Fusion Power: Breaking the Coulomb Barrier

[Potaire]

Fusion for power??

I am guessing that the Coulomb Barrier is the major problem? What exactly is necessary to breach the Barrier, as far as the amount of energy, heat, velocity of proton, etc, etc. If the Barrier COULD be breached without destroying the city at the same time, what other problems are there? Converting the MeV inside the system into voltage outside of the system, maybe?? Keeping the reaction going for sufficient time periods? Interesting stuff!

 

[Astronuc]

I am guessing that the Coulomb Barrier is the major problem? What exactly is necessary to breach the Barrier, as far as the amount of energy, heat, velocity of proton, etc, etc. If the Barrier COULD be breached without destroying the city at the same time, what other problems are there? Converting the MeV inside the system into voltage outside of the system, maybe?? Keeping the reaction going for sufficient time periods? Interesting stuff!

Indirectly.

The problem is that to get fusion going, particularly for more favorable aneutronic reactions, high plasma temperatures are required. The limitation is actually on 1) plasma pressure, which is limited by the strongest superconducting magnets we can build and by the strength of the supporting structures, and 2) energy losses which increase with temperature.

Plasma pressure and energy losses are also a function of plasma ion/electron density.

Plasma pressure ~ nkT where n is the particle (ion/electron) density, T is temperature, and k = Boltzmann's constant.

The energy losses are more complicated because there are different mechanisms, e.g. cyclotron radiation, brehstrahlung, recombination, etc. each of which is affected differently as a function of temperature, particle density, and Z (nuclear charge). Cyclotron radiation is due primarily to electrons.

 

[berkeman]

BTW, here's a good discussion thread on the status of fusion power generation from a little while back here on the PF:

https://www.physicsforums.com/showthread.php?t=162537



.

 

[Potaire]

Very fast question I didn't want to waste a whole new thread on---

what is the proper term for an element in it's "pure" isotope--the isotope when the neutrons and protons are equal in number--it's natural and most common (I'd think) state?

 

[cesiumfrog]

No such thing.

You're thinking of all radioactive states as some kind of abomination, for you to contrast with "stable" states (like black and white.. that approach leads to outlawing nuclear industry and stem cell research, or permitting slave trade ). Instead think of all possible nuclei as being "equal" and radioactive, although some have such short half-lives we don't even see them, and some have such long half-lives you might never see one decay, and others lie anywhere in between (like shades of grey). Yes, the number of protons in a nucleus determines the number of electrons it attracts, and hence the way it behaves chemically (the element). And it's natural that nuclei which don't last as long aren't seen as often, but no reason there there couldn't be several (practically) equal-stable-ness isotopes of a single element, none could claim to be the more "pure" element (though the term you asked for is likely either "(most) stable isotope" or "most (naturally) abundant isotope").. and incidentally, isotopes are generally more stable if they have more neutrons than protons (more glue per repulsive charge).

 

[LastGuyOnTheLeft]

[...] and incidentally, isotopes are generally more stable if they have more neutrons than protons (more glue per repulsive charge).

Just a comment, this is true for heavier elements. Lighter elements (for example carbon) like N=Z more.

 

[arivero]

I am guessing that the Coulomb Barrier is the major problem?

I'd bet for cross-section as a problem too. But yep, we try H + H instead of C+C, so Coulomb seems to be an important issue.

 

[arivero]

Very fast question I didn't want to waste a whole new thread on---

what is the proper term for an element in it's "pure" isotope--the isotope when the neutrons and protons are equal in number--it's natural and most common (I'd think) state?

I can not think of a name now. It is only natural if isospin is preserved, and electromagnetic charge breaks isospin. But within isospin symmetry, all the nuclei in the series from Z=1, N=k to Z=k, N=1 are in the same footing.

The nucleus with the number of neutrons and protons exchanged is usually called a "Mirror nucleus" of the original one.

The line of the most stable nuclei along all the NZ plot is called the "stability line". Perhaps you are asking for this. As other have pointed out, the stability line only coincides with the diagonal N=Z for lowest values of N,Z.

 

[Astronuc]

Very fast question I didn't want to waste a whole new thread on---

what is the proper term for an element in it's "pure" isotope--the isotope when the neutrons and protons are equal in number--it's natural and most common (I'd think) state?

