The Pebble Bed Reactor Today

Preface: All big coal power plants use 1,000°F steam for higher efficiency.  Conventional nuclear reactors cannot produce steam hotter than 550°F, so they cannot be used to convert coal power plants to nuclear.  1,700°F high-temperature pebble bed reactors will work just fine.

Reminder: Coal produces heat by the chemically destructive process of burning.  Uranium produces heat by the atom destructive process of fission.  Per pound, uranium produces about 3 million times as much heat as coal but, unlike burning, fission produces no Global Warming CO2.

Introduction: Pebble Bed Reactors   An Alternative Atomic Energy.  They are very different from conventional nuclear power plant reactors.  These High-Temperature Gas-cooled Reactors (HTGR) have been around since the beginning of the atomic age.  Being helium gas-cooled, rather than water-cooled, they are incapable of a steam explosion.  Like air-cooled engines compared with water-cooled engines, they run hotter, are larger for the same amount of power, and, while naturally safer, are not the money-makers conventional nuclear reactors are.  Over time, their more expensive fuel offsets both their lower initial construction costs and faster build time savings.

Pebble beds are not your father's reactor.  They are a simple, fundamentally different kind of reactor that limits its own maximum temperature via an unalterable natural process, and, since the reactor lacks steam pressure, pebble beds are unable to explode like steam boiler reactors.  Just as the advanced electronics technology for computers is in the microcomputer chip itself, the advanced nuclear technology for pebble bed reactors is in the pebble itself.  Introduction to TRISO Pebbles

Pebble Personality: Blowing on TRISO pebbles cools them, causing them to make more heat.  Not unlike a pile of charcoal briquettes in your grille (but very different physics).  Doppler broadening causes the pebbles to have an extremely strong negative temperature coefficient.  The hotter they get, the fewer BTUs of heat they make.  A counter-intuitive concept.

If the cooling stops for any reason and if, for some reason, the control rods are not dropped to shut the reactor off, the pebbles go to a "hot idle" condition that produces fewer BTUs of heat than the space around the reactor will absorb, thus keeping the pebbles from getting hotter.  (The reactor has a three foot thick insulation lining made of graphite on the inside that holds both the heat and radiation in.)  This natural behavior of the pebbles makes the reactor inherently safe - no "meltdown" potential for a properly designed, built, and installed reactor.  Cooling the pebbles down again will cause the pebbles to go out of "hot idle" and to make a lot of heat (BTUs) again.  Like the "bed" of hot briquettes in your back yard grille, a "bed" of TRISO pebbles is incapable of exploding.

Pebble Power: HTGR reactors will run on all the usual fission fuels but often run on thorium, which is much more available than uranium, with small amounts of uranium mixed into the thorium.  More expensive than simple uranium pellet-filled conventional reactor fuel rods, HTGR fuel .pdf  fuel fabrication begins with tiny sand-size grains of thorium and uranium.  These grains are then triple coated (layers of pyrolytic carbon - silicon carbide - pyrolytic carbon)  [named "TRISO"] and then about 10,000 of these almost microscopic grains are formed into either tennis-ball size spheres ( Pebbles ) or stapler-size blocks ( Prisms ).  Developed long before the era of terrorism, this extremely tough pyrolytic fuel encapsulation configuration inadvertently happens to make it extremely difficult for a terrorist to get to the few uranium particles mixed in among the much greater quantity of no-terror-value thorium particles.

Past Pebbles: The 2006 South African PBMR Pebble Bed Modular Reactor is an advancement beyond the good running 1983 German  THTR-300   pebble bed reactor  but as of this moment the PBMR doesn't have a substantial run time track record.  It was a great loss for the world that the follow-on THTR-500 pebble bed was never built in the early 1990s and run for a decade to see if it really would have been better than its THTR-300 predecessor.  The troublesome late 1960s United States' General Atomics Fort St Vrain block fuel reactor, along with the late 50s Peach Bottom I  were the U.S. HTGR power plants.  Going even further back in time, the first  MAGNOX  power station, a high temperature CO2 gas-cooled reactor, the 1956 British Calder Hall, was the world's first commercial nuclear power station. 

Present Pebbles: Contemporary operating HTGR reactors are China's HTR-10  pebble bed at Tsinghua University and Japan's 30 mWt HTTR  prismatic at Oarai, Japan.  Not yet operating is the GT-MHR prismatic reactor, an American-Russian project.  The scope of the Russian Minatom GT-MHR prismatic reactor program includes construction of a GT-MHR plant at Seversk (formerly Tomsk-7) to destroy a portion of the Russian inventory of surplus weapons plutonium and to produce electricity for the surrounding region.  (10% of the electricity powering the monitor you are reading now is from destroying weapons plutonium - the Megatons to Megawatts Program - or about half of the nuclear electricity presently being generated in the United States - nuclear is 20% of all U.S. electricity.)

