Interesting summary.
I have one question though.
Why do you think a pebbel bed reactor should be more succesful for ship propulsion than, for example, the traditional PWR's used in that niche?
Obviously the impossibility of meltdown is one pro, but couldn't that also be done with passive cooling, like in the AP1000?
And has anyone done any research on reprocessing fuel pebbles?
Indeed. It appears to me that it would be very simple to carve off the bulk of a spent fuel pellet, using abrasive jets or electrical discharge machining. You could leave just a small coating of SiC/graphite around the actual spent fuel and then store that.
Just out of curiosity, Rod... what would allow burnups as high as 750,000 MWD/ton?
The big advantage for pebble bed gas cooled reactors is in the part of the system that actually converts heat into useful energy.
Though steam plants are venerable and well proven, they have certain limitations that have made them not competitive against diesel engines and gas turbines in the fossil fuel market.
The same limitations apply if the heat source is nuclear. Those limitations change considerably if the heat engine is a Brayton (gas turbine) cycle. It is not possible for light water reactors to produce their heat at a temperature high enough for an efficient Brayton cycle machine, that is why I like the high temperature gas cooled reactor.
Carving off graphite from a fuel element where you have 10,000 or so 1 mm diameter particles in a 6 cm diameter graphite and fuel matrix fuel ball can be challenging.
Complete burn-up would be 1,000,000 MW days per ton. That would be a bit tough to achieve, but it might very well be possible to achieve a 75% burnup if the coatings can hold and it is possible to keep adding fresh fuel in strategic locations to keep the reactivity sufficiently high.
Rod
What about using supercriticalwater reactors? Then you could get nice fuel elements with no reprocessing issues, 45 % thermal efficiency and you could probably also throw in passive cooling à la AP1000.
Obviously, no one has ever managed to build a SCWR as of yet due to material issues, but the GEN-IV programs are doing their best as we speak.
Starvid:
I do not know anything about supercritical water reactors.
If you are talking about high temperature reactors that produce high enough temperatures to use a supercritical steam plant as the power producting secondary, I believe that the Toshiba 4S system is designed along those lines. That plant is a liquid metal cooled reactor that can produce steam temperatures in the 500-600 C range, much higher than light water reactors.
I know steam, like steam, but do not think it is well suited for the applications that we are interested in pursuing.
I also have no interest in science projects - when someone tells me that there is just a material issue to solve, I shy away. Our interest at Adams Engines is in applying technology that has already been developed and proven to actually produce power for people to use.
BTW - Engineer-Poet, one of my readers, a student from Italy, pointed me to a good source of information about "deep burn" in reactors using TRISO coated fuel particles. The company making the presentation, General Atomics, has been in the gas cooled reactor technology field for about 50 years and actually built two plants in the US in the early days of nuclear power.
Here is the link (you might need to piece it together to make it work.)
http://www.iaea.org/inis/aws/fns...ste-
2005_DB.pdf
Rod
The SCWR is presented here http://en.wikipedia.org/wiki/SCWR
It is basically a direct cycle LWR, like a BWR, but due to the water being supercritical it wouldn't boil, or something like that.
It is still basically a science project (mainly Japanese IIRC) and has certain materials and chemistry issues (uh-oh). It won't come around for quite some time. 10-20 years maybe, far away at least.
But sadly I have a feeling we won't see civilian nuclear ships in the next 10 years either, though they will come eventually. Let's say peak oil hits in 2010-2015 and then we'll see the first nuclear ship 5-10 years later.
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By the way, another exciting retro/futuristic reactor design is the molten salt reactor.
Check it out at http://en.wikipedia.org/wiki/
Mol...en_salt_reactor
What about using magneto hydrodynamics dynamo for a first stage (which would have to be very hot for a closed cycle) and a Brayton cycle for the second stage?
http://en.wikipedia.org/wiki/
Mag...odynamic_dynamo
As I mentioned, one of the problems with using these now is that very high temperatures are required to ionize the seed material, which nuclear just happens to be capable of producing if you can contain the high temperature reaction.
Another problem with the classical MHD is the extremely high temperature discharge you have left on the other side, but that's been solved with using that heat in another cycle, such as the Brayton cycle as I mentioned above.
The relevance directly with MHD in this thread is that obviously water cooled reactors would have a really tough time producing the temps we are talking about, but a gas cooled pebble bed shouldn't have a problem as long as it doesn't melt it's way out of the container.
Although the carbon in the pebbles can safely go very much hotter than its design temperature, one of the pebble-bed reactor's strong points is that fission cannot. So don't assume just because it's carbon that it can feed gas that is extremely ionized, just because it's crazily hot, to an MHD throat.
Purely carbon-moderated reactors tend progressively to shut themselves off as you try to raise their temperature closer to about 1,500 K. This may help helium/carbon reactors rapidly load-follow: if you force more helium through them, they get cooler, but not as much as the increased flow rate would suggest because being cooler makes them speed up.
--- Graham Cowan, former hydrogen fan
boron: internal combustion, nuclear cachet
Actually my most serious concern was that the melting temp and vaporization temp of most of the materials I can think of in a reactor are BELOW 1500C. Also, from what I know of MHD's, they are using Cesium seed gas because it has a low ionization temp so it's the "low bar" we can shoot for. I wasn't even thinking of ionizing Helium or Nitrogen, because IIRC, they have to get much much hotter to ionize.
When I was thinking of this my main concerns were liquifying or vaporizing the fissile material and chemical reactions causing problems, like eating away the heat exchanger or bonding with the fissile material, moderator, or containment cell (Cesium is very reactive, which is why it's so easy to ionize).
The reason this fascinates me is that chemical reactions have an inherent peak temp they can reach, and very few can get hot enough even at perfectly stoichiometric combustion to work an MHD, even with a seed gas like Cesium in a closed cycle, and those are in high demand in other areas (Methane-home heating). Moderator issues aside as you point out above, nuclear can get much much hotter in theory. High temps are exactly what's needed to increase the Carnot efficiency (with cycles that can work with the high temps), and MHDs fit neatly into the equation for the icing on the cake.
If fusion ever gets off the ground, I've heard that the first stage may be an MHD because the products will be superhot plasma, exactly what an MHD needs. The second cycle will still have immensely hot effluent to use in a traditional Carnot based heat engine.
Rod, the fuel pellet design on page 4 of the Gudowski presentation contradicts one of your claims. The fuel is held in the center of the pellet, not throughout the volume; it appears that it would be feasible to machine off the outer two layers and reduce the total volume by perhaps a factor of 4, or simply carve through it and expose the spent material for reprocessing.
It was also my impression that fission products are neutron absorbers and poison chain reactions. Could you really achieve such high burnups?
Engineer-Poet:
That is a common misperception. Most fission products are neutron rich already and have little affinity for absorbing more neutrons. There are only two or three that have any effect on reactivity at all, and most of that effect is transitory due to the half lives and other production/depeltion terms in the differential equation that solves for the reactivity effect.
Rod
Misperception?
Virtually all fission fragments start out neutron-rich; that's why they do negative beta-decay. After a few beta decays, for some mass numbers -- 149 is a big one, IIRC -- they're not so neutron-rich any more.
--- Graham Cowan, former hydrogen fanboron: internal combustion, nuclear cachet
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