Talk:Pebble bed reactor

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[edit] Not a single picture or diagram in this article of the structure or design of a PBR

What? For something like this, you really should have a picture or diagram of what it looks like or how it's designed. Definately an oversight. —Preceding unsigned comment added by 74.99.27.53 (talk) 16:48, 24 September 2007 (UTC)

[edit] Removed text

This discussion of LWRs seemed off topic to me. Ray Van De Walker 04:44, 31 May 2006 (UTC)

Conventional design

The design of any "conventional" nuclear power plant is generally similar to any other: the nuclear reactions in the reactor core provide heat, which is used to heat a working fluid that is in turn used to drive turbines attached to electrical generators. Additionally, some form of neutron moderator is needed to slow the fast neutrons released by the fission reactions to a slower speed that will react with other atoms in the fuel.

Water can be used as both a working fluid and a moderator. Although it is not particularly effective at either role, its economics and well understood engineering made it almost universal in commercial nuclear reactors. Water is not particularly efficient at carrying away heat due to its relatively low boiling point and the problems when attempting to pump steam. For this reason the water flowing in the core is kept under high pressure, which raises the boiling point. The hot water generated in this "primary cooling loop" is then pumped into a heat exchanger, transferring heat into the "secondary cooling loop", which is allowed to boil in order to produce steam for the reactor. In some designs a third loop is also used, cooling the secondary loop via large cooling towers.

The only widely-used modification to this basic design is the heavy water reactor, which uses heavy water in the primary cooling loop. Heavy water is a much more efficient neutron moderator, which permits reactions in the fuel that would otherwise not be possible. This allows the heavy water reactor to use its fuel more efficiently, or more accurately, use fuel that is considerably less processed, in theory lowering fuel costs. In practice the savings have proven difficult to achieve; heavy water is extremely expensive to produce, and the lower energy density of the fuel makes the reactor as a whole less powerful for any given size.

In order to reduce this complexity, a number of experimental designs in the 1970s replaced the cooling water with an inert gas, which drove the turbines directly. None of these attempts were commercially successful, for a variety of reasons that were unrelated to the concept itself. Gas cooled reactors remained a promising design, one that no-one appeared interested in investing the time and money needed to develop into a practical design.

This is not just off-topic, it also contains oversimplifications and outright errors, and is arguably WP:OR in places. Much better removed! Andrewa 14:02, 24 June 2007 (UTC)

[edit] Condensers

Someone changed the references to 'secondary heat exchangers' to 'condensers'; this is inaccurate because PBRs do not use a steam cycle, so there is no steam to condense into water. I revised the texts to restore the heat exchanger language.

[edit] Expense of heat exchangers

Nitrogen and air are almost identical, so a turbine designed for air should work well almost without changes. Though AAE's design might require a larger secondary condenser, this might not be a practical problem with a sea-water-cooled condenser, or a small stationary installation that can afford a small cooling tower.

I'm not clear what that means, "might not be a practical problem". Does it mean that a design with seawater cooling will be impractical, or that the problem would be easily solved? - Tzarius 01:13, 13 May 2005 (UTC)

I wrote the original text. The issue is the size and expense of the heat exchangers. Water has a much larger heat capacity than air, and therefore smaller heat exchangers for the low temperature heat exchanger that AAE's design requires for a heat sink. User:Ray Van De Walker

Edited sentence stating conventional reactors cannot provide more then base power "because they require so long to change steam production rates". It is a false statement. An increase in electrical demand results in an increase in steam demand to keep the turbine spinning at the same frequency. The increase in the amount of steam produced drops the temperature of the reactor coolant. Since reactors are designed with a negative temperature coefficient of reactivity, the cooler temperature increases fissioning and increases power. All of this occurs within seconds. There is no massive time lag that prevents them from providing peak power; they do so every day on navy ships when electrical demand changes with every watch rotation.

Hmmm... see

http://www.eia.doe.gov/emeu/cabs/safr_nuke.html

(September 2002) or more recently

http://www.scienceinafrica.co.za/2003/june/pbmr.htm

(June 2003)

both of which indicate that the plant at Koeberg is two PWRs and that it is merely proposed to build a PBMR there.

There also seem some discrepancies between the technical details of this article and the above links.

Andrewa 18:40 20 Jun 2003 (UTC)

Removed a comment about a criticality excursion at Windscale. There was no such thing. Wigner energy was discovered after the Windscale reactor was built. The response was to instigate a deliberate program of Wigner energy release, by deliberately taking the reactor to a high temperature at which the energy would be released. Thermocouples were installed to monitor this process, but one by one they failed. On this occasion, the thermocouples in part of the core had all failed, and the operators didn't realise that the release had started in this area of the core. They took the reactor to power again to try to start the release. The combination of the Wigner release that had already started and the deliberate overheating of the core designed to start the release set the moderator on fire. Andrewa 04:39, 25 May 2004 (UTC)

I've recently done some work on expanding and tidying the entry for wigner energy - perhaps you would care to check/correct it? Ian 12:50, 17 Jul 2004 (UTC)

minor typos

... they advocate a system that reduces the partical pressure of helium in the coolant loop (should this be particle?)

... so a turbine deisgned for air should work well almost without changes (typo)

If you feel a change is needed, feel free to make it yourself! Wikipedia is a wiki, so anyone — including you — can edit any article by clicking the edit this page tab at the top of the page. You don't even need to log in, although there are several reasons why you might want to. Wikipedia convention is to be bold and not be afraid of making mistakes. If you're not sure how editing works, have a look at How to edit a page, or try out the Sandbox to test your editing skills. New contributors are always welcome. -- Grunt   ҈  00:04, 2004 Sep 3 (UTC)

[edit] China PBR

Today China announced plans for using PBR's to produce much of their energy. After a little bit of reading a good paragraph or two could be added to this article. --Ignignot 00:34, Sep 3, 2004 (UTC)

[edit] Conflicting info?

This section has the sentence:

"Some authorities say that pyrolytic graphite can burn in air, and cite the famous accidents at Windscale and Chernobyl—both graphite-moderated reactors."

yet the wiki article on Wigner energy linked to in this article (http://en.wikipedia.org/wiki/Wigner_energy) says: "Despite some reports, Wigner energy buildup had nothing to do with Chernobyl:"

AlienDonkey —The preceding unsigned comment was added by 81.159.183.87 (talk) 03:11, 15 January 2007 (UTC).

Both those statements appear to me to be true, although some authorities is a good example of weasel words. Wigner energy was the initial problem that caused the Windscale fire, but not a major factor at Chernobyl. Burning graphite (but not pyrolitic graphite) was the main cause of the release of radioactive material at both, which is why some authorities talk about burning pyrolitic graphite. (Other differences: At Windscale, there was also burning uranium metal fuel, while at Chernobyl the fuel was already oxide, which doesn't burn; At Chernobyl, there was also a major initial release before the fire had started, while at Windscale the whole release was caused by the fire.) Andrewa 09:22, 25 June 2007 (UTC)

[edit] Modular?

