Nuclear Power Industry News

The Evolution of the Pebble Bed Reactor

 - By Duncan Williams -

One of the more promising nuclear reactor technologies known today is the pebble bed nuclear reactor.

Offering many advantages over conventional reactors, the pebble bed reactor gets its name from the type of the nuclear fuel it consumes.  Just like conventional reactors, the pebble bed reactor can fission uranium, thorium and/or plutonium as its nuclear fuel.  But the similarities end there.  Instead of forming the fuel into plates or pellets as in conventional reactors, the pebble bed reactor fuel is manufactured into spheres, known as “pebbles,” slightly smaller than a tennis ball.  Simply stacking the pebbles together in a pebble bed in the reactor can form a critical geometry that initiates the nuclear fission process.  Instead of being cooled by water as in a conventional reactor, helium gas is used to cool the spherical fuel elements.

The most widely used spherical fuel is made up of thousands of coated particles known as tristructural-isotropic (TRISO) particles.  As shown in the diagram, the center of the particle is typically uranium dioxide, known as the fuel kernel, and is .5 mm in diameter.  The fuel kernel is coated with a layer of porous carbon which serves to capture any fission product particles emitted from the fuel kernel.  Three additional layers of carbon are then applied to each particle: an inner layer of pyrolitic carbon; a mid-layer of silicon carbide; and an outer layer of pyrolitic carbon.  These layers provide the structural support necessary to endure irradiation during the fission process.  Thousands of these TRISO particles are then embedded in a graphite matrix and formed into a sphere.  An outer layer of graphite is then added to fully form the 60 mm fuel sphere.

One of the major advantages of the pebble bed reactor is that it is extremely unlikely that it will ever suffer a catastrophic meltdown at high temperatures.  Instead of relying on mechanical or chemical safeguards to avoid meltdowns as in conventional reactors, the pebble bed reactor relies on the Doppler broadening effect and its relationship to the uranium contained in the spheres.  The spherical fuel elements contain both uranium-238 as well as low-enriched uranium-235.

Unlike the uranium-235, which is likely to fission upon the absorption of a neutron, uranium-238 will simply absorb the neutron without any further fissioning  At low temperatures, uranium-238 only absorbs neutrons having only certain energy levels, thereby allowing most of the neutrons to be absorbed by uranium-235 thus sustaining the fission reaction.  At higher temperatures, however, the Doppler effect causes the uranium-238 to absorb neutrons having a much wider range of energy levels.  This leaves less neutrons available for absorption by the uranium-235 which effectively halts the fission process as well as the temperature rise in the reactor.  The pebble bed reactor will “idle” near 1600 C, well below the vaporization temperature of the spherical fuel elements (4000 C).

Another advantage of the pebble bed reactor over conventional reactors is that much more energy can be extracted from the heat produced by the fission process, thereby greatly increasing the thermal efficiency of the reactor.  Because gas-cooled reactors can operate at much higher temperatures than conventional reactors, it is possible to extract up to 50% of the available energy from the heated helium gas.  The most advanced nuclear reactors today using water as a coolant typically have a thermal efficiency between 35-40%.

Unlike conventional nuclear reactors, pebble bed reactors do not need to be shutdown in order to be refueled.  Refueling in conventional reactors requires long shutdown periods while implementing costly procedures to replace used fuel elements.  In contrast, new spherical fuel elements are simply placed in the top of the pebble bed.  Mechanical systems in the pebble bed reactor cause new fuel pebbles to slowly move lower in the core as it is used so that the oldest spheres can easily be extracted through a tube at the bottom of the reactor.

Another major advantage of pebble bed reactors is that the high temperature helium used as a coolant can be used in various industrial processes.  For example, the high temperature gas can be used in fuel refinement, the production of plastics, as well as the production of fertilizer.  The high temperature helium gas can also be used for the production of hydrogen, which in turn is used for treating metals, processing food, as well as preventing an alternative fuel source in the form of hydrogen fuel cells.  In fact, the U.S. Department of Energy’s Next Generation Nuclear Plant project is currently studying the effectiveness of locating gas-cooled nuclear reactors near the aforementioned industrial facilities in order to take advantage of the high-temperature process heat.

