Nuclear Forensics Summer School 2010radchem.nevada.edu/classes/nfss/labs/Plutonium Uranium... ·...
Transcript of Nuclear Forensics Summer School 2010radchem.nevada.edu/classes/nfss/labs/Plutonium Uranium... ·...
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