These reactors are often referred to as "Modular Pebble Bed Reactors." What makes them modular? --NeuronExMachina 06:52, 4 Sep 2004 (UTC)

Their size. The typical reactor has been >1000 MW. Which is a lot of power to take in one lump--and to pay for up front. These (proposed) reactors run 100-200 MW. Buy and build standardized units and plug them into the grid as your demand increases. Being smaller they're quicker to build and can be more-or-less mass-produced, which should lower the cost and increase the reliability.
—wwoods 07:27, 4 Sep 2004 (UTC)

So the individual reactors are themselves seen as modular, as opposed to them being constructed of modular parts? --NeuronExMachina 07:36, 4 Sep 2004 (UTC)

Exactly. Though the "pebbles" could also be considered modular parts, I suppose.
From the Wired article:
"If Wu's pebble-bed 'thing' is, well, hot, it's because Chinergy's product is tailor-made for the world's fastest-growing energy market: a modular design that snaps together like Legos. Despite some attempts at standardization, the latest generation of big nukes are still custom-built onsite. By contrast, production versions of INET's reactor will be barely a fifth their size and power, and built from standardized components that can be mass-produced, shipped by road or rail, and assembled quickly. Moreover, multiple reactors can be daisy-chained around one or more turbines, all monitored from a single control room. In other words, Tsinghua's power plants can do the two things that matter most amid China's explosive growth: get where they're needed and get big, fast.
—wwoods 08:25, 4 Sep 2004 (UTC)
Yep, and they are also modular in the sense that they are expandable and reconfigurable: when China gets to the "roll-out" stage, they can get sites set up quickly with a minimal reactor configuration, have the setup team iron out the bumps and the personnel issues and move on, then when there are more reactor modules available they can be added to bring the sites up to full strength. As demand continues to grow, they can add more. If a single reactor needs to be shut down for inspection, the turbine can continue generating electricity from the other reactors on-site. One of the (many) problems with the big "monolithic" reactors was that they were all-or-nothing: if you needed to shut one down for service or repair, you lost a big chunk of output for the duration, so it wasn't something you did lightly: the temptation would be to keep the reactor running even when you really shouldn't. Having a set of smaller, independent reactor modules on-site sounds more flexible. ErkDemon 17:22, 22 May 2007 (UTC)

[edit] Missing aspect

I am missing an important detail in the article, as, to my understanding, there were mainly political reasons to stop the pebble bed reactor in Germany. IMHO there were interests of major companies (Siemens, partially owned by the German government) not to continue the development of that technology. And Rudolf Schulten had a contract with BBC, as far as I remember.

If I once find the time, I'll try to dig the details and add some aspects to the article.

Dr. M. Schulten

There was also the unfortunate decision to concentrate on thorium fuel in the large-scale prototype THTR. It was understandable at the time, as uranium was expensive and expected to get more so. But it's interesting that the Chinese HTR-10 reactor is based on AVR reactor rather than THTR. Japan and China have both invested heavily in buying the data from both of these early pebble bed reactors (and also of course from the (continuing) Soviet (now Russian) LMFBR program).
Please do contribute what you know... But beware of WP:OR. If there is doubt about the verifiability (as Wikipedia describes it) of what you know, you might also like to consider contributing to my Oral History Wiki, or to any number of similar blogs. Andrewa 04:15, 16 May 2007 (UTC)

[edit] What makes PBMR safer?

I am not a nuclear engineer. But, I notice something odd in the article. In one occasion, the fail-safe design of PBMR is linked to 'doppler effect[Doppler_effect]' rather than 'doppler broadening[Doppler_broadening]'. Would any person knowledgable in this area double check this fact. --81.107.219.6 00:12, 5 November 2006 (UTC)

The article doesn't explain why the 'doppler broadening' effect which makes PBMR inherently fail-safe doesn't apply to more common reactor types. The article says that the temperature sensitivity is a property of U238. But U238 is used in all uranium reactors. So why aren't all reactors fail safe due to rising temperatures causing a decrease in fission leading to a safe equilibrium? What's different? Just the Uranium not being so packed together? Are the pebbles themselves important to this?

My guess is that water-cooled reactors operate at far lower temperatures, so the doppler-broadening is insignificant. Mackerm 02:23, 25 July 2005 (UTC)

>> While this may well be true, surely the problem arises when you have an uncontrolled chain reaction, leading to temperatures well above normal operating range. I don't understand why this doppler broadening doesn't prevent that.

Yes, they shut down, but there still is enough decay heat that the fuel will melt which leaves a Three Mile Islandish cleanup problem. The pebble bed reactor shuts it self down and can remove the decay heat as well, so there is no cleanup cost and the reactor can be restarted. Jrincayc 02:12, 6 January 2006 (UTC)
All US and Western European reactors (at least power producing ones) have the property of having a negative temperature coefficient, that is, when the tempurature goes up the reactivity goes down. So, the same effect happens in other reactors. The difference with PBRs is that a) they have less excess reactivity to start with and b) the temperature can safely increase by a greater factor (600 degrees C as opposed to maybe 100 degrees). Jrincayc 04:49, 1 December 2005 (UTC)

I believe that it is three things. First is the thermal mass of all that carbon. It takes a long time to heat it up. Second is the very high failure temperature of the fuel, much higher than its normal operating temperature. Third, at high temperatures, one can rely on radiation cooling. pstudier 02:41, 2005 July 25 (UTC)

I'm afraid I don't follow any of the three explanations. Can you go into more detail?
The main fear with a nuclear reactor is that the fuel will get so hot it melts or vaporizes and then escapes its containment. With any well designed reactor it is easy to stop the chain reaction, and even if one doesn't, any well designed reactor will slow or stop its chain reaction when it gets hotter. (Chernobyl failed both of these criteria.) Even if the chain reaction stops, the fuel is so radioactive right after shutdown that it can heat up and melt. This is what happened at Three Mile Island. With the PBMR, it takes longer to heat the fuel because of the large mass of carbon, and it can get a lot hotter before it melts. pstudier 23:33, 2005 July 31 (UTC)

The "Doppler Broadening" concept is a new one on me. My understanding is that the configuration of the fuel (tennis-ball sized spheres in a stack, each containing coated grains of fissile material) has the property that as it heats up the distance between the grains of material increases sufficiently to reduce the density of the materials below criticality. The spheres expand because they're hot, but then their contents and neighbors aren't close enough to sustain the "chain reaction". This is a direct result of the shape and arrangement of the fuel. In other reactors the fuel elements have room to expand as they heat up and don't move relative to each other as they do. Safety mechanisms or human intervention are required to keep the reaction under control.Crag 16:15, August 2, 2005 (UTC)

>> The argument about "expanding spheres" would make sense for one sphere in isolation, but surely does not make sense when you have a bin full of them. In particular, surely it can't make a difference for the spheres in the middle, where the neutron flux is highest ?

