Uranium Split

[Carl Fredrik Ahl]

Hi,

I wonder why uranium (don't remember what isotope) splits when a nuetron hits it. I also wonder how the initial neutron get shot at the first uranium.

[Janus]

Elements high on the periodic table contain lots of neutrons and protons in their nucleus.  This makes them less stable.  Uranium 235 is one of those elements.

Also, an excess of neutrons to protons can add to instability.    Most isotopes undergo simple radioactive decay (alpha or beta) to reach a more stable state.

Uranium 235 does this. It decays by alpha decay into Thorium 231.

But because of the particular arrangement of neutrons and protons in its nucleus, it can  also split into two smaller nuclei and some free neutrons by in an act called spontaneous fission instead.  This very rare event can happen all by itself.( All radioactive decay processes are statistical by nature, and some nuclei can take more than one route. You can never say with certainty which route a particular nuclei will take, only the odds of it following one or the other*. Spontaneous fission is very low on the probability list. ) 

If one of the free neutrons from this fission is absorbed by another U-235 nucleus, it briefly becomes a U-236 nucleus.  The U 236 nucleus is even more unstable and much more likely to decay by fission. This of course can lead to further induced fission in other nuclei.

So the introduction of the neutron makes an already unstable nucleus even more unstable. And the  fission inducing neutron could have been produced by the spontaneous (non-induced) fission of a U 235 Nucleus.

 

 

* Even with fission, there is a small chance of breaking into three smaller nuclei rather than two.

[Carl Fredrik Ahl]

38 minutes ago, Janus said:

Elements high on the periodic table contain lots of neutrons and protons in their nucleus.  This makes them less stable.  Uranium 235 is one of those elements.

Also, an excess of neutrons to protons can add to instability.    Most isotopes undergo simple radioactive decay (alpha or beta) to reach a more stable state.

Uranium 235 does this. It decays by alpha decay into Thorium 231.

But because of the particular arrangement of neutrons and protons in its nucleus, it can  also split into two smaller nuclei and some free neutrons by in an act called spontaneous fission instead.  This very rare event can happen all by itself.( All radioactive decay processes are statistical by nature, and some nuclei can take more than one route. You can never say with certainty which route a particular nuclei will take, only the odds of it following one or the other*. Spontaneous fission is very low on the probability list. ) 

If one of the free neutrons from this fission is absorbed by another U-235 nucleus, it briefly becomes a U-236 nucleus.  The U 236 nucleus is even more unstable and much more likely to decay by fission. This of course can lead to further induced fission in other nuclei.

So the introduction of the neutron makes an already unstable nucleus even more unstable. And the  fission inducing neutron could have been produced by the spontaneous (non-induced) fission of a U 235 Nucleus.

 

 

* Even with fission, there is a small chance of breaking into three smaller nuclei rather than two.

Thx for the answer.

47 minutes ago, Janus said:

Elements high on the periodic table contain lots of neutrons and protons in their nucleus.  This makes them less stable.  Uranium 235 is one of those elements.

Also, an excess of neutrons to protons can add to instability.    Most isotopes undergo simple radioactive decay (alpha or beta) to reach a more stable state.

Uranium 235 does this. It decays by alpha decay into Thorium 231.

But because of the particular arrangement of neutrons and protons in its nucleus, it can  also split into two smaller nuclei and some free neutrons by in an act called spontaneous fission instead.  This very rare event can happen all by itself.( All radioactive decay processes are statistical by nature, and some nuclei can take more than one route. You can never say with certainty which route a particular nuclei will take, only the odds of it following one or the other*. Spontaneous fission is very low on the probability list. ) 

If one of the free neutrons from this fission is absorbed by another U-235 nucleus, it briefly becomes a U-236 nucleus.  The U 236 nucleus is even more unstable and much more likely to decay by fission. This of course can lead to further induced fission in other nuclei.

So the introduction of the neutron makes an already unstable nucleus even more unstable. And the  fission inducing neutron could have been produced by the spontaneous (non-induced) fission of a U 235 Nucleus.

