Pebble bed reactor history and future
At the request of several listeners and blog readers, Shane and I decided to focus this podcast on the graphite and heavy metal “pebbles” that form the basic fuel element for a number of currently proposed reactor types.
We talk a bit about the inventor of the pebble bed concept – Rudolf Schulten – and about the success of the demonstration and testing reactor built using his fuel elements – the German AVR. We spend a good deal of time talking about the follow-on to the AVR- the Thorium High Temperature Reactor (THTR) and about the circumstances that led to its early demise.
Then we move on to more contemporary versions – the South African PBMR, the Chinese HTR and the planned follow-ons to that prototype and the Adams EngineTM. If you want to see some good information on all kinds of high temperature gas reactors, check out Professor Andrew Kadak’s presentation at http://web.mit.edu/pebble-bed/Presentation/HTGR.pdf
Hope you enjoy the show. For those that were kind enough to suggest the topic, we hope that we answered your questions in our own geeky way.
Sorry we got a bit long winded, we plan to cut back down to about 40 minutes – unless, of course, we get really excited about another topic.
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I’ll stick my neck out and attempt to answer Rod’s question regarding smaller sized spheres, which he posed at the end of the program. If the spheres are smaller, there is more total surface area to the fuel and therefore easier heat transfer, assuming that the total volume of fuel in the reactor stays the same. There was a recent article about hollow PWR fuel pins which would give the same kind of heat transfer advantage. Smaller and more numerous spheres would also give more contact points, sphere to sphere, in the core. This might reduce the heat transfer area a little. But if the fuel maintains its shape and is sufficiently smooth, that problem is hopefully negligible.
One reason I can see why you wouldn’t want to go to smaller pebbles is because that would result in removing more pebbles each day for inspection. I’m not a big fan of the PBMR because of the gumball machinery needed and the possible failures which might result from broken or chipped pebbles. The German AVR had some sort of stuck pebble incident, didn’t it? I suppose the reliability of the system and durability of the fuel is something best determined during demonstration plant testing.
One question I have long wondered about. What sort of test is used on the pebbles during the on-line refueling? As I understand it, each removed pebble is examined to determine its burn-up. If it hasn’t reached an appropriate level of burn-up, then the pebble is returned to the core. How do they quickly determine if a pebble has reached its limit?
T. Peters (if that T. stands for Tom – I need to have you get in touch with Wayne Turmel of the Cranky Middle Manager show. He has been dying for an interview with Tom Peters. . .)
You are correct in your analysis that smaller pebbles can provide more heat transfer area. If the volume of the core remains constant, the volume of the fuel and the volume of the gas in the core will also be constant for spherical fuel elements.
With regard to pebble removal – that is not necessary in our design. We use a fixed bed core and make up for fuel depletion and poison build up in other ways.
Sorry, I’m not that T. Peters.
I did a little searching on the internet regarding my question about how they determine the burn-up of each pebble in the PBMR online refueling system. It appears they plan to use a gamma ray spectrometer to measure the concentration of Cs-137 inside the fuel. They can then correlate a particular Cesium level to burn-up. This seems straight forward, enough.
T. Peters:
Interesting. That is one aspect of the other pebble bed projects that I have never learned much about.
i think that the early German projects (AVR and THTR) did it a different way. I never found much detail about their system, but I did find a decommissioning report on the THTR. (http://www.iaea.or.at/inis/aws/htgr/fulltext/29059899.pdf)It included the following paragraph:
“Also the nearly hands on decommissioning of the small burn-up measuring reactor, used for distinguishing fuel absorber and and graphite elements and measuring the burn-up of fuel elements containing 3.6 kg of highly enriched U-AI fuel, took place just after finishing core unloading in 1995.”
That sounds like they had a small critical assembly (similar to a research reactor) outside of the main reactor. I imagine that the world’s current phobia about using HEU – even when it is arguably the best technical solution to a problem like having a small critical assembly – has caused the change.
Hello.
I’d like to understand what is the actual state of the art of “dry” nuclear
waste reprocessing
systems like pyroprocessing,expecially those who can deal with oxide fuel
(for example at Russian RIAR).What are their efficiency in extracting
actinides,for example U or Pu and MA in/out and their typical
decontamination factors? What their typical size/footprint ? What about
their estimated cost?
