
#1
May2113, 03:07 PM

P: 23

Hello. I have some questions regarding Uranium 235 and its instability.
Whenever you hear about nuclear reactions, you almost always hear Uranium 235 linked with it as being quite suitable for splitting because it is unstable (At least I do). I have been vaguely exploring nuclear transmutations and what you might call "Nuclear Alchemy" and forming new elements via nuclear fusion and fission. I know the amount of protons in an atom determines what element it is and a certain isotopic ratio of protons to neutrons determines if it is unstable or not, and I always hear how Uranium 235 is the perfect element for nuclear fission because it is unstable and breaks easily. I know nuclear fusion and fission are quite hard to perform on normal larger atoms due to the amount of energy required. For example, all the energy in the sun is enough to only make iron as the largest element from nuclear fusion and it requires much larger events (Supernovas) to transmute larger elements. To my questions. Why are unstable elements such as Uranium 235 so easy to perform nuclear fission on and make two new random elements? Why can't other elements such as Lead, Iron and Copper be made unstable and thusly used in nuclear fission to make new elements? Are there any cheats or catalysts that would allow nuclear fusion to be accomplished without all the energy of the sun required to simply transmute Hydrogen into Helium? Thank you. (Edit: Additional question... What is the difference between Uranium and Uranium 235 specifically?) 



#2
May2213, 12:06 AM

P: 240

You may wish to study the concept of the binding energy per nucleon to get answers for your questions regarding Fission. See for example:
http://en.wikipedia.org/wiki/Nuclear...binding_energy 



#3
May2213, 02:46 PM

P: 23

Thank you. But I know that separating atoms takes an incredible amount of energy considering they are bound by an incredible amount of energy. But why is Uranium 235 an exception? The Binding force for U235 must still be present in order to get the amount of energy we can from it (Atomic bombs, Nuclear reactors), so why is it so much easier to perform nuclear fission on?
What I mean to say is, why can't we just send a nucleon into any old atom and get nuclear fission? I know it has something to do with being unstable, but what does the isotopic ratio matter anyways, neutrons don't even have a charge? 



#4
May2213, 04:12 PM

P: 353

Why Unstable UraniumHowever, instead of separating U235 into constituent protons and neutrons, you can separate it into just two nuclei, so that each of these has a large binding energy  and their combined binding energy is bigger than the binding energy of the initial U235. For example, you can separate U235 into nuclei of Th231 and He4, and have energy left over. 



#5
May2213, 04:18 PM

P: 240

In reference to the binding energy per nucleon curve in the wiki page I linked above, all the nuclides on the right to the isotope Fe56 are unstable and try to decrease their mass number (A) by undergoing either alpha decay or spontanious fission.
Now we can "induce" fission by bombarding these heavey nuclides with neutrons. The fact that these neutrons are neutral makes them optimum since they can penetrate the atom without being scattered due to electrmagnetic interactions and hence can approach the nucleus. U235 is a good nuclear fuel because it has a high probablity to interact with slow neutrons (the probability is generally termed cross section, and slow netrons are generally termed thermal). This means that it can udnergo fission if you bombard it with a slow neutron. As a result of the fission reaction, more neutrons will be generated. These newly born neutrons can almost always induce fission again in the remaining U235 nuclides because they usually have an energy sufficient to induce the fission (recall thermal neutron is enough to induce fission in U235). U235 is not unique in this, heavy nuclides with odd mass number have the same property. These are generally called "fissile". On the other hand nuclides such as U238 are called "fissionable but nonfissile" because you can induce fission in these nuclides but using fast neutrons (> 0.5 MeV). 



#6
May2213, 04:43 PM

P: 353

Between 100 milliard years and 100 million years, the isotopes are: K40 (1,25) Rb87 (49) Sm146 (0,103) Lu176 (37,8) Re187 (41,2) Th232 (14) U235 (0,71) U238 (4,47) And of these 8, 5 (all the low mass ones) decay into stable nuclei by single decay  one alpha for Sm146, one beta for all others, incl. K40 which also can undergo electron capture and positron emission. The 3 long lived isotopes on the isle of stability are unique in being longlived isotopes that decay into shortlived isotopes undergoing a radioactive decay chain. 



#7
May2213, 05:16 PM

P: 23

I just noticed I'm asking a ton of questions, so forgive me if I annoy anyone with my lack of knowledge. 



#8
May2213, 06:01 PM

P: 240





#9
May2213, 06:06 PM

P: 240

http://www.amazon.com/IntroductionN...eering+Lamarsh The first two chapters (and may be chapter 4 as well) can get you right on the track to learn the language of fission. Having said that, questions are always welcome :) 



#10
May2213, 06:34 PM

P: 23

Thank you! I'll do some more research.




#11
May2213, 08:22 PM

Admin
P: 21,628

Actually, it's U236* (= U235 + n) which is unstable, the * indicating an excited nucleus. U234* (U233 + n) is similarly unstable, but there is a probablity that U236* or U234* will decay by IT (γ  emission), and remain stable. One can find trace amounts of U234 in natural U, and a fair amount of U236 in recycled U.
U235 is an isotope of U. 



#12
May2313, 11:17 AM

P: 353

There is a reason odd isotopes of uranium and plutonium are fissile and even isotopes are not. Pairing energy.
Basically, even numbers of neutrons are more strongly bound than odd number of neutrons. The energy needed to fission U236 is similar to the energy needed to fission U239. But the energy released in adding a slow neutron to U235 is larger than energy released in adding a slow neutron to U238. So if a slow neutron is captured by U238, the resulting U239 does not have enough energy for fission, and has to get rid of the energy by emitting gamma ray. If, however, a slow neutron is captured by U235, the energy is big enough to cause fission of U236, and most of time does, though it still often emits gamma ray instead of fission. If a fast neutron is captured by U238, the additional energy from the kinetic energy of the neutron may be enough to cause fission. 



#13
May2313, 02:30 PM

P: 5,634

GT: If you have not yet read here
http://en.wikipedia.org/wiki/Nuclear_fission it will provide you some additional insights like this one: 


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