 

 

* Even with fission, there is a small chance of breaking into three smaller nuclei rather than two.

Thx for the answer. How is the initial neutron shot at the uranium?

[John Cuthber]

If you are running a nuclear reactor then you can just wait for random "spontaneous fission" where the nucleus falls apart without being hit  by anything.

 

If you are making a nuclear bomb, then you don't have long enough to wait.
They rely on neutron sources based on things like mixtures of radium (which emits alpha particles) and Beryllium (which emits neutrons when hit by alpha particles.
And, if you want to make sure there are neutrons...

https://en.wikipedia.org/wiki/Startup_neutron_source

 

 

 

[Janus]

7 hours ago, Carl Fredrik Ahl said:

Thx for the answer.

Thx for the answer. How is the initial neutron shot at the uranium?

I answered that in the post.  Not all instances of U 235 fission needs to be induced by absorbing a neutron.  One of the ways Uranium 235 can decay is to spontaneously undergo fission. It not the most likely way, but  you have some 2.5e21 Atoms per gram of Uranium. Even with nuclear fuel only enriched to 5% U 235,  you still are left with 1.25e20 nuclei;  a large enough number that even with only a 2e-7% chance of any given nuclei decaying by spontaneous fission, you are going to have a good number of nuclei undergoing such fission per minute.

[Carl Fredrik Ahl]

7 hours ago, Janus said:

I answered that in the post.  Not all instances of U 235 fission needs to be induced by absorbing a neutron.  One of the ways Uranium 235 can decay is to spontaneously undergo fission. It not the most likely way, but  you have some 2.5e21 Atoms per gram of Uranium. Even with nuclear fuel only enriched to 5% U 235,  you still are left with 1.25e20 nuclei;  a large enough number that even with only a 2e-7% chance of any given nuclei decaying by spontaneous fission, you are going to have a good number of nuclei undergoing such fission per minute.

Ok thanks for the answer. How do they prevent uranium from spontaneously undergo fission in an atomic bomb before they want the explosion?

13 hours ago, John Cuthber said:

If you are running a nuclear reactor then you can just wait for random "spontaneous fission" where the nucleus falls apart without being hit  by anything.

 

If you are making a nuclear bomb, then you don't have long enough to wait.
They rely on neutron sources based on things like mixtures of radium (which emits alpha particles) and Beryllium (which emits neutrons when hit by alpha particles.
And, if you want to make sure there are neutrons...

https://en.wikipedia.org/wiki/Startup_neutron_source

 

 

 

Thx for the answer. So they can use beryllium in atomic bombs to start the fission reaction of uranium. How do they isolate beyllium from the uranium before the moment that they want the explosion?

[swansont]

3 hours ago, Carl Fredrik Ahl said:

Ok thanks for the answer. How do they prevent uranium from spontaneously undergo fission in an atomic bomb before they want the explosion?

Thx for the answer. So they can use beryllium in atomic bombs to start the fission reaction of uranium. How do they isolate beyllium from the uranium before the moment that they want the explosion?

They don't prevent it. The Uranium or Plutonium pieces are separated into subcritical masses. It's only after they are combined that you have neutron multiplication to larger numbers. Same with the initiator. The alpha source and the beryllium are not kept next to each other. Geometry ensures they produce an insignificant neutron flux before the bomb is set off.

[Janus]

6 hours ago, Carl Fredrik Ahl said:

Ok thanks for the answer. How do they prevent uranium from spontaneously undergo fission in an atomic bomb before they want the explosion?

 

While spontaneous fission is always occurring and in turn producing some induced fission reactions,   This isn't enough to cause a chain reaction if you don't have enough uranium 235 contained in a small enough region. For pure U 235, this  requires a sphere  52 kg in mass.   With a smaller amount, too many of the produced neutrons make their way out of the sphere without encountering a nucleus.¹   If you break the 52 kg into small pieces and separate them this has the same effect, it allows for more neutrons to escape without inducing a reaction.

