Fusion Now

John · Senior Member

Fusion Now

iprimap · Veteran Member

John,

Please consider what Rod Adams writes about the subject in the following essay:

Fusion vs Fission: Difficult vs Easy http://www.atomicinsights.com/AI_03-04-05.html

When I was a child in the 1960s scientists stated that commercial fusion power was 50 years away. The time is 40 years later and commercial fusion power is still 50 years away.

Commercial fission power provides the greatest hope for solving mankind's energy needs without environmental pollution. With reprocessing spent nuclear fuel and using thorium, there is enough available fission fuel to last mankind tens of millennia. Fission can provide low cost, pollution free electricity, hydrogen gas to replace petroleum for vehicular fuels, and the means whereby mankind can set out to explore and colonize the solar system. Fission is available right now. In 50 more years scientists may well be saying what they did in the 1960s. We need to use what is available and safe right now, and not place false hopes in an unproven technology. With global warming from green house gas emissions from fossil fuel burning and the depletion of the world's oil resources, there is no other logical choice.

Regards,

Paul

John · Senior Member

Fusion may well be 50 years away at the rate our politcal leaders are dealing with the matter. At the time of the great fusion budget cut in 1994 the U.S. government was saying that it had spent $4 billion. (With the reduced level we might have spend another $3 billion since then for a totoal of about $7 billion.) There are corporate investments and foreign efforts which are no in advance of our own. Right now the goverments are debating the detail of the planned ITER. The last I've is the want to cut the budget in half too.

My point is that over the time frames in question $7 billion isn't much. Any major weapons systme like a tactical fighter would cost well of $20 billion to develop. The fusion effort is hardly a nation commitment like project Apollo. I've even heard that the funding is so sparse that its is hard to get top quality people to work on the subject. (Granted high energy physicics is more fun but it doesn't that the practical payoff.)

In the lower right hand corner of the page I'm posting and you will see a chart that show continuous progress in energy output until the 1994 budget cuts (with a hoped for ITER result in the future).

http://fusioned.gat.com/images/pdf/what_is_fusion.pdf

What is notable is the lack of real progress since the mid-1990s. The reason for this is the Congressional budget cuts (pre-Gingrich as speaker too.) that shut down our best experienents and did give enough funds to build a real next step after the TFTR. Now its twelve years later and we are still talking rather than acting.

I think the cold fusion fraud/farce has done a lot to sap the focus on the real goal. Plus, I a feeling that some special interests really don't want success. Note that it was the breakeven experiments with the TFTR what occurred just before the budget got slashed.

On the issue of exiting nuclear technology, let me say that I have no problem with using fission power plants. I was total against the policy of no more nuclear plants after Three Mile Island. We should start building new nuclear power plants to fill the gap between now and when fusion becomes commercial which will be at least 20 years from the point of commitment. I would also increase oil exploration ANWR and off shore.

10kBq Jaro · Senior Member

I think - and this has been said before by others - that the problem often lies in how people define "success" in thermonuclear fusion.

Getting a lot of energy out of the reaction is one thing, and its likely that ITER or the next project after it, might succeed in producing as much energy as is pumped in to keep it going.

But this has nothing to do with being successful as a technology that can be made sufficiently practical, reliable and economical for use by commercial utilities.

An obvious case in point is the fact that you've got an enormous container with a hundred-million-degree temperature on the inside, and supercooled helium on the outside, with a huge flux of high-energy neutrons dumping heat energy all around.

To gain a sense of perspective, consider an interesting comparison with a fission-type reactor that uses an arrangement that is in some respects similar : the Canadian CANDU reactor.

It is able to use natural, un-enriched uranium quite effectively, because it uses a relatively small amount of high-pressure hot coolant, while the neutron moderation function is carried out by a separate system using a relatively large reservoir of cooled heavy water moderator (this in contrast to PWRs, where the hot water in the pressure vessel provides both the cooling and neutron moderation function simultaneously).