Large, commercial-size pebble bed powered projects under way at this time are electricity generating complexes in South Africa (ESKOM at Capetown - 4,000 to 6,000 mWe from 20 to 30 PBMR gas turbine generating units) and China (Rongcheng - 3,600 mWe from 19 PBR steam turbine generating units).  Power companies are not in the business of doing new things and often get stung when they do.  That's why a demonstration facility such as J. R. Whiting is essential to get them to even THINK about what this web site is suggesting.

Future Pebbles: As the HTGR community is well aware, there is also a great deal of development work going on right now with perhaps 20 different reactors including several different HTGRs.  The 600 mWt prismatic ANTARES / GT-MHR and the GENERATION-IV HTRS are considered to be significant future reactors.

The 400 mWt pebble bed PBMR appears to me to be both the closest fit and the closest to being a real product in a form that could be rapidly mass-produced at reasonable cost and applied to existing fossil fuel power plants in the event a world-wide emergency rapid CO2 mitigation project is activated after we come to understand the answer wasn't blowing in the wind.   to  NRC New Reactor Certifications 

Producing Pebble Bed Reactors: Pebble bed reactors are not your father's reactor and shouldn't take nearly as long or cost nearly as much to build. I may be wrong about this, but I don't think pebble bed reactors have the super-large castings and forgings that make up conventional reactors.  These huge parts add so much to a reactor's cost and build time.  Production of some of the biggest parts needed to build a conventional reactor is limited to several of the world's major industrial giant countries (some question if the United States is still a "able-to-do-it-all" member of this select fraternity). 

If a country is capable of producing the largest - 50,000 horsepower plus - marine diesel engines, it should be able to produce all the metal parts needed to manufacture PBMR reactors.  If a country is capable of producing the hydraulic actuators and avionics computers that go in the very largest jet liners, it should be able to do the pebble handlers and control instrumentation.  As to critical materials, I estimate the world is producing sufficient conventional and nuclear-grade graphite these days to supply the 3 foot thick, 20 foot OD, 88 foot tall, reactor vessel thermal/radiation insulating linings and neutron reflectors for about 500 PBMR reactors a year.

We are not talking about a toy or something that's a week-end project to build.  A PBMR reactor is 20 feet in diameter and 88 feet tall.  The reactor is lined with over 3,500 tons of graphite neutron reflectors and shielding blocks, 800 tons of which are nuclear grade graphite,  These numbers seem big when comparing with your basement hot water heater but are modest when you compare with conventional reactors and coal power plants which can have 30 story tall boilers.  The world produces enough graphite to build about 500 pebble bed reactors a year.  Nuclear Graphite from coal   

It also should be pointed out that the supercritical water heater and the superheated steam generator are not trivial devices and will represent a substantial portion of the conversion cost.  The biggest steam generators in the world are for PWR nuclear plants and it's not unusual for them to cost 100 million dollars.  To our advantage is the fact we have 50+ years experience in this area so those subtle metallurgy problems are well-known.  Neither device will be exposed to radiation.

Pebble People: Massachusetts Institute of Technology (Andrew C. Kadak, PhD., & Co.) has a pebble bed reactor project.  China's Tsinghua University has the HTR-10 teaching pebble bed reactor. China's Huaneng Power International, Inc., is supposed to be active in pebble bed reactors.  United States' General Atomics is the GT-MHR company.  South Africa's Pebble Bed Modular Reactors, (Pty) Ltd. is the PBMR company.  Find a pebble bed expert at:  http://www-fae.iaea.org/index.cfm   Rod Adams of Adams Atomic Engines, Inc., comes to mind as a pebble-person.  There has to be a herd of pebble people out there.  Run an ad in the nuclear profession engineer and equipment magazines.

Single page refreshers:

http://en.wikipedia.org/wiki/Brownian_motion   Brownian motion.  http://en.wikipedia.org/wiki/Doppler_broadening   Doppler Broadening.  (Me too.)

Links to additional details:   What Are Nuclear Pebbles Like?    How Are Nuclear Pebbles Made?    Wikipedia 

A 7oz nuclear pebble showing its 0.3oz of inner micro spheres of uranium and thorium→

This is in addition to the conventional atomic fission control system in a pebble bed reactor. 