Bring the pieces suddenly together and you have a critical mass that will produce your nuclear explosion. ²

 

¹ you can get away with using less if you surround the uranium with a neutron reflector which bounces the neutrons back through the mass, giving them another chance to interact with a nucleus.

² This presents its own problems.   As the pieces are brought closer together, the rate of induced fission events increases.  This creates an increasing energy output from the Uranium.  If they aren't brought together properly, or fast enough, this release of energy can be enough to blow the pieces apart from each other before they can get close enough to form that fully critical mass and produce the explosive fission event.   The bomb will fizzle out.   This type of bomb has to be designed properly so that the critical mass is formed before the individual pieces are blown apart again.  It's not a matter of keeping the bomb from exploding, it's getting it to explode properly.

[Carl Fredrik Ahl]

16 hours ago, Janus said:

While spontaneous fission is always occurring and in turn producing some induced fission reactions,   This isn't enough to cause a chain reaction if you don't have enough uranium 235 contained in a small enough region. For pure U 235, this  requires a sphere  52 kg in mass.   With a smaller amount, too many of the produced neutrons make their way out of the sphere without encountering a nucleus.¹   If you break the 52 kg into small pieces and separate them this has the same effect, it allows for more neutrons to escape without inducing a reaction.

Bring the pieces suddenly together and you have a critical mass that will produce your nuclear explosion. ²

 

¹ you can get away with using less if you surround the uranium with a neutron reflector which bounces the neutrons back through the mass, giving them another chance to interact with a nucleus.

² This presents its own problems.   As the pieces are brought closer together, the rate of induced fission events increases.  This creates an increasing energy output from the Uranium.  If they aren't brought together properly, or fast enough, this release of energy can be enough to blow the pieces apart from each other before they can get close enough to form that fully critical mass and produce the explosive fission event.   The bomb will fizzle out.   This type of bomb has to be designed properly so that the critical mass is formed before the individual pieces are blown apart again.  It's not a matter of keeping the bomb from exploding, it's getting it to explode properly.

Thx for the answer! I also wonder how they make it that the first uranium shoots out 2 neutrons and the second 3 neutrons and the third 4 neutrons and so on. How do they do that? I know it's necessary to hit more and more uranium and get the donimo effect. I just don't know how it works.

[Sensei]

46 minutes ago, Carl Fredrik Ahl said:

Thx for the answer! I also wonder how they make it that the first uranium shoots out 2 neutrons and the second 3 neutrons and the third 4 neutrons and so on. How do they do that? I know it's necessary to hit more and more uranium and get the donimo effect. I just don't know how it works.

One path of Uranium fission is:

Uranium-235 + n⁰ -> Barium-141 + Krypton + n⁰ + n⁰ + n⁰ .... (a few neutrons, not constant quantity) (some unstable neutron-rich Krypton and Xenon isotopes are decaying via neutron emission)

Either Barium-141 and Krypton are unstable and before they reach stable isotope they undergoes plentiful of decays.

Newly created free neutrons are repeating cycle with other fissile material such as Uranium-235.

 

 

Edited November 8, 2018 by Sensei

[swansont]

2 hours ago, Carl Fredrik Ahl said:

Thx for the answer! I also wonder how they make it that the first uranium shoots out 2 neutrons and the second 3 neutrons and the third 4 neutrons and so on. How do they do that? I know it's necessary to hit more and more uranium and get the donimo effect. I just don't know how it works.

Thermal fission of U-235 releases 2.43 neutrons, on average. As Sensei has implied, there are a number of possibilities for the reaction. Some give 3 neutrons or more. But they don't "make it" do that. U-235 is used because that's how it behaves. If it didn't, they would use something else.

edit: Pu-239, for example, yields ~2.87 neutrons per fission. U-233 yields ~2.48
https://www.nrc.gov/docs/ML1214/ML12142A078.pdf

[Carl Fredrik Ahl]

1 hour ago, swansont said:

Thermal fission of U-235 releases 2.43 neutrons, on average. As Sensei has implied, there are a number of possibilities for the reaction. Some give 3 neutrons or more. But they don't "make it" do that. U-235 is used because that's how it behaves. If it didn't, they would use something else.

edit: Pu-239, for example, yields ~2.87 neutrons per fission. U-233 yields ~2.48
https://www.nrc.gov/docs/ML1214/ML12142A078.pdf

Ok, does this mean that the initial neutron has enough kinetic energy to get two neutrons out of the first uranium and the two neutrons from the first uranium has enough kinetic energy to get three neutrons out of the second uranium and so on?