My own idea is,instead to re-burn them in fast reactors (see integral fast
reactor program) that are costly to buil and difficult to operate with
multiple recycles (I suppose due to low burn-ups achievable),to burn Pu and
minor actinides in very high burn-up (e.g. > 700 MWg/kg HM),good neutron
economy thermal reactors with only one reprocessing (pyroprocessing) cycle.I
think that pebble bed reactors developed in South Africa or generally HTGR
based could have these features
http://www.technologyreview.com/read_article.aspx?id=12727&ch=energy
http://web.mit.edu/pebble-bed/Presentation/HTGR.pdf
Finally,I found a lot of Authors are really developing this point of
view,i.e.
thermal actinides transmutation versus fast reactors.
http://aaa.nevada.edu/pdffiles/nov1104/leon.pdf
http://www3.inspi.ufl.edu/icapp04/program/abstracts/4038.pdf
http://www.physor2004.anl.gov/PHYSOR%20Program%20(FINAL).pdf (pag.50)
Clearly,an other approach I thought about is to load pebble bed cores with
two different kind of Triso,of course in the right proportions:only thorium
Triso and
only Pu and Ma Triso in order to “consume” all o nearly all Pu/Ma fuel and
to reprocess only thorium Triso,if needed,or simply recirculate them in the
reactor if not needed (low poison or parasitic absorptions?)
A big question I know,but I’d appreciate any comments,
suggestions or opinions. Thanks.
Alessio
Wow – a series of great questions that you seem to have already started to answer for yourself. Thank you for sharing the links – they are quite interesting.
Your idea is certainly something worth development. I happen to agree very strongly with the direction that your thoughts are going – many of the issues that remain to be solved with regard to nuclear power become far less important if reactors can achieve the kinds of burn-ups that you mention. Uranium supply becomes almost infinitely large compared to the demand, waste storage requirements shrink by several orders of magnitude, and proliferation concerns almost disappear. Long life cores reduce the requirements for regular shutdowns and increase the earning potential for nuclear power plants.
Have you visited the Adams Atomic Engines, Inc. web site yet? Just imagine the usefulness of a high burn-up reactor with a simple energy conversion system that is placed in an area that needs clean water, abundant power, and some left over heat.
That’s what the future holds, I think.
Thanx Rod for your quick answer.Yes I just visited your interesting site
Don’t misinterpret me,I’m a huge fan of breeders in any form,molten salts or liquid metals,but they aren’t
now in costruction or commercial operation in western countries;south africans instead will start
to built pebble beds in the next september.So then,I’m asking myself: “what
can we do with existing infractures – or next to build – to burn nuclear
waste actinides?”.Forget for a while fast reactors,breeders,Ads accelerators
and of course molten salt reactors,altough I agree with you they are very
promising
Of course reprocessing TRISO fuel seems to be more costly than LWR assemblies.In my vision,at the begining we could
start to burn
LWR fuels in pebble beds/htr reactors at least together with low eneriched
uranium use ,after that HTR could burn burn its “own waste”
I’m not really a fan of pebble bed technology precisely because of the inherent stability of the pebbles. It seems to me that this technology is inherently “once through.”
In my mind, the Pressurized Water Reactor has been an enormous success and for the short term offers the best strategy for burning plutonium and minor actinides. There are a number of advanced fuel cycles proposed for doing this. Notable are strategies like CORAIL and similar approaches. These use existing infrastructure.
It seems to me that one of the biggest players in the so called “nuclear waste” game is not the minor actinides or even the plutonium. It’s the uranium. I really think that plutonium burning is well enough established for the short term – the single cycle of plutonium now broadly used around the world MOX – is well enough understood.
I think we could go a long way toward reducing the (largely irrational) public concern over so called nuclear waste by using a simple strategy – which is to run recovered uranium through heavy water reactors of the CANDU type. This would offer several advantages. First it would demonstrate that spent fuel is a resource, not a difficulty. Secondly it would provide a reservoir of uranium that is highly enriched with U-236. Once we have such uranium – and who can doubt that we can address the reactor physics issues – we have uranium that will always produce plutonium that has some Pu-238 in it, even some Pu-236 from the Np(n,2n) reaction. This will, of course, have significant non-proliferation value. This would kick the props from under certain anti-nuclear arguments.
All of us here know that most of the anti-nuclear arguments are absurd on their face. The major stumbling block to nuclear power, although it is certainly relaxing, is not technical at all, but perceptual. We have an almost unlimited reserve of nuclear technologies from which to choose. Probably it is the case for the short term that we should include perceptual issues as much as technical issues in selecting those best suited for the short term.
For the long term I believe we will need some reactors with fast spectrums. Fast reactors are just superior for minor actinide burning. Personally, I don’t have a problem with curium and californium – I think Californium can be a very useful element. But fast reactors will allow us to determine that we have exactly the amount of Californium we can use, and no more. For us to realize this degree of control, we will have to have fast reactors.
