• • • • • • •

Readings: Uranium chapter:



http://radchem.nevada.edu/classes/r Chemistry in the fuel cycle



Uranium



Solution Chemistry

     

Separation Fluorination and enrichment Oxide Metal Focus on chemistry in the fuel cycle



Speciation (chemical form) Oxidation state Ionic radius and molecular size Utilization of fission process to create heat



Heat used to turn turbine and produce electricity Requires fissile isotopes



233 U, 235 U, 239 Pu



233 Need in sufficient concentration and geometry U and 239 Pu can be created in neutron flux 235

 

U in nature Need isotope enrichment Ratios of isotopes established



234: 0.005±0.001, 68.9 a

 

235: 0.720±0.001, 7.04E8 a 238: 99.275±0.002, 4.5E9 a

•

RFSS: Part 2 Lecture 11 Uranium Chemistry and the Fuel Cycle Fission properties of uranium



Defined importance of element and future investigations

  

Identified by Hahn in 1937 200 MeV/fission 2.5 neutrons

1

•

Nuclear Fuel: Uranium-oxygen system

A number of binary uranium-oxygen compounds



UO

 

Solid UO unstable, NaCl structure From UO 2

*

heated with U metal Carbon promotes reaction, formation of UC



UO 2



Reduction of UO 3 ºC

  *

CO, C, CH 4 , or C 2 H 5 OH can be used as reductants O 2 presence responsible for UO 2+x formation Large scale preparation

* *

UO 4 , (NH 4 ) 2 U 2 O 7 , or (NH 4 ) 4 UO 2 (CO 3 ) 3 Calcination in air at 400-500 ºC

* *

or U 3 O 8 with H H 2 at 650-800 ºC UO 2 has high surface area 2 from 800 ºC to 1100

2

•

Uranium-oxygen

U 3 O 8

 

From oxidation of UO



2 in air at 800 ºC

a

phase uranium coordinated to oxygen in pentagonal bipyrimid

b

phase results from the heating of the

a

above 1350 ºC



Slow cooling phase

3

Uranium-oxygen

•

UO 3



Seven phases can be prepared

•

A phase (amorphous)



Heating in air at 400 ºC

*

UO 4 .

2H 2 O, UO 2 C 2 O 4 .

3H 2 O, or (HN 4 ) 4 UO 2 (CO 3 ) 3



Prefer to use compounds without N or C

 a

-phase



Crystallization of A-phase at 485 ºC at 4 days



O-U-O-U-O chain with U surrounded by 6 O in a plane to the chain



Contains UO 2 2+

 b

-phase



Ammonium diuranate or uranyl nitrate heated rapidly in air at 400-500 ºC

 g

-phase prepared under O 2 6-10 atmosphere at 400-500 ºC

4

Uranium-oxygen

• •

UO 3

 

hydrates 6 different hydrated UO 3 compounds UO 3 .

2H 2 O Anhydrous UO 25-70 ºC 3 exposed to water from

 

Heating resulting compound in air to 100 ºC forms

a

-UO 3 .

0.8 H 2 O

a

-UO 2 (OH) 2 [

a

UO



3 .

b

H 2 O] forms in hydrothermal experiments -UO 3 .

H 2 O also forms

5

• • •

Uranium-oxygen single crystals

UO



2 from the melt of UO 2 Arc melter used



Vapor deposition 2.0 ≤ U/O ≤ 2.375



Fluorite structure powder Uranium oxides show range of structures



Some variation due to existence of UO 2 2+ in structure



Some layer structures

6

• • •

UO

2

Heat Capacity

Room temperature to 1000 K



Increase in heat capacity due to harmonic lattice vibrations



Small contribution to thermal excitation of U 4+ localized electrons in crystal field 1000-1500 K



Thermal expansion induces anharmonic lattice vibration 1500-2670 K



Lattice and electronic defects

7

Vaporization of UO

2

• •

Above and below the melting point Number of gaseous species observed



U, UO, UO 2 , UO 3 , O, and O 2

 

Use of mass spectrometer to determine partial pressure for each species For hypostiochiometric UO 2 , partial pressure



levels comparable to UO 2 O increases dramatically at O/U above 2

8

•

Uranium oxide chemical properties

Oxides dissolve in strong mineral acids



Valence does not change in HCl, H 2 SO 4 , and H 3 PO 4



Sintered pellets dissolve slowly in HNO 3



Rate increases with addition of NH

*

4 F, H 2 O 2 , or carbonates H 2 O 2 reaction



UO 2 + at surface oxidized to UO 2 2+

9

Solid solutions with UO

2

• • • •

Solid solution



crystal structure unchanged by addition of another compound



mixture remains as single phase



ThO 2 -UO 2 is a solid solution Solid solutions formed with group 2 elements, lanthanides, actinides, and some transition elements (Mn, Zr, Nb, Cd)



Distribution of metals on UO 2 fluorite type cubic crystals based on stoichiometry Prepared by heating oxide mixture under reducing conditions from 1000 ºC to 2000 ºC