I believe the key to the "Doppler broadening" is the way the cross-section of U-238 varies with neutron energy.[1] Higher temperature in the reactor effectively broadens the band of neutron energies, which means a higher fraction of the fast neutrons are absorbed, cutting off the chain reaction. This doesn't apply to the ordinary reactor, because the moderator cools the neutrons down to the thermal neutron range too quickly to matter.
Disclaimer: IANANE. —wwoods 06:25, 19 September 2005 (UTC)

More on "Doppler broadening". U-238, like many isotopes, has an affinity for absorbing neutrons that is inverse to the relative velocity of the neutron. This is called a 1/v absorption curve. But very heavy isotopes (including U-238) also have several sharp spikes in the probability of absorption that occur as specific energy levels. These 'spikes' lead to a phenomenon known as 'resonance capture'. If the neutron energy is at one of these 'resonance' energy levels, there is a much higher probability of absorption that removes the neutron from the fission chain. As the fast neutrons born from fission are thermalized, they lose energy with each collision with moderator atoms. If after one of these collisions it has an energy level (relative to the heavy U-238 atom) in a resonance peak, it is very likely to be absorbed.

The resonance energy spike is very 'narrow', but the neutron's kinetic energy is relative to the U-238 atom which is also moving due to thermal vibration. So the exact energy depends on if the U-238 atom is moving towards, across, or away from the neutron as it approaches. In any realizable fuel grain, there are a huge number of U-238 atoms, all vibrating randomly with thermal energies in random directions. When the fuel is cold, the U-238 atom velocity is low and regardless of direction that an individual atom is moving, and the neutron must have energy very close to the resonance peak to be in danger of absorbtion. Otherwise, it 'escapes' resonance capture. When graphed, the probability of absorption versus neutron energy has a very narrow 'peak'. But as fuel heats up, the range of energies for the U-238 atoms widens. So a neutron whose energy was not at the resonance level before, is more probably going to find a U-238 atom whose velocity is 'just wrong' that will make up for the fact that the neutron energy was near but not exactly at the resonance level. So neutrons at velocities near the resonance are more likely to be absorbed now. The result is the peak in absorption probability versus neutron energy is now 'broader'. The reason is the shift in relative neutron energy caused by the Doppler affect when U-238 atoms are moving faster. Hence the term, 'Doppler Broadening'.

The end result is, as the fuel heats up, more neutrons are absorbed in U-238 while slowing down from their 'fast' energy levels to thermal energy levels and are thus removed from the chain reaction. And the reactor fission process shuts down.

>> The above makes sense, but I don't understand why the same does not apply to a traditional fission reactor.

PBMR reactors are safer because of several reasons. The graphite material that encases the fissionable material (tennis balls as they seem to be refered to) act as a moderator to the critical reaction. There are also several control rods that can be used in the walls of the reactors to completely stop the reaction from continuing. The difference in the new design of the PBMR and the earlier designs is that these control rods are not pushed through the pebble bed but are used in the walls of the reactor for control purposes. By not allowing the rods to be pushed through the pebble media, fracturing, breaking or otherwise deteriorating the "tennis balls" is almost totally eliminated. Another inheritant safety design of the PBMR is that the graphite encasement of the fissionable material will not deteriorate or "burn up" even at the maximum temperature attainable in the reactor. The design takes into account the maximum attainable temperature by limitting the consentration of the fissionable matterial as well as the cooling properties of the helium used in the reactor. Furthur onsite safety of spent fuel is achieved by the encasement of the fissionable material in the "tennis ball". The design of the "tennis ball" is such that it will not or cannot be ruptured by the pressure of the deteriorating material inside. Because of all the design safety features in the PBMR, a release of radioactive material is practically elliminated. Please see http://www.pbmr.co.za/ for more information on PBMR technology. Sandman76 September 18, 2005

Helium is lighter than air, so air can displace the helium if the reactor wall is breached. Pebble bed reactors need fire-prevention features to keep the graphite of the pebbles from burning in the presence of air. Luckily, these are not difficult. This sounds a little awkward, aside from the language, there is no explanation nor a link to such an explanation of why "these are not difficult". Sounds a little Non-NPOV IMHO. It sounds like an interresting subtopic. What happens when there is a leak and the atmosphere enters the reactor? Somewhere in the article the possibility to flood parts of the building are mentioned: wouldn't this be a form of cooling that could cause the pebbles to start producing more heat again? What are the different possible worst-case scenario's? --MrPrince 07:41, 13 December 2005 (UTC)

Well, the graphite should not burn even when in an air environment [2]. A worst case scenario would probably be blow up the pressure vessel, the containment vessel and a dam upstream. The extra water would provide extra moderation and would also cool down the reactor which combined would probably cause the reactor to go prompt supercritial. This might very well cause Chernobylish amounts of of radiation release. For the same amount of effort, you could easily kill more people by attacking a chemical plant or bridge or something, so the pebble bed reactor is relatively safe. Jrincayc 16:24, 26 December 2005 (UTC)
I don't see any way you can blow up something with burning hot graphite balls. This is not powder, and you have no way to go from ball to powder except mechanical crushing. Additionally, the graphite and the fuel oxides won't even melt when supercritical. That's why you won't get Chernobyl at all.Rwst 10:35, 3 March 2006 (UTC)
Sorry for not being clearer. You would need to use chemical explosives to cause the explosion. Jrincayc 15:30, 3 March 2006 (UTC)

[edit] Conflicting information

The article states two things: That the German PBR research was shut down in the wake of Chernobyl in 1988 and, right at the end of the article, the German PBR research was shut down in 1986 due to a 'jammed pebble' releasing radation into the environment.

Which was it? It can't have been both considering the 2 year gap. Could we have some more information on the jammed pebble incident?

They experienced a 'jammed pebble' and ended up damaging it in 1986. This resulted in a small release of radioactivity just days after the Chernobyl accident. The plant received much criticism for not being totally open about the release. In 1988 the decision was made to permanently close the facility.

I think you might want to check out the THTR-300 article. ALthough I have not looked at it too much in depth yet, it appears to be accurate. Lcolson 03:19, 7 December 2005 (UTC)

[edit] New South African PBMR

"PBMR contract for new reactor. South Africa's PBMR company has awarded a contract for engineering, procurement and construction management to SLMR - a Canadian-South African joint venture - for its demonstration Pebble Bed Modular Reactor at Koeberg. Construction is envisaged from 2007, and a second round of environmental hearings is under way at present. Meanwhile the BNFL share in PBMR has been passed to Westinghouse and negotiations are under way with other possible investors to enable Eskom to reduce its stake from 30% to 5%. (Published in) Nucleonics Week 17/11/05, UX Weekly 14/11/05." Simesa 11:43, 1 December 2005 (UTC)

[edit] Pebble bed reactors: a nuke for everybody, including UBL.

These pebbles are mass produced radiology dirty bombs and instant gratification for Al-Kaida. Because there are so many of them it is impossible to inventory them exactly. Steal one, put it in your pocket, travel to NY, crack it with a hammer and drop it on the streets. Result is a billion dollar clean-up operation and mass panic among the people. With widespread pebble bed reactor use everybody will have his/her personal own glowing ball from the black market.