Ar-36 (Z=N=18) is the heaviest stable nuclide for which Z=N, although Ca-40 (Z=N=20) might be considered stable since it's half-life is > 3 x 10²¹ yrs. Heavier elements have N > Z for stability much for the reason indicated by cesiumfrog - that the neutrons dilute the coulomb repulsion while providing the nuclear binding forces.

One can review the properties of nuclides at -
http://www.nndc.bnl.gov/chart/ - use Zoom 1 to see the details.

 

[Potaire]

jeez--just thought there might be some cute little name is all--didn't mean to get into who is radioactive and who objects to being 'labeled" radioactive and what "normal" means, or anything like that. I just thought the might be a name for the isotopes like H1, He4, Li6, C12, etc, etc, etc. But, interesting info anyways---thanx.

 

[mgb_phys]

Other than stable, I don't think there is a prticular name.
For larger elements they are still stable when there is a small surplus of neutrons.

 

[Morbius]

Ar-36 (Z=N=18) is the heaviest stable nuclide for which Z=N, although Ca-40 (Z=N=20) might be considered stable since it's half-life is > 3 x 10²¹ yrs. Heavier elements have N > Z for stability much for the reason indicated by cesiumfrog - that the neutrons dilute the coulomb repulsion while providing the nuclear binding forces..

Astronuc,

Just to add to what Astronuc has stated, it is also instructive to look at the Semi-Empirical Mass Formula
which is an empirical fit to the binding energy:

http://en.wikipedia.org/wiki/Semi-empirical_mass_formula

Both neutrons and protons add to the binding energy via the "volume" term - the first term proportional to
A - which is the sum of the number of protons and number of neutrons.

However, the protons also bring in an "unbinding" energy due to their mutual repulsion. That's the
third term which is negative, so it is "unbinding" and is proportional to Z(Z-1). The more protons
are added this "unbinding" energy is going becoming larger roughly with the SQUARE of Z.

If you add a new proton to a nucleus with Z protons - then there will be Z/2 new pairs of proton-proton
repulsions. That's where the Z(Z-1) term comes from. For each of Z protons; there are (Z-1) other
protons to repel. This double counts the number of pairs - so you divide by 2; but that is absorbed in
the leading coefficient.

So for large atomic number nuclei - large Z - the addition of protons leads to a proportionately larger
increase in "unbinding" energy. Adding more neutrons doesn't have that effect; so neutrons work like
"glue".

Dr. Gregory Greenman
Physicist

 

[Potaire]

OK, cool. Perhaps in my mind I will use a term like the "standard" isotope of Whatever. Thanx all.

 

[Astronuc]

It's not so much that there is a standard isotope of a particular element, but rather certain isotopes are more abundant than others.

Some elements have only one stable isotope, e.g. Be, F, Na, Al, P.

Ruthenium (Ru) has 7 stable isotopes, Mo and Pd have 6, although some consider Mo-100 stable, which would be a 7th stable isotope.



This is cool - http://en.wikipedia.org/wiki/Isotope_table_(complete)

 

[berkeman]

This is cool - http://en.wikipedia.org/wiki/Isotope_table_(complete)

Oh, WOW! I've never seen the Periodic Table shown like that. How cool is that! Thanks Astro.

 

[Potaire]

I just receiced the same chart in book form--exact same only runs bottom left to upper right, instead of upper left to bottom right.
http://www.chartofthenuclides.com/

This online version is interactive and WAY, WAY, WAY cool:
http://www.nndc.bnl.gov/chart/

 

[Morbius]

Oh, WOW! I've never seen the Periodic Table shown like that. How cool is that! Thanks Astro.

berkeman,

Actually it's NOT the Periodic Table. The Periodic Table graphically shows the Elements - the building
blocks for the science of Chemistry. However, in the Periodic Table, there is a SINGLE entry for any
given Element. There's only one entry for Tungsten, for example. That's because all isotopes of a
given Element behave identically in their chemical properties.

The table Astronuc posted is a Table of the Nuclides. It gives an entry for each isotope of each element.

Although different isotopes of the same Element are chemically identical; their nuclear properties are
NOT identical. Deuterium [ D2 or H2 ] , the heavy form of Hydrogen has a lower propensity to absorb
neutrons than its chemical twin "Protium" or ordinary light Hydrogen. That's why "heavy water" has
different nuclear properties than "light water".