Although there is a renewed interest in pebble bed reactors, the technology is certainly not new.  The first pebble bed reactor was created in Germany, and was known as the German Arbeitsgemeinschaft Versuchsreaktor (AVR) - loosely translated as the working-group experimental reactor.  This small 10 MW experimental enriched-uranium reactor operated successfully from 1966-1988 in Julich, West Germany.  Many of the first patents in the United States relating to pebbled bed reactors are based off of this initial German design.  For example, one of the earliest known American patents relating to pebble bed nuclear reactors is U.S. Patent No. 3,960,656, issued on June 1, 1976.  This patent describes a funnel device in which nuclear pebbles (29) are placed in the top and then slowly roll down to the bottom of the reactor as used pebbles are taken out of the bottom outlet (26).

The fission process in the AVR was controlled by inserting devices known as control rods into the bed of pebbles.  Control rods are typically made up of a material that absorbs neutrons that are emitted from the pebbles so that they cannot be used to continue the fission process.  However, inserting the control rods can place a large amount of stress on the surface of the pebbles causing them to crack and release the radioactive nuclear fuel contained inside.  Various early U.S. patents filed by German corporations, drawing from lessons learned in the AVR, address this issue.  For example, U.S. Patent No. 3,971,444, issued on July 27, 1976, and assigned to a German corporation (HKG), describes a pebble bed reactor containing different diameters of pebbles in order to minimize stress on each pebble when a control rod is inserted.  As can be seen in the diagram, the pebbles (2,3) have different diameters, which allows more spacing in between the pebbles.  This allows the control rod (4) to glide through the pebbles producing a minimal amount of stress.

Another patent U.S. Patent No. 4,010,070, issued on March 1, 1977, and assigned to Interatom Internatioinale Atomreaktorbau GmbH, describes a hollow helical spiral rod (1) containing a neutron absorbing material (1a).   As the rod is twisted into the pebbles, much like a corkscrew, the spheres are simply moved out of the way instead of absorbing any stress.

Based on the successes of the AVR design, Germany went on to build a much larger reactor using a combination of enriched uranium and thorium as its nuclear fuel.  Known as the thorium high temperature reactor (THTR-300), the 300 MWe (750MWth) reactor operated from 1983-1989.  However, the political climate soured on nuclear power technology and Germany abruptly ended its pebble bed projects.

In 2000, China began construction of its first experimental pebble bed reactor.  Modeled after the German AVR, the 10 MWt Chinese High Temperature Gas-Cooled Reactor (HTR-10) began operating in 2003 at the Institute of Nuclear Energy Technology at Tsinghua University in Beijing.  The HTR-10 uses spherical fuel elements containing coated nuclear fuel particles and is cooled by helium gas.  This small reactor yields such promising results that the Chinese government announced plans to begin construction on two more much larger pebble bed reactors in 2009 at the Shidaowan plant in the Shandong Province in China.  Two 105 MWt reactors, known as the High Temperature Reactor- Pebble Bed Modules (HTR-PM), should become operational in 2013.  If all goes well, China plans on developing approximately 30 of these reactors by 2020.

Here in the United States, the Massachusetts Institute of Technology (MIT) has long been designing a dual-zoned pebble bed reactor, although there are no plans actually build it.  On March 8, 2005, the U.S. Patent office issued Patent No. 6,865,245 to MIT.  As shown in the diagram, the patent describes a pebble bed reactor made up of a central column of reflector pebbles (10) made of graphite, surrounded by fuel pebbles (18) made from graphite coated uranium dioxide.  The reflector pebbles slow the neutrons escaping from the fuel pebbles and reflects them back into the fuel pebble (18) so that the neutron can be absorbed by the uranium dioxide in order to continue the fission process.  This design allows for the fine tuning of the power distribution throughout the core resulting in a more even burning of fuel throughout the reactor.  For example, removing reflector or fuel pebbles decreases the power density, while adding reflector or fuel pebbles will increase it.

The safety of pebble bed reactor technology makes it ideal as a small portable power supply.  Accordingly, U.S. Patent No 5,309,492, issued on May 3, 1994, and assigned to Adams Atomic Engines, Inc., describes a closed gas turbine system wherein air is heated by an enclosed pebble bed reactor.  As shown in the diagram, opening the throttle valve (14) allows helium gas that was heated by the pebble bed reactor core (62) to flow to the turbine engine (18).  The more the throttle valve (14) is opened, the faster the turbine (18) spins resulting in more power produced.  In order to stop the turbine, the throttle valve (14) is shut, which stops the circulation of helium through the core (62) as well as to the turbine (18).  The inherent stability of the pebble bed reactor will cause the temperature in the core (62) to rise to a certain temperature and then idle due to the Doppler effect.