Considering the huge mess little cobalt radiocative pellets caused in the famous "1983 Juarez" incident, we must consider what these golf-ball sized uranium-thorium pebbles could do to the living. Whatever is round gets easily rolled away and then it is hard to collect them, which becomes a nightmare for radiating items. That was first thing I learned in elemantary school when we played with those little coloured glass balls. For Juarez see: http://en.wikipedia.org/wiki/List_of_civilian_radiation_accidents#1980s

As for pebble bed powered vehicles proposed, this is what happens when they crash: http://www.bravia-advert.com/includes/vid/bravia_60_sec_high.mov 195.70.32.136 11:34, 7 December 2005 (UTC)

What kind of doses would a spent pebble actually give? Jrincayc 03:25, 8 December 2005 (UTC)
If you steal an unirradiated pellet, it would not hurt anyone if you took it to new york and crumbled it and put it on a street. It would have to be ingested (inhaled) since uranium decays by releasing alpha particles. If it was irradiated by neutron radiation it would likely be so radioactive it would kill anyone holding it in a matter of seconds and couldn't be "put in a pocket". If an irradiated pellet was lost, it could quickly be found due to its rad signature (i.e. use a geiger counter) and the fact that people would be looking for it. The Jurez incident you linked to is from a medical application of radiation. Hospitals actually lose radioactive sources ecause the stafff are not trained as well as the staff at nuclear power plants. This would be nearly unheard of at a nuclear power plant. No-one would seriously consider making a car with a nuclear reactor core (except fools in the 50's that had no idea how a reactor works). Personally, I think the above comments were meant as troll bait, because they do not account for common sense solutions. As for the exact radiation dose, I'm not sure, but the fuel would most likely have to fulfill the requirement of being "self protecting" meaning it would likely give off 1000s of REM/hr.Lcolson 02:32, 9 December 2005 (UTC)
Well, you could wait until the pebble cools down enough that it kills the person with it in their pocket after a week or month and not after a day. But compared to regular nuclear fuel, the pebble will be a lot safer since the TRISO coating would tend to protect against inhalation and ingestion hazards leaving only gamma emmisions. I agree with you that this is might be a troll. Jrincayc 03:11, 9 December 2005 (UTC)
>irradiated pellets can be found with geiger counter
Not everbody walks with a geiger counter in the pocket, not even the average policeman, ambulance or firefighter. A few years ago a russian businessman was assasinated by the mafia in Moscow. They arranged to have about a kilo of isotopes hidden in his leather office seat and that killed him in little more than a month. There is no assurance someone with a pebble would be noticed based on rad signature alone. 195.70.48.242 18:27, 19 December 2005 (UTC)
>It would quickly be found the fact ... that people would be looking for it
This is the catch. There are several tens of thousands of golfballs in the reactor so you cannot reasonably assume that people will put great effort into locating a single or a few missing one. Maybe Jesus and the good shephard would do that, but real-world people are lazy and likely to fix up documents and inventory lists rather than starts searching seriously. They may never realize that the few stuff was stolen they would assume it is rounding error and must be in some drawer in the corner.
That is so ridiculous I can't even believe you could think of it. Fort Knox has millions of gold bars. Do you think they would just say "oh it was probably a rounding error" if they lost one? Would they try to cover it up? Pebbles are at least worth their weight in gold. The reactor has machinery that would immediately notice if one was missing, and probably send up alarms. I don't understand how you could accidently leave nuclear fuel "in a drawer someplace." I agree with the above posters this is probably a troll. The fact is, there are much easier ways to get a significant quantity of radioactive material. --Ignignot 18:58, 19 December 2005 (UTC)

The fuels in the PBR (ie. uranium, thorium) emit alpha rays and have a very long half life (all >108 years), therefore the radioactivity of them is low. Also, they cannot be used in a nuclear weapon (not Prompt critical. For breeder reactors with uranium 233 of plutonium 239 (reactor grade), various technical difficulties will probably prevent a nuclear weapon from being built using the pebbles. Polonium

Like the previous comment says, there is no security risk if a pebble is stolen. However, the notion that the pebbles are very valuable and expensive is nonsense. Gold costs $18300/kg, while uranium costs $26.39/kg (according to Wikipedia articles on those elements). Uranium is the most expensive part of the pebbles (graphite (even the high grade needed) and the other materials are not very expensive). At most, they would be as expensive as the uranium. Even then, the pebble is 693 times cheaper. While there is some cost involved in making the pebble, I cannot see it reaching the cost of gold. The pebbles will be safe, useful, and inexpensive. Polonium 20:46, 5 March 2006 (UTC)

[edit] Don't forget the residue

Have just expanded the bracketed phrase <nuclear waste> in the criticism section of the page. I feel it is a major snag that has been ignored so many times in the past that it deserves to be spelt out in full. It is not a major change to the text, the expansion just serves to highlight the magnitude of the legacy. Square brackets below denote my replacement for the bare phrase 'nuclear waste'

" Like most nuclear reactors, pebble bed reactors produce [radioactive waste which must either be safely stored for many human generations, reprocessed (more difficult after this method of reaction) or disposed of by a method yet to be devised.] The waste is more difficult to reprocess for further use due to the extra coatings. The fuel from other types of reactors is easier to reprocesses. "

Dave Jackson

The concept of the PBMR is a once-through cycle, without reprocessing. If you want to reprocess, you want to use another technology.
You might even find other pebble bed reactors more suitable. The PBMR is not the only pebble bed design, just the one receiving attention currently. Andrewa 15:44, 24 June 2007 (UTC)

[edit] Fast ?

I removed the section Fuel Cycle that can minimize nuclear waste because every high temperature gas reactor that I have heard of, except a proposed Generation IV design, is a thermal reactor. Thermal designs can not breed like IFR's. In any case reprocessing the pebbles is difficult and I know of no proposals to do so. One would have to either burn the carbon or crush the pellets and dissolve out the fuel. pstudier 18:29, 20 January 2006 (UTC)

A few points here:
-Most of the Gen. IV designs are fast reactors.
-It is feasibly possible to breed with thermal reactors by using fertile thorium, which I believe is what India is trying to do.
-I have seen proposals to minimize nuclear waste with a PBMR from General Atomics, and I am willing to bet others have too. I believe recycling the pebbles is a possibility, but also the PBMR could use reprocessed spent fuel from LWRs. I don't know what was in the section you removed, but what was in there may have been appropriate to keep in.--Ajnosek 18:10, 14 February 2006 (UTC)
I would guess that 20 years of AVR operation in Germany should have been enough to find a way to effectively get back U-233, but no. I also think that this failure was one of the reasons the project was dismantled, the other being that the accident happened just one month after Chernobyl (it was published somewhat later, no wonder).Rwst 10:22, 3 March 2006 (UTC)
Part of the THTR-300 project was to investigate a thermal breeder cycle using Thorium feedstock, and that involved reprocessing. One reason for its early demise was the depressed price of uranium made it pointless. There's no reason that pebble bed fuel can't be designed for reprocessing, but PBMR fuel isn't. Just another reason that it's important not to assume that all pebble bed reactors are PBMRs. They're not.
The comparison to the IFR is misleading. There's no reason a thermal reactor can't have a breeder ratio greater than 1. There are good reasons for thinking that a PWR or PWR won't ever be set up to breed, but other classes of thermal reactor can be, and several designs are currently being investigated to do exactly that. Andrewa 15:57, 24 June 2007 (UTC)