The coolant circulates inside pressure tubes (which also contain the fuel rods) at around 300 C, while the moderator is kept at about 80 C, through the use of pumps and heat exchangers that dump the low-grade heat to the environment.

The colder the moderator, the slower the neutrons, and the higher the fission x-section of the uranium nuclei in the fuel.

Now if we decided to go to a moderator cooled to liquid helium temperature, like the fusion reactors, our CANDU reactor would become far more effective at fissioning uranium -- we could get a much higher fuel burn-up before having to discharge the "spent" fuel. Probably something close to what PWRs get, with five times higher U235 content.

But the flux of fast neutrons (and gammas) from fission reactions dumps several dozen megawatts of heat into the moderator.

It would be utterly ridiculous to try to maintain the moderator at near-absolute zero temperatures, with that much heat going in.

You couldn't just dump the heat to the environment, because the environment is much hotter -- you would need a vast refrigeration system, consuming an enormous amount of power (not to mention its capital cost).

Only the fusion community could come up with such a design and call it a "success."

Utilities wouldn't buy it in a million years.

This is what's meant by "Fusion vs Fission: Difficult vs Easy."

PS. as an interesting historical note, recall that scientists in war-time Nazi Germany experimented with reactors made of lumps of uranium embedded in a large block of dry ice (i.e. frozen CO2). It was a brilliant idea, since dry ice is a lot easier for producing pure material, free of contaminants that absorb neutrons -- in contrast to graphite, which requires a lot of technology to produce "nuclear grade" material. On top of that, the low temperature of the dry ice makes for a very effective neutron moderator. Given enough time & effort, the German scientists would no doubt have succeeded in getting their reactor to go critical. But guess why we don't see dry ice reactors operating today ?

John · Senior Member

I see the point very well. The are some radical thermodynamic issues in both cases. As far as dry ice well other approachs proved better. One does not need dry ice to make a successful fission reactor. We may well need superconductivity in the thermonuclear case. The other thing is that the 100 million K temperatue in the plasma is no in physical contact with the container so you only have radiation coupling while moderators in fission reactors you conduction of head. So it is the x-ray energy that has to be removed. This is one reason for D-T over the D-He3 is that overate in a region were the x-ray is massive.

The neutrons in a D-T fusion reators are your main usable energy while He4 energy is used to reheat the plasma. Lithium is then used to capture the neutrons and breed additional tritium at the same time. D-T reactor is at heart a breeder (or it really is a dead end). So when you get to the cryogenic part you are on the outside with layers of cooling and insolation reducing the burden of cooling the magnets. I don't think it is nearly the same as using a dry ice a moderator like the Germans tried.

This is a real challenge I do not disagree. I'm just trouble that the whole things is beening left to an international organization in the case of the ITER. If they succeed there is a real question economics and reliability. However, I do have a little hidden agenda here that I'll reveal. I'm also most interested in fusion for space propulsion and unless a working reactor is demonstrated by the energy department people I rather doubt NASA would even consider using it for space beyond "pie the sky" paper studies like the Breakthough Propulsion Project some years back.

10kBq Jaro · Senior Member

John wrote:

I rather doubt NASA would even consider using [fusion] for space beyond "pie the sky" paper studies like the Breakthough Propulsion Project some years back.

True, but while they may not use fusion as such, they may use some of the technology developed for it, in particular magnetic confinement -- applied to the nozzles of VASIMR rocket engines.