Once uranium prices rise, and they will, fast reactors will increasingly become more economic. I think that there are a lot of excellent fast reactor technologies out there. Sodium is just OK with me, but I like Lead-Bismuth and I think that people have not seriously investigated the idea of fast molten salt reactors. People like to think that MSR’s are all thermal breeders with lithium/beryllium salts, but there is no reason that they have to be of this type. If you really think about it, the size of the periodic table makes the neutron spectrum tunable in the molten salt system. We’ve just scratched the surface.
Nathan:
Thank you for your thoughtful comment.
One thing that I would like you to consider is the fact that the idea of a pebble bed reactor is not limited to the concept that was developed in Germany and is currently the basis for the South African and Chinese programs. There are an infinite number of ways to adjust the cycle, some of which harden the neutron spectrum in the core. I would prefer to take any discussion of specifics off line.
My preference for high temperature gas reactors stems more from my interest in using them as heat sources for Brayton Cycle machines than from a displeasure with the reactor characteristics of light water reactors. There are a lot of reasons for my interest, but they can be summarized by pointing to the machinery of preference for converting fossil fuels into useful power – wherever possible, the choice in the last 30 years has been to abandon steam plants for a different alternative – namely either Otto cycle piston engines, Diesel piston engines or Brayton Cycle gas turbines.
I cannot figure out how to make fission work with piston engines, but it seems like a natural heat source for a direct Brayton Cycle. I may be dead wrong and end up broke and despondent, but I am pretty sure that I have found a way to significantly reduce the capital cost associated with nuclear fission power plants.
Rod
Well Rod, I certainly hope you don’t end up broke and despondent. You’re a great guy and a great thinker.
If I’m familiar with your business model, and I’m not sure I’m totally up to date, you are planning to make small reactors and I think there are plenty of places where this niche could be exploited. I certainly don’t want to imply that one size fits all. There is a world of reactor types that have not yet been explored. I am sure Adams Atomic Engines could make a world of money in this niches, remote cities, commercial shipping, and small process plants of various types requiring process heat.
Still, I remain concerned for the long term about the stability of the TRISO fuel. As I understand it – and I haven’t looked into the details to be honest – the fuel has enormous chemical stability. This limits the amount of materials that can be recovered. I don’t know how well the Triso balls can be broken up- mechanical destruction might allow for the recovery of some materials. I could be wrong, but the plan for spent TRISO fuel seems to be to take it directly to a repository without further treatment.
My outlook on so called “nuclear waste” is NOT to dispose of it however. I cannot think of a very many fission products or actinide that it without potential use. I have spent some decades thinking about the chemistry of these elements and I believe strongly that these materials should be chemically isolated and stored in readily accessible places for use by future generations.
This is why I’m a giant fan of molten salt technology because it allows you to play with the chemistry of the fuel while still on line in a very straight foreward way.
But I think Triso fueled reactors are definitely preferable to any fossil fuel technology now in use. I think the most important task before humanity right now is to get rid of fossil fuels. While I may sound like I’m quibbling, I do support what you are doing, and I really, really, really want to see you succeed, because I believe in the importance of your work.
Gas cooled reactors will play a big role of course in the synthetic motor fuel industry. I am preparing a report on the status of sulfur-iodine projects around the world for DKos – where my (increasingly easy) task is to convince the vast majority of Democrats that nuclear power is not only the safest and cleanest form of energy there is, but that it is absolutely essential for the world’s energy future. (I hope you’ll drop by over there and take a look at my article when I put it up.) Everybody working on the SI process around the world, so far as I am aware, is working on TRISO fuel.
I think the stability problem [i]could[/i] be solved as there are limits to the stability of silicon carbide and a chemical process for dissolution is certainly imaginable. I imagine that fluorine chemistry might be suitable, although I wouldn’t want to use any process that adds carbon tetrafluoride to the atmosphere. It’s too stable and its a very, very, very potent GHG.
About recycling actinides in fast reators,personally I don’t think that nuclear waste transmutation in fast reactors is a smart idea.
Past economic failures of European and American breeders programs suggest us that fast reactor are much more costly and complex to built and operate than light water reactors (and maybe any other kind of thermal one).
So I continue to believe that recycle actinides in a high burn-up,good neutron economy thermal (not fast) reactor,like pebble beds proposed in South Africa,is a good strategy for a once-through waste transmutation with of course the help of an efficient pyroprocess dry actinides reprocessing technology
This is why I’m a giant fan of molten salt technology because it allows you to play with the chemistry of the fuel while still on line in a very straight foreward way.
Hi Nathan,
I am also very interested in salt-based reactors (both based on fluorides and on chlorides). The chloride reactors are especially interesting for reactors where you need a very fast spectrum. I’ve collected a lot of papers on the subject at my blog and invite you to take a look at them.