[edit] Flamability of graphite

On the page is a discussion on the flamability of graphite. In the book "Nuclear Graphite", 1962 by Nightingale, it discusses the flamability of graphite in hydrogen and in oxygen and in air. Graphite is substantially less flamable than other carbon componds such as coal. The reaction involving oxygen is exothermic so self sustaining reactions are possible. However, whether it is actually self sustaining depends on the pressure, temperature and air flow conditions of the air. Nuclear Graphite gives a few graphs of different conditions. In short, you can get graphite to burn in regular atmospheric air if you give it the right pressure, air flow and tempetature, but for different pressures and temperatures, it will not burn. On General Atomics page[3], they claim that graphite is non-flammable for the typical conditions for a high temperature gas reactor. Self sustained reaction are possible for graphite that is kept in a region with high neutron flux at temperatures below the annealing temperature. Basically, what happens is that the neutrons will deposit energy as they slow down in the graphite. Then, this energy will add to the energy availble for burning. This caused the fire in the graphite used in the reactor in England. This is not a problem for pebble bed reactors since they use a temperature that is high enough to keep this energy from being stored in the graphite. I have not found any reports on this available on the internet. Jrincayc 14:07, 9 April 2006 (UTC)

A less combustible form of graphite --

Another consideration is that not all pyrolytic graphite is created equal. The pebbles contain porous, low-density pyrolytic graphite, which has a large surface area and consists of unoriented graphite planes. This makes it relatively combustible. The surfaces of the pebbles, in contrast, are made of dense, highly-oriented pyrolytic graphite. It is non-porous, which makes it less combustible, but it also has another very positive characteristic: its "orientation".

Graphite consists of stacks of sheets of carbon atoms bonded to their neighbors but not bonded to the sheets above and below. The surface of a these sheet is remarkably inert, so oxidation would (I think) be confined to edges and defects. This would make oxidation very slow, in which case air cooling would dominate the heat balance, keeping the material too cool to burn. The pebbles are coated with this material, and the planes are parallel to the surface. Thus, most of the surface may have a negligible oxidation rate, even if other forms of pyrolytic graphite could burn under the same conditions. Harold f 00:51, 10 August 2006 (UTC)

[edit] Efficiency: As a function of nuclear waste

NPR aired a piece this morning on South African work towards pebble bed reactors. http://www.npr.org/templates/story/story.php?storyId=5345501

In it, a detractor suggests that pebble bed reactors generate more radioactive waste than a fission reactor, on a per-killowatt basis. Is this currently true? If so, what might be done to resolve the issue? A seperate subsection on efficiency would be fascinating, comparing the KW/$ and KW/(gram of waste), and construction costs to fission reactors. It would be doubly interesting if the functions for KW/$ and KW/(gram of waste) included the cost and benefit of recycled fuel.

Also, on a related note, can the spent fuel of the pebble bed reactor be recycled? Mrzaius 22:41, 17 April 2006 (UTC)

The pebble bed reactor generates more kilograms of waste per kilowatt-hour than a light water reactor, but about the same radioactivity when measured in Becquerel per kilowatt-hour. The pebble bed fuel includes the graphite moderator which dilutes the fuel. Waste repository capacity is usually limited more by heat than volume so the burden of disposal should be comparable. The graphite fuel is a more durable form than the uranium dioxide fuel from a LWR and can be placed in dry storage immediately after removal. So it is arguable that the waste is easier to handle than LWR waste in a once through process. To recycle pebble bed waste, one would either have to burn the graphite, being careful not to release radiation in the smoke, or grind the pellets to allow one to dissolve the waste, and then dispose of the graphite. To the best of my knowledge neither process this has never been demonstrated. pstudier 23:18, 17 April 2006 (UTC) It has been suggested that an Emotionalist might tip a barrel of gasoline into the "can" and ignite the pebbles --Truegbruno 05:46, 5 July 2006 (UTC)G Bruno (the truegbruno)

My understanding was that you aren't supposed to "recycle" the pebbles's nuclear material in the usual way: the special encapsulation is supposed to be "for life", including disposal. Sealed unit, no user-serviceable parts. This is part of the attraction of the pebble design: it's supposed to be an idiot-proof, mass-production, streamed commodity product. With fuel rods, you have to take them to a special plant and chop them up and expose their horrid radioactive innards before reencapsulating the worst bits for burial, with "pebbles" you don't.ErkDemon 16:05, 22 May 2007 (UTC)

[edit] nuclear proliferation

From the article: "There is considerable opposition to the PBR from environmentalists and lobbey groups such as Earthlife Africa and Koeberg Alert who are concerned about its environmental impact and nuclear proliferation."

Pebble bed reactors are rather better for nuclear proliferation than a standard reactor for two reasons:
  1. They have low excess reactivity, so it is hard to hide anything that absorbs neutrons, since it will show up in the amount of fuel that is used. I.E. if you put a blanket of say u-238 to try and produce plutonium then you will need to use more fuel since the neutrons will be absorbed by the blanket. This is not noticeable in conventional reactors since you can just pull the control rods out farther if you want more neutrons.
  2. The pebbles are harder to reprocess than conventional fuel. You have to remove the silicon carbide before you can get at the plutonium that is produced. It's probably not impossible, but it is harder than a typical fuel rod from a light water reactor.

Do Earthlife Africa and Koeberg Alert give any reason for their fears about nuclear proliferation? Jrincayc 03:27, 30 June 2006 (UTC)

Discussing PBMRs is a bit like rearranging the deck-chairs on the Titantic. I've campaigned against nuclear energy for about 15 years and the debate always comes down to an ethical position regarding the use of uranium. For those deaf to the casualities resulting from uranium mining, and who ignore the considerable evidence supplied by the medical community who contend mining uranium ore is bad for ones health, there is something in the act of fission itself that conjours up dreams of riches, wealth and power. For those who see radioactive emmissions from conventional nuclear power plants as unacceptable and who believe there is no such thing as a safe dose of radiation, the nuclear industry is guilty of contaminating and polluting the planet. In other words, nuclear proliferation doesn't stop with bombs, but necessarily includes the entire uranium cycle. If we are to avoid nuclear war, we must put a halt to the entire industry.
I can't speak for Earthlife Africa or Koeberg Alert but they have a quite a bit of material online.Ethnopunk 13:39, 30 June 2006 (UTC)


