Ezio3.1415 said:
1.Uranium-235, after taking a neutron turns into Xe and Sr and 2 neutrons or it turns into Ba and Kr and 3 neutrons or many other things. How it is decided that Ur nucleus will change into what?
2.Why aren't we still using fusion to produce energy?
3. Isomers-what's the reason behind the nucleus having same things (p,n,e) being in different energy states?
Each fission event is random because the state of the U-235 atom and the absorption of the neutron (and formation of the composite nucleus) involve randomness (stochasticism).
http://www.world-nuclear.org/education/phys.htm
The absorption of a neutron by U-235 produces an excited U-236 nucleus. About 16% of the events result in a γ-decay without fission. Then U-236 may decay, or it may absorb a neutron and form U-237, which may decay to Np-237.
When U-236 fissions, it splits into two nuclei of Z
1 and Z
2, where Z
2 = 92-Z
1. The atomic masses of the two nuclei are A
1 and A
2, where A
1+A
2+2,3 n = 236. Two or three neutrons may be released in the reaction, but there are 'delayed' neutrons, which are released from some of the neutron-rich fission products. Delayed neutrons represent less than 0.7% of neutrons of each generation, and the are critical to the control of a nuclear reactor.
The most probably fission reaction (based on yield) is one producing Te-134 (Z=52), with the complementary nuclide being, Zr-100 (Z=40), and two neutrons. Other probable results are:
Xe-140, 139, 138 with Sr 94, 95 with 2 or 3 neutrons (as appropriate, i.e., A1+A2+#n = 236),
Ba-144, 143 with Kr-89, 90 with two or three neutrons (as appropriate).
In a fission reactor, the neutron field is far from monoenergetic, and in fact, there is a spectrum of neutron energies from 0.01 eV up to several MeV or about 8 orders of magnitude of energy in units of eV. Some fission occur in U-238 due to fast neutrons - about 8 to 10% of fissions. Most fissions occur in U-235 until sufficient Pu-239 has been produced to compete for neutrons with U-235. The accumulation of Pu-239 and other fissile transuranics is continuous, and the proportion of fissions in transuranics is continually increasing until discharge.
As Drakkith indicated, fusion has not been used for commercial energy production. The physics is straightforward, the engineering is a challenge.
Nuclei have different energy levels due to internal arrangement of nucleons. Some may exist in a metastable energy state, i.e., they have a relatively long half-life, e.g., Kr-85m or Xe-135m, and those nuclei decay via isomeric transition (gamma decay). Z and A do not change. Those nuclei could decay by beta decay, which is a competing process.
