Alternative Nuclear Power

8/11/2019 Alternative Nuclear Power

1/16

Alternative Nuclear Power: Pebble Bed Reactor

Written by Philip Proefrockon 11/12/11

This article is part of a series on alternative possibilities in nuclear power.Previously at EcoGeek:Alternative Possibilities in Nuclear Power

Pebble Bed ReactorThe pebble-bed reactor was supposed to be another intrinsically safe, and "melt-down proof" design."Pebble bed reactors are helium-cooled, graphite-moderated reactors in which the fuel is in the form oftennis ball-sized spherical "pebbles" encased in a graphite moderator. New fuel pebbles are continuouslyadded at the top of a cylindrical reactor vessel and travel slowly down the column by gravity, until theyreach the bottom and are removed." Cooling uses an inert gas such as helium, rather than a liquid, whichsimplifies many of the reactor systems.

"The use of helium and graphite allows the reactor to burn the fuel efficiently and to operate at muchhigher temperatures than conventional light water reactors." Since the pebble bed reactor was alreadydesigned to operate at very high temperatures, and since its cooling medium was a gas, rather than aliquid, the control systems for a pebble bed reactor could be much simpler. The largest problems that

need to be dealt with for a boiling water reactor - overheating andcoolantboiling away - are notconcerns for a pebble bed reactor. The pebble bed also produces less power as the temperature rises, sothe design is effectively self-limiting.

The pebble bed design offers some operational advantages, such as allowing the reacor to operate

constantly without needing to be shut down periodically for refueling. As each pebble makes its waythrough the system and is drawn out at the bottom of the reactor, it can be tested and either reinsertedat the top of the reactor (the average pebble would cycle through the reactor about ten times before itwas expended) or withdrawn if it was spent. New fuel pebbles could also be added when needed to keepthe reactor operating.

Early experimental work with pebble bed reactors was carried out in Germany beginning in the 1960s.Pebble bed reactors were thought to be a promising next step in reactor design. But several issues

operational made the pebble bed design less than ideal. Contaminated graphite dust is created from thepebbles from friction as they move down through the reactor. Tests carried out with dummy pebbles also

found overheating conditions inside the reactor. The volume of radioactive waste from a pebble bedreactor is larger than that from other designs, which presents moreof a problem when dealing with

spent pebbles. And decomissioning the reactor may have higher costs because of the radioactivity of thereactor components.

Because of these problems, theGerman project was abandonedby the 1980s, and rights to carry on thework were obtained by a series of South African companies. However, after years of development, the
http://www.ecogeek.org/component/content/article/3657http://www.ecogeek.org/ask-the-ecogeek/3656http://www.ecogeek.org/ask-the-ecogeek/3656http://www.ecogeek.org/ask-the-ecogeek/3656http://www.thebulletin.org/web-edition/features/the-demise-of-the-pebble-bed-modular-reactorhttp://www.thebulletin.org/web-edition/features/the-demise-of-the-pebble-bed-modular-reactorhttp://www.thebulletin.org/web-edition/features/the-demise-of-the-pebble-bed-modular-reactorhttp://www.thebulletin.org/web-edition/features/the-demise-of-the-pebble-bed-modular-reactorhttp://www.ecogeek.org/ask-the-ecogeek/3656http://www.ecogeek.org/component/content/article/3657


2/16

work on developing a pebble bed reactor has pulled curtailed


3/16

plants, and set aside another $1.25 billion for an experimental reactor to be built here in the Idaho

desert. The reactor will be the centerpiece of a modern-day Manhattan Project, with scientists from

around the world working together to revolutionize the production of nuclear power.

Nuclear Shortcut

At the heart of every reactor is fuelusually uraniumundergoing a chain reaction that generates heat

and fast-moving neutrons. A coolantdraws away the heat and uses it to spin a turbine to generate

electricity, and a moderator slows the neutrons to keep the reaction under control. Any material used in

building a reactor has to withstand the heatas well as intense pressure and a constant barrage of

neutronsfor the reactor's projected lifetime. To prove that a new alloy can last 25 years, you could put

it in a furnace for 25 years and bombard it with neutronsor, if you don't want to wait that long, you can

use the ATR.