Wow, what a red-herring rant. This comment will probably not bring anything good to this article, but perhaps I can help clear some misconceptions. Uranium mining is no worse than any other type of underground mining. In-situ mining is less harmful to workers, and by-product mining merely gets uranium from other ore that was already mined for other purposes. Open pit mining also doesn't expose workers to as much radon as would underground mining. From what I've heard, uranium niners have no higher or lower of an accident/death rate than other types of mining. Oh, by the way, I also heard that more radioactivity is realeased from coal power plants due to the uranium that is naturally present in coal than from a nuclear powerplant. And as mentioned earlier in this talk page, you won't be able to simply walk out with a pebble, they will be tracked.Lcolson 18:36, 3 July 2006 (UTC)
Sounds a bit what they used to say about the Asbestos industry. "Asbestos is safe for you and we should be using it everywhere including on our roofs, our gutters, our fences and in our kitchens." You are assuming that coal-mining is healthy and that we should continue to use fossil fuels. Isn't the real question, how are we going to convert dirty industrialists and their minions to clean, environmentally friendly energy, like hydrogen-gas go-generation fuel cells for one.Ethnopunk 11:59, 4 July 2006 (UTC)
No, it's not like Asbestos. That industry was in a state of denial after they knew the facts. Uranium miners have some risk of radiation poisening, but no more than any other. But coal mining being dangerous has never been a reason not to mine coal, it is the hazzard to the ecology (CO2, soot, etc). We can't stop now to use coal, but we can with more nuclear. Coal, btw, is THE major contributor toward murcury in the envriorment since this is thrown out whenever coal is burned, another reason to phase it out. 216.203.27.99 23:30, 4 February 2007 (UTC)dwaltersMIA


Back on topic, do you have any idea or source for Earthlife Africa or Koeberg Alert believing that PBMR's contribute to nuclear proliferation? Ethnopunk, I am more than happy to discuss nuclear power in general somewhere other than talk:pbr. Jrincayc 14:54, 4 July 2006 (UTC)

[edit] fossil fuel

SA, we are told, does not have enough fossil fuel. What about all the coal? Paul Beardsell 06:34, 26 September 2006 (UTC)

[edit] imported exporter

SA will import Uranium from Russia, we are told. But also, we are correctly told, SA has its own uranium which it exports. Both true? Paul Beardsell 06:35, 26 September 2006 (UTC)

Having the ore and havng the fuel are two different things. This is what the controversy around Iran is about. Niger, in West Africa, is a huge exporter of processed uranium ore (called "yellow cake"). But it has zero capability to turn this into usuable fuel.

Fuel cycle in South Africa

Uranium production in South Africa has generally been a by-product of gold or copper mining. In 1951 a company was formed to exploit the uranium-rich slurries from gold mining and this grew into Nufcor, which in 1998 became a subsidiary of AngloGold Ltd. It produces over 1000 tonnes U3O8 per year.

Originally fuel for Koeberg was imported, but at the height of sanctions the Atomic Energy Corporation (AEC) was asked to set up and operate conversion, enrichment and fuel manufacturing services for Koeberg. These have now been closed down. Enrichment was undertaken at Valindaba, 60 km north of Johannesburg, by a unique aerodynamic Helikon vortex tube process developed in South Africa. Since this was not economic both centrifuge and molecular laser isotope processes were being explored when operations ceased. The semi commercial plant was of 300,000 SWU/yr capacity.

The AEC became the Nuclear Energy Corporation of South Africa (Necsa).

Eskom now procures conversion, enrichment and fuel fabrication services on world markets.

Since 1965 the AEC/Necsa has operated a 20 MW tank-type research reactor - Safari-1 - at the Pelindaba nuclear research centre. Since 1981 it used 45% enriched fuel elements manufactured locally from locally-enriched uranium, though the pilot enrichment plant producing this closed in 1990.

216.203.27.99 23:35, 4 February 2007 (UTC)David Walters

[edit] made correction under Chinese section

Added "... of which PBMRs will be a major component." to the section on China which indicated they are planning on building 300 reactors. Most of the nuclear generation will NOT be PBMR, they will be bigger base load units of the 1000+ MW variety.

216.203.27.99 23:42, 4 February 2007 (UTC)David Walters

[edit] HTR 2006 papers online

The High Temperature Reactor 2006 conference papers are now online at http://htr2006.co.za/index_actual.php?site_action=downloads This may help with finding missing citations. Jrincayc 04:23, 27 February 2007 (UTC)

[edit] Recent visit to Chinese Reactor

ABC Catalyst Program incl video I am not sure how useful any of this information is, but it may be of interest. 220.233.94.28 03:54, 4 March 2007 (UTC)

[edit] Article name and content

The PBMR is not the only type of pebble bed reactor. I'm not quite sure how to handle this. Andrewa 18:03, 12 May 2007 (UTC)

[edit] Other graphite moderated pebble bed reactors

For example, AVR Reactor and Thorium High Temperature Reactor were both pebble bed reactors, but not PBMRs. Andrewa 15:21, 15 May 2007 (UTC)

[edit] Other moderators

In its early days, the Australian Atomic Energy Commission concentrated its research work on beryllium moderated pebble bed reactors, although no prototype was ever constructed. See also EBOR. Andrewa 19:25, 16 May 2007 (UTC)

[edit] Action

In that nobody speaks, and that PBMR was a one-way disambiguation that needed fixing, I've created a new stub at pebble bed modular reactor and I'm working my way through this one to introduce consistent terminology. See Talk:PBMR. Andrewa 07:33, 20 June 2007 (UTC)

[edit] Chicken McNuggets

For anyone who's wondering about the change in emphasis between the old reactors and the PBR designs, the PBR is using the "Chicken McNugget" business model, you put all your high technology investment up-front into the material manufacturing (the "pebble factory"), and then you can set up cheap franchised outlets wherever you want, taking delivery of the high-tech pebbles, bunging them into cheap reactors, and serving up hot and tasty electricity using staff who don't have to be geniuses, and who don't get the chance to do anything potentially dangerous.

Someone who used to design structural steelwork for UK reactors once said that they were designed like cathedrals: every one was different. It's like the restaurant business: you can have a single restaurant with specialist equipment and staff, all working together to create a great product and avoid food-poisoning disasters, but when it comes to franchising it, you have problems: You can't easily duplicate key staff, monitoring quality control through the chain is difficult (so you get stupid accidents) and you don't get bulk deals on the key equipment. You may get an economy of scale regarding know-how, but not for staff or hardware, because you aren't intending to build that many restaurants, and all your equipment is still very specialised.

The alternative is the McDonalds approach: to redesign the entire business model around mass production and franchising. Design once, build many (but spend a lot of money on the design). MacDonalds is the home of the Chicken McNugget: It takes a product that is variable and messy, and needs to be handled with care (chicken) and it turns it into a centrally-produced, encapsulated, idiot-proof format. You move all your high-tech staff and plant and specialist equipment into a central location where you use computers and robots and liquid nitrogen and god knows what else to churn out "nuggets", then you ship the nuggets to your local franchises, where the cheap staff dunk them in hot oil and serve them. The local staff never get to touch a knife or a piece of raw chicken themselves.