[swansont]

23 minutes ago, Carl Fredrik Ahl said:

Ok, does this mean that the initial neutron has enough kinetic energy to get two neutrons out of the first uranium and the two neutrons from the first uranium has enough kinetic energy to get three neutrons out of the second uranium and so on?

The neutron energy is typically very small, and is not required to initiate the reaction. The reaction itself releases energy, and since the fission products have excess neutrons as compared to stable nuclei, neutrons are emitted. (a small fraction of the neutrons are emitted by the fission products as they beta decay. These are called delayed neutrons)

Neutrons in a thermal reactor are slowed down (using a moderator, which is something that removes kinetic energy, such as carbon, or water), because higher-energy neutrons (aka fast neutrons) are actually less likely to induce a fission.

There is not a progression of two neutrons, and then three, etc. It's stochastic. 

[Janus]

3 hours ago, Carl Fredrik Ahl said:

Ok, does this mean that the initial neutron has enough kinetic energy to get two neutrons out of the first uranium and the two neutrons from the first uranium has enough kinetic energy to get three neutrons out of the second uranium and so on?

A nuclear fission reaction doesn't occur by one atom at a time.   First fission event produces 2 neutrons (or sometimes 3). Each of these neutrons go on to induce fission in two other atoms.   Each of these atoms produce 2 neutrons each, so now you have 4 free neutrons,   each capable  of inducing fission in a nucleus.

So it goes something like this:

1 atom produces 2 neutrons leading to

2 atoms producing 4 neutrons (2 each) leading to

4 atoms producing 8 neutrons, leading to

8 atoms producing 16 neutrons. etc.

 

[swansont]

1 hour ago, Janus said:

A nuclear fission reaction doesn't occur by one atom at a time.   First fission event produces 2 neutrons (or sometimes 3). Each of these neutrons go on to induce fission in two other atoms.   Each of these atoms produce 2 neutrons each, so now you have 4 free neutrons,   each capable  of inducing fission in a nucleus.

So it goes something like this:

1 atom produces 2 neutrons leading to

2 atoms producing 4 neutrons (2 each) leading to

4 atoms producing 8 neutrons, leading to

8 atoms producing 16 neutrons. etc.

 

Also note that if the system is at criticality, then, on average, only one neutron per fission causes another. The rest are captured or leak out of the system. The system described above is highly supercritical.

And the "generations" depiction Janus has provided is another simplification (it's a schematic). The reactions would not actually be simultaneous.

[Janus]

7 hours ago, swansont said:

Also note that if the system is at criticality, then, on average, only one neutron per fission causes another. The rest are captured or leak out of the system. The system described above is highly supercritical.

And the "generations" depiction Janus has provided is another simplification (it's a schematic). The reactions would not actually be simultaneous.

Or, As I should have probably said, the two neutrons produced by a fission event could potentially induce two more fission events. 

[Carl Fredrik Ahl]

17 hours ago, Janus said:

A nuclear fission reaction doesn't occur by one atom at a time.   First fission event produces 2 neutrons (or sometimes 3). Each of these neutrons go on to induce fission in two other atoms.   Each of these atoms produce 2 neutrons each, so now you have 4 free neutrons,   each capable  of inducing fission in a nucleus.

So it goes something like this:

1 atom produces 2 neutrons leading to

2 atoms producing 4 neutrons (2 each) leading to

4 atoms producing 8 neutrons, leading to

8 atoms producing 16 neutrons. etc.

 

Thanks for the answer!, now I understand