RRFM 2009 Transactions - European Nuclear Society

was ~ 1-2.5 and 1-2 μm correspondingly. UO 2 layer was being produced on fuel particles by
oxidizing them at air, the layer thickness was ~ 0.3 μm.
Foregoing fuel compositions were being used for fabricating the mini-elements of diameter ~
10.88-11.0 and height ~ 5.07-5.28 mm.
Mini-elements were being irradiated in the interior of the fuel assembly of IVV-2M reactor
during 118 days at mean heat flux of 60 W/cm 2 . The mean fission rate was equal to 4.0×10 14
fis./(cm 3·c). The mean fission density of 4.1 ×10 21 fis./cm 3 and the equivalent 235 U burn up of
60% have been achieved to the irradiation end. The fuel composition temperature during
irradiation process did not exceed 90ºC.
Post-irradiation tests confirmed our suggestions [2,3]:
• There were not visible failures on the mini-elements, the bulges on the claddings
were not observed. Cracking of the composition was not observed in all cases.
• Coating U-Mo particles by Nb and Zr-1% Nb alloy reduces the swelling of minielements
(of the basic U-Mo/Al composition) with technically clean Al-0.3%Si matrix
by 20-27%, and oxidizing – by ~45%.
• Using the Al-12%Si alloy as the matrix and the particles without the coating leads to
the swelling reducing by comparison with the basic composition by 20%, and using
the UO 2 coatings – more than two times.
• Coating the fuel particles by barrier metallic layers hinders to penetration of U and Mo
atoms and also fission fragments from fuel to matrix.
• Gas-filled pores were observed in the basic fuel composition both in the fuel particles
and in the interaction layer which width was equal to ~5 μm.
• The width of the interaction layer in U-9%Mo/UO 2 /Al fuel composition is less by ~10%
only by comparison with the basic one, but pores are absent in this layer.
• The interaction layer in U-9%Mo/Nb/Al fuel composition consists of two parts: the
Internal one (remnants of Nb coating) with width of 1 μm and external one (Nb 3 Al,
Nb 2 Al, NbAl 3 combinations) which width is equal to 3 μm. There are no pores in these
layers.
• The width of the interaction layer in U-9%Mo/Zr-1%Nb/Al fuel composition is ~1 μm.
There are no pores in this layer also.
• The mean width of the layer observed around the fuel particles in U-9%Mo/Al-12%Si
fuel composition is equal to ~ 4.3 μm. This value is less by ~15% by comparison with
the basic fuel composition. Gas-filled pores were not observed.
• The mean width of the layer around the fuel particles in U-9%Mo/ UO 2 /Al-12%Si fuel
composition is equal to ~3.3 μm. This value is less by ~30% by comparison with the
basic fuel composition. Gas-filled pores are absent also.
Thus, all barrier coatings which we used decrease the rate of (U,Mo)Al x layer growth. It
should note that according to data of the post-reactor tests the thickness of oxidic layer of 0.3
μm is insufficient for protection against the interaction between the Al matrix and components
of the fuel particles.
Based on these data and results of the calculations [4], we suggest the fuel elements of
impregnated type for using as fuel elements for Russian research reactors instead of
elements fabricated by the extrusion technique:
• the impregnated fuel element of monolithic type with the Zr-1% Nb alloy cladding;
• the fuel composition – the U-9% Mo particulates coated by the Nb protective layer;
• the Al-12% Si alloy is used as the matrix material impregnated into the fuel element.
The increased charge of fuel particulates (up to 65% of volume) is the main advantage of
impregnated type fuel elements, this value is more than two times larger than analogous
characteristