# Atomic Power

1. Oct 11, 2005

### Line

Exactly how does it work when you split an atom? A neutron is shot at a nucleus. Does the nucleus split apart because of the force of the neutron hiting it? I saw something about the neutron sticking to the nucleus and it becomes unstable and splits.

Also wonder how do you accelerate a neutron if it has no charge? I would think using an electron would be alot easier or is it to small to cause a nucleus to break?

I'm also now understanding how prt of the nucleus becomes energy and how it even has energy. WHat kind of energy is it containing? Is it kinetic? Standard models show the nucleus standing still while the electrons whirl all around it. The nucleus is probrably moving too. and I just can't understand fission, fussion, and antimatter reactions. Am I right ,just by hitting an atom part of it become energy?

2. Oct 11, 2005

### Pengwuino

I believe there are different ways of getting power out of nuclear reactions. I may very well be wrong (and i'm probably the last person who should be answering) but in nuclear reactors, you have naturally decaying fuel that doesn't need the extra neutron. In a nuclear detonation however, you have an atom that is somewhat unstable and then you add a neutron. This makes it incredibly unstable and starts the chain reaction and BOOM. The reason they wouldn't use an electron as far as i can tell is that 1) i'm not sure if it would make it unstable and 2) it would be very difficult to overcome the coulomb interaction.

3. Oct 11, 2005

### Nomy-the wanderer

To understand how a part become san energy:
Well of course there's the basic E=mc², when a nucleus split, there's a missing mass usually, when u calculate the mass for the resulting nuclei, and this missing mass turns into energy using the above equation...

It's no longer called atomic power btw, it's the nuclear power, the nucleus is everything in these reactions...

Because u need a lot of information hyperphysics is a great site...
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

4. Oct 11, 2005

### Line

uH huh but I'm not understansing the physics behind it.

Is it the force of the particle slaming into one another that causes the sonversion into energy? Like you slame to peices of wood together they'll break into smaller peices. If you slame 2 molecules together they'll make smaller peices. Well particles are the smallest known peices of matter. It seems since you can't beak them down further the only way they go is to become energy.

5. Oct 11, 2005

### Pengwuino

That's not how the process works. A nucleus will become unstable and a fission reaction will occur when a neutron is introduced. The nucleus, without hte neutron, is somewhat stable, but when you add the neutron, it becomes very unstable and will undergo fission. It's not about smashing something into something else like you would smash a baseball into a glass mirror.

6. Oct 11, 2005

### Nomy-the wanderer

7. Oct 11, 2005

### ZapperZ

Staff Emeritus
When a "stable" atom absorbs another neutron, what you have typically is an unstable isotope (i.e. an atom with the SAME number of proton, but with different number of neutrons or atomic mass number). This isotope then decays via a nuclear reaction.

In a typical nuclear reactor, a slow (or what they call thermal neutron) is captured by the uranium atom. This unstable uranium atom then decays via a fission process that releases energy. You can't just shoot a neutron at the uranium to achieve this exact process, because a fast neutron will not give the same type of reaction. That's why the the whole reactor is in water, because water has hydrogen that has about the same mass a neutrons and can effectively slow down these neutrons.

So no, it is not simply a "hit the atom with a neutron and BAM" type of a process.

Zz.

8. Oct 11, 2005

### Staff: Mentor

It turns out that there are a few atoms which are unstable with respect to fission, the most common being U-233, U-235 and Pu-239. Some heavier isotopes of Pu and heavier transuranics will undergo spontaneous fission.

As ZapperZ mentioned, neutrons are absorbed by uranium, prinicipally U-235 in a water reactor. The hydrogen in the water slows the fast neutrons (E > 1MeV) released in the fission process so that they are more readily absorbed by U-235. Commercial power reactors, generally known as Light Water Reactors, use this process, and the neutrons are called 'thermal' neutrons because they are generally in thermal equilibrium with the environment in the reactor. U-235 have a relatively low rate of spontaenous fission, whereas Pu-239 and heavier isotopes have higher rates.

When U-235 absorbs a neutron, it becomes U-236. Most of the time, U-236 (the nucleus is 'excited', i.e. has energy in excess of its stability) will fission, but it could in same cases emit a gamma-ray and decay to a lower energy state (Reprocessed uranium contains some fraction of U-236). Similarly Pu-239 absorbs a neutron and becomes an excited Pu-240, which may fission, or in some cases emit a gamma-ray to become more stable. Successive n-captures will result in the production of Pu241 and Pu242, which will decay to Am241 and Cm-242 through beta dacay. Pu242 may absorb a neutron becoming Pu-243 which decays to Am-243.

Last edited: Oct 11, 2005
9. Oct 11, 2005

### Andrew Mason

As has been pointed out, neutron capture by a U235 nucleus is the trigger that causes U235 to become unstable, undergo fission and release energy. It may be helpful to think of that trigger as being half way down the back of a golf ball cup and is activated by being hit by a golf ball. The ball has to have just the right speed to fall into the hole and strike the back. If it is too fast, it goes over the hole. If it is too slow, it doesn't reach the back of the cup.