Pebbles control their own fission using a natural physics phenomena that takes advantage of the natural thermal "Brownian movement" of atoms.  It's called "Doppler Broadening." 

People can't alter how a pebble controls its own temperature and a pebble bed reactor is naturally immune to thermal runaway.  Here's why:

It's all about  RELATIVE velocities of atoms and free neutrons adding up to a certain "near-perfect" RELATIVE velocity that will cause free neutrons flying about in the reactor to be readily captured by non-radioactive Uranium-238 atoms. 

Free neutrons flying about (and slowing down as they do so because they are passing through a graphite 'speed moderator') in a reactor cause atomic fission (splitting) when they happen to collide with a radioactive Uranium-235 (when they have slowed) or a radioactive Plutonium-239 (also when they have slowed) atom.  This produces heat and more "prompt" and "delayed" neutrons flying about that could collide with other radioactive atoms. The well-known slow chain reaction.

Heat causes atoms to vibrate a bit.  This movement is called "Brownian movement".  The greater the back-and-forth Brownian movement of a non-radioactive Uranium-238 atom, the greater the chances are that the atom will "eat" a passing fast-moving free neutron like "Pac-man," thus diverting that neutron from striking a radioactive Uranium-235 or a radioactive Plutonium-239 atom and making some more heat and even more free neutrons flying about.  (After a while, that now overweight (pregnant?) non-radioactive Uranium-239 atom will decay (gestate?) into a radioactive Plutonium-239 atom - this is the process called atomic "breeding" - and, now radioactive, this atom will fission if it is struck by some other free neutron flying about inside the reactor).

Since a pebble's Uranium is over 90% non-radioactive Uranium-238, the chances of a free neutron escaping capture to make heat and more free neutrons become very poor as the pebble gets hotter and hotter and the "reach" of the non-radioactive U-238 atoms becomes greater and greater.  This increased "reaching" action as temperature goes up is called "Doppler Broadening".

Since free neutrons come from radioactive atoms splitting, and strong Doppler Broadening dries up the supply of free neutrons, atom splitting in hot pebbles slows way down with the pebble going into a state not unlike the "hot idle" of a car engine - hot, but not running hard.  (Some describe this state as the "Reactor going to sleep.")  Blowing a coolant like air on the very hot pebble cools it down, (wakes it up) suppressing Doppler Broadening, thus revving up the atomic fissions again, causing the pebble to make more heat as it attempts to get back up to hot idle again - about 2,500°F.  Somewhat like blowing on a bed of hot charcoal briquettes in your grille.

As with conventional reactors, a pebble's radioactive Uranium-235 becomes consumed and natural neutron poisons like xenon gas build up in the pebble, so it becomes "tired" after a couple of years, not putting out much heat anymore even though there's still a lot of uranium left in it.  Like all other so-called "waste" nuclear fuels, pebbles would then be crushed into powder and recycled to recover the still-unused 9+% Uranium-238, along with harvesting the plutonium that was produced for MOX fuel in either conventional reactors or for use in the production of MOX pebbles, rather than Uranium-235 pebbles.  Plutonium-239 resembles Uranium-235 when it 'burns,' but produces only about half as many delayed neutrons, thus needing different enrichment ratios than a uranium pebble.

Note: Being an electronic/electrical engineer, I know nothing about nuclear physics or fuel element design.  I would like to pass along the observation that similar dimensions, materials, and materials ratios appear in other pebble and prism designs by others so I have to suspect that just any old design along these lines might not produce a pebble that runs as predictably.

To repeat, this time from Wikipedia, using their words:

"Nuclear technology:  In a nuclear reactor, this effect reduces the power generated as the reactor temperature increases.

When a reactor gets hotter, the accelerated motion of the atoms in the fuel increases the probability of neutron capture by U-238 atoms. When the uranium is heated, its nuclei move more rapidly in random directions, and therefore see and generate a wider range of relative neutron speeds. U-238, which forms the bulk of the uranium in the reactor, is much more likely to absorb fast neutrons. This reduces the number of neutrons available to cause U-235 fission, reducing the power output by the reactor.

In some reactor designs, such as the pebble bed reactor, this natural negative feedback places an inherent upper limit on the temperature at which the chain reaction can proceed. Such reactors are said to be "inherently safe" because a reactor failure cannot generate a criticality excursion. It is worth noting, however, that because of decay heat emitted from the decay of fission products, a meltdown is still theoretically possible if the ability to cool the reactor is lost, and thus the reactor design must be designed to prevent a loss of coolant accident." - From Wikipedia.

designed to prevent a loss of coolant accident." - From Wikipedia.