With cheap local staff and simple local plant, and there's a limit to how badly they can mess things up, because their job has already been made as simple as possible. You try to think of everything beforehand and streamline the local processes to be as close to idiot-proof as possible. The guys with the tough job are the ones back at the nugget factory (or the pebble factory) -- the cost and consistency of what happens there dictates the viability of the entire chain. If the mass-produced designs are right, and the product is designed well, the system runs smoothly and efficiently. If there's a problem at the nugget plant, it affects the whole chain. So it's critically important to get everything right (or nearly right) before you take the thing into mass-production.

This is why we haven't already had a big PBR roll-out: they want to hold off and make sure that they get it right, and then if a few pilot plants show that the model works, and there aren't any further cost or reliability issues with the pebbles ... poom ... the things will be everywhere. But the integrity, durability, reliability, manufacturing safety and cost of the pebbles themselves is critical. ErkDemon 16:22, 22 May 2007 (UTC)

A good essay. It leaves a few things out:
  • The standardisation of reactor designs isn't just a PBMR phenomenon, all reactor designers are doing it. It's part of the maturing of the industry, which may be about to put the "green" hiccup in the past and enter the boom that everyone in the 1950s thought was only 20 years away. I said may.
  • The pebble technology has applications in other reactor designs. If I were directing research, I'd be very interested in investigating a (slightly) higher-temperature Magnox-style plant using the cladding techniques that are involved in the pebbles... and looking at that sort of thing is very much part of the generation IV reactor project. The AGR would have been a world-beater if their original intention of using beryllium-based cladding had worked... but it didn't.
  • There are a number of sleeping problems political still to be encountered... the traditional ones of course are bombs, wastes and accidents. All three have the potential to scuttle the PBMR in favour of other designs (remember we are talking here about politics, not engineering):
    • Bombs - the pebble-bed concept lends itself to online refuelling, and even if the fuel pellets are reprocessing-proof, what's to stop you slipping in some fertile pellets that aren't? Made for it.
Because of online refuelling, you can keep the excess reactivity low. So, if someone is adding extra fertile pellets in, they will soak up noticeable amounts of neutrons, which will increase the amount of fuel used, so producing plutonium will either be slow or will be noticed. Jrincayc 01:52, 29 May 2007 (UTC)
That's the argument that is generally used. There are three problems with it. Firstly, it's a technical argument. Even if valid, frankly these subtleties have generally been lost on the antis. Secondly, it ignores the possibility that the operators will not care that they're being noticed, as happened with India. Thirdly and most important IMO, the point is not so much that the proliferation risk of a PBMR is all that high, it's rather that the risk however small is far greater than that posed by (say) a PWR or BWR - types which have both been resisted on the grounds of proliferation risk (however foolishly) - which gets us back to the politics. Our article on Nuclear fuel currently reads Reprocessing of spent commercial-reactor nuclear fuel has not been permitted in the United States due to nonproliferation considerations. That is high-burnup spent PWR and BWR fuel we are talking about there! Andrewa Andrewa 01:07, 31 May 2007 (UTC)
    • Wastes - the PBMR concept not only abandons recycling of uranium and transuranics, it also throws away the moderator. First proposed nuclear fuel cycle in history to produce more nuclear waste per megawatt-hour than a once-through Westinghouse PWR.
    • Accidents - Graphite-moderated reactors quite simply have the worst accident record of any moderator type, so far. Windscale, Chernobyl. TMI was a hiccup by comparison.
But I like your analysis. Interesting times. Andrewa 01:58, 26 May 2007 (UTC)
Though interesting, this is supposed to be a talk page about the article, not a discussion forum about the topic. There are lots of forums out there, only one Wikipedia though. Pro crast in a tor 10:06, 29 May 2007 (UTC)
Sorry, Wikipedia is not that special. theanphibian 19:16, 29 May 2007 (UTC)
I think that's a valid point... but one of the special things about Wikipedia is that we do have articles on current events and controversial topics, and we try to be NPOV even there. And it's a challenge at times! Where to draw the line? I think this essay is helpful, and suitable for a talk page, because I think it will help us to improve the article. But, if that's not the case, then you're quite right, it doesn't belong here. There are lots of interesting things that don't belong here. The acid test is simple: Will it help us to improve the articles? Andrewa 00:35, 31 May 2007 (UTC)
The topic was meant to prompt a few thoughts or discussions that might be useful to the development of the main page. But it ended up longer than intended, and if it isn't considered to be sufficiently useful I've no objection to it being deleted. All is happy! ErkDemon 00:33, 12 June 2007 (UTC)
As I said above, it will probably be archived eventually. Even if not, it will probably remain in the history.
Another interesting question that it raises... is the McNugget approach successful? In the 1970s I shared a house with other undergraduate university students, and one was working as a cook in a fast food chain. He would never eat there, he looked rather green when any of the rest of us did, and occasionally told stories which don't bear repeating to try to convince us not to either. The problem seeemd to be that the job was so simple, there was no sense of responsibility for the product. Some chains now appear to be quite intentionally working to correct this, to introduce some responsibility and accountability and even pride into jobs at all levels.
On the other extreme, one of the common strands to several nuclear reactor accidents (in particular SL-1 and Chernobyl) is that the operators had been told and believed that they couldn't break the reactor. The Soviets had been saying since the mid-1950s that their control systems were foolproof; This was their justification for building reactors of a type that the rest of the world thought dangerously unstable, and they were very proud of the achievement. And they'd evidently communicated this to the management and staff of Chernobyl NPP to the point that nobody was worried about conducting an experiment in the absence of anyone trained in nuclear engineering. The collegues of the three victims of the SL-1 accident have replied to suggestions that one of the victims might have deliberately lifted the control rod assembly in order to commit suicide by pointing out that in their training they were repeatedly told how safe the whole rig was, and that they doubted that any of them knew of the danger that this action posed. Presumably, the same belief in the inherent safety of SL-1 was behind the (in hindsight appalling) decision to have these people unbolting and manually operating the control rod without having a nuclear engineer present to supervise.
It's impossible to make anything foolproof, because fools are so ingenious. Andrewa 02:54, 3 July 2007 (UTC)
Firstly, the safety of HTRs is impeccable. Their safety has even been tested several times with accident conditions were ANY other nuclear reactor would have had a meltdown. Full control rod pull-out coupled with a complete shutdown of all cooling has been demonstrated several times in the AVR and the HT-10. HTRs are safe because no operator action is required to prevent the release of radioactive fission products.
There are two hypothetical accident cases situated around the inability to switch off the blowers. In these cases the staff has several hours time to switch off the blowers. That's why the ONLY safety grade equipment within the entire reactor are the switch-off controls for the blowers. And even then, there are multiple ways to switch something off, you know. Just get a fire axe and whack the power cable. You've got several hours time to do that...
Got a leak? No problem. Get a mechanic and weld a piece of metal above the hole once the core is depressurized to minimize future damage from air ingress. BTW, constructing a dangerous case wrt. to air ingress is more difficult than you think, it doesn't just happen with a hole...
The safety of the HTR is that if even only a fraction of the equipment works, and a severe accident happens, the staff can first have a lunch, then go down to the pub for a drink and then afterwards go on weekend trip, and NOTHING would happen. Sure the reactor would be scrap, but no release of fission products would occur. The only way that the core can overheat, and cause about one in a million fuel particles to break, resolves around the inability to switch the blower off during extremely severe accidents. And I am talking here about a terrorist-style-bombing-of-selected-components-extremely-severe accident...
Secondly about fuel waste. HTRs need much, much less fuel due to their insane burn-ups AND the uniformity of that burn-up within the spent fuel. Theoretically, one could mill the spent fuel and then do the usual reprocessing afterwards, but due to the extremely high burn-ups of HTRs this is highly uneconomical. Burn-ups around 200GWd/tHM in HTRs btw. translates to about 70-80% of all energy being created by fissioning breeded Plutonium. There is a reason why the US chose this reactor design to destroy Russian Plutonium: its simply insanely good in utilizing nuclear fuel. --Dio1982 (talk) 19:42, 31 January 2008 (UTC)
I just want to say, this is by far, one of the most interesting "article" I had read on wikipedia. It is quite POV, I agree, but man does it provide insight. I guess that's the problem with POV, what some consider to be insightful would be considered to be total BS by others. 129.173.242.237 19:49, 5 November 2007 (UTC)

