About Pebble Bed Reactors

A new (to the U.S.) and sometimes controversial nuclear reactor is the Pebble Bed Modular Reactor (PBMR).  Proponents point out that the reactor is smaller, less expensive, cleaner and safer than other nuclear plants.  Pebble bed reactors offer the opportunity to reduce the complexity of the plant because the number of safety systems required is reduced significantly due to the inherent safety of the technology.  Rather than depending on the economics of scale to reduce cost, the economics of production volume and simplified design are used.  Opponents of nuclear power say it produces more nuclear waste than existing nuclear reactor designs.  Both sides of this discussion have valid points.

The PBMR is classified as a high temperature gaseous reactor (HTGR or HTR) because it operates at temperatures up to 900°C compared to an operating temperature of typically 340°C in pressurized water reactors (PWR), the most common type of reactor.  The higher temperature results in a higher efficiency, calculated to be up to 44%, about one-third more than conventional reactors . 

The reactor is hopper like vessell filled with graphite pebbles, each about 60 mm in diameter and filled with thousands of Uranium Oxide (UO2) fuel particles with diameters of less than 1 mm.  Each fuel particle is coated with two layers of pyrolytic carbon, silicon carbide, and porous carbon. The coatings retains the gaseous fission products.  Graphite in the form of pebbles or fuel rods is used as a moderator to control the reaction.  Heat that is generated by the fission reaction is transferred to the helium gas that flows through the reactor.  From the reactor the helium goes directly to gas turbines that convert the thermal energy in the gas into electricity.  The gas is then cooled, compressed and reheated before being returned to the  reactor.

The reactor is continuously fed with fuel pebbles from the top and the used fuel is removed from the bottom.  After each pass though the reactor the pebbles are tested to determine how much fissionable fuel is left.  Those with enough fuel left are returned to the top, each cycle through the reactor takes six months.  Each pebble that is removed is replaced with a new pebble.  Each pebble passes through the reactor six times and lasts about three years before it is exhausted.

The PBMR reactors are made in relatively small modules that are factory made, saving assembly labor over field assembled units and allowing faster assembly in the field.  Either single modules can be used to generate power, or more likely, several modules are combined to make a power plant of more conventional size, 500 MW to 1,300 MW.

Safety

The pebble bed technology is considered by proponents extremely safe, with passive safety features that require no human intervention and that can't be bypassed.  According to its proponents, a meltdown, the bane of conventional nuclear reactors, can't happen in a pebble bed system. There is, they say, no chance of overheating caused by radioactive decay because of the resistance to high temperature of the billions of fuel particles contained within the graphite balls.

Each pebble, within the vessel, is a 60 mm sphere of pyrolytic graphite filled with the fissionable material. The shell melts at 3000°C, more than twice the design temperature of most reactors. It slows neutrons very well, is strong, inexpensive, and has a long history of use in reactors. The graphite act as a renewable moderator for the reactor, and are strong containment vessels.

A pebble-bed reactor can have all of its supporting machinery fail, and the reactor will not crack, melt, explode or spew hazardous wastes. It simply goes up to a designed "idle" temperature, and stays there. In that state, the reactor vessel radiates heat, but the vessel and fuel spheres remain intact and undamaged. The machinery can be repaired or the fuel can be removed.

The reactor is cooled by an inert, fireproof gas, so it cannot have a steam explosion as a light-water reactor can.

There is discussion as to whether a containment vessel for the reactors is necessary or not because of the inherent safety of the system.  The South African plant will have a concrete containment vessel.

Opponents of the PMBR say that it creates more nuclear waste than conventional reactors, which is true.  They also point out that "the inherent safety" is largely theoretical and remains to be proven.  Specifically there are serious questions as to being able to have adequate quality control in the manufacturing of the pebbles and the possibility of having hot spots in the reactor due to uneven distribution of the pebbles. Proponents argue that even though there is more waste it takes up much less volume and is therefore easier to control, transport or dispose.  The fuel is consumed to a greater extent than fuel in conventional reactors and therefore there is less fissionable material that could be extracted from a given amount of spent PBMR fuel.  See Harding paper for critique of safety issues.