Again: There are no humans, computers, or devices of any kind in a pebble's nuclear fission control loop.  Just unalterable natural physics.

Practical pebbles: The hotter a pebble gets, the more Brownian movement atoms have, so the more Doppler Broadening reduces the pebble's ability to fission and make lots of heat.  Pebbles are ceramic and can withstand temperatures over 3,600°F.  A pebble bed reactor is hottest in its idle state (afterheat) - typically 2,500°F -  when heat is not being taken from it.  This high temperature "ceiling" causes Doppler Broadening to suppress almost all fission.  The pebble goes from full cool output, perhaps 500 watts of heat at 800°F, to full hot idle, perhaps just 25 watts of heat at 2,500°F.

When first encountered, this idea will strike one as being very counter-intuitive.  Pebbles must be stimulated by cooling to cause them to make large quantities of heat.

The high surface area relative to volume, and the low power density in the core can make the pebble bed a very thermally "lossy" reactor with proper design and the higher the temperature, the faster the few "hot idle" watts being made move on out.  At idle temperature, the reactor, basically just a sheet metal silo full of pebbles and a pebble monitor machine, can't get hotter and melt down because its design causes it to loose its idling heat through natural heat heatsinking - radiation and conduction - into its surroundings.  So it goes into a "hot idle" equilibrium naturally with no devices or human action needed.  There is no steam or anything else under high pressure in the reactor to explode.

The harder you work a pebble bed reactor by pulling heat out of it, the cooler it becomes and the less Doppler Broadening suppresses neutron fission activity - thus causing it to make even more heat.  To say again in different words, the cooler a pebble becomes, the harder it tries to get "idle" hot again.  This behavior is called a "Negative Temperature Coefficient" and is the main reason pebble bed reactors are considered "Always walk-away safe".

Again: At 7 MegaWatts thermal per cubic yard, pebbles are not very energy-dense when compared to conventional reactors, i.e., they are considered large for the amount of heat they make, perhaps comparable to a coal boiler in size for the same heat.

The much cooler conventional liquid-cooled reactors also demonstrate a little Doppler Broadening that helps cruising a bit, but not nearly enough for either complete self-control or safety purposes.  Another effect with light-water conventional reactors that also helps cruising is when at operating temperatures, if the temperature of the water increases, its density drops, and fewer neutrons passing through it are slowed enough to trigger further reactions. That negative feedback stabilizes the reaction rate.

What Are Nuclear Pebbles Like?

My wife says Nuclear Pebbles remind her of the fictional Dilithium crystals used to propel the Star Trek ship "Enterprise". 

Each tennis ball size pebble is almost an individual nuclear reactor, which in turn, is filled with about 15,000 nearly microscopic nearly nano-reactor specks 0.5 millimeter in diameter - smaller than a grain of salt. 

The thermal behavior of a nuclear pebble is a little counter-intuitive.  They DON'T TURN OFF.  They are ALWAYS  "running" and, when there's a pile of them big enough to make a "critical mass", are "hot" - from both from a thermal and a radiation standpoint.

The modern pebble concept was perfected by Professor Dr. Rudolf Schulten in the 1950s.  The modern pebble concept was enabled by his understanding that engineered forms of silicon carbide and pyrolytic carbon were quite strong, even at temperatures as high as 2000 °C (3600 °F).  It's calculated that under enclosed conditions with other running pebbles around it, a pebble could get as hot as 1,600 °C with no cooling going on, leaving at least a 400 °C (750 °F) safety margin.

http://en.wikipedia.org/wiki/Rudolf_Schulten

(Chorus:) ♫To make a pebble put out more heat, you have to cool it down.♫

Nuclear pebbles are spheres that fission more or less depending upon how much heat is being taken out of them.  The hotter they get, the less they fission, and, as a consequence, the less heat they make.  The more heat you take out, the more heat they produce.  Within limits, of course.

Explanations of how this is achieved from two different sources:

"As a pebble gets hotter, the rapid motion of atoms in the fuel decreases probability of neutron capture by U-235 atoms.  This effect is known as Doppler Broadening*.  Nuclei of heated uranium move more rapidly in random directions generating a wider range of neutron speeds.  U-238, the isotope which makes up most of the uranium in the reactor, is much more likely to absorb the faster moving neutrons.  This reduces the number of neutrons available to spark U-235 fission.  This, in turn, lowers heat output.  This built-in negative feedback places a temperature limit on the fuel without operator intervention."