"It is like a time machine," says Duling, the facility's former deputy director. The reactor uses uranium

enriched to 92 percent (anything more than 20 percent is considered weapons-grade) to generate a

quadrillion neutrons per square centimeter per second100 to 1000 times greater than commercial

reactors. By cranking up the neutron dose, the ATR can simulate as much as 40 years of wear and tearon a new fuel or alloy in a single year.

The test reactor is a simple water-cooled model built in 1967. But by tuning the pressure, temperature

and chemistry inside its core, scientists can use it to reproduce the conditions in just about any other

type of reactor. Recently, they tested chunks of graphite to see whether it's safe to extend the life of

Britain's antiquated Magnox reactors. INL staff are now gearing up for an even bigger challenge: testing

parts for proposed Generation IV reactors, which would leap technologically two steps ahead of the Gen

II designs operating commercially in the United States today.

Despite concerns about catastrophic accidents and radioactive waste disposal, Gen II plants "are cost-

effective and working well, and safety continues to improve," says James Lake, INL's associate director.Yet, no new reactors have been ordered in the States since the industry's peak sales year of 1973. Simple

economics quashed further growth.

Thanks to the 2005 congressional incentives, a dozen utilities around the country have once again started

the lengthy process of applying to build nuclear plants. If all goes smoothly, they could produce power by

the middle of the next decade. These reactors would be Generation III and III+ designsevolutionary

improvements on today's Generation II reactors, which use water in some form as both a coolant and a

moderator.

But, according to the DOE, what is really needed are even safer, cheaper reactors that produce less

waste and use fuel that's not easily adapted for weapons production. To develop this kind of reactor, 10

countries, including the United States, joined forces in 2000 to launch the Generation IV International

Forum. A committee of 100-plus scientists from participating countries evaluated more than 100 designs;

after two years, they picked the six best. All of the final Gen IV concepts make a clean break from past

designs. Some don't use a moderator, for instance. Others call for helium or molten lead to be used as

coolants.

How It Works: Generation II and III Reactors:


4/16

All 103 nuclear power plants now operating in the United States employ light-water reactors, which use

ordinary water as both a moderator and a coolant. The next wave of nuclear plants has taken theseGeneration II concepts to the next level, improving both safety and efficiency. Utilities plan to begin

building Generation III reactors by the end of the decade.

In a Gen II Pressurized Water Reactor, water circulates through the core [1]where it is heated by the

fuel's chain reaction. The hot water is then piped to a steam generator, and the steam spins a

turbine[2]that produces electricity. The Gen III Evolutionary Pressurized Reactor improves upon this

design primarily by enhancing safety features. Two separate 51-in.-thick concrete walls [3], the inner

one lined with metal, are each strong enough to withstand the impact of a heavy commercial airplane.

The reactor vessel sits on a 20-ft. slab of concrete with a leaktight "core catcher," [4]where the molten

core would collect and cool in the event of a meltdown. There are also four safeguard buildings [5]with

independent pressurizers and steam generators, each capable of providing emergency cooling of thereactor core


5/16

Uranium in graphite "pebbles" may fuel future reactors.

Pebble Power

Kevan Weaver, like most of the lab's 3500 employees, works in a sprawling group of campus-likebuildings on the outskirts of Idaho Falls. Standing in his third-floor office, the fresh-faced nuclear

engineer holds what could be the future of nuclear power in his hand: a smooth graphite sphere about

the size of a tennis ball. It could take years to weigh the pros and cons of all six Gen IV designs, Weaver

says, but Congress can't wait that long. In addition to replacing the aging fleet of Generation II reactors,

the government wants to make progress on another front: the production of hydrogen, to fuel the dream

of exhaust-free cars running independent of foreign oil.

