1. NOVEL TRENDS IN FUEL AND MATRIX ALLOYING TO REDUCE INTERACTION1
VNIINM
A.A. BOCHVAR ALL-RUSSIA RESEARCH INSTITUTE OF INORGANIC MATERIALS
NOVEL TRENDS IN FUEL AND MATRIX ALLOYING TO REDUCE INTERACTION
A.M. SAVCHENKO, I.I. KONOVALOV, S.A. ERSHOV, E.K. MALAMANOVA
UMo/Al dispersion fuel interaction is the main reason of delaying RERTR Program.
Interaction leads to unacceptable pillowing and swelling of the fuel plate.

2. 2 Arbitrarily 4 interaction stages might be distinguished.
Fission fragments release from fuel into matrix. Formation of damage rim around fuel granules Al
UMo Al
UMo IL Al
UMo
2. Growth of interaction layer in matrix
direction with fission fragments displaced
to interaction layer – matrix boundary.

3. 3
3. Fission fragments piling up on interaction phase – matrix boundary,
especially, in heat flux direction, “shears” form at the interfaces with
Al matrix as well as FGP concentrate within pores. Al
heat
flow
UMo Al
heat
flow
UMo
4. FGP build-up in voids, pressure increases, formation of pillows.

4. 4 4 ways to reduce interaction is under consideration now. They are:
Alloying of fuel.
Alloying of matrix.
Creating protective barriers.
Application of monolithic UMo fuel.
Fuel and matrix alloying is the most optimal version in terms of decreasing interaction as well as of technology.

5. 5
Alloying of Fuel
A concept of UMo fuel alloying has been elaborated that consists in producing a two phase structure alloy, viz., basic -(U-Mo) phase and intermetallic phase. Intermetallic phase shall have the maximal content of uranium, a low molybdenum content, be well compatible with aluminium, be highly irradiation resistant and precipitate along grain boundaries. The latter condition allows fuel particle production via grinding.   

6. Structures of alloyed UMo two-phase alloy; a, b – U + 6.2Mo + 1.2Si;
-(U-Mo) phase
Intermetallic phase a b c d
Structures of alloyed UMo two-phase alloy; a, b – U + 6.2Mo + 1.2Si;
c, d – U + 6.8Mo + 1.0Si + 0.2Al

7. 7
After grinding weakly interacting with Al intermetallic phase is primarily available at fuel particle surfaces, forming some kind of a protection barrier. a b
Microstructure of fabricated fuel composition with UMo alloy,
a – (U+6.2Mo+1.2Si)/Al as fabricated; b – (U+6.8Mo+1.0Si+0.2Al)/Al - 6 hour
annealed at 600 С

8. 8 Thermodynamic approach
The main challenge in alloying UMo fuel is a decrease in -(U-Mo) phase stability at the expense of a reduced molybdenum content when alloying elements are added. a b
Types of phase diagrams of UMo alloy with elements that form intermetallic compounds with uranium, a – system with ternary compounds (U-Mo-Al) [3], b – isothermal section of U-Mo-C phase diagram at 1500 С

9. 9
In U-Mo-C system ternary compounds should is not to form close to the uranium angle of phase diagram, therefore Mo remains in solid solution of  U-Mo phase a b c
Distribution of elements of U-6.5Mo alloy with carbon impurity in characteristic X-ray radiation a – secondary electrons, b – Mo, c - C

10. Fabricating fuel alloys by powder metallurgy method10
Fabricating fuel alloys by powder metallurgy method
Advantages:
Feasibility of producing thermodynamically non-equilibrium structures of fuel alloy (inert to Al interaction), that provides decreased emergence of Mo from -phase;
2. A qualitatively different composition of powder surface layer after grinding that simplifies protective thermochemical treatment, particularly, oxidation;
3. Produced alloys are adequately brittle and readily ground.

11. 11
Sintered structures of alloys prepared from initial powders of U, Mo, Si and other elements and process dynamics studied by differential thermal analysis. a b
Microstructure of alloys produced by powder metallurgy method,
a – (U+6.5Mo+1.4Si); b – (U+6.5Mo+1,1Si+0.6Fe)

12. 12
Alloying of Matrix
Three trends of matrix alloying are under consideration now:
1. Alloying with magnesium;
2. Alloying with silicon to obtain optimal structure of interaction layer;
3. Alloying with high temperature resistant additives to decrease diffusionability of aluminium.
It is a qualitatively novel option and most promising. But the main problem is related to technology and method of introducing alloying additives.

13. 13
Aluminium alloys of SAP (sintered aluminium powder) type in the form of particles mm in size were used as matrix. They contain 6-19% Al2O3 as intergranular thin layers.
Aluminium insoluble oxide coats have to serve as diffusion barriers against diffusion of aluminium into UMo fuel.

14. 14 a b d c
Structures of U9Mo + SAP fuel compositions, a – as fabricated, b, c, d - as 4 h annealed at 6300C, c - fuel granules pressed into aluminium cladding, d - fuel granules in fuel element centre in SAP matrix

15. 15 Combination of methods 1. Alloying of fuel + alloying of matrix.
2. Alloyed fuel powder + surface treatment O2
UO2 
Intermetallic phase present at surface has other physical and chemical properties and thermal-chemical treatment will result in qualitatively new coats, particularly oxide coats.

16. A.A. BOCHVAR ALL-RUSSIA RESEARCH INSTITUTE OF INORGANIC MATERIALS16
VNIINM
A.A. BOCHVAR ALL-RUSSIA RESEARCH INSTITUTE OF INORGANIC MATERIALS
Conclusion
Novel approaches to fuel and Al matrix alloying are discussed that are aimed to reduce interaction between UMo fuel and matrix. A concept of U-Mo fuel alloying has been elaborated that consists in producing an alloy of two phase structure, viz., basic (U-Mo) and intermetallic phases on grain boundaries, which allows fuel particle production via grinding. Using a thermodynamic approach and phase diagrams, optimized systems were selected that meet those requirements.
Method of U-Mo fuel alloying by powder metallurgy, i. e. sintering powders of initial components followed by grinding resultant ingots is under developments.
Novel trend in alloying matrix to decrease diffusionability of aluminium (alloying with high temperature resistant additives) is under consideration. It consists in applying aluminium alloys of the SAP type (sintered aluminium powder) as matrix. Aluminium insoluble oxide interlayers in a structure have to serve as diffusion barriers against diffusion of aluminium into U-Mo fuel. Preliminary technological studies and compatibility tests demonstrated the potential of this approach.

17. A.A. BOCHVAR ALL-RUSSIA RESEARCH INSTITUTE OF INORGANIC MATERIALS17
VNIINM
A.A. BOCHVAR ALL-RUSSIA RESEARCH INSTITUTE OF INORGANIC MATERIALS
Acknowledgements
The authors gratefully acknowledge CEA and ANL for promoting our activity and IAEA for valuable support.