To repeat, from a more detailed source: "[*Doppler broadening is a temperature feedback mechanism in which the absorption resonances of U-238 in the 6-100 eV range broaden as the temperature increases, resulting in greater resonant neutron absorption.  As more neutrons are captured by U-238 atoms, fewer are available for U-235 fission at thermal energies (i.e. around 1/40 eV), reducing the reactivity.  Since neutrons must undergo more collisions with carbon than with hydrogen to reach thermal energies, there are more neutrons in the resonant absorption range for graphite-moderated homogeneous systems than for water-moderated homogeneous systems, so the graphite system would feel the Doppler effect more strongly.]"

http://en.wikipedia.org/wiki/Doppler_broadening  *Wikipedia page devoted to Doppler Broadening only.

http://en.wikipedia.org/wiki/Pebble_bed_reactor#Pebble_bed_design  *Wikipedia provides an explanation of how Doppler Broadening is used to increase or decrease the nuclear fission going on inside a pebble as the pebble's design adjusts its heat output under increasing or decreasing work loads (cooling).

Doppler Broadening isn't just for pebbles.  "Note that Doppler broadening is already effective in Westinghouse-type Pressurized Water Reactors  (e.g. French plants), it's a passive feedback loop that alone is 100% efficient in maintaining core temperature within a few degrees of the design goal during "cruise" operation." -- Pierre,  from "Nuclear renaissance in Europe, part 1," by Starvid
 

(Chorus:) ♫To make a pebble put out more heat, you have to cool it down.♫ 

A pebble bed reactor (PBR) is simple to make.  You take a barrel of pebbles, and, since air blows easily through a barrel full of spheres, blow room air into the top, and air a bit less than 1,600 °F air will come out the bottom.  Like your furnace, we're just blowing air around.  In real life, inert helium, or, more likely, nitrogen is more likely to be used.  There is no reactor steam, so there's no high pressure nuclear reactor boiler that can explode.  1,800 °F is about as hot as your car's combustion chamber or a coal-burning power plant's boiler fire.  Air that hot can also be used to turn coal into a vapor (pyrolysis) that can be liquefied (with the help of catalysts) into oil or gasoline.  Or drive a gas turbine generator directly to make electricity without needing to use a steam cycle.  This is called a Brayton cycle.  http://en.wikipedia.org/wiki/Brayton_cycle  The pebble bed world is obsessed with using pebble beds in this manner because its much more efficient than conventional (Your Father's) steam reactors.

http://www.memagazine.org/contents/current/features/harness/harness.html  Discussion about the danger of high pressure steam boilers and our boiler laws.

The energy density of pebble bed reactors is 1/30th that of a regular nuclear reactor, so there's much less energy packed into a small space. 

Pebble bed reactors are intended to be typically 1% to 15% as powerful as a water cooled nuclear power plant.  PBRs are designed to be mass-produced at low cost in modules (hence, the name "Pebble Bed Modular Reactor" or PBMR) that can be grouped together to supply large amounts of power if needed.  The intent of the PBMR Company in South Africa is to electrify Africa without the enormous expense of building an electrical grid by having a lot of small stand-alone power plants at small towns all over the continent.

Other early applications will be to use PBMR heat for water purification and sea-going ship engines (Romawa, B. V., Holland), This author thinks a 10 megawatt thermal (20,000 or about 3 cubic yards of pebbles) railroad engine would be a good application for the toroid reactor Adams Engine if combined with the secondary coolant discharge to the atmosphere idea of the Romawa ship engine (Adams Atomic Engines, Inc, U.S.A.), replacements for large industrial coal-burning boilers such as oil refinery, hydrogen gas production, and oil production from non-oil fossil fuels (PBMR, South Africa). 

The Chinese have a research pebble bed reactor unit up and running at Beijing's Tsinghua University.  Other than at the Massachusetts Institute of Technology, not very much seems to be happening with pebble bed reactors in the United States.

How Are Nuclear Pebbles Made?:

How nuclear pebbles are made:  To avoid interpretation distortion, the following was extracted from a paper posted on PBMR's web site.

http://www.pbmr.co.za/download/FuelSystem.pdf

"PBMR fuel is based on a proven, high-quality German fuel design consisting of low enriched uranium triple-coated isotropic (LEU -TRISO) particles contained in a molded graphite sphere.  A coated particle comprises a kernel of uranium dioxide surrounded by four coating layers (see diagram on web page).