As a result, the frontrunner for the initial $1.25 billion demonstration plant in Idaho is a helium-cooled,

graphite-moderated reactor whose extremely high outlet temperature (1650 to 1830 F) would be ideal for

efficiently producing hydrogen. There are a couple of designs that could run that hot, but the "pebble

bed," so named for the fuel pebble that Weaver holds, is attracting particularly intense interest.

A typical pebble-bed reactor would function somewhat like a giant gumball machine. The design calls for

a core filled with about 360,000 of these fuel pebbles"kernels" of uranium oxide wrapped in two layers

of silicon carbide and one layer of pyrolytic carbon, and embedded in a graphite shell. Each day about

3000 pebbles are removed from the bottom as fuel becomes spent. Fresh pebbles are added to the top,

eliminating the need to shut down the reactor for refueling. Helium gas flows through the spaces

between the spheres, carrying away the heat of the reacting fuel. This hot gaswhich is inert, so a leak

wouldn't be radioactivecan then be used to spin a turbine to generate electricity, or serve more exotic

uses such as produce hydrogen, refine shale oil or desalinate water.

The pebbles are fireproof and almost impossible to use for weapons production. The spent fuel is easy

to transportand store, though there still remains the long-term problem of where to store it. And the

design of the nuclear reactor is inherently meltdown-proof. If the fuel gets too hot, it begins absorbing

neutrons, shutting down the chain reaction. In 2004, the cooling gas and secondary safety controls were

shut off at an experimental pebble-bed reactor in Chinaand no calamity followed, says MIT professor

Andrew Kadak, who witnessed the test.

Pebble-bed reactors also could be far more cost-effective than Gen II plants, which had an average

construction time of more than nine years. Even proposed Gen III designs have an estimated build time

of more than five years. Kadak's group at MIT has developed a pebble-bed design in which every part is

small and light enough to be shipped by train and truck, so the components could be mass-produced

off-site.

"Our whole approach is that you don't construct a reactor, you assemble it," Kadak says. "Think about

LEGOs: You just clip them together." This could shorten construction time to as little as two years; if a

part breaks, the module containing it could be replaced quickly. Kadak envisions small 250-megawatt

reactors, with additional units added to meet demand, making the initial cost lower than that of current

1000-megawatt giants.

Starting next year, both China and South Africa intend to build full-scale prototype pebble beds based on


6/16


7/16

Fourth-generation nuclear power plants differ radically from current reactors by replacing water coolants

and moderators, reaching higher temperatures, and gaining the potential to create hydrogen, as well as

electricity.

One of the six Gen IV designs under consideration is the meltdown-proof pebble-bed reactor, which usesgrains of uranium encased in balls of graphite as fuel. Helium gas is heated as it circulates through a

vessel of these pebbles [1]and then powers a turbine [2]to generate electricity. A heat

exchanger [3]can transfer heat from the helium to adjacent facilities [4]for the production of hydrogen.

The plant relies on "passive safety": If the cooling system fails, the nuclear reaction grinds to a halt on its

own.

Fast Breeders

Though the pebble-bed reactor is promising, other Gen IV designs have distinct advantages, too. Three

of the six under consideration are fast neutronreactors; the term refers to the high speed of the

neutrons ricocheting around the reactor core when there is no moderator to slow them down. When fast

neutrons collide with fuel particles, they can actually generate more fuel than they burn. Such breederreactors were developed in the late 1940s, but remained more expensive than other designs. These

reactors have more appeal today because they also can burn up the longest-lived radioactive isotopes in

their fuel, producing waste that stays dangerous for hundreds of years instead of hundreds of

thousands.

These fast reactor concepts differ in the material they use to cool the reactor core. One uses gas, another

sodium, and the third, molten lead. But, so far, all three designs are still more expensive and further from

completion than the other top contenders. One solution, Weaver says, would be to carry two different

designs forward: "a thermal reactor like the pebble bed for the near term, and a fast reactor for the far

term."