In summary, the Table of the Nuclides is more than just an alternate presentation of the Periodic Table.

It contains MORE information.

Dr. Gregory Greenman
Physicist

 

[Potaire]

Exactly, Dr. I highly recommend checking out the interactive Table of Nuclides I mentioned above--link provided. It is awesome! And I agree with Dr. about the usefulness of the Periodic Table--great for Chemistry, but of limited value in our applications.

 

[Rade]

It's not so much that there is a standard isotope of a particular element, but rather certain isotopes are more abundant than others. Some elements have only one stable isotope, e.g. Be, F, Na, Al, P. Ruthenium (Ru) has 7 stable isotopes, Mo and Pd have 6, although some consider Mo-100 stable, which would be a 7th stable isotope. This is cool - http://en.wikipedia.org/wiki/Isotope_table_(complete)

Question. Note that for the isotope chart:
http://en.wikipedia.org/wiki/Isotope_table_(complete)

we find no stable isotopes at n= 19, 21, 35, 39, 45, 61, 89, 115, 123 (for n from 0-126)
and no stable istopes at z = 43 and 61 (for z from 1-82)

Does anyone know theory that helps explain these patterns--seems strange that so many gaps in stability exist in the building of atoms. How are these gaps explained by shell model and/or collective model (or any other model of atomic nuclei) ?

 

[Astronuc]

Question. Note that for the isotope chart:
http://en.wikipedia.org/wiki/Isotope_table_(complete)

we find no stable isotopes at n= 19, 21, 35, 39, 45, 61, 89, 115, 123 (for n from 0-126)
and no stable istopes at z = 43 and 61 (for z from 1-82)

Tc (Z=43) and Pm (Z=61) are interesting cases.

Another part of the puzzle is the stability of even Z and the lack of stability of odd Z.

Z=39, 41, 45, 49, 53, 55, 57, 59, 63, 65, 67, 69 all have only one stable isotope, where as the neighoring even Z elements have multiple stable isotopes.

This might help somewhat -
Nuclear mass and stability - http://book.nc.chalmers.se/KAPITEL/CH03NY3.PDF
Nuclear structure - http://book.nc.chalmers.se/KAPITEL/CH11NY3.PDF

From
http://book.nc.chalmers.se/KAPITEL/CONTENT.HTM

 

[quetzalcoatl9]

That's because all isotopes of a
given Element behave identically in their chemical properties.

not to nitpick, but in fairness this is only approximately true.

 

[Astronuc]

I believe that chemcially, i.e. they way isotopes of one element iteract with other elements, isotopes are considered to have identical chemical behavior.

Now physically there is a difference, and that difference is exploited with respect to isotopic separation processes such as the Silex process.
http://en.wikipedia.org/wiki/Silex_Process

 

[quetzalcoatl9]

yeah but not exactly...for example, when we solve for the 1s wavefunction of hydrogen it is the reduced mass that factors in and so a change in the mass of the nucleus will have a (albeit small) effect on the 1s orbital. while the difference is small, it can probably be detected spectroscopically (not entirely sure about that tho). while i don't know anything about silex, it seems that this is how the excitation is different, no (that is, a small degree of field coupling occurring between the nucleus and the electrons, leading to fine structure)? i would consider electron excitation to be within the realm of chemistry.

also, ANY chemical process that involves vibronic coupling will be affected by isotopic differences.

also, the vibrational modes of diatomics will obviously be different (markedly so for say DD or TT vs. HH), and this may affect various chemical and/or physical processes.

 

[Astronuc]

yeah but not exactly...for example, when we solve for the 1s wavefunction of hydrogen it is the reduced mass that factors in and so a change in the mass of the nucleus will have a (albeit small) effect on the 1s orbital. while the difference is small, it can probably be detected spectroscopically (not entirely sure about that tho). while i don't know anything about silex, it seems that this is how the excitation is different, no (that is, a small degree of field coupling occurring between the nucleus and the electrons, leading to fine structure)? i would consider electron excitation to be within the realm of chemistry.

also, ANY chemical process that involves vibronic coupling will be affected by isotopic differences.

also, the vibrational modes of diatomics will obviously be different (markedly so for say DD or TT vs. HH), and this may affect various chemical and/or physical processes.