In the fabrication process, a solution of uranyl nitrate is sprayed to form microspheres, which are then gelled and calcined (baked at high temperature) to produce uranium dioxide fuel “kernels”.  The kernels are then run through a chemical vapor deposition (CVD) oven – typically using an argon environment at a temperature of 1,000 ºC (1,832 ºF) – in which layers of specific chemicals can be added with extreme precision.

For PBMR fuel, the first layer deposited on the kernels is porous carbon, which allows fission products to collect without over-pressurizing the coated fuel particles.  This is followed by a thin coating of pyrolytic carbon (a very dense form of heat-treated carbon), a layer of silicon carbide (a strong refractory material), and another layer of pyrolytic carbon.

The porous carbon accommodates any mechanical deformation that the uranium dioxide kernel may undergo during the lifetime of the fuel as well as gaseous fission products diffusing out of the kernel.  The pyrolytic carbon and silicon carbide layers provide an impenetrable barrier designed to contain the fuel and radioactive fission products resulting from nuclear reactions in the kernel.  Some 15,000 of these coated particles, now about a millimeter in diameter, are then mixed with graphite powder and a phenolic resin and pressed into the shape of 50 mm diameter balls.  A 5 mm thick layer of pure carbon is then added to form a “non-fuel” zone, and the resulting spheres are then sintered and annealed to make them hard and durable.

Finally, the spherical fuel “pebbles” are machined to a uniform diameter of 60 mm, about the size of a tennis ball.

Each fuel pebble contains 9 g of uranium and the total mass of a fuel pebble is 210 g - about 8 ounces.  The total uranium in one entire pebble bed reactor fuel load is 4.1 metric tons"

A typical American automobile weighs around 2 tons.

Maximum output of a nuclear pebble has been stated as about 500 watts.

http://www.nea.fr/html/science/meetings/ARWIF2004/0.05.pdf  U.S. Study on pebble details.

Cost of an individual nuclear pebble was stated as being around $10 to $15 US or about 0.5 cents per kiloWatt hour.  I suspect it will soon be higher since new nuclear plants are being built all over the world and the main secondary source of nuclear fuel, old weapons warheads, is becoming depleted.  The other major secondary source of uranium, nuclear waste, which is 95% recyclable, is supposed to be discarded and buried rather than re-used and ultimately  "burned" down to almost nothing radioactive.

An NPR Radio clip from a nuclear pebble factory:  http://www.loe.org/shows/segments.htm?programID=05-P13-00039&segmentID=3

Pebble beds are not the long-term answer for electricity generation.    I see pebble beds as the everyday, common usage, by common people, heat source for process and other industrial heat uses.  I would think that any form of new coal-burning power plant, or, for that matter, any combustion use of any fossil fuel for other than vehicular fuel would be outlawed.  We must understand that fissile Uranium-235 is in ample supply only as matches, not as firewood.  Future new power plants need to be of the breeder type so that we will always have ample quantities of the secondary fissile materials - U-233 and, if necessary, Pu-239 - available forever.  Its available for the mining and breeding in U-238 and Th-232 form.

The pebbles described above define a slow reactor using a lot of less than totally desirable graphite (it caught fire at Chernobyl Unit 4) as a moderator.  I've caught wind of a fast reactor version of a pebble being looked at by General Atomics partnering with some Russian group that uses little or no graphite as a moderator along with other materials as inter-TRISO media.  They are getting a very high burn up on the first pass but its still far from complete.

HTRs can potentially use thorium-based fuels, such as HEU or LEU with Th, U-233 with Th, and Pu with Th. Most of the experience with thorium fuels has been in HTRs. General Atomics say that the MHR has a neutron spectrum as such and the TRISO fuel so stable that the reactor can be powered fully with separated transuranic wastes (neptunium, plutonium, americium and curium) from light water reactor used fuel.

The Thorium Instead of Uranium Option

Recycling Used Pebbles:

Global Warming or Nuclear Proliferation?

Most of the big CO2 polluting countries also have nuclear power. A more realistic scenario is business as usual with each country doing more of what it is doing already with conventional nuclear fuel rods and would simply add spent pebbles to their existing programs.  Most of the big CO2 polluting countries that could make good use of pebble nuclear technology already have nuclear power plants and world-acceptable nuclear fuel policies and handling procedures in place already.  http://en.wikipedia.org/wiki/Nuclear_power_phase-out  -- Links to policies of almost all nuclear countries.