"Near term" is relative: Last year's EnergyPolicy Act doesn't require a final decision on construction of

the demonstration plant until 2014, a cautious timeline that frustrates the program's boosters. In the

meantime, researchis pressing on in the Idaho desert and in Idaho Falls, where the Thursday night

entertainment is the monthly dinner meeting of the nation's largest chapter of the American Nuclear

Society. In the parking lot, bumper stickers read, "Split an atom, save a tree."

The Great Nuclear Debate

Now's the Time

Patrick Moore, Chair and Chief Scientist, Greenspirit Strategies Ltd.

When I helped found Greenpeace in 1971, my colleagues and I were firmly opposed to nuclear energy.

But times have changed. Nuclear energy is the only non-greenhouse gas-emitting power source that can

effectively replace fossil fuels and satisfy growing demand.

Nuclear energy is affordable.The average cost of producing nuclear energy in the United States is

less than 2 cents per kilowatt-hour, comparable to coal and hydroelectric.


8/16

Nuclear energy is safe.In 1979, a partial reactor core meltdown at Three Mile Island frightened the

country. No one noticed that Three Mile Island was a success; the containment structure prevented

radiation from escaping and there was no injury among the public or workers.

Spent nuclear fuel is not waste.Recycling spent fuel, which still contains 95 percent of its original

energy, will greatly reduce the need for treatment and disposal.

Nuclear power plants are not vulnerable to terrorist attack.The 5-ft.-thick reinforced

concretecontainment vessel protects contents from the outside as well as the inside.

Nuclear weapons are no longer inextricably linked to power plants.Centrifuge technology now

allows nations to produce weapons-grade plutonium without a reactor. Iran's nuclear weapons threat, for

instance, is distinct from peaceful nuclear energy.

Nuclear reactors offer a practical path to the hydrogen economy.Excess heat from the plants,

instead of fossil fuels, can be used for electrolysis. It also can address the increasing shortage of fresh

water through desalinization.

Together with a combination of solar, wind, geothermaland hydroelectric sources, nuclear energy can

play a key role in producing safe, clean, reliable baseload electricity.

Now's the Time

Anna Aurilio, Legislative Director, U.S. Public Interest Research Group

Nuclear energy is too expensive, too dangerous and too polluting. And, despite claims from industry, it's

not necessary either for our future electricity needs or to meet the very real challenge of global warming.

Worldwide, renewable alternatives such as wind, solar and geothermal power, along with small

decentralized heat and power cogeneration plants, already produced 92 percent as much electricity asnuclear power did in 2004and those sources are growing almost six times faster.

In a post-9/11 world, nuclear facilities will always be tempting targets for

terrorists.Government studies have highlighted the weaknesses in our current safeguards.

Even without attackers, danger is ever present.In 2002, inspectors at the Davis-Besse nuclear

power plant in Ohio found a hole that had corroded almost all the way through a pressure vessel, leaving

less than an inch of steel preventing the release of radioactive steam.

No country in the world has solved the problem of how to dispose of high-level radioactive

waste.Even the most optimistic advanced reactor designs will continue adding to the lethal mountain ofwaste already produced.

Nuclear energy is not our best bet to reduce global warming emissions.That argument only

makes sense if coal is the only other option. That's a false choice, and it ignores the rapidly developing

range of energy-efficient, clean, renewable energy sources.

For 33 years, no one has ordered or built a nuclear plant, for very good economic reasons. Now Congress

and the nuclear industry are distorting the market with new subsidies. They're pushing a technology with


9/16

serious health, safety and economic risks, and in doing so diverting research dollars away from better

alternatives

Write this part

Nuclear's next generation

Inside story: A group of six new blueprints for nuclear power stations promise advances in safety and

efficiency. How do they differ from existing designs?

