Uranium to Electricity:The Chemistry of the Nuclear Fuel Cycle

Dr. Frank A. SettleVisiting Professor of Chemistry Washington and Lee University

Lexington, VA 24450

Presentation

• Background

• Components of the Fuel Cycle

•Front End

• Service Period (conversion of fuel to energy)

• Back end

• Storage

• Reprocessing

•Alternatives and Economics

•Proliferation Concerns

How is the 2007 Israeli air strike on a Syrianreactor connected to the nuclear fuel cycle?

Conversion Losses27.10

Global Electricity Consumption

China & India

Generating Capacity

The 15 Wedge Approach to Energy Demands (Scientific American, 9/06)

Double Nuclear Capacity

The Nuclear Fuel Cycle

(natural uranium)(Low enriched uranium

LEU 3-5% U-235)

The Front End of the CycleFor Light Water Reactor Fuel

Uranium

• URANIUM is a slightly radioactive metal that occurs throughout the earth's crust.

• It is about 500 times more abundant than gold and about as common as tin.

• It is present in most rocks and soils as well as in many rivers and in sea water.

• Most of the radioactivity associated with uranium in nature is due to other materials derived from it by radioactive decay processes, and which are left behind in mining and milling.

• Economically feasible deposits of the ore, pitchblende, U3O8, range from 0.1% to 20% U3O8.

Uranium Mining

Both excavation and in situ techniques are used to recover uranium ore.

• Open pit mining is used where deposits are close to the surface and underground mining is used for deep deposits, typically greater than 120m deep.

• An increasing proportion of the world's uranium now comes from in situ leaching (ISL), where oxygenated groundwater is circulated through a very porous ore body to dissolve the uranium and bring it to the surface. ISL may use slightly acidic or alkaline solutions to keep the uranium in solution. The uranium is then recovered from the solution.

• The decision as to which mining method to use for a particular deposit is governed by the nature of the ore body, safety and economic considerations.

• In the case of underground uranium mines, special precautions, consisting primarily of increased ventilation, are required to protect against airborne radiation exposure.

Uranium Mine in Niger (Sahara Desert)

Uranium Metallurgy

“Yellowcake”

“Yellowcake”

DOE classifies the tailings or waste produced by the extraction or concentration of uranium or thorium from their ores as 11e(2) byproduct material. More than 200 pounds of byproduct material are typically produced for each pound of uranium. After extraction of uranium from the ore, the tailings contain much of their original radioactivity in the form of alpha-emitting uranium, thorium230, radium226, and daughter products such as radon222 gas. The total radioactivity present in mill tailings can exceed 1,000 picocurie per gram. Toxic heavy metals, including chromium, lead, molybdenum, and vanadium, are also present in this byproduct material in low, but significant, concentrations

Tailings from Uranium Mining and Milling

Uranium Global Resources

World Uranium Production

Conversion

• The product of a uranium mill is not directly usable as a fuel for light water nuclear reactors. Additional processing, generally referred to as enrichment, is

required for these reactors. This process requires the conversion of uranium to gaseous uranium hexafluoride.

• At a conversion facility, uranium is first refined to uranium dioxide, which can be used as the fuel for heavy water reactors that do not require enriched uranium. Most is converted into uranium hexafluoride for enrichment. It is shipped to the enrichment facility in strong metal containers. The main hazard of this stage of the fuel cycle is the use of hydrogen fluoride.

or centrifugation

COMURHEX – Malvesi, France U3O8 → UF4

COMURHEX – Pierrelatte, FranceUF4 → UF6

Enrichment• Natural uranium consists, primarily, of a mixture of two isotopes (atomic forms) of

uranium. Only 0.7% of natural uranium is "fissile", or capable of undergoing fission, the process by which energy is produced in a nuclear reactor. The fissile isotope of uranium is uranium 235 (U-235). The remainder is uranium 238 (U-238).

• In the most common types of nuclear reactors, a higher than natural concentration of U-235 is required. The enrichment process produces this higher concentration, typically between 3.5% and 5% U-235. This is done by separating gaseous uranium hexafluoride into two streams, one being enriched to the required level and known as low-enriched uranium. The other stream is progressively depleted in U-235 and is called 'tails'.

• Two enrichment processes exist in large scale commercial use, each uses UF6 as feed: gaseous diffusion and gas centrifuge. They both use the physical properties of molecules, specifically the 1% mass difference, to separate the isotopes. The product of this stage of the nuclear fuel cycle is enriched uranium hexafluoride, which is reconverted to produce enriched uranium oxide.

Centrifuge EnrichmentFeed

Enriched exit

Depleted exit

U235F6is lighter and collects in the center

(enriched)

U238F6 is heavier and

collects on the outside walls

(Depleted/Tails)

Feed to

Next Stage

The gas centrifuge process has three characteristics that make it economically attractive for uranium enrichment:

Proven technology: Centrifuge is a proven enrichment process, currently usedin several countries.

Low operating costs: Its energy requirements are less than 5% of the requirements of a comparably sized gaseous diffusion plant.

Modular architecture: The modularity of the centrifuge technology allows for flexible deployment, enabling capacity to be added in increments as demand increases.

