# Nuclear fission

i have some basic doubts...why is that nuclear fission is done only on heavy nucleus..what stops to perform nuclear fission on lighter nucleus..what are the lighter nucleus that are used for fission if any?

Astronuc
Staff Emeritus
Binding energy basically prevents fissioning of lighter nuclei.

The lightest nucleus readily fissioned by low energy neutrons is U-233. Th-232 can be fission by fast neutrons.

Technically, when B-10 absorbs a neutron, it fissions into L-7 and He-4, but that doesn't yield much energy/unit mass.

One can obtain spallation reactions, but that doesn't really mean fission.

malawi_glenn
Homework Helper
Yes, fission of nucleus lighter than Fe, no energy is gained. And fusion above Fe you dont gain energy.

You see that the peak is around Fe-56; most energy / nucleon in average.

The idea of nuclear-power is to use this binding energy to get a net gain of energy =)

Last edited by a moderator:
how the temperature and energy release is calculated in fission and fussion...i mean,any general formula for the calculation of temperature and energy released in fission/fussion for any element...

malawi_glenn
Homework Helper
what has temperature with anything to do?

on what level do you want the answer?

The first level is that you compare the total (mass and kinetic) energy of the nuclies before and after reaction. The total mass will be smaller or larger after the reaction (if you fissile a nucleus heavier than iron). The "lost" mass is then transformed into kinetic energy of the reaction products.

Yes, fission of nucleus lighter than Fe, no energy is gained. And fusion above Fe you dont gain energy.

You see that the peak is around Fe-56; most energy / nucleon in average.

The idea of nuclear-power is to use this binding energy to get a net gain of energy =)

cant you not gain energy? you just transfer it or something

and dont you need about 300 million degrees to have fissio/fussion (forgot which one is where you combine two molecules) be a viable source without wasting energy put into it.

Last edited by a moderator:
what has temperature with anything to do?

on what level do you want the answer?

The first level is that you compare the total (mass and kinetic) energy of the nuclies before and after reaction. The total mass will be smaller or larger after the reaction (if you fissile a nucleus heavier than iron). The "lost" mass is then transformed into kinetic energy of the reaction products.

i mean the heat released...

so are u saying that only the energy released can be found...what are the different energies released, is it only heat ...is there anyway so that we can find the energy released which is in the form of heat...

the neutrons released in the fission bombards other nuclear fuel also,so it would be a chain reaction...would the neutrons released in fission bombard the fission products and would it continue bombarding the fission products again and again..is this possible...Or neutrons bombard only the nuclear fuel and not the nuclear fission products...

malawi_glenn
Homework Helper
cant you not gain energy? you just transfer it or something

and dont you need about 300 million degrees to have fissio/fussion (forgot which one is where you combine two molecules) be a viable source without wasting energy put into it.

No this is more complex that this, but still very elementary. When you fuse two protons, you gain 2.22 MeV which is the binding energy of the deutron. Look at this picture:
When you go towards iron from H, you bind the nucleus harder and harder, so when you fuse togheter nucleis up to Iron, energy is gained. You get more energy than you put in. But if you fuse two nucleis heavier than iron, the system is more loosley bound, and you get less energy than you put in.

Standard thing you should know in introductory nuclear physics.

Last edited by a moderator:
malawi_glenn
Homework Helper
i mean the heat released...

so are u saying that only the energy released can be found...what are the different energies released, is it only heat ...is there anyway so that we can find the energy released which is in the form of heat...

Dont know how that is done in theory, only empiracally. Well approx 80% of the energy is carried away by the fissile products, the rest is shared by the neutrons, gamma etc

I would recommend K.S Kranes "Introductory nuclear phyics" book, chapter 13.

No this is more complex that this, but still very elementary. When you fuse two protons, you gain 2.22 MeV which is the binding energy of the deutron. Look at this picture:
When you go towards iron from H, you bind the nucleus harder and harder, so when you fuse togheter nucleis up to Iron, energy is gained. You get more energy than you put in. But if you fuse two nucleis heavier than iron, the system is more loosley bound, and you get less energy than you put in.

Standard thing you should know in introductory nuclear physics.

ya i have no clue what that link is trying to say. i kinda get that h4 is like the most productive.

ya i have only taken an intro to physics. need more math to move up.
but wasn't someone trying to do (fission/fusion forgot which one is which) with lasers and either (break apart or bond again forgot which goes where) the nuclei together kinda forgot still interesting though.

