Nuclear Forensics Summer School 2010

Laboratory Experiment #3

Uranium and Plutonium Separation

Introduction

1. Recycling Process

There are many countries such as France, Germany, China, and Japan who are planning to

reprocess their waste if they have not already started. France currently has a closed fuel cycle,

represented in Figure 1. A closed fuel cycle is where the uranium makes a full circle from fuel

pellet to reprocessing back to another fuel pellet.

Figure 1 Closed Fuel Cycle

Currently, the United States has an open fuel cycle which means all the waste generated from

our nuclear power plants will not be reprocessed and needs to be stored in some type of long

RECYCLINGMOX FUEL

FABRICATION

Nuclear Repository

Waste

Other

Plutonium 0.9 %

Minor Actinides 0.1%

Cs and Sr 0.3%

Long-lived I and Tc 0.1%Other Long-Lived Fission

Products 0.1 %

Stable Fission Products 2.9%

Uranium 95.6%

term storage. As seen in Figure 2, spent fuel is 96% recyclable where the last 4% of fission

products would need to be put into long term storage.

Figure 2 Spent Nuclear Fuel

To ensure long term usage of nuclear energy, recycling of the uranium is essential to

maximize the reserves. Since plutonium is produced due to neutron capture by fertile 238U about

30 kg of fissile plutonium is produced in a Pressurized Water Reactor (PWR) while it generates 1

TWh of electricity.1 Since uranium and plutonium have fairly similar chemical characteristics

there needs to be a way to separate the uranium from the plutonium to reprocess. The science and

technology of used fuel reprocessing started in 1944 and has been continuously evolving since

then.

The workhorse of the reprocessing used nuclear fuel is the Plutonium Uranium Extraction

process (PUREX) which selectively extracts plutonium and uranium from highly radioactive

fission products. This not only enables reuse of uranium for future fuels but allows proper

management of radioactive waste as well. This extraction system takes advantage of Uranium

and Plutonium’s oxidation states. The process steps include, as seen in the flowsheet, Figure 3:

(i) Fuel pins are chopped and dissolved in nitric acid, where feed clarification and

adjustments of chemical conditions of the solution for solvent extraction occur.

(ii) Extraction of U(VI) and Pu(IV) into the organic phase by 30% TBP (tributyl

phosphate) in n-dodecane. This leaves a bulk of the fission products in the aqueous

phase.

PuO2

Liquid effluents

UNLOADINGOFF-GASTREATMENT

OFF GASTREATMENT

IodineKr-XeKr

Storage

GAS TREATMENT

VENTILATION

STORAGE POOL

SHEARING DISSOLUTION

CLAR

IFIC

ATIO

MN

1st CYCLETBPEXT.

2ndcycle

3rdcycle

2ndcycle

3rdcycle

Punitrate

Unitrate

UO3

Pu

U HLWFP, Np, Cm, Am

CONCENTRATIONDENITRATION

Interim liquid storage

Interim storage of glass blocks in wells

Interim storage under water

TREATMENT

SLUDGE

GASEOUS EFFLUENTS

Solid compounds

LiquidEffluents

SOLID WASTES

OXIDES +

CLADS

FUEL ASSEMBLIES

WASTES

Vitrification

OxalatePrecipitation

DISSOLUTIONAm-241

HF T

REAT

MEN

TRE

DUCT

ION

UF4

UO2

F2 C

OM

BUST

ION

UF6

RECY

CLIN

G

PuO2

UO2

(iii) Washing/scrubbing of the organic phase with nitric acid, this removes some

unwanted fission products that were previously co-extracted with uranium and

plutonium.

(iv) Stripping plutonium from uranium occurs by reduction of Pu(IV) to Pu(III) which

leaves the organic phase to the aqueous, where back extraction of pure uranium can

then occur with dilute nitric acid.

Figure 3 PUREX process flowsheet

2. Liquid-liquid Extraction

Extraction is the separation of a substance from one phase by another phase. This is widely

used industrially and modestly. For instance, when brewing coffee the hot water extracts the

caffeine from the ground coffee beans. The extraction method used in PUREX is liquid-liquid

where the organic phase extracts the radionuclides of interest from an immiscible aqueous phase.

For a successful extraction of a compound from one liquid to another the two liquids

must be immiscible, like oil and water. Water is immiscible with most organic solvents. There

are some solvents that are only partly immiscible with one another; unfortunately, this would

lower the efficiency of the extraction.

After mixing the immiscible liquids, the compound of interest will distribute itself

between the two solvents. The amount of compound transferred is affected by the solubility of

the solute in each solvent. The ratio of the concentrations of the solute in each solvent at a

particular temperature is a constant called the distribution ratio:

(Eq. 1)

Where solvent1 and solvent2 are immiscible liquids and solvent2 is the solvent in

which the compound (solute) of interest is more soluble.

In calculations of distribution coefficients, we assume that the solute neither ionizes in nor reacts

with either solvent. Because a ratio is involved, the concentrations may be in any units, as long

as the two concentrations are the same units. To a rough approximation, the ratio of

concentrations of the above equation is the same as the ratio of the solubility of the compound in

the two solvents, measured independently.