The undisputed global leader of commercially developed pebble bed reactors is a South African company called the Pebble Bed Modular Reactor Proprietary, Ltd. (PBMR).  On March 24, 2009, PBMR submitted a letter of intent to the U.S. Nuclear Regulatory Commission to submit a design certification in 2013 for the DOE’s Next Generation Nuclear Plant project.  PBMR is currently actively experimenting with improved fuel sphere technology for its pebble bed designs. 

For example, U.S. Patent Application No. 20070263762, published on November 15, 2008, and owned by PBMR, describes an improved TRISO fuel particle that includes a 40 micrometer diamond outer coating.  After combining thousands of these diamond-coated fuel particles into a sphere, a final 240 micrometer coating of diamond is deposited onto the fuel sphere.  These diamond coatings apparently allow the fuel particles to withstand pressures of up to 4,350,000 psi, allowing them to be compressed into spheres without fear of being crushed.  The diamond coating on the outer surface of the fuel results in the sphere being more resistant to oxidation, thus minimizing the creation dust that may have otherwise been released as the spheres are jostled inside the reactor.

U.S. Patent Application No. 20090080591, issued on March 26, 2009, and issued to PBMR, describes another type of coating to be placed on the uranium fuel kernel.  Instead of placing multiple layers on the uranium oxide fuel kernel, this application describes placing one coating containing a fluorinated mixture of silicon, silicon carbide, magnesium and diamond.  This coating will retain fission products released from the fuel kernel up to temperatures as high as 1900 C.

U.S. Patent Application No. 20090129533, issued on May 21, 2009, and issued to PBMR, describes yet another improvement to the fuel spheres.  As shown in the diagram, the uranium dioxide fuel kernel (12) is coated with up to seven layers of pyrolitic carbon (16) alternated with six layers of silicon carbide (18), with each layer having a thickness of 10 micrometers at most.  Between each layer is a 2-4 micrometer transition zone containing a mixture of the pyrolitic carbon and silicon carbide.  These coatings allow the fuel particle to withstand a pressure of up to 58,015 psi at 1800 C for about 30 days.

As can be seen from these recent patent applications, advances in material science are continuously improving pebble bed reactors.  Although a slow evolution, pebble bed reactors have advanced to the forefront of the exciting world of nuclear power.

 Last Week's Column:

Under The Hood With Duncan Williams - South Korean APR 1400 Reactor Plant Surprises UAE Officials

About Duncan Williams
Duncan Williams graduated from the University of Florida in 1994 with a B.S. in Physics, and a minor in mathematics.  Upon graduation, he was commissioned  in the U.S. Navy where he completed training in the Navy’s Nuclear Propulsion program.  He then served onboard an aircraft carrier, the USS Theodore Roosevelt, as a reactor control division officer.  Onboard, he was responsible for the operation and maintenance of the electrical and mechanical components that make up the reactor control systems.  This includes the control rod drive mechanisms, the reactor safety and emergency systems, the reactor coolant pump systems, and the ion exchangers.  He also developed and implemented ship-wide reactor safety drills in order to educate sailors in reactor safety.

Duncan then transferred to the U.S. Naval Academy, where he served as a senior instructor teaching Thermodynamics to senior cadets.  While serving as an instructor at the Naval Academy, Duncan attended night law school at the George Washington University Law School.  After receiving his J.D. in 2004, he resigned his commission and began working as an intellectual property associate with Kenyon & Kenyon LLP.  While at Kenyon & Kenyon, he drafted numerous patents relating to medical devices, electronic devices, telecommunications, as well as other technologies.  He also has experience in all stages of patent litigation, and has represented numerous Fortune 500 companies in protecting their intellectual property rights.  Duncan is currently an intellectual property associate at Blank Rome LLP.

If you have questions, comments, or know of a patent that you think Duncan should review E-mail Duncan Williams>> duncan@nuclearstreet.com