Dec 10th 2009 |From the print edition

eyevine

DWIGHT EISENHOWER observed in his Atoms for Peace speech in 1953 that nuclear technology

originally developed for military purposes could also be put to peaceful uses, namely generating

electricity. His speech led to the dissemination of nuclear technology for civilian purposes and the

establishment of the first nuclear power stations. Many of these early reactors, built during the cold

war, made a virtue of the dual use nature of nuclear technology. Designs were favoured that could

create weapons-grade material as well as electricity.

Today those priorities have been reversed. America and Russia are taking steps to reduce theirstockpiles of nuclear weapons, and the international community is trying to prevent their acquisition

by new states. Under America's Megatons to Megawatts programme, weapons-grade material from

retired warheads is being broken down to provide fuel for civilian nuclear power stations. With 53

new reactors under construction around the world and dozens more planned, the main difficulties

facing nuclear scientists now are to reduce the threat of proliferation, improve efficiency and do

something about the growing stock of nuclear waste in indefinite temporary storage.
http://www.economist.com/printedition/2009-12-12http://www.economist.com/printedition/2009-12-12http://www.economist.com/printedition/2009-12-12http://www.economist.com/node/15048703http://www.economist.com/node/15048703http://www.economist.com/printedition/2009-12-12


10/16

These new priorities favour new sorts of reactor. Taking the lead in the development of the next

generation of reactors is an international programme called the Generation IV International Forum

(GIF), a collaboration between the governments of America, Argentina, Brazil, Britain, Canada,

China, France, Japan, Russia, South Africa, South Korea and Switzerland, plus Euratom, the EU's

nuclear body. Established in 2001, the GIF has drawn up a shortlist of six of the most promisingdesigns, which range from updated versions of existing reactors to radically different approaches.

Technology Quarterly

Greenery on the march

Glue bones

Electrical potential

Making ink bulletproof

Who pays for the pipes?

A question of character

Better ways to collaborate

And the winners were...

Powering the drive

Read all about it

Nuclear's next generation

Fields of automation

Son et lumire meets surgery

Beyond the ether

Reprints

Related topics

Georgia Institute of Technology

Europe

World politics

Russian politics

European politics

All nuclear reactors rely on nuclear fission, a process discovered in the 1930s. When certain heavy

atoms are struck by a neutron, they absorb it, become unstable and split apart. This results in two

lighter atoms, and two or three neutrons are ejected. The process releases large amounts of energy,

much of it in the form of the kinetic energy of the fast-moving fission products. This energy is

converted to heat as the fission products slow down. If the ejected neutrons hit other atoms nearby,

those too can break apart, releasing further neutrons in a chain reaction. When enough neutrons

produce further fissionsrather than escaping, bouncing off or being absorbed by atoms that do not

splitthe process becomes self-sustaining.
http://www.economist.com/node/15048783http://www.economist.com/node/15048783http://www.economist.com/node/15048727http://www.economist.com/node/15048727http://www.economist.com/node/15048719http://www.economist.com/node/15048719http://www.economist.com/node/15048773http://www.economist.com/node/15048773http://www.economist.com/node/15048801http://www.economist.com/node/15048801http://www.economist.com/node/15048809http://www.economist.com/node/15048809http://www.economist.com/node/15048827http://www.economist.com/node/15048827http://www.economist.com/node/15048819http://www.economist.com/node/15048819http://www.economist.com/node/15048837http://www.economist.com/node/15048837http://www.economist.com/node/15048695http://www.economist.com/node/15048695http://www.economist.com/node/15048711http://www.economist.com/node/15048711http://www.economist.com/node/15048761http://www.economist.com/node/15048761http://www.economist.com/node/15048791http://www.economist.com/node/15048791http://www.economist.com/rightshttp://www.economist.com/rightshttp://www.economist.com/topics/georgia-institute-technologyhttp://www.economist.com/topics/georgia-institute-technologyhttp://www.economist.com/topics/europehttp://www.economist.com/topics/europehttp://www.economist.com/topics/world-politicshttp://www.economist.com/topics/world-politicshttp://www.economist.com/topics/russian-politicshttp://www.economist.com/topics/russian-politicshttp://www.economist.com/topics/european-politicshttp://www.economist.com/topics/european-politicshttp://www.economist.com/topics/european-politicshttp://www.economist.com/topics/russian-politicshttp://www.economist.com/topics/world-politicshttp://www.economist.com/topics/europehttp://www.economist.com/topics/georgia-institute-technologyhttp://www.economist.com/rightshttp://www.economist.com/node/15048791http://www.economist.com/node/15048761http://www.economist.com/node/15048711http://www.economist.com/node/15048695http://www.economist.com/node/15048837http://www.economist.com/node/15048819http://www.economist.com/node/15048827http://www.economist.com/node/15048809http://www.economist.com/node/15048801http://www.economist.com/node/15048773http://www.economist.com/node/15048719http://www.economist.com/node/15048727http://www.economist.com/node/15048783


