INTERNATIONALTHERMONUCLEAR

EXPERIMENTALREACTOR

During the 1930s, Hans Bethedid the theoretical work thatexplained how fusion re-actions in the sun are the

source of the sun's energy, via thisreaction:

D2 + P >He^ + ri' + 17.46 MeV

This means that when deuterium(hydrogen with a nucleus of oneproton and one neutron) fuses with tri-tium (hydrogen with a nucleus of oneproton and two neutrons) it yieldshelium (two protons and two neu-trons), a neutron, and 17.36 MeV ofenergy. This result led some scientiststo consider the possibility that fusioncould be a way to generate electricity,especially when they realized that afusion reaction produces about threetimes as much energy as a fission re-action. In fact, a kilogram of U-235,the fuel for a fission reactor, yields 8.8X 10'̂ Joules of energy, whereas a kilo-gram of mixed D-T fuel produces ap-proximately 3,38 X 10'̂ Joules.

The challenge on Earth has been tocontrol the enormous amount of inputenergy needed to initiate the fusionreaction, maintain those conditions toprovide a continuous source of power,and still control the energy thatemerges from the fusion reaction.Many have considered the problemtoo impossibly complex, but a cadreof stubborn scientists and engineershas been addressing the difficultiessystematically for the past 50 years.Although not completely solved,many of the more complex and dif-ficult issues have been elucidated, and the next logicalstep is to build a reactor to apply what has been leamed:Hence, the International Thermonuclear ExperimentalReactor (Fig. 1).

This article describes the operation of a fusion reactor*Fellou> of ASM International

ADVANCED AAATERIALS & PROCESSES/FEBRUARY 2008

Fig. 1 — The ITERfusionreactor. For reference, the height ofthis device is approximately 60 feet.

Fusion power has been adream for decades, and now

a consortium has begunconstruction ofthe

International ThermonuclearExperimental Reactor.

James Marder"^ASM lutcruational

Materials Park, Ohio

BhaktaRath''Naval Research Laboratory

Washington, D.C.

Stephen ObenschainNaval Research Laboratory

Washington, D.C.

in general, discusses the Joint EuropeanTorus program, and covers the many

materials issues that must be ad-dressed in the ITER, the latest fusiontechnology.

The fusion reactionIn a fusion reactor, one deuterium nu-

cleus can fuse with another (D-D), or adeuterium can fuse with a tritium nu-cleus, (D-T). Kerry Lawson, at the Har-

well Laboratory of the Atomic EnergyResearch Establishment in England

in the 1950s, defined the conditionsfor a reactor:"The minimum temperature at which sucha system could operate may be found byequating that portion ofthe reaction energy. carried by the charged particles to the radia-tion loss. This temperature is 3x10^ degreesfor the D-D reaction and 5x10' degreesfor the T-D reaction."

At 50 million degrees, the tempera-ture of the deuterium-tritium plasmais so high that the nuclei can be con-fined only by a strong magnetic field.The most efficient shape for plasmaconfinement under these conditionsis a doughnut (torus), so that end-effect energy losses are eliminated.The generic name for these reactorsis "tokamak," Russian for doughnut.In a tokomak, the electric charge ofthe nuclei and large external electro-magnets confine the plasma. Notethere is another approach called in-ertial fusion, where the fusion fuel iscompressed and heated so quicklythat no confinement is needed (e.g.,laser fusion). This approach will notbe discussed here, but it has similar

promise and technical challenges as magnetic fusion.Prof. Lawson investigated not only temperature, but also

the amount of pressure and the length of time duringwhich the temperature and pressure were maintained,which he called the "pulse time." He defined the "tripleproduct" of time-temperature-pressure for ignition of a

39

10-

= 0 .01 -

fusion reaction. In the tokomak, the ionized gasis heated by several means, and is constricted byelectromagnets that generate strong magneticfields to increase its temperature and pressure.

A great deal of progress has been made towardssimultaneously obtaining the temperatures andconfinement times in the ionized D-D or D-T fuelrequired for fusion. The conditions that have beenproduced in experimental fusion reactors areshown in Eig. 2. IF IK is the Tokomak Fusion TestReactor, JET is the Joint European Torus, and theconditions found in these and other experimentalreactors are shown. The box shows the tripleproduct needed for ignition.

