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Nuclear Fuel Cycles

Ronald A. Knief XE Corporation

I. Overview I. Unique Features I. Uranium Fuel Cycle IV. Recycle V. Alternative Uranium Fuel Cycles VI. Plutonium Fuel Cycle VII. Thorium Fuel Cycle VIII. Other Fuel Cycles IX. Support Activities

Chain reaction Sustained reaction wherein neutrons cause fissions, which in turn produce more neutrons, which cause the next generation of fissions.

Critical Condition where a fission chain reaction is stable with production balancing losses at a nonzero neutron level.

Enrichment Process in which isotopes are separated by physical means; applies primarily to separating 235U from natural uranium, but also to separating deuterium (e.g., as heavy water) from hydrogen in water.

Fertile Material, not itself fissile, capable of being converted to fissile material following absorption of a neutron.

Fissile Material capable of sustaining a fission chain reaction.

Fission Process in which a heavy nucleus splits into two or more large fragments and releases kinetic energy.

Radioactivity Emissionofparticulateorelectromagnetic radiation from an energetically unstable nucleus.

Reactor Combination of fissile and other materials in a geometric arrangement designed to support a neutron chain reaction.

Recycle Reuse of nuclear fuel material that has been separated from fuel previously used in a reactor.

Reprocessing Process of separating nuclear fuel and waste constituents contained in spent reactor fuel.

THE USE OF nuclear fission fuels for energy production depends on a fuel cycle that takes uranium ore from the ground, prepares fuel for use in a nuclear reactor, and handles the used fuel material and the byproduct wastes. This fuel cycle is more complex and extensive than those associated with other fuels, due to unique features of the fission chain reaction and steps. (Fusion, a form of nuclear energy that has not achieved commercial status, has its own fuel cycle.)

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656 Nuclear Fuel Cycles

The current basis for commercial application of nuclear energy in the fission process. A neutron striking a heavy nucleus such as uranium-235 (235U) may cause it to fission or split into two or more parts, release energy, and give off additional neutrons and other radiations. Desired aspects of the reaction are large energy release (more than 50 million times as great as that from burning a carbon atom in a fossil fuel) and neutrons that can cause additional fissions and lead to a sustained chain reaction in a system called a reactor. Materials that can support a chain reaction by themselves are said to be fissile, while those that are fertile can be converted into fissile materials when struck by neutrons. A balanced steady-state chain reaction, which can produce energy at a constant-rate, is said to be critical.

Disadvantages of the fission process for energy production relate to the particulate and electromagnetic radiations emitted at the time of fission and to the radioactivity (i.e., the property of emitting radiation with a characteristic time frame or half-life)o ft he fission fragments and their products. These and other concerns are addressed by the design and operation of the steps in the nuclear fuel cycle.

Each fuel used for energy production is characterized by a fuelcycle.Typicalcycles,suchaswithfossilfuels,include

1. exploration 2. mining or drilling to extract the resource 3. processing or refining to remove impurities and otherwise prepare the fuel for use 4. energy production 5. waste disposal 6. transportation among other steps

The nuclear fuel cycle is more complex due to the following unique features associated with the fission energy source.

1. Uranium-235 (235U), the only naturally occurring material that can support a chain reaction, is less than 1% abundant in natural uranium. 2. Two other fissile materials, 233U and 239Pu (plutonium-239), are produced by neutron bombardment of fertile 232Th (thorium-232) and 238U, respectively. 3. All fuel materials contain small to large amounts of radioactivity. 4. A neutron chain reaction (criticality) could occur in fuel materials located outside of a reactor.

FIGURE 1 Generic nuclear fuel cycle material flow paths. [Knief, R. A. (1992). “Nuclear Engineering: Theory and Technology of Commercial Nuclear Power,” 2nd ed., Taylor & Francis/ Hemisphere, New York.]

5. The fission chain reaction used for commercial power generation has potential application to a nuclear explosive device.

Each of these concerns influences one or more of the fuel-cycle steps.

A schematic representation of a generic nuclear fuel cycle is shown in Fig. 1. The steps preceding reactor use of the fuel are classified as front end and characterized by small amounts of radioactivity per unit mass of material handled. Following reactor use, the resulting highly radioactive fuel is handled in the back end steps of the cycle. Fuel-cycle elements not appearing by name on Fig. 1 are transportation (between other steps), radiation safety, criticality safety, and material safeguards.

Major fuel-cycle features are described next, starting with those for the uranium fuel cycle employed by the current generation of popular pressurized-water reactors (PWRs) and boiling-water reactors (BWRs), known collectively as light-water reactors (LWR). Subsequent sections consider recycle of uranium and plutonium in LWRs, alternative uranium fuel cycles, plutonium and thorium fuel cycles, other fuel-cycle concepts, and support activities.