11/16

The technology underpinning civilian nuclear power-generation has not progressed much since the

1950s when a small number of prototype commercial reactors were first brought online. Based on

the military reactors developed for weapons programmes and naval propulsion, these generation I

systems pioneered the pressurised water reactor (PWR) design, which is the basis for most of the

generation II nuclear reactors now in operation. In a PWR ordinary water, kept at a high pressureto prevent it from boiling, is used both to cool the reactor core and to moderate the nuclear reaction

by reducing the speed of the neutrons in order to maximise their ability to cause further fissions.

According to the International Atomic Energy Agency (IAEA), of the 436 nuclear reactors in

operation today, 356 are either PWRs or boiling-water reactorsa simplified version of the same

design.

The vast majority of current reactors use a once through fuel cycle, in which each batch of fuel

spends a single term in the reactor core, and the leftovers are then removed and placed in storage.

This spent fuel presents a storage problem, but it also offers an opportunity. According to the World

Nuclear Association, an industry body, the spent fuel recovered from a reactor still contains around

96% of the original uranium, as well as plutonium that has been formed in the core. If the nuclear

renaissance takes off at the rate that many are predicting, this inefficient use of the uranium fuel is

likely to prove unsustainable, says Bill Stacey, a professor of nuclear engineering at the Georgia

Institute of Technology.

The original series

In the near term most new reactors will continue to be PWRs. A forthcoming crop of generation III

and generation III+ reactors buildon the light-water design with new safety mechanisms. Some can

also run on mixed oxide (MOx) fuel, which is produced by reprocessing spent fuel to extract the

plutonium and uranium and combining them to make a new fuel. But although MOx is currently used

in around one-third of French reactors, the idea of reprocessing is controversial and has yet to gain

widespread international support. Critics say it is uneconomic and increases the risk of proliferation.

The technology underpinning civilian nuclear power-generation has not progressed much since the

1950s

The six most promising generation IV designs identified by the GIF from an original list of over 100

concepts depart markedly from the light-water moderated, once-through models that dominate the

existing fleet. Even those reactors that draw upon aspects of current designs add some new twists.

Start with the supercritical water-cooled reactor(SCWR). Although it uses water as the

coolant, like existing designs, the water is at a much higher temperature (above 374{degree}C) and

pressure. Under these conditions the water exists in a single, supercritical phase, rather than as

liquid or steam. This eliminates the need to transfer heat from the coolant water to steam (via a

secondary heat-exchanger) to drive a steam turbine, as is the case with current PWRs. Instead,

supercritical water from the core drives a turbine directly.


12/16

Doing away with the need for separate pumps, pressurisers and steam generators results in higher

thermal efficiency: 45% rather than the 33% of existing PWRs, according to Idaho National

Laboratory. The simplicity of the design should also make it cheaper. The GIF estimates that an

SCWR could be built at a cost of $900 per kilowatt of generating capacityabout a quarter of the

expected cost of current generation III+ reactors. Some industry observers, however, are scepticalthat these cost savings can be achieved.

