the neutrons down.
Completely correct. I can only add that in typical eleastic collision (all these neutron moderation collisions are elastic) the energy of the 17 MeV neutron is equally shared on average if it collides with another nucleus of one atomic mass unit - I.e. with a proton or another neutron. If it collides with heavier nucleus, say a carbon nucleus it will mainly bounce off with little energy transfer to the carbon nucleus, so many more collision will be require to "thermalize it." Knowing this long ago made me wonder why ordinary Hydrogen could not be used, instead of heavy water. The H2 would need to be both higher pressure and a larger volume to do the moderation job and would absorb some neutrons to become deuterium (perhaps too many and kill the reaction?) If that is the problem, why not flow the H2 thru graphite? (It can take high temperatures in H2 atmosphere) I.e. why not have deuterium and perhaps tritium production in fission reactor for D/T fusion reactors some day? The hot high pressure H2 (perhaps some HD or even HT) would seem like a good way to extract the fission energy, even with work being recovered in a H2 driven turbine, followed by waste heat dump to cool more (than the expansion in turbine did) before recompress to working pressure and then back for the neutrons to make the H2 hot again in a closed loop cycle (even the electric generation in the post heat dump cooled H2 flow so no shaft seal problems.) This is an old (and somewhat obvious) idea of mine. What is wrong with it? Does anyone know? It is not done or even talked about, so surely there is something wrong with the idea.
A few things: US reactors are H moderated. (light water actually) As you yourself guessed, H captures more neutrons. That's why US fuel rods must used partially enriched Uranium (I forget the exact percentage, several percent U235 iirc). Canada (the CANDU reactors) use deuterium water, and so can use naturally occuring uranium. Nobody likes to use graphite anymore, except for nuclear rocket designs and the like where there is no realistic alternative. Just too risky. When a carbon atom rebounds it leaves a dislocation injury that doesn't always 'heal'. Over time, this can store large amounts of energy in the crystal lattice. If a hot spot in the graphite exceeds a certain critical temperature, if aneals the local injuries releasing heat. The effect can spead rapidly superheating the graphite, and is implicated at both the Windscale accident in GB and at Chernobyl.
Thanks. Ok, why not use heavy water steam instead of H2 under pressure thru the turbine etc. as I described in prior post? Perhaps even only slightly enriched Uranium (sort a gasious Candu?) It would no doubt need to be with more volume for the steam than the heavy water to do the same moderation, and that may be an economic disadvantage. That is why I suggested partial moderation by carbon. Perhaps the carbon could be long rods with periodic passage of electric power thru them to limit the defect stored energy build up (keeping the moderator volume reasonable by "carbon assist")?
Live steam + white hot carbon = a potentially really bad day. I know you know why, think about it Billy. Please Register or Log in to view the hidden image! Graphite moderated reactors (are there any still in use?) are periodically anealed, but the procedure has always been problematic. The process tends to be unpredictable and gets out of hand. And there are other factors. Graphite, for reasons I don't know, tends to be variable in its effectiveness as a moderator as temperature varies, becoming a better moderator as it heats up. Likely to do with themal expansion affecting carbon lattice spacing and neutron transparency. This positive feedback is undesirable, to say the least. Getting back to your idea of H2 through graphite, that has in fact been done. Many of the Kiwi/Nerva nuclear rocket engines do just that. Some of the biggest problems are the temperature dependant moderation exibitted by graphite and also complex temp press density relationships in the working gas (H2) that also affect moderation and make 'throttling' a major headache. I think that is why pressurized water is what is used most for commercial reactors. The density of the water is fairly constant throughout the core, giving the system better stability and control.
There are still 12 RBMKs in operation spread across four different plants, all in the FSU tributaries. Plus dozens of small specialized piles for research and/or fuels production.