PS. we have very little heat conduction between the hot pressure tubes and the cool moderator in CANDU reactors, because we have a gas annulus around the pressure tubes, with a second, so-called callandria tube, surrounding that (i.e. a set of concentric tubes, with a small gap between them). So most of the heat transfer really is by neutron & gamma radiation across the two sets of tubes.... This, incidentally, is what causes the moderator water in the reactor to light up with blue Cerenkov radiation, as shown in my animation :

http://www.cns-snc.ca/branches/quebec/CANDU/CANDU_control_animation.gif

http://www.cns-snc.ca/branches/quebec/CANDU/CANDU_PHT_anim_no_bkgrnd.gif

-- Edited by 10kBq Jaro at 18:52, 2006-01-29

G R L Cowan · Veteran Member

10kBq Jaro wrote:

PS. as an interesting historical note, recall that scientists in war-time Nazi Germany experimented with reactors made of lumps of uranium embedded in a large block of dry ice (i.e. frozen CO2). It was a brilliant idea, since dry ice is a lot easier for producing pure material, free of contaminants that absorb neutrons -- in contrast to graphite, which requires a lot of technology to produce "nuclear grade" material. On top of that, the low temperature of the dry ice makes for a very effective neutron moderator. Given enough time & effort, the German scientists would no doubt have succeeded in getting their reactor to go critical. But guess why we don't see dry ice reactors operating today ?

Dry ice is too cold. But if one were to invert the CANDU arrangement, or anyway pressurize the cold moderator, one could have 80-Celsius 200-bar CO2 with a density of, according to http://webbook.nist.gov/chemistry/fluid/ , 0.59389 kilograms per litre. Pretty thin, so probably one would want to put more effort into cooling it than with D2O; maybe to 40 Celsius and 0.84 kg/L.

Now, what's keeping the fuel down to 300 Celsius? Hmm, maybe there's a whole lot of fuel zipping through on little rail cars ...

Actually I guess it would all be pressurized,with the dense CO2 regions thermally insulated from, but at the same pressure as, the rest.

--- Graham Cowan, former hydrogen fan
Fire without monoxide

10kBq Jaro · Senior Member

Hi Graham,

I'm not sure I understand the second part of your post.

But the fuel is of course much hotter than 300 C -- its just that the coolant is circulated fast enough to keep it at about 300 C, in order to avoid gross boiling and dry-out on the surface of the fuel -- which would drastically cut heat transfer, at that pressure, and increase the likelihood of fuel failure (meaning a small, local leak of fission products into the cooling circuit).

I don't have the exact figures handy right now, but in fact the fuel temperature, at the center of the individual ceramic UO2 fuel pellets, is something close to 2000 C.

G R L Cowan · Veteran Member

10kBq Jaro wrote:

Hi Graham,
I'm not sure I understand the second part of your post.
But the fuel is of course much hotter than 300 C -- its just that the coolant is circulated fast enough to keep it at about 300 C, in order to avoid gross boiling and dry-out on the surface of the fuel -- which would drastically cut heat transfer, at that pressure, and increase the likelihood of fuel failure (meaning a small, local leak of fission products into the cooling circuit).
I don't have the exact figures handy right now, but in fact the fuel temperature, at the center of the individual ceramic UO2 fuel pellets, is something close to 2000 C.

Yes, I should have said 300 Celsius at the surface of the fuel pin.

I was trying to think about what would change if CANDU were changed to CANCO2U. 200 bars of pressure on strongly cooled CO2 can make it fairly dense, so I figured the low-pressure D2O at 80 Celsius would change to 200-bar CO2 at 40 Celsius. But then the fuel, also needing a pressure envelope, should logically share the same one. It will be cooled by supercritical CO2. The moderator channels will need to be thermally insulated from the fuel channels even though no pressure wall separates them.

--- Graham Cowan, former hydrogen fan
Fire without monoxide

John · Senior Member

Here is a link to the official ITER website. I hadn't seen this site before but it looks like my schedule is fairly realistic, i.e. 10 years to the first prototype. I guess the question remains can this international organization stay on schedule give its vulnerability to the whims of the member governments. In general the information on this site is good news to me!

http://www.iter.org/index.htm

I was in particularly encouraged by the fact that they are including the tritium breeding that is essential to making a working D-T fusion reactor.