Joint European TorusThe JET (Joint European Torus) provided a

large-scale test bed for tokomak fusion. Becauseof the hundreds of millions of degrees and the ex-treme pressures that are the prerequisite to fusion,pulse times of only milliseconds were achievedin fusion experiments when JET began operation.However, in its first year JET achieved a pulsetime of one second, and pulse times of four sec-onds at a fusion output level of four megawattswere achieved by the 199O's. In fact, JET personnelreported that the plasma stability and control weregood enough to maintain the power generatingconditions for at least 30 seconds, essentially con-tinuous operation.

Although JET did not reach a Q value of one,where the power needed to maintain the plasmais equal to the energy produced by the fusion re-action, it was apparent that a change in the deu-terium-tritium ratio fuel would have achievedthis milestone.

The JET incorporated two essential elements ofthe next generation of fusion reactors: Tritiumis a component of the fuel, and beryllium servesas the plasma facing material.

Beryllium is a low-activation element with a highmelting point, so that the heat generated by theplasma does not e\'aporate the first wall easily. Anyberylliuni vapor generated does not contaminateand poison the reactions in the plasma. Berylliumand tritium handling facilities were tested in theJET, as prototypes for the follow-on ITER.

ntuT > 3xlOS"Km ^s

0.1 1 10Central ion temperature, T; (keV)

100

Fig. 2 — Experimental fusion reactors hai-e been designed since 1965.

40

ITER challengesThe results from the JET experiment led directly

to the ITER, the Intemational Thermonuclear Ex-perimental Reactor. JET was a product of a Euro-pean consortium of 20 countries, including Eng-land, France, and Germany. The ITER hasparticipants from the European Union, Japan, thePeoples Republic of China, the Republic of Korea,the Russian Federation, and the United States.Whereas JET was located in Oxfordshire, Eng-land, the ITER is being built in Cadarache, in thesouth of France.

ITER is planned to generate 500 MW of fusionpower for extended periods of time (thousandsof seconds) with Q^IO, ten times more than theenergy input needed to maintain the plasma. Itwill be the first fusion experiment to producemore energy than consumed to heat the plasma.However ITER will not be a high duty-cycle de-vice that produces electrical power. That will bea follow-on device called DEMO, which willdemonstrate fusion power. ITER is intended tobe a tool to help solve the engineering issues thatfusion power raises. The physics problems of fuel,energy, and plasma management have beenshown to be soluble by JET and its predecessors.

Despite the encouraging results from the JETmachine and the predictions for ITER, it will likelybe approximately fifty years before fusion powercan be commercialized. However, the potentialbenefits of fusion should make it worth the wait.

Fuel availabilityThe most practical fusion reaction for power

generation appears to be the D-T reaction. Otherfusion reactions are possible, such as the proton-proton fusion in many stars, and the D-D reactionof most fusion experiments. However, these re-quire longer confinement times and still highertemperatures, and so the first commercial fusionreactors are likely to be D-T reactors.

Deuterium is really quite abundant naturally.About one part in 5000 of the hydrogen in sea-water is deuterium. This amounts to over 10'̂ tonsof deuterium available naturally. A single gallonof seawater would produce as much energy as300 gallons of gasoline.

However, the situation with respect to tritium isquite different. Tritium is radioactive and has a halflife of about ten years. Tritium must therefore beman-made, but can be manufactured by irradi-atiiig lithium-6 with slow neutrons. Approximately7.6% of natural lithium is lithium 6. Alternatively,lithium 7 can be irradiated with fast neutrons toprovide the tritium required.

Breeding tritium will probably become abyproduct of the fusion reaction, if lithium servesas the coolant for the first wall material, in whichcase its irradiation will produce fuel for futureoperation.

ADVANCED MATERIALS & PROCESSES/FEBRUARY 2008

Year

- 1 9 8 0

-1970

1965

The abundance of deuterium and lithium im-plies that fuelling fusion reactors should not be aproblem within any foreseeable time frame (thou-sands of years, limited only by known easily ac-cessed reserves of lithium). In addition, the hugeamount of energy liberated in fusion will allow itto be a primary energy source, complementingexisting fission reactors. By eventually replacinggreenhouse-gas-producing fossil fuel plants, itwill provide environmentally friendly centralpower generation.