The Hard Part: Recycling.  I never said pebbles were the lowest-cost way to go nuclear.  Coal Yard Nukes are intended to be a quick way for the world to get back on the nuclear electricity track as a way to end unnecessary Global Warming just to make electricity.  I'm only discussing the basic uranium pebble here - the one that has a fuel cycle almost identical to your conventional neighborhood nuclear power plant.  Thorium and breeder pebbles are similar, but not identical.  http://en.wikipedia.org/wiki/Nuclear_reprocessing 

The "Full Recycling" model described in the December, 2005, issue of Scientific American magazine by William H. Hannum, Gerald E. Marsh and George S Stanford, is the most expensive, but most practical approach.  They claim less than 1% of the radioactive material escapes being used for making electricity and only tiny quantities of short term (500 years rather than 10,000 years) wastes need to be buried.

The low-hanging nuclear fruits on Mother Nature's tree of energy are: Uranium-235 (not plentiful, radioactive, and the "match" for all nuclear fire), Uranium-238 (plentiful, but has to be made radioactive, and should be thought of as "nuclear firewood"), and Thorium-232 (extremely plentiful, but has to be made radioactive, and should also be thought of as "nuclear firewood"). 

If we continue to burn just the matches, and at this time, pebble beds fall into the category of match-burners, we'll become unable to start nuclear fires in as little as 200 years.  "Full Recycling" supports the breeding U-238 and Th-232 into radioactive Plutonium-239 and radioactive Uranium-233 and then always recovering and re-using them as an essential part of ensuring mankind unlimited electricity forever. 

Nuclear recycling facilities are about the size of a small automobile assembly plant.  It doesn't seem unreasonable that most countries would do their own recycling, if only to keep the cash flow involved domestic.  Disposal of the waste is actually a simple and inexpensive task that any country can afford to do.

The Easy Part: Permanent Burial of the really spent nuclear fuel (actinides only) in a disposal shaft is an inexpensive and easy thing for any country to do.  Using an oil rig, drill an 8 inch (a common and cheap size - some other size may be economically optimal in real life) oil well pipe 6 miles into the ground, creating a 1,800+ cubic foot disposal volume (about a 12 foot cube) in the pipe between miles 6 and 5, plug the bottom end with 50 feet of reinforcing bars and concrete, batch poured over several days to seal any leaks,  fill mile 6 to mile 5 with the actinides as a slurry, let settle a month, then capping the last 50 feet with reinforcing bars mixed with concrete batch poured over time, break the pipe at mile 5 with an attached shaped explosive cutting charge, then withdraw the remaining pipe to use for making the next disposal shaft. 

Anyone trying to retrieve that worthless stuff will have to re-drill a 5 mile shaft to better than plus or minus 2 inches accuracy and then deal with 50 feet of concrete and steel.  Assuming we allow one square yard per 8 inch disposal pipe, we can put about 5,000 such disposal shafts on a single acre.  How's that for being kind to the environment?  Since Yucca Flats is already set up for nuclear disposal, we could set up the disposal oil rig in a several acre space just beyond the administration building's parking lot so the rig crew could use the building's cafeteria and toilets.

The Reactor Silos:  What They Are Like and How They Work.

Water is a wonderful way to turn heat energy into mechanical energy because when you turn water into steam, it expands its volume 1,600 times.  If steam is not allowed to expand in volume,  its pressure will go up drastically.  That's where all that piston-pushin power in a steam locomotive comes from.

 Below: How a pebble bed steam boiler power plant is set up.  Compared to conventional nuclear, pebble nuclear silos would be considered 'light.' 

Pebble bed reactors are much better suited for "load following" than conventional reactors and should approximate a coal boiler's load following abilities in a superheated steam application.  Steam control for load following (not shown in this drawing) will come from a computer coordinated feedwater throttle valve, a variable-speed "reactor gas circulating blower", a variable steam generator (boiler) gas bypass valve, and some bypass piping to return the hot gas directly to the reactor.  Temperature and flow sensors to keep the computer and the operator informed of how things are going are also necessary.  To repeat, strange as it sounds, running the hot gas directly back into the bed of pebbles and making them even hotter causes pebbles to reduce their atomic activity.  Doppler Broadening (a nano-technology phenomenon if there ever was one) will make pebbles go toward, but not quite to, zero atomic activity as they approach their "hot idling" temperature.  (A retrofit to an existing coal-fired plant might also show an economizer   http://en.wikipedia.org/wiki/Economizer   .)

Because pebbles are so hot, a gas - usually helium, nitrogen, or carbon dioxide - is used to carry the pebble's heat to the power plant's steam boiler.  Water's supercritical temperature - where it stops behaving like a vapor and begins behaving like a gas - is around 700°F. 

Inlet temperature of a pebble bed reactor is about 950°F, and its outlet temperature is almost 1,700°F.  That duplicates the 1,000°F superheated steam produced by coal fire while running the pebbles in their thermal "sweet spot."

Note: The pebble bed power plant shown in the drawing is set up as a conventional coal-fired superheated steam boiler power plant.  750°C = 1382°F, 530°C = 986°F, 250°C = 482°F, 35°C = 95°F, 25°C = 77°F.  I suspect this drawing may have been inspired by the now decommissioned German THTR-300 - a thorium-powered high-temperature pebble bed nuclear reactor rated at 300 MW electric that actually powered the German grid during the 80's.  http://en.wikipedia.org/wiki/THTR-300 

BOILERS: Modular pebble bed reactors were designed from the start to be inexpensively mass-produced in factories and shipped in standard shipping containers.  There could be a variety of boiler types but current fossil-heated designs may be best for rapid power plant conversion - since speed, rather than efficiency is the most important issue.  Boiler design will depend upon whether the boiler has reheat stages in addition to the typical drum with superheater stages.  A supercritical pressurized water system, much like a conventional PWR reactor, but at supercritical temperatures and pressures is also a possibility since we have working temperatures as high as 1,700°F.

The German THTR-300 thorium pebble bed (above) had a boiler (actually, a ring of small boilers) designed for steam but they used helium pressurized to about 1,300 psi instead at low or no pressure to carry the heat from the pebbles to the boiler.  In a fossil-fuel burning plant, these velocities are determined by the combined efforts of the forced draft and the induced draft blowers.  While boiler design and fabrication  is a routine heavy industry activity involving at least several months, the first pebble bed heated supercritical water heater will likely take more design and fabrication time since doing anything the first time takes longer.   http://www.iaea.org/inisnkm/nkm/aws/htgr/abstracts/abst_iwggcr15.html    IWGGCR-15: Technology of steam generators for gas-cooled reactors.

Note: In the past, some high temperature gas-cooled reactors have used multiple small boilers arranged in a ring around the reactor vessel.  This produces a very large surface area for the volume that contains the heat exchanging surfaces.  I don't know what current thinking is on this design detail but, since the amount of heat is large and supercritical water, rather than steam is involved, I'm inclined to think that as there are efficiency constraints (there's a lot of heat to be lost and we want it to run as cool as possible) rather than size constraints, a large single unit might be the appropriate design starting place for both ends of the hot water loop.

(Photograph of the interior of a pebble bed reactor silo from a January, 2002, Scientific American magazine article by J. A. Lake, et al.)  Not unlike a huge charcoal grille with air blowing down through it.  Aerodynamically, pebbles behave somewhat like a sintered metal air filter and have acceptably low drag.

REACTOR CONTAINMENT: For a power plant conversion, pebble bed reactors should be installed in individual underground silos instead of above ground.  The moisture in the earth provides a good radiation barrier along with being a good heat sink, something pebble beds need.  PBMR pebble bed reactors are 20 foot in diameter tubes, about 88 feet tall, made of thick, radiation-proof sheet metal, capable of running at very high temperatures like a boiler, are hermetically sealed in very heavily reinforced underground concrete silos.  Since nothing in direct contact with radioactive materials is under pressure, the traditional concrete and steel steam explosion containment vessels needed by conventional reactors are unnecessary, thereby providing a huge saving in both cost and construction time.

Pebble beds do not have a fuel cycle as such.  Pebbles are circulated through the pebble bed reactor in much the same manner as a fluid.  Each pebble is checked individually for how strong it is and is removed and replaced when too "tired".  10 to 15 trips through the reactor over a two to three year time are said to be typical.  Additional "Moderating" pebbles containing only graphite are also added to "dilute" the critical mass to fine-tune the reactor's heat output and burnup rate.

Tired pebbles are removed pneumatically and transported to a central storage area where they are kept spaced from each other to keep a critical mass from forming and automatically packaged in thermally insulated, radiation-proof casks for shipment to a recycling plant.

(Left) From MIT's pebble bed reactor web site.  Pneumatic pebble handling system that removes, measures for remaining energy, and then either returns a useable, removes a spent pebble or removes fragments of a broken pebble.  A broken pebble was a problem in the 1980's-era German THTR-300 pebble bed reactor.  http://web.mit.edu/pebble-bed/

As with conventional fission nuclear fuel, only about 5% of a pebble's atomic energy is actually consumed during a power run, so pebbles will have to be crushed to powder like quarry rocks and recycled for their remaining energy as is done world-wide with conventional nuclear waste. 

High-Temperature Gas-Cooled Reactors - In Greater Depth