[edit] SAFETY - Pebble Heat Cycling

The designs of the active pebble container (reactor) itself looks like a tall funnel. If the reactor was to cool down, would the pebbles pack themselves tighter due to thermal contraction, and as a result shift down from gravity?

What happens when it heats up again - Are the pebbles strong enough to push the entire stack above them back up  ? Will it burst the walls of the funnel or crack a few pebbles ?

They need to run one of these things with inert fuel for a while to make sure no pebbles get stuck or cracked. Simulate the heat by burning propane or something.

65.104.118.99 04:07, 11 June 2007 (UTC)


Maybe a bundle of vertical tubes that have two-stage sphincter valves at the bottom of each tube. The top valve opens first, allows the entire stack of balls to drop, and then is closed off above the bottom ball. Next, the lower sphincter opens up and allows only one ball to drop out. Guaranteed.

18:58, 18 June 2007 (UTC)

The spheres are in a close packed lattice. If they ever expanded or contracted sufficiently to disrupt this arrangement, there would be all sorts of problems. The design assumes and attempts to assure that they never do.

There's been a lot of work done on the way the lattices of spheres react to various situations... how balls progress down the stack as you take some out the bottom and add others to the top, how much they can expand or contract before it impacts the lattice, all those sorts of geometric things. Big subject! And the PBMR being an annular core has very special geometry.

The exact shape of the fuel space is being chosen with all of this in mind, to provide a stable, close-packed stacking that cycles all the fuel spheres through the outlet port(s), in a predictable fashion that supports the proposed fuel change schedule.

Part of fuel testing should be to make sure that the balls can withstand all likely movements and movement patterns, particularly in view of the proposed lack of secondary containment. A good question to ask the vendors... They have an enquiry email I think. Andrewa 09:19, 25 June 2007 (UTC)

Andrewa is right that massive amounts of tests have been conducted on pebble flow and geometry. But this problem can be effectively counted as solved since the 60s. The knowledge and means is there to mitigate any potential problems. One shouldn't forget that there are three reactors (AVR, THTR, HTR-10) which have used pebbles and none of them have had problems relating to the pebble packing lattice.--Dio1982 13:50, 27 June 2007 (UTC)

[edit] Is Doppler Broadening a Unique Safety Feature?

At the top of the safety feature list is the doppler broadening effect. They way this effect is explained, it should happen anytime you have hot U238 in the fuel. This would seem to apply to almost any other reactor design. Why is it touted as particularly important in PBR's

Sign yourself, and it is an unique feature. Of course, all reactors observe the effect, but the relative size of their's inner core and density make the effect irrelevent. However since radioactive material is in such smalle quantity per pebble, the effect become relevent. The natural geometry and design of the Pebble Bed Reactor makes it an unique feature. 129.173.242.237 20:32, 5 November 2007 (UTC)
This has a lot to do with the core physics.
The doppler broadening effect is significant in any reactor. It is the prime means, besides of negative feedback from steam voids, to prevent an overheating of the core if excess reactivity is accidentally inserted into the core by a loss of boron or accidential control rod removal. Doppler broadening is also the prime reason why the core will shut down during severe accidents where the shit is truly hitting the fan (think Chernobyl bad). BUT, the effect is relatively small since any water cooled reactor will not deviate a lot from its steady state temperature, hence the doppler broadening effect is very small. "Active" measures of shut down control rods and/or Boron control is necessary to kill the fission reaction during more "normal" accidents.
By definition HTRs may not rely on any active measures to remain safe. If you look at the available physical processes that would "eat up" neutrons if the reactor becomes too hot, there are not many options. To be honest there are only two big ones: moderator steam voids and doppler broadening. Since HTRs are gas cooled, that only leaves you with doppler broadening.
And in order to get any meaningfull doppler broadening effects, you need huge temperature variations here. We are talking about the +600°C range here. What happens at these high temperatures, is that some slow thermal neutrons get bumped by 1200°C carbon moderator atoms into a much higher energy state (backscattering). It just so happens that this higher energy state is situated just around the first absorption harmonic of U238. This actually adds a tremendous 100pcc of negative reactivity to the core.
The neat thing about HTRs relying completely on doppler broadening to shut down is that you can do instantaneous load following without any external inputs. The core is held at its equilibrium temperature no matter what. If the coolant gases enter the core too hot, the core will produce less power, if they enter too cold, it'll produce more power make up the difference.
The downside is that you can't really completely switch the reactor off. Even at shutdown, the reactor will produce some minor amount of fission energy (~1-2MW of thermal power) to keep the core at a lower equilibrium temperature. This is a huge problem for people designing these reactors to get license approval.
Safety wise this is btw, not a problem. The reactor vessel walls will be just too hot to touch, nothing else. But the reactor would need months to cool down to room temperature, unless the core is evacuated which takes about a month on its own. --Dio1982 (talk) 17:34, 11 March 2008 (UTC)

[edit] Reference 9 is missing

I just tried to access reference 9 but it appears to have 404d... 87.112.74.244 (talk) 11:46, 3 January 2008 (UTC)

[edit] Citation needed on public radiation release

In section Pebble_bed_reactor#Safety_features, it says:

Later problems with the AVR reactor resulted in a small release of radiation to the public.

I really think we need a citation for this event to clarify just how safe or unsafe these reactors are. I would also like to see information on exactly how "small" it was, if that information can be found.

The best reference I found in Googling is at the end of a page on how pebbles circulate within the reactor, but it is still very vague, and not a very authoritative source. Another potential reference is Reactor Watchdog Project's info page on PBMR, which has more detail, but also comes from a site with an obvious anti-nuclear spin.

It is also mentioned in THTR-300#Decommissioning, but I don't see a citation there either.

Different sources identify the event as happening in 1986, 1988, or May 4, 1985.

LeBleu (talk) 00:44, 4 February 2008 (UTC)