Centrifuge Cascade

F6

F6

Loading uranium hexafluoride containers

Gaseous diffusion plantPaducah, Kentucky

Fuel Fabrication• Reactor fuel is generally in the form of ceramic pellets. These are formed from

pressed uranium oxide which is sintered (baked) at a high temperature (over 1400°C). The pellets are then encased in metal tubes to form fuel rods, which are arranged into a fuel assembly ready for introduction into a reactor. The dimensions of the fuel pellets and other components of the fuel assembly are precisely controlled to ensure consistency in the characteristics of fuel bundles.

• In a fuel fabrication plant great care is taken with the size and shape of processing vessels to avoid criticality (a limited chain reaction releasing radiation). With low-enriched fuel criticality is most unlikely, but in plants handling special fuels for research reactors this is a vital consideration.

UF6 Gas to UO2 Powder to Pellets

Fuel Pellets

Nuclear Fuel Assembly

Fuel Pellet

Fuel Assembly for Light Water Reactor

Fuel Assemblies are Inserted in Reactor Vessel

Nuclear Power Reactor

PWR Reactor Vessel

• 41 feet tall

• 14 feet ID

• 8.5 inch thick walls

• 665 tons

U-235

Pu-239

Pu-240Amount

Time in reactor

Removal of fuel elementsfor reprocessing

Production of plutonium in a nuclear reactor

Back End of the Fuel Cycle(Open vs. Closed Cycles)

Composition of Spent fuel Rods from a Light Water Reactor

Material Initial Fuel Spent Fuel Type of WasteTransuranic elements 0.000 0.065% TRUU-236 0.000 0.46%Pu isotopes 0.000 0.89% TRUFission products 0.000 0.35% High LevelU-235 3.3% 0.08%U-238 96.7% 94.3%

The actinides are the fifteen elements with atomic numbers 89 to 103.

Fates of Spent Fuel

Open Cycle

Closed Cycle

The spent fuel removed from the reactors continues to release heat and is still radioactive. It is, for those reasons, that the fuel is initially stored under water in the spent fuel storage pools.

Spent Fuel Storage Pools

Dry Cask Storage on Reactor Sites

Open Cycle Storage – Current Status in USfor Typical Power Reactors

Transport of Spent Fuel

Carlsbad, New Mexico – Waste Isolation Pilot Plant (WIPP)

Since 1999 this site stores transuranic waste from clothing, tools, rags, residues, debris, soils, and other items contaminated with radioactive elements mostly plutonium.

Solidifying high-level waste in borosilicate glass for long term storage in a repository

Reprocessing – Closed Fuel Cycle

Recovery of uranium and plutonium from spent fuel

Reduce volume and radioactivity of waste

France, the UK, Japan, and Russia currently reprocess spent fuel

Pu Recovery by Bismuth Phosphate Process

• Pu is found in low concentrations (<250 ppm) in reactor products.

• Weapons grade Pu must be chemically pure (< 1 part in 107 parts Pu).

• The Pu recovery for total process was 95% with < 1 part impurity in 107.

Pu(s) + X(s) HNO3 Pu4+(aq) + Xy+(aq)H2SO4

Pu4+(aq) + Xy+(aq) + Bi3+(aq) Pu3(PO)4(s) + Xy+(aq) + BiPO4(s)

Pu3(PO)4(s) + BiPO4(s)HNO3

oxid. agentPu6+(aq) + Bi3+(aq)

Pu6+(aq) + Bi3+(aq)

H3PO4

H3PO4 Pu6+(aq) + BiPO4(s)

Pu6+(aq)H2O2 PuO2

2+(aq) Pu(s)reducingagent

X(s) = fission products or uranium; y+ = oxidation state

Plutonium was redissolved and further purified using LaF2 in place of BiPO4(s)

Mixed Oxide Fuel (MOX)

MOX is produced from the output of reprocessing plants and is a mixture of plutonium and uranium oxides with a composition of 3% to 7% PuO2 and the rest UO2. The MOX is then mixed with ordinary LEU uranium-oxide fuel for use in lightwater reactors. Mixture is 1/3 MOX and 2/3 LEU.

By 2001, over 20 power reactors in France were using MOX for one third of their fuel In the US, MOX fuel is being used as a means of disposing of Pu fromdismantled nuclear weapons in the US and Russia.

Fuel Reprocessing Plant, Marcoule, France

US Global Nuclear Energy Partnership

Nuclear v. Alternatives ($/MWh)

nuclear coal gas wind solar

capital 50 30 12 60 250

O&M 15 5 3 10 5

fuel 5 10 25-50 0 0

total 70 45 40-65 70 250

+ $100/tC 0 25 12 0 0

new total 70 70 52-77 70 250

Front-end Costs

unit cost units/kg $/kg $/MWh

Uranium $50/kg 10 kg $500 1.3

Conversion $5/kg 10 kg $50 0.1

Enrichment $100/SWU 6 SWU $600 1.5

Fabrication $250/kg 1 kg $250 0.6

Total $1400 3.5

Assumes fuel with 4.4% U235 and burnup of 50 MWtd/kg, tails assay of 0.3% U235, and efficiency of 33%

Back-end Costs

$/kg $/MWh

Wet storage included in capital, O&M

Dry storage $200 0.5

Geologic disposal $400 1.0

Total $600 1.5

Total front + back $2000 5.0

HEU

Pu-239

Iranian Nuclear Complex

Presentation

• Background

• Components of the Fuel Cycle

•Front End

• Service Period (conversion of fuel to energy)

• Back end

• Open (Storage)

• Closed (Reprocessing)

•Alternatives and Economics

•Proliferation Concerns

More Information at http://alsos.wlu.edu