Last edited by a moderator:
malawi_glenn
Homework Helper
There is not so much math needed to understand this on this level. I can ensure you that (almost) every intro book on modern physics cover this.

nice.
ya thats my problem right now.
need to start physics but cant untill calc
cant do calc till done with trig
cant do trig untill done with allg
so ya...
a few years from now and i will be helping you :P

are the fission products random...is there any way we can predict the fission products and the energy released..any general formuala or relation for fission...
what would be the temperature of nuclear fission reactors...

Last edited:
malawi_glenn
Homework Helper
yes the fission products are random, they follow a distribution. Se for example:

http://www.science.uwaterloo.ca/~cchieh/cact/nucfig/fissionyield.gif

So therefore, the expression for how much energy that is relased is (semi)emperically measured.

In a fission reactor using water as moderator. the water is boiling. The rods with the Uran is approx 800K, if i remember correct, was three years scince I had nuclear reactor science on the shedule =)

yes the fission products are random, they follow a distribution. Se for example:

http://www.science.uwaterloo.ca/~cchieh/cact/nucfig/fissionyield.gif

So therefore, the expression for how much energy that is relased is (semi)emperically measured.

In a fission reactor using water as moderator. the water is boiling. The rods with the Uran is approx 800K, if i remember correct, was three years scince I had nuclear reactor science on the shedule =)

the temperature u mentioned is low i think...cant the fission reactions produce million kelvin temperature..is it so,only fusion can produce million kelvin of temperature...

malawi_glenn
Homework Helper
.. you have made a misstake now.

Millions of kelvin is NEEDED to have a fusion reactor.

Think once more, can you contain medium of 1million kelvin? No, you need strong magnetic fields. Do you use magnetic-cages in fission reactors? No.

The point is I dont remember if it is 1200K or 800K...

stars have million kelvin temperature because of fusion..if fusion can produce this much temperature then why cant the fission produce million kelvin temperature...am confussed..

malawi_glenn
Homework Helper
Fission and fusion if two totaly sperate things.

Fission you split a nucleu and due to the relased binding energy parts fly away and heat the rod (heat is motion/vibration, when nucleis are split, the fragments are beeing breaked in the rod, and that makes heat) and the rod will heat the water. As a water boiler, you have a metal rod, which gets warm, like 500K, and that makes the water boil.

The stars are hot in its interior due to huge gas compression due to gravitational collapse. And in order to overcome the coloumb barrier (two similar charges repel each other), lots of energy/velocity of the protons is needed. So in order do have many partilces(protons) undergo fusion is to have high heat (heat is average velocity/kinetic energy of particles). So in stars, the temperature from grav-collapse (the gas law tells you that if you compress gas, temp will rise) can make the hydrogen ions (protons) undergo fusion, wich releases energy (photons) that can halt the gas from beeing gravitationally collapsed into a degenerate remnant or black hole. The sun is 15millions kelivn in the center and 6000K on the "surface".

But on earth, we cant have a big thing like the sun on earth, and if you have 15million K in a container... things melt... (melt at approx 6000K so we cant even reach 15million K in an ordinary container). So one uses strong magnetic fields, you know that charges in motion in magnetic fields are bent. So you contain this hot hot plasma in magnetiv cages on earth.

I dont have time to give you a more lengthy answer than this, I suggest you wait for more people to reply or either (better) consult textbooks from your library in introductory nuclear physics and stellar physics.

is it possible atleast theoritically to bombard a lighter nuclues than uranium and having Atomic number >60 if we use a very high energy neutron...so that there wont be any need for uranium in future if this is possible...

malawi_glenn
Homework Helper
is it possible atleast theoritically to bombard a lighter nuclues than uranium and having Atomic number >60 if we use a very high energy neutron...so that there wont be any need for uranium in future if this is possible...

I dont know if nucleis with so low mass-number is instable due to neutron fission. And it also depends of you want to use thermal neutrons or higher energy neutrons, or maybe even have protons as probes.

Anyway, i know research is going on to have a Thorium isotope instead of Uranium, there is much more thorium in the world + it dont give long lived daugther nucleids which are radioactive. Also transmutation of "burned out" uranium is under research. There is a lot of info if you google. And Astronuc is a guru about these things.

But I am very sure that we will get fusion working before uranium runs out. But still, we need to take care of the Uranium-waste. My opinion.

Thank you for all replies..