History

A 15 megawatt (electric) demonstration reactor, the AVR, was built in Germany. The unit's started operation in 1966 and ran successfully for 21 years, and was decommissioned in 1988 in the wake of the Chernobyl disaster.  China has licensed the the AVR technology and is developing a pebble-bed modular reactor for power generation.  A 10 MW prototype is at Tsinghua University in Beijing.  Construction of a 200 megawatt production plant is scheduled to begin 2007.

Pebble Bed Modular Reactors Pty. Ltd. in South Africa is developing a modular PBMR.   The reactor is undergoing pre-certification in the U.S.  Eskom, the South African power utility has received  administrative approval to build a prototype, but has been delayed by environmental proceedings.  The following are highlights of the plants design

• Up to 450,000 fuel pebbles recycle through the reactor continuously
• The pressure vessel is lined with graphite and there is a central column of graphite as reflector. Control rods are in the side reflectors and cold shutdown units in the center column.
• Performance includes great flexibility in loads (40-100%), with rapid change in power settings.The pebble bed reactor's design can be throttled in real time to meet peak electric power loads just like conventional reactors, where power follows steam demand in seconds.
• The plant has passive safety systems embedded which cannot be circumvented by human intervention.  Should a worst-case scenario occur, no human intervention is required in the short or medium term.

They believe that production units will cost about $1000 per kw of capacity competitive with fossil fueled power plants and well below the $3000 per kW average cost of conventional reactors in the U.S.

Westinghouse Electric and British Nuclear Fuels plc are assisting with the South African program.  According to the Pittsburgh Tribune-Review, a spokesman for Pebble Bed Modular Reactors Pty. Ltd. in Pittsburgh for meeting with Westinghouse said that packages of a four modules are the most cost-efficient.  Each reactor is designed to be off-line for three weeks once every six years.  A power plant consisting of two four-packs, giving you 1,320 MW, is designed to never have more than one unit down for scheduled maintenance at one time.  Pebble bed power plants require a much smaller safety zone: 1,320 feet. The typical nuclear reactor in this country utilizes a 10-mile zone.

MIT has a program to develop a technical and economic basis for the PBMR to determine whether it can compete with natural gas fired power plants and meet meet safety, proliferation resistance and waste disposal concerns.  They have designed a completely modularized plant in which all of the plants components are modularized and then assembled in the field.  They do not use direct hydrogen in the power turbines, rather intermediate heat exchangers to isolate the reactor helium from the power turbines.  There evaluation is not complete but so far they see no reason that their concept cannot meet its stated objectives.

Resources:

PBMR (Pty) Ltd., South African Pebble Bed Modular Reactor Company, Centurion, South Africa
Pebble Bed Reactor Technology, Eskom, Johannesburg, South Africa
Pebble bed reactor, Wikepedia
Pebble Bed Modular Reactors - Status and Prospects, Jim Harding, Feb. 2004
Nuclear reactor technology advances, Pittsburgh Tribune-Review, October 12, 2005
Modular Pebble Bed Reactor, MIT website
Advanced Modularity Design for the MIT Pebble Bed Reactor, Andrew C Kadak and Marc V Berte, MIT, Sept. 2004
Nuclear Power for the Future, Michael Freemantle, Chemical & Engineering News, September 13, 2004
New Reactor Designs, EIA/Nuclear/Nuclear Features/New Reactor Designs
Nuclear Power Reactors, Uranium Information Center Ltd, Melborne, Australia - an excellent website that defines the various generations of reactors and how they work.

Technorati tags: nuclear power, nuclear reactor, energy

Comments

[JesseJenkins]

Thanks for another great post.

These reactors seem to solve the saftey concerns that plauged older nuclear designs (at least well enough to put my mind at ease in that category). So that still leaves the three areas the MIT study you mentioned plans to look at: economic competitiveness, nuclear proliferation concerns and waste disposal concerns.

