# Presentation: 213813

## Stability of Actinide-bearing Solids: Bulk vs. Nanoscale Properties

**Alexandra Navrotsky**
**UC Davis**

## Issues

**Processing of advanced fuels (MOX, nitride, inert matrix, thorium cycle)**
**Materials compatibility**
**Radiation damage and chemical degradation in reactor**
**Disposal**
**Leaching and environmental transport**
***All need knowledge of ***
***thermodynamic properties***

## Examples

**Fuels:**** UO****2****-PuO****2****-ZrO****2****-LnO****1.5 ****fluorites**
**Corrosion products:**** ****Uranyl and actinyl minerals**
**Environmental contamination:**** ****Transport by actinide colloids and iron oxides**** **
**Waste forms:**** ****Tailored materials for separated waste streams**

## High temperature oxide melt calorimetry

**High temperature oxide melt calorimetry**

**Custom built Calvet-type calorimeter**

**P-Name-Meeting_XX/XX/XX.ppt**

## UO2-PuO2-ZrO2-LnO1.5 fluorite structured phases

**Basis of normal, MOX, and inert matrix fuels**
**Oxygen vacancies, high ionic conductivity, defect clustering**
**Considerable thermodynamic stabilization possible**

## Slide 6

## UO2 - YO1.5 AND UO2 - CaO

**Yttria-urania solid solution shows significant stabilization relative to constituent oxides. This agrees with the previously reported vapor pressure data (****Aitken&Joseph, 1966)****.**

- 	Data is to be presented on joint 19th IUPAC International Conference on Chemical Thermodynamics and 61st Calorimetry Conference. Paper “Energetics of UO2-solid solutions with CaO and Y2O3 ” (Mazeina L., Navrotsky A., Greenblatt M., et al) is in preparation

## Uranyl and Actinyl Phases

**In contact with aqueous solutions in air major oxidation states are U(VI), Np(V) and Pu(IV) **
**A series of increasingly stable complex uranyl phases form by oxidation of UO****2**** and reaction with ions in groundwater**
**Np and Pu incorporation in uranyl phases versus new structures **

## Uranyl Carbonates:       UO2CO3

**Uranyl Peroxides:           (UO****2****)O****2****(H****2****O)****4****,**** **
**Uranyl Oxide Hydrates: (UO****3****)(H****2****O)****2****, (UO****2****)(OH)****2**
**	****	****	****	****            ****Ca(UO****2****)****6****O****4****(OH)****6****(H****2****O)****8****, ****	****	****	****	****                                        Ca(UO****2****)****4****O****3****(OH)****4****(H****2****O)****2****	**
**	****	****	****	****            ****Na(UO****2****)O(OH), Na****2****(UO****2****)****3****O****3****(O**
**     ****Uranyl -Phosphates:       Cu(UO****2****)****2****(PO****4****)****2****(H****2****O)****8****	**** **
**	****	****	****	****           ****Ca(UO****2****)****2****(PO****4****)****2****(H****2****O)****6**
**	****	****	****	****           ****Sr(UO****2****)****2****(PO****4****)****2****(H****2****O)****6**
**	****	****	****	****           ****K(UO****2****)(PO****4****)(H****2****O)****3**
**	****	****	****	****           ****Rb(UO****2****)(PO**_**4**_**)(H****2****O)****3**** **
**     ****Uranyl Silicate:               (UO****2****)****2****(SiO****4****).2H****2****O**
**And many many more !!**

**Uranyl minerals**

## Studtite, a Uranyl Peroxide Mineral

**(UO****2****)O****2****(H****2****O)****4**** (crystalline, 298 K)**
**ΔH ̊****298 K**** = 22.3 ± 3.9 kJ mol****-1**** **

**UO****3**** (crystalline, 298 K) + 3 H****2****O (l, 298 K) + H****2****O****2**** (l, 298 K) = **
**(UO****2****)O****2****(H****2****O)****4**** (crystalline, 298 K)****	****	****	****	****	****	**
**ΔH ̊****r****298 K**** = -75.7 ± 4.1 kJ mol****-1**** **

**Studtite, a Uranyl Peroxide Mineral**

_***Formation from Oxides and Oxygen***_

_***Formation from Peroxide***_

## Nanoscale Phenomena

**High surface area (100+ m****2****/g) means high reactivity and sorption capacity**
**Competition between surface enrrgy and closely balanced energetics of various polymorphs means different crystal structures can be stable at nanoscale. *****More metastable bulk polymorph has lower surface energy***** **
**These polymorphs will have different properties, e.g. actinide adsorption**

## Energetics of Nanocrystalline Zirconia

## Zirconia energetics

| Phase | Transformation Enthalpy, kJ/mol | Surface Enthalpy, J/m2 |
| --- | --- | --- |
| Monoclinic | 0 | 6.5 ± 0.2 |
| Tetragonal | 9.5 ± 0.4 | 2.1 ± 0.05 |
| Amorphous | 34 ± 4 | 0.5 ± 0.05 |

## Schematic of a) standard enthalpies of formation of goethite, akaganeite and lepidocrocite, and b) anhydrous and c) hydrated surface enthalpies of goethite, akaganeite and lepidocrocite. Hatch marks show propagated error. All values for akaganeite and lepidocrocite overlap. 

**Schematic of a) standard enthalpies of formation of goethite, akaganeite and lepidocrocite, and b) anhydrous and c) hydrated surface enthalpies of goethite, akaganeite and lepidocrocite. Hatch marks show propagated error. All values for akaganeite and lepidocrocite overlap.*** *

## Tailored waste form, e.g. Na2Nb2-xTixO6-0.5x  for short lived isotopes, e.g. Cs, Sr

## Plutoniumwaste form 

**composition diagram**

- * Ce-pyrochlore as a surrogate for Pu-pyrochlore

## Conclusion

**Thermodynamic data for uranium phases are just beginning to be solidified**
**Thermodynamic data for higher actinide phases are almost nonexistent, at both trace and bulk concentrations**
**Advanced fuels mean new materials which need thermodynamic characterization**