DEC 11

This article is part of a series on alternative possibilities in nuclear power.
Previously at EcoGeek: Alternative Possibilities in Nuclear Power

Pebble Bed Reactor
The pebble-bed reactor was supposed to be another intrinsically safe, and "melt-down proof" design. "Pebble bed reactors are helium-cooled, graphite-moderated reactors in which the fuel is in the form of tennis ball-sized spherical "pebbles" encased in a graphite moderator. New fuel pebbles are continuously added at the top of a cylindrical reactor vessel and travel slowly down the column by gravity, until they reach the bottom and are removed." Cooling uses an inert gas such as helium, rather than a liquid, which simplifies many of the reactor systems.

"The use of helium and graphite allows the reactor to burn the fuel efficiently and to operate at much higher temperatures than conventional light water reactors." Since the pebble bed reactor was already designed to operate at very high temperatures, and since its cooling medium was a gas, rather than a liquid, the control systems for a pebble bed reactor could be much simpler. The largest problems that need to be dealt with for a boiling water reactor - overheating and coolant boiling away - are not concerns for a pebble bed reactor. The pebble bed also produces less power as the temperature rises, so the design is effectively self-limiting.

The pebble bed design offers some operational advantages, such as allowing the reacor to operate constantly without needing to be shut down periodically for refueling. As each pebble makes its way through the system and is drawn out at the bottom of the reactor, it can be tested and either reinserted at the top of the reactor (the average pebble would cycle through the reactor about ten times before it was expended) or withdrawn if it was spent. New fuel pebbles could also be added when needed to keep the reactor operating.

Early experimental work with pebble bed reactors was carried out in Germany beginning in the 1960s. Pebble bed reactors were thought to be a promising next step in reactor design. But several issues operational made the pebble bed design less than ideal. Contaminated graphite dust is created from the pebbles from friction as they move down through the reactor. Tests carried out with dummy pebbles also found overheating conditions inside the reactor. The volume of radioactive waste from a pebble bed reactor is larger than that from other designs, which presents more of a problem when dealing with spent pebbles. And decomissioning the reactor may have higher costs because of the radioactivity of the reactor components.

Because of these problems, the German project was abandoned by the 1980s, and rights to carry on the work were obtained by a series of South African companies. However, after years of development, the work on developing a pebble bed reactor has pulled curtailed<, and the company is now concentrating solely on high-temperature industrial applications (such as coal gasification) for the technology. At present, China remains the only country working on developing pebble bed technology.

Links: Bulletin of the Atomic Scientists
European Nuclear Society
Pebble bed reactor (Wikipedia)

Comments (10)

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How the current reactors compare........
written by James Aach , December 11, 2011

The pebble bed reactor is an interesting concept, but as the article discusses, there are often unexpected problems going from the drawing board to the grid. The same is certainly true of the current generation of reactors. I think we'll make better decisions about our energy future if we first understand our energy present. I've worked in the US nuke industry for 20+ years and wrote "Rad Decision," a techno-thriller novel covering the good and the bad. It is available free online - just Google the title. I have no sponsors and there are no advertisements. See the homepage for reviews. The featured plant is similar in design to Fukushima and the climatic event is similar to that tragedy. (Whoopee, I guess.)

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...
written by Andrew Meadows, December 12, 2011

Graphite isn't very strong material so yeah, it would tend to break apart, especially under heat and bombardment by slow neutrons.

I don't understand the claim that the coolant can't boil off. It is pretty easy to lose Helium gas. What to do when the helium is gone? Pump nitrogen through?

The stability of the reaction when it gets too hot... I don't understand that either. I assume some physicists ran the numbers using perfectly hard spheres and an exact melting point? Seems to me you'd end up with soft graphite spheres as things heated up. At the bottom of the pile you'd get local hot spots, distortion, and welding which would cause the pebbles to fall apart much faster than normal, maybe even clogging the funnel at the bottom.

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Greenies killed Germany's pebble bed
written by Marje Hecht, December 13, 2011

Germany's pebble bed program was shut down because of Green political opposition, including terrorism, not because of any "problems" with the technology.
The same Greenies went after South Africa's PBMR program, aided by the short-sightedness of the present government. You can find some of the documentation here:
http://www.21stcenturysciencetech.com/Articles 2008/F-W_2008/HTR_4.pdf

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Helium Loss
written by John Frescki, December 15, 2011

I read an extensive article in "Wired" of all places about 2 years ago on these reactors. If I remember correctly (IF), in the event of helium loss increased temperature and pressure would effectively force the graphite balls apart, and the seperation would create the self-limiting nature of the reaction.

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Hopper Jamming
written by Aiyoung, December 15, 2011

One immediate concern i'd have with this design is the possibility of the hopper that delivers the fuel could jam.

See this video of a 2-D experiment of a hopper to see what i'm talking about.

http://youtu.be/lWSJwZhqoQw

Hopper jamming and granular materials is a pretty active area of research.

The researchers who made the above video recently published a paper on that work.

http://www.physorg.com/news/2011-12-stress-clogs-coffee-coal.html

While hopper jamming is an irritation in production of pharmaceuticals and coffee and grain industries, it'd be a nightmare to have to unjam a hopper full of radioactive fuel!

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Waste (of time?)
written by Seb, December 19, 2011

Well, it'd be good that nuclear gets safer but that still doesn't solve be main major problem with nuclear: What do we do with the wastes? No real solution so far as been found even though nuclear technology is used widely.
If only the billions spent in that stupid technology would have been spent in energy consumption reduction, efficiency and renewable, we'd already have a world without coal or nuclear. Sad.

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Liquid Floride Thorium Reactor
written by Brad, January 10, 2012

There have been some impressive things from the re-evaluation of thorium based nuclear power that was initially dropped in the nuclear weapons development. A former NASA engineer, Kirk Sorensen has been a huge part of educating the public about this technology.

I highly recommend one of the LFTR Remix videos on Youtube and/or checking out energyfromthorium.com

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electronics recycling
written by Jeff Birks, February 06, 2012

Fission is only a step towards fusion, and fusion has far less waste than fission. If steps like this make nuclear power more acceptable then it will help attract funds towards research fission.

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French using vitrified nuclear waste to heat schools?
written by Charlie Goodman, February 07, 2012

I can't find it now but a couple years ago I heard or read about France using nuke waste sealed in 1m3 glass cubes with copper pipes wrapped around and buried not that deep under primary and secondary schools in France to heat the buildings. Was that a dream? Anyone else hear or know about this?

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Thorium
written by Blake, March 30, 2013

One of the more interesting pebble bed reactors that used to exist, I read about recently, was one that had both fissile uranium and fertile thorium pebbles in it.

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