Physical processes and molecular vibration would certainly be affected by mass differences, particularly for the lightest elements like H. However, I don't believe chemical properties are affected, particularly stoichiometry.

The key distinction is that isotopes cannot be separated chemically, but must be separated by physical processes such as mass spectrometry, diffusion, or differences in light absorption properties. I believe ionization (and excitation) is considered a physical process not a chemical one, and perhaps the distinction is somewhat arbitrary and superficial.

 

[theCandyman]

I did hear once from my inorgainc chemistry professor that he was working with heavy water and it behaved differently from light water in an experiment he was doing with mixing concrete. I'm not sure that would count as a chemical effect though.

 

[quetzalcoatl9]

I did hear once from my inorgainc chemistry professor that he was working with heavy water and it behaved differently from light water in an experiment he was doing with mixing concrete. I'm not sure that would count as a chemical effect though.

heavy water is one of the few compounds where the isotope effect on it's chemistry is readily evident. the OD vs. OH bond dissociation energy is a few percent different. light water at STP has a pH of 7.0 vs. DOD has a pH of 7.4 under the same conditions.

 

[quetzalcoatl9]

The key distinction is that isotopes cannot be separated chemically, but must be separated by physical processes such as mass spectrometry, diffusion, or differences in light absorption properties. I believe ionization (and excitation) is considered a physical process not a chemical one, and perhaps the distinction is somewhat arbitrary and superficial.

Astronuc,

On the wiki page for MLIS, it is stated:

Instead of vaporized uranium of AVLIS, the working medium of the MLIS is uranium hexafluoride, which requires much lower temperature to vaporize. In every stage, the stream of UF6 is irradiated with an infrared laser operating at the wavelength of 16 µm. The mix is then irradiated with another laser, infrared or ultraviolet, are selectively absorbed by the excited 235UF6, causing its photolysis to UF5 and fluorine.

http://en.wikipedia.org/wiki/Molecular_laser_isotope_separationif i understand this process correctly, then it would seem that the isotope is "selected" by preferentially exciting the vibrational modes of the species of interest. once the vibrational mode of that population is excited a UV beam is applied to excite the electronic state. it would then seem (and this is a big guess here) that there is a pathway between the two excited state surfaces. that is, there is ample vibronic coupling such that the electronic transition frequency for the vibrationally-excited species is different from the non-vibrationally excited species. if this were not true, then there would seem to be no reason for applying an IR beam. is this how the process actually works? clearly the born-oppenheimer approximation breaks down here.

 

[Paulanddiw]

heavy water is one of the few compounds where the isotope effect on it's chemistry is readily evident. the OD vs. OH bond dissociation energy is a few percent different. light water at STP has a pH of 7.0 vs. DOD has a pH of 7.4 under the same conditions.

I believe that if you drank enough heavy water, it would posion you.

 

[TestUser12]

I believe that if you drank enough heavy water, it would posion you.

You'd need to replace 25-50% of the water in the body with heavy water in order to kill yourself. (That's a lot)

See http://en.wikipedia.org/wiki/Heavy_water#Toxicity_in_humans

Also, you would also be poisoned if you drank enough light water. (Somewhere around a couple of gallons in a few minutes)

See http://en.wikipedia.org/wiki/Water_intoxication

I fail to see your point.

 

[Morbius]

yeah but not exactly...for example, when we solve for the 1s wavefunction of hydrogen it is the reduced mass that factors in and so a change in the mass of the nucleus will have a (albeit small) effect on the 1s orbital. while the difference is small, it can probably be detected spectroscopically (not entirely sure about that ...

quetzcalcoatl9,

not to nitpick - but the issues you raise about the wavefunction being different are true - but
those are usually classified as being differences in the physics NOT the chemistry.

We can separate isotopes by weighing them - as in the case of electromagnetic separation,
gaseous diffusion, ... but those differences are considered physical, not chemical.

We can separate isotopes by laser isotope separation; because as you point out the reduced mass
is used in calculating the wavefunctions; hence the ionization potentials are different - however,
again - those are called physical differences; NOT chemical.

When I or most scientists say "chemical" - we mean the chemical structure - how many valence
electrons, and what reactions are possible. In those cases, isotopes ARE chemically identical.

Dr. Gregory Greenman
Physicist