Powders mixed by co-precipitation or mechanical mixing of powders Written as M y U 1-y O 2+x



x is positive and negative

10

Solid solutions with UO

2

•

Lattice parameter change in solid solution



Changes nearly linearly with increase in y and x



M y U 1-y O 2+x



Evaluate by change of lattice parameter with change in y

*

δa/δy



a is lattice parameter in

  

Can have both negative and positive values δa/δy is large for metals with large ionic radii δa/δx terms negative and between -0.11 to -0.3



Varied if x is positive or negative

11

•

Tetravalent M y U 1-y O 2+x



Zr solid solutions



Large range of systems

 

Solid solutions of UO

2 y=0.35 highest value Metastable at lower temperature

•

Tri and tetravalent M y U 1-y O 2+x



Cerium solid solutions



Continuous for y=0 to y=1

 

For x<0, solid solution restricted to y≤0.35

*

Two phases (Ce,U)O 2 and (Ce,U)O 2-x x<-0.04, y=0.1 to x<-0.24, y=0.7



0≤x≤0.18, solid solution y<0.5



Th solid solution



Continuous solid solutions for 0≤y≤1 and x=0

• 

Air oxidized hyperstoichiometric

*

y 0.56 to 1 at 1100 ºC

*

y 0.26-1.0 1550 ºC



For x>0, upper limit on solubility

*

y=0.45 at 1100 ºC to y=0.36 at 1500 ºC Tri and divalent



Reducing atmosphere



x is negative

 

fcc structure Maximum values vary with metal ion



Also has variation with O 2 partial pressure

*

At 0.2 atm., y=0.383 at 700 ºC to y=0.068 at 1500 ºC



Oxidizing atmosphere



Solid solution can prevent formation of U 3 O 8



Some systematics in trends

*

For Nd, when y is between 0.3 and 0.5, x = 0.5-y

12

U-Zr oxide system

13

•

Solid solution UO

2 Oxygen potential



Zr solid solution



Lower than the UO 2+x system

*

x=0.05, y=0.3



-270 kJ/mol for solid solution

 

-210 kJ/mol for UO 2+x Th solid solution



Increase in

D

G with increasing y Compared to UO 2

 

difference is small at y less than 0.1

Ce solid solution



Wide changes over y range due to different oxidation states



Shape of the curve is similar to Pu system, but values differ

*

Higher

D

G for CeO compared to PuO 2-x 2-x

14

• • •

Metallic Uranium

Three different phase

 a, b, g

phases



Dominate at different temperatures Uranium is strongly electropositive



Cannot be prepared through H 2 reduction Metallic uranium preparation



UF 4 or UCl 4 with Ca or Mg

 

UO 2 with Ca Electrodeposition from molten salt baths

15

Metallic Uranium phases

• • •  a

-phase



Room temperature to 942 K



Orthorhombic

• 

U-U distance 2.80 Å



Unique structure type

 b

-phase



Exists between 668 and 775 ºC



Tetragonal unit cell

 g

-phase



Formed above 775 ºC



bcc structure Metal has plastic character

 a ‐phase U-U distances in layer (2.80±0.05) Å and between layers

Gamma phase soft, difficult fabrication

3.26 Å 

Beta phase brittle and hard Paramagnetic Temperature dependence of resistivity Alloyed with Mo, Nb, Nb-Zr, and Ti

16 b -phase

•

Intermetallic compounds

Wide range of intermetallic compounds and solid solutions in alpha and beta uranium



Hard and brittle transition metal compounds



U 6 X, X=Mn, Fe, Co, Ni



Noble metal compounds



Ru, Rh, Pd

*

Of interests for reprocessing



Solid solutions with:



Mo, Ti, Zr, Nb, and Pu

17

Uranium-Aluminum Phase Diagram Uranium-Titanium Phase Diagram

18

Chemical properties of uranium metal and alloys

• • • • •

Reacts with most elements on periodic table



Corrosion by O 2 , air, 2 Dissolves in HCl



Also forms hydrated UO 2 during dissolution Non-oxidizing acid results in slow dissolution



Sulfuric, phosphoric, HF Exothermic reaction with powered U metal and nitric Dissolves in base with addition of peroxide



peroxyuranates

19

Review

• • • •

How is uranium chemistry linked with the fuel cycle What are the main oxidation states uranium Describe the uranium enrichment process



Mass based



Laser bases Understand the fundamental chemistry of uranium as it relates to:



Production

  

Solution chemistry Speciation Spectroscopy

20

Questions

• • • • • • • • • •

What are the different types of conditions used for separation of U from ore What is the physical basis for enriching U by gas and laser methods?

Describe the basic chemistry for the production of U metal What are the natural isotopes of uranium Describe the synthesis and properties of the uranium halides How is the O to U ratio for uranium oxides determined What are the trends in U solution chemistry What atomic orbitals form the molecular orbitals for UO 2 2+ What else could be used instead of 235 U as the fissile isotope in a reactor?

Describe two processes for enriching uranium. Why does uranium need to be enriched?

21

Questions

• •

Respond to PDF Quiz 11 Post comments on the blog



http://rfssunlv.blogspot.com/

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