A U235 nucleus undergoing fission, produces very high energy neutrons. In order for these neutrons to trigger other fission events (ie. fall into and strike the back of the cup), these neutrons have to be slowed down (to about 1/1000th of their original speed). If they collide with something much more massive, they just bounce off and retain most of their energy. If they strike something close to their own mass, they will lose much more energy per collision. Since hydrogen has virtually the same mass as a neutron, a few successive collisions with hydrogen nuclei will reduce the speed dramatically. H1 has a tendency to capture these free neutrons whereas heavy hydrogen or deuterium H2 does not. So heavy water is a much better moderator.
AM

Last edited: Oct 11, 2005
10. Oct 11, 2005

### Staff: Mentor

Well, yes and no. Actually hydrogen, H1 is the best moderator in terms of its moderatation ability. A neutron can lose almost all of its kinetic energy if it happens to have perfect head on collision with a proton. Most of the time though, it has a slightly off-center scattering collision.

However, as Andrew correctly pointed out, H can also absorb neutrons, especially at thermal energies, and it then becomes D (H2). This is one reason that UO2 fuel in light water reactors (LWRs) must be enriched well above the 0.71% by weight of U-235 in natural U. Current licensed limits are 5%, although special provisions are made for slightly higher enrichments for special types of fuel, e.g. fuel used in special experimental reactors. Such fuel is fabricated at special facilities under special licensing.

Fuel must be enriched not only because neutrons are absorbed by H, but also by core structural materials, fission products which accumulate in the fuel as it operates, and also because it is consumed. Modern LWR fuel operates to exposures (burnups) of up to 50-55 GWd/tU for peak assemblies, and up to peak fuel rod exposures of approximately 62 GWd/tU. Peak pellets might reach 65-67 GWd/tU. Under special tests for demonstration purposes, commerical fuel has been irradiated to burnups of 85-90 GWd/tU, and IIRC near 100 GWd/tU; however this is limited to a few special fuel rods. In contrast, fast reactor fuel in FFTR was irradiated to maximum burnups of up to 180-200 GWd/tU. A burnup of 9.7 GWd/tU is approximately 1.0% FIMA (fissions per initial metal atoms).

The Canadians opted to develop heavy water reactors, which go by the name CANDU (Canadian Deuterium Uranium). The CANDU reactors have used natural U, also in the form of UO2, however more recently AECL has introduced CANDU with a slight increase in enrichment for higher exposure (burnup).

Last edited: Oct 11, 2005
11. Oct 11, 2005

### Line

Ok but hwo do you get a neutron to accelrate in the first place. I mean protons and electrons have electric charges which allows them to be manipulated. Neutron is neutron so how do you control it?

12. Oct 11, 2005

### ZapperZ

Staff Emeritus
You don't. The neutrons that initiate this came out of the fission process itself - that's why it is a self-sustaining chain reaction. It already has a kinetic energy when it is liberated during the nuclear reaction.

Zz.

13. Oct 11, 2005

### Staff: Mentor

Neutrons are simply liberated in the fission process - usually two or three, but sometimes 4 per fission. They have a range of energies above 1 MeV.

Otherwise, neutrons can be generated from a reaction between a high energy alpha particle (several MeV - from a radioisotope of Pu or Po) and Be or an interaction of high energy (> 1.5 MeV) gamma-ray with Be.

Sb-Be sources are used as startup sources in commerical nuclear plants so that there is a source of neutrons for the in-core and ex-core neutron detectors to detect while the core is subcritical or at very low power.

A third source of neutrons, other than fission are a handful of fission product radionuclides which produce delayed neutrons. These neutrons account for about 0.7% of neutrons in a nuclear reactor (particularly while the system is critical) and that fraction is sufficient to provide for control of the fission process.

14. Oct 12, 2005

### Line

OK got that, but I'm not getting why matter becomes energy. I would think if you broke a nucleus that 2 remaining parts would be eqaul to the once whole.

15. Oct 12, 2005

### Grogs

No! If this were the case, fusion and fission wouldn't happen, stars wouldn't burn, and we wouldn't be here. If you take a bunch of protons and neutrons and cram them together to form a nucleus, the resulting nucleus will have less than the sum of all the protons and neutrons that went into it. We call this difference the mass defect. The more tightly bound and stable the nucleus is, the larger the value of the mass defect and the less the resulting nucleus weighs.

When a U-235 atom fissions, it usually splits into 2 lighter nuclei plus 2-3 neutrons. The sum of all the masses of the nuclei and neutrons released will be less than that of the original atom that fissioned. This mass difference between the products and the reactants shows up as the kinetic energy of the products, which is what we use to heat the water which goes on to produce electricity.

16. Oct 12, 2005

### Line

Ok but I'm not understanding that. So just split in nucleus or ram 2 together some mass turns up missing?

17. Oct 12, 2005

### ZapperZ

Staff Emeritus
18. Oct 12, 2005

### Andrew Mason

In terms of the number of thermal neutrons available D2O is by far the best moderator. If one takes the neutron scattering cross-section multiplied by the energy lost per collision and divided by the neutron capture cross-section, D2O is about 80 times more effective as a moderator than H2O.

AM

19. Oct 12, 2005

### Staff: Mentor

Yes, I would agree that using the moderating ratio ($\xi\frac{\Sigma_s}{\Sigma_a}$) as the criterion, there is an advantage to D2O.

On the other hand, this advantage is somewhat mitigated by the fact that neutrons have a much greater diffusion length in D2O, so the escape probability is much greater for a D2O moderated system than for a H2O moderated system.

Last edited: Oct 12, 2005
20. Oct 12, 2005

### Line

That site says the mass of the nucleus is les s than the sum of the neutrons and protons. Makes no sense.

SO when mass converts to energy is it an entire particle or just part of a neutron or proton that disappears?