Radioactive wasteRadioactive waste in fusion reactors is a dif-

ferent, and with proper design, far less troublingissue than it is in fission reactors. The fuels deu-terium and lithium are not radioactive, and nei-ther is helium, the product of fusion. Most of theneutrons produced by the fusion reaction are ab-sorbed by the lithium breeder blanket. Materialssuch as beryllium, which are not activated by neu-trons, are used for many of the components thatface the plasma. However, elements such as ironand aluminum in the beryllium tiles that face theplasma can be rendered radioactive by the neu-tron flux. The radioactivity is relatively short-Ii\'ed in comparison with the problem posed byspent fission fuel elements. Components that areretired from service would be considered safe afterabout 100 years of storage.

Materials choicesThe conditions of temperature and pressure for

the materials of construction of the reactor are ex-treme, to say the least. Most of the energy in theDT reaction is contained in the neutrons that aredeposited in the lithium blanket. However, sig-nificant energy remains in the ions that impactmaterial walls. Remo\'ing this heat becomes amajor challenge, particularly in the high neutron-flux environment of the reactor.

For a fusion power reactor, the most importantcharacteristics of the materials close to the plasmaare that they tolerate a high fluence of 14 MeVneutrons, yet produce only a small amount of ra-dioactive waste. Thus, low-activation materialsare beneficial, and must be developed if fusion isto provide a commercial energy source.

The Plasma Facing Material must be able towithstand high temperatures, neutron flux, andthermal shock loads (in the case of a sudden shut-down). With judicious use of existing low nio-bium and cobalt stainless steel grades, most ra-dioactive waste in ITER (except, essentially, fromthe blanket) can be cleared for service a centuryfrom the end of operation. This sounds like a longtime, but fission waste must be stored for hun-dreds of thousands of years before the radioac-tivity decays to a safe level.

For ITER, austenitic stainless steels appear to be

Fig. 3 — The main }ET building at night, from 1991. Image courtesy EuropeanFusion Development Agreement (EFDA)-JET.

the most likely candidates for critical structural ap-plications, ITER will produce daniageof 3 dpa (dis-placements per atom) in the austenitic stainlesssteel first wall. For a commercial fusion reactor andthe reactors that follow ITER, damage above 300to 500 dpa is expected over a 30-year service life.These walls can be replaced periodically every fewyears, but this taxes the capability of austeniticsteels, in which swelling is observed at 30 dpa. Al-ternative first wall materials include low activa-tion ferritic steels and SiC connposites. These ma-terials can withstand doses in excess of 150 dpawithout swelling, and may provide a solution tothe most severe fusion material challenge.

Many ITER conditions are highly relevant forreactor design choices. ITER will be the first fa-cility with a complete fusion neutron spectrum,so the materials tests will yield empirical infor-mation on fusion damage, rather than as devel-oped in a fission-based model.

Other areas in which ITER will provide the firststructural information include plasma-facing ma-terials compatible with plasma purity and withheat removal from the plasma. Materials for un-derlying heat sinks and structures, as well ascoolants, will be evaluated. The magnet structure,because of its large size, needs to be assembled bywelding, and welds introduce weak points sus-ceptible to fatigue.

By far the most prevalent structural material inITER is austenitic stainless steel. The materialchoice is oriented toward industrially availablematerials and manufacturing technologies thatcan satisfy ITER general goals. This material haslargely been already qualified for nuclear struc-tures through the fission and fast breeder devel-opment programs, and can therefore be qualifiedfor ITER at relatively low cost.

Among other materials, beryllium, tungsten,and carbon fiber composites (CFC) are suitablefor various regions of the first material wall facingthe plasma. These materials are joined by variousmethods to copper-alloy heat sinks, which in turnare joined to stainless steel supporting structures.

In the magnets area, a niobium-tin alloy super-conductor is used for the highest field compo-nents, and niobium-titanium elsewhere. In the di-agnostic field, the key issue is to choose materials,that maintain optical, electrical, or stmctural prop-erties while absorbing high radiation doses. •

For moreinformation:James Marder,ASM International,Materials Park,OH 44073;jim. marder®asminlernatianal.org; www.asminternotianal.org.

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