There are many different conditions that can change the distribution ratio, which include:

temperature, acid concentration, extractant concentration, and mixing time. Concentration of the

nitric acid and TBP are often studied for liquid-liquid extraction, Figures 4 through 6 show

commonly found distribution ratios for varying conditions. In some extraction processes, co-

extractants are used to aid in the extraction of a specific radionuclide.

Figure 4 Variation of distribution ratio of Pu(III) with varying % uranium saturation of 30% TBP in n-dodecane3

Figure 5 Variation of distribution ratio of Pu (III) with [HNO3] using 30% TBP in n-dodecane3

Figure 6 Distribution ratio of U(IV) between 30% TBP in dodecane and aqueous HNO34

3. Molecular interactions

A commonly used compound for liquid-liquid extractions is 18-Crown-6, seen in Figure 7.

Crown ethers can complex with the compound of interest in many different forms. The most

commonly seen is that a single atom will complex in the interior of the ring. For this

conformation you can change the size of the ring to fit to different sized elements. Another

conformation is a sandwich with a single atom surrounded by two of the crown ethers.

Figure 7 Structure of 18-Crown-6

The extractant used in the PUREX process interacts differently than the crown ethers.

Since uranium and plutonium are positively charged it will ionic bond to the oxygens on the

tributyl phosphate, in Figure 8. This will give a 1:1 ratio of TBP:U. Plutonium will follow the

same pattern as they have the same oxidation state when bonded with TBP.

Figure 8 Tributyl Phosphate

Experimental Procedure

Equipment

UV-Vis Alpha Spectroscopy Centrifuge Vortex Mixer 15 mL centrifuge tubes Glass pipettes Plastic bulbs

1. Technique

Label all centrifuge tubes with their content and amount. Write down concentration and activity of solutions. Add 2 mL of n-dodecane and 2 mL DI water into a 15 mL centrifuge tube. Close

tube before putting on the vortex. Double check the cap, then vortex for 1 minute. Put the tube in the centrifuge, for 2 minutes at 1500 rpm. Make sure there is a

counter weight of comparable size and amount. Using a glass pipette and plastic bulb, carefully separate the aqueous phase and

the organic phase into separate tubes. It is essential in this step to make sure to not spill or splatter any material as in future steps a spill or splatter will cause contamination of the work area.

Make sure to label the tubes.

2. Uranium Extraction Label all centrifuge tubes with their content and amount. Write down all concentration and activity of the solutions. Add 2 mL of 30% TBP in n-dodecane and 2 mL of 0.05 M UO3(NO3)2 in 4 M

HNO3. Close tube before putting on the vortex. Double check the cap, then vortex for 1 minute. Put the tube in the centrifuge, for 2 minutes at 1500 rpm. Make sure there is a

counter weight of comparable size and amount. Using a glass pipette and plastic bulb, carefully separate the aqueous phase and

the organic phase into separate tubes. Label tubes Measure aqueous and organic phase for the Uranium concentration in the UV-Vis.

3. Plutonium Uranium Extraction Label all centrifuge tubes with their content and amount. Write down all concentration and activity of the solutions. Add 2 mL of 30% TBP in n-dodecane and 2 mL of 0.05 M UO3(NO3)2 + 5 Bq Pu

in 4 M HNO3. Close tube before putting on the vortex. Double check the cap, then vortex for 1 minute. Put the tube in the centrifuge, for 2 minutes at 1500 rpm. Make sure there is a

counter weight of comparable size and amount. Using a glass pipette and plastic bulb, carefully separate the aqueous phase and

the organic phase into separate tubes. Label tubes. Measure aqueous and organic phase for the Uranium concentration in the Alpha

Spectroscopy.

Outline Results

1. Uranium Extraction Results Determine the amount of concentration of uranium in each phase by using the

calibration provided and Beer’s Law:

Where A= absorbanceε = molar absortivityb = cell widthc = concentration

Determine the distribution ratio of uranium by using equation 1. Determine amount of activity in initial aqueous solution. Determine the percent of activity lost in the extraction.

2. Plutonium Uranium Extraction Results Determine the amount of concentration in the organic and aqueous phases which

were previously counted by the alpha spec. Determine the amount of activity in initial aqueous solution. Determine the percent of activity lost in the extraction.

Questions

1. How does this apply to nuclear forensics and practical applications?2. What would you change about the experiment to approve the forensic capabilities?3. What are all the possible oxidation states of both uranium and plutonium?4. For nitric acid and n-dodecane, will the aqueous or organic phase be on the bottom?

Why?

References

1. Sood, D. D., Patil, S. K., Chemistry of Nuclear Fuel Reprocessing: Current Status, Journal of Radioanalytical and Nuclear Chemistry, Vol. 203, No. 2 (1996), pg. 547-73

2. Fessenden, Feist, Organic Laboratory Techniques, Brooks/Cole, 3rd Edition, 2001

3. Sagar, Veena, Chetty, K Venugopal and Sood, D. D. Extraction of Plutonium(III) by TBP in presence of Uranium, Solvent Extraction and Ion Exchange, Taylor & Francis, 18:2, 307-17

4. Sawant, R. M., Rastogi, R. K., Study on the extraction of U(IV) relevant to the PUREX process, Journal of Radioanalytical and Nuclear Chemistry, Vol. 229 (1998), pg. 203-6