Given that it builds on existing reactor designs, and also borrows from supercritical fossil-fuel boilers,

which are also an established technology, the SCWR is likely to be one of the first generation-IV

designs to be implemented. The GIF is aiming to have a demonstration version ready by 2022. But

several technical challenges remain. In particular, says William Cook of the University of New

Brunswick in Canada, current reactor materials that do not crack corrode excessively, while

materials that do not corrode excessively crack. New alloys will be needed that do not crack or

corrode under stress.


13/16

The second design with roots in existing technology is theVery High Temperature

Reactor(VHTR). It has a once-through uranium cycle, but instead of water it uses graphite as the

moderator and helium gas as the coolant. (Helium has the advantage that it is chemically inert and


14/16

has only a limited tendency to become radioactive when exposed to neutrons.) As its name

suggests, the VHTR is designed to run at very high temperatures, heating the coolant to around

950{degree}C, compared with 315{degree}C for a standard PWR, making it more thermally efficient.

Like the SCWR, the VHTR will require the development of new materials. Although the helium

coolant presents fewer corrosion problems than supercritical water, creating core materials and fuelcasings that can withstand the high temperatures involved is a daunting task. Nevertheless, the

VHTR has sufficiently impressed the Obama administration, which in September announced $40m

in funding for research and development of the Next Generation Nuclear Plant, a reactor based on

the VHTR design.

Unlike the SCWR and the VHTR, which build on current reactors, the other four generation-IV

designs take a completely different approach to the nuclear-fuel cycle. Three of them are fast

neutron reactors, which do not include a moderator to slow down freeneutrons during the fission

process. With more free neutrons flying about, fast reactors can consume or burn up existing

nuclear waste, a characteristic that endears them to waste-reduction advocates who see them as a

means of closing the nuclear fuelcycle.

In keeping with the Janus-faced nature of nuclear technology, however, fast reactors can also be

used to produce or breed new fissile materialconverting uranium-238 into the notoriously dual-

purpose plutonium, for example. Opponents of fast reactors worry about the costs and proliferation

risks. But the prospect of being able to extract useful energy from nuclear waste, and also reduce its

volume and toxicity, give fast reactors obvious appeal. The three shortlisted fast-reactor concepts

sodium-cooled, gas-cooled and lead-cooledare differentiated primarily by their use of coolant.

Each has its own pros and cons.

The most successful of the three designs to date has been the sodium-cooled fast

reactor(SFR), which has racked up the highest number of reactor-years of operation in prototype

form. One of the merits of the SFR is that we really can build one, says Robert Hill of America's

Argonne National Laboratories. He points to the Russian BN600, a reactor that has been running

since the 1980s. Sodium is favoured as a coolant because of its good heat-transfer properties, its

ability to operate at lower pressures than other coolants and its relative transparency to fast

neutrons, which means it does not interfere in the fission process, says Dr Hill. According to the

IAEA, Russia, South Korea and India are all currently operating versions of the SFR, and China is

due to bring a prototype online in mid-2010.The gas-cooled fast reactor(GFR), in contrast, has yet to be demonstrated on a commercial

scale. But many see it as a better bet than the SFR due to its technical similarity to generation III

gas-cooled designs. Like the VHTR, the GFR uses pressurised helium both to cool the reactor core

and drive a turbine, yielding higher thermal efficiency than systems with a secondary heat-transfer

loop. As with a VHTR, the other advantages of a gas coolant, says Tom Wei, a senior engineer at

Argonne, include its non-corrosive characteristics and its capacity for use at high temperatures (the


15/16

GFR would operate at around 850{degree}C). But, like the VHTR, the GFR will require new

materials to enable its cladding and fuel assemblies to withstand such high temperatures.

The third fast-reactor concept uses molten lead as the coolant, an approach historically favoured by

the Soviet military, which used early lead-bismuth cooled fast reactors to power its submarines.