At ~$1,000 kW, it seems like it should be economically competitve. As far as nuclear proliferation concerns, do you know anything about what quantities of plutonium this reactor design produces?

Waste concerns seem to be the big problem here. I have yet to hear a satisfactory plan for disposing of the waste. The previous post you wrote on recycling nuclear waste seemed to have potential though. This plant seems to more completely consume its fissionable fuel than typcial reactors. How would this affect the potential to recycle the waste from this type of reactor? Would it still be feasable/desirable to recycle the waste rather than store it in geological vaults?

I'm still not sold on nuclear. With the safety issue 'solved' that's a step in the right direction but we're gonna have to find a suitable solution for the waste problem before I'm gonna start advocating nukes over clean coal (IGCC w/ co2 sequestration)
. Why not use a domestically plentiful source of fuel that doesnt produce radioctive waste that will stick around for generations?

[Engineer-Poet]

One glaring error in the post:
***
The gas is then cooled, compressed and reheated before being returned to the reactor.
***
That should probably say "The gas is then cooled and compressed before being reheated in the reactor."

I think the waste issue is overblown. Each pellet is a very big "fruit" around a rather small "pit", where all the fissionables are. If it were up to me, I'd let the pellets cool for ten years or so and then use abrasive-jet machining or electrical-discharge machining to remove most of the bulk of the pellet cladding, leaving a bit of graphite around the spent uranium and drastically reducing the volume. The "pits" could be buried or reprocessed.

[James Fraser]

Eskom describes the process as follows:

"Helium at a temperature of about 500 ¦C is introduced into the top of the reactor.
After the gas passes between the fuel balls, it leaves at the bottom at a temperature of about 900 ¦C.

This gas passes through three turbines.

The first two turbines drive compressors and the third generator, from where the power emerges.

At that stage the gas is about 600 ¦C.It then goes into a recuperator where it loses excess energy and leaves at about 140 ¦C.

A water-cooled precooler takes it down further to about 30 ¦C.

The gas is then repressurised in a turbo-compressor before moving back to the regenerator heat-exchanger, where it picks up the residual energy and goes back into the reactor."

A very similar description is on the PBMR website. You can follow the process easily on the included flow diagram - too bad the temperatures are not on the diagram. Restating the description to put the emphasis on the gas temperature entering the reactor: The gas is leaving the generator turbine at 600 C and is cooled in the recuperator to 140 C, the gas leaving the high pressure compressor is heated additionally in the recuperator to 500 C before entering the reactor. That would imply that the temperature leaving the high pressure compressor was on the order of 100 C which seems a little low, but 500 C would be too high. This situation results from the very high operating temperature in the reactor.

[Engineer-Poet]

Point. But recuperators/heat exchangers are usually mentioned as a separate process instead of generic "reheat".

Helium is particularly good for this, because the ratio of specfic heats (gamma) is about 5/3 as opposed to diatomic gases which are about 7/5 at room temperature. This translates to a greater temperature change per unit of pressure change in compression and expansion.

[Orion]

The waste disposal problem is somewhat overblown. In the first place far less "ash" is produced than in conventional fuel power plants and these vent directly to the atmosphere. Coal-fired power plants, especially those using anthracite coal, also spew radiation, more annually than was released in the Three Mile Island accident.

Secondly and more importantly critics assume that the waste will eventually seep into the environment, causing death and mayhem. Not likely; by the time Yucca starts leaking under the worst-case scenario we'll have long since pulled all the nuclear material back out for reprocessing and subsequent disposal elsewhere. At some point within the next 200 years we'll start shooting the stuff into the sun where it can radiate to the environment to its heart's content and no one will care.

If we make the assumption that civilization collapses before we get to this point and people forget how to dispose of the stuff, the repositories are in remote locations and extremely difficult to get to w/o advanced technology anyway and the kind of disaster that would destroy civilization would ALREADY have caused so much damage to the ecosystem that a little radiation in the ground water around the repositories would be little more than "bouncing the rubble". The survivors of said worldwide catastrophe, if any, would barely notice.