The uranium fuel cycle (e.g., Fig. 1) for the light-water reactors (LWRs) may be implemented as an “open” cycle, which does not proceed past interim spent fuel storage. A completed or “closed” cycle would include reprocessing,

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Nuclear Fuel Cycles 657

FIGURE 2 Nuclear fuel cycle material flow sheet for a typical pressurized-water reactor (PWR) without fuel recycle. [From Pigford, T. H. (1978). In Report to the APS by the Study Group on Nuclear Fuel Cycles and Waste Management, Part I. Rev. Mod. Phys. 50(1).] with or without recycle of uranium or plutonium, and waste disposal.

Each major step of the LWR fuel cycle is described below. Typical mass flows for an open LWR cycle are shown in Fig. 2. Capacities of typical fuel cycle facilities are provided in Table I. Estimates of world-wide uranium resources and reported capacities of various fuel-cycle facilities are provided in Table I.

A. Exploration

Uranium exploration typically begins with geologic evaluation to identify formations similar to those known to contain the ore. Chemical and radiological testing confirm probable sites.

Prospect sites are drilled for core samples, which are subjected to detailed evaluation through various nonde-

TABLE I Typical Capacities for Nuclear Fuel-Cycle Facilities Facility Capacity (tonnes/year)

Underground mine 14 U Surface mine 140 U Mill 807 U

UF6 conversion 15,0 U Enrichment 2400 U

Uranium fabrication 1500 U Mixed-oxide fabrication 360 U+Pu Spent-fuel storage 3500 U+Pu (total capacity) Reprocessing 2000 U+Pu

From U.S. Nuclear Regulatory Commission. (1976). Final Environmental Statement on the Use of Recycle Plutonium in Mixed Oxide Fuel in Light Water Reactors (GESMO). NUREG-0002. Washington, D.C.

structive or destructive analyses. For areas selected for mining, core samples provide a basis for detailed mapping of the ore bodies.

B. Mining

Assays of less than 0.25% U3O8 equivalent in the ore have been typical in the United States (although significantly higher values found in locations such as in western Africa, Australia, Canada, and elsewhere). Even though the very low assay means that hundreds of units of ore must be mined for each unit of uranium recovered, the reference value still provides 30 to 50 times the energy production per unit mass mined as is typical of coal.

Uraniumorebodiestendtobe“spotty”(i.e.,thinandnot too wide or long) and widely separated by low-grade ore or barren sands. Surface or open-pit mining techniques are employed for shallow deposits with relatively soft overburden. Underground methods are used for deeper deposits or those covered with very hard rock strata.

C. Milling

Milling or refining operations are often conducted close to the mines to minimize the amount of transportation required for the bulky ore. The low assay of the uranium ore dictates against the use of standard metallurgical techniques in favor of complex chemical methods.

One approach to separation of uranium from the ore consists of the following.

1. Crushing and grinding the ore to desired size. 2. Leaching in acid to dissolve the metallic constituents away from the sand-like residues that constitute the bulk of the ore. 3. Ion-exchange or solvent-extraction operations to separate uranium from other metallic constituents. 4. Production of ammonium diuranate (called yellowcake due to its color).

The large volume of ore residues, or tailings, must be disposed of.

D. Conversion

Reactor applications require very-high-purity uranium, especially in terms of impurities with a large tendency to absorb, and thereby remove, neutrons needed to sustain the fission chain reaction. Initial purification of yellowcake is obtained by solvent extraction. Treatment of the concentrate with hydrofluoric acid and elemental fluorine

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658 Nuclear Fuel Cycles

Fuel-Cycle

F acilities c

Reasonably

Assur ed

Uranium

Reser v e sb

(1000 tU)

A way-fr om-r eactor storage capacity (tHM)

1998 Refining/

Uranium Mixed-oxide

Hea vy water

Uranium

Milling con v ersion

Enrichment fabrication fabrication

Repr ocessing pr oduction pr oduction d capacity capacity capacity capacity capacity capacity

Under gr ound capacity

Country

< $80/lb

$80–$130/lb

(tU) (tU/y)

(tU/y) (tSWU/y)

(tHM/y) (tHM/y)

W e t

Dry (tHM/y) r epositories

(t/y)

Algeria 26 Ar gentina

Armenia

Australia

Belgium 15

Brazil 162

Bulg aria

Canada 331

Central

African Republic

Chile d China,

Peoples

Republic Columbia

Czech

Republic 500

Denmark 27

Egypt

Estonia 1 Finland

I W orld wide Uranium

Resour ce Estimates and Fuel-Cyc

Pr ocessing

Capacities a

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Nuclear Fuel Cycles 659

France

Gabon 6

German y

Greece 1 Hung ary

India

Indonesia

Iran [ ] Italy

Japan

Kazakhstan

K orea, South

K y r gystan

Me xico

Mongolia 62 Morocco Namibia

Netherlands

Niger 70

Norw ay continues

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660 Nuclear Fuel Cycles

P akistan

Peru 2 Portugal

Romania

Russian 145

Federation

2/1M 1/0

Slo v akia

Slo v enia

Somalia 7 South

Africa

Spain

Sweden

Switzerland

T aiw an

T ajikistan 1 T urk e y

Ukraine

United

Kingdom

550[hm] 1300

500 el

F acilities c

Reasonably

Assur ed

Uranium

Reser v e sb

(1000 tU)