Since the late 1990s there has been renewed interest in the lead-cooled fast reactor(LFR),particularly in Europe. A distinctive advantage of the LFR concept is its potential to be adapted to

smaller battery designs, which can be manufactured as self-contained systems with a lifetime

core. Such reactors could provide a way to extend civilian nuclear power to new countries without

giving them access to the sensitive parts of the nuclear-fuel cycle.

Although a commercial fleet of fast reactors would be attractive from a waste-management

perspective, it presents its own set of proliferation-related problems. According to Charles Ferguson,

a nuclear expert at the Council on Foreign Relations, a think-tank, the commercial adoption of fast

reactors would require near real-time monitoring capabilities via secure video links to ensure that

the reactors were not being used to make weapons. Getting countries to agree to such intrusive

measures, he says, would be very difficult.

In the belly of the beastGetty Images

The sixth shortlisted design, themolten salt reactor(MSR), works by dissolving nuclear fuel in a

fluoride solution, which acts as both the fuel and the coolant in the reactor core. The molten salt,

which has good heat-transfer properties and can be heated to temperatures above 1,000{degree}C

without boiling, is moderated using graphite. The circulation of the fuel in this way eliminates the

need for fuel fabrication and allows for continuous online reprocessing. It also makes the design well

suited to the use of existing fissile material, which can be easily blended into the fuel mixture. And

like fast reactors, the MSR can be designed to burn up many of the longer-lived byproducts of the

fission process, resulting in nuclear waste that is much less radioactive than that produced by the

once-through cycle.

One form of MSR, the liquid fluoride thorium reactor (LFTR), has garnered particular enthusiasm

among those who regard thorium as an attractive replacement for uranium and plutonium in the fuel


16/16

cycle. (Thorium is both cheaper and more abundant than uranium.) According to Kirk Sorensen, an

engineer at NASA who also runs a blog on the merits of the thorium cycle, natural thorium provides

at least 250 times more energy per unit than natural uranium. However, unlike fissile uranium,

natural thorium must be seeded with external neutrons in order to get it to fission. Another obstacle

for the MSR is finding materials capable of withstanding hot, corrosive, radioactive salt.

Flicking the switch

Which of these designs will prevail in the coming decades? After all, not all the generation-IV reactor

concepts are likely to make it to commercialisation. Ideally, the strongest approaches will win out

through natural selection, says Thierry Dujardin at the OECD's Nuclear Energy Agency (NEA) in

Paris. But with each of the designs closely connected to different national research programmes

and international variations within each of the categoriesgovernments are unsurprisingly reluctant

to see their particular projects sidelined.

Harold McFarlane at the Idaho National Laboratory reckons the VHTR and SFR are almost ready tomove out of the research phase and into the design stage. Others share this view: the British

government has identified the VHTR, GFR, and SFR as high-priority designs, and Japan, France

and America agreed last year to work together on SFR prototypes.

Dr Ferguson thinks the prospects of the entire generation-IV programme are contingent on the level

of investment allocated to nearer-term projects. Do we commit to generation III or do we leapfrog to

generation IV? he asks. Two important considerations for answering his question are regulatory

compliance and economic viability. With regard to the former, the NEA's Multinational Design

Evaluation Programme is considering an international licensing scheme to standardise safety

requirements for the new reactors. As for the latter, the success of generation IV reactors is likely to

hinge on large amounts of government support.

In the near term this support should take the form of increased research-and-development funding,

says Dr Stacey of Georgia Tech. In the longer term, governments have an important role to play in

the provision of loan guarantees, which are vital for overcoming engineering and first of a kind

risks, says Joe Turnage at Unistar, a commercial nuclear joint-venture between Constellation

Energy, an American utility, and EDF, a French one. But whatever the next generation of nuclear

power-stations looks like, it is clear that the research being done around the world to develop such a

variety of new reactors, rather than new nuclear weapons, has fulfilled Eisenhower's wish, back in1953, that the miraculous inventiveness of man shall not be dedicated to his death, but consecrated

to his life.

Documents

Alternative Nuclear Power