A way-fr om-r eactor storage capacity (tHM)

Re fi ning/

Uranium Mixed-oxide

Hea vy water

Uranium

Milling con v ersion

Enrichment fabrication fabrication

Repr ocessing pr oduction pr oduction d capacity capacity capacity capacity capacity capacity

Under gr ound capacity

Country

< $80/lb

– $130/lb

(tU) (tU/y)

(tU/y) (tSWU/y)

(tHM/y) (tHM/y)

W e t

Dry (tHM/y) r epositories

(t/y)

()

I Continued

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Nuclear Fuel Cycles 661

United

States

Uzbekistan

V enezuela d V iet

Nam

Zambia d Zaire 2 Zimbabwe 2 T

(Adjusted)

(Adjusted) RAR

(Adjusted) (Adjusted) EAR

Cat. I

Cat. I

KEY a Source:

International

Atomic

Ener gy Agenc y (IAEA) web-site w-nfcis.iaea.or e xcept as noted

(see footnotes b and b Source:

“ Uranium

1997: Resources,

Production and Demand

1998 Edition,

” Nuclear

Ener gy Agenc

Or ganization for Economic

Cooperation and

De v elopment,

P aris,

Sept. 1998.

T otals are for

Reasonably

Assured Resources and Estimated

Additional

Resources

Cate gory

Capacity

In O peration;

P lanned;

Under C onstruction;

S hutdo wn/ D ecommissioned d Sour ce :

Uranium

Institute w .uilondon.or e Estimated

Additional

Resources

Cate gory

I (EAR-2) or Speculati v e Resources from a abo

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662 Nuclear Fuel Cycles produces UF6 (uranium hexafluoride) and fluoride compounds of the other constituents. Using a fractional dis- tillation process, the other fluorides are driven off, since they are more volatile than the UF6. Thefinalproductofthisconversionstepinthefuelcycle

ishighlypureUF6.Thiscompound—agasattemperatures above 56◦F at atmospheric pressure—is readily employed in several methods for enriching uranium in its fissile 235U isotope.

E. Enrichment

Natural uranium, the mixture that exists in nature, is composed of 0.711 wt.% 235U and 9.3 wt.% 238U. Most reactor concepts call for a higher fraction of the fissile 235U, such as the 2–5 wt.% slightly enriched uranium used in LWR systems. This necessitates isotope separation or enrichment.Sincebydefinitionthesetwoisotopesareboth forms of the element uranium and cannot be separated by ordinary chemical means, physical means of enrichment have been implemented. Several enrichment methods rely on the small differ- ence in mass between UF6 molecules of the two isotopes. According to the kinetic theory of gases, each molecule has the same average kinetic energy (=12mv2), so that the lighter molecule will have the greater speed, and the heav- ier a lesser speed. Other enrichment methods are based on differences in absorption of laser radiation by the energy levels of the respective nuclei or on slight shifts in chemical equilibrium reactions.

Enrichment processes for any materials may be characterized by the amount of separation that occurs in a single unit or stage. A separation factor α may be defined as

where e and d are the initial and final 235U isotopic fractions in the material streams, designated as “enriched” or “depleted,” respectively. Since α is often small, it is usually necessary to couple many individual units or stages. The two output streams from the system as a whole are for the enriched product and the depleted tails.

1. Gaseous Diffusion

The gaseous diffusion enrichment method employs cylindrical barriers of precisely controlled porousity against whichUF6 isforced.AsshownbyFig.3,thelighter235UF6 molecules, with their slightly greater average speed, tend to pass preferentially through the barrier, leaving behind the heavier 238UF6 molecules. The enriched stream collects in the outer portion of the stage in Fig. 3 with a theoretical or ideal separation factor

FIGURE 3 Schematic of typical gaseous diffusion enrichment stage. •, 235U; ❡, 238U. [Courtesy of U.S. Department of Energy.] of only α =1.0043. This low value leads to a requirement formanypassesthroughthestageorarrangementofmany stages in cascades of the type shown by Fig. 4. In practice, about1200stagesarerequiredtoenrichnaturaluraniumto the3wt.%235Utypicaloflight-waterreactorfuel.Thetails composition is generally in the range of 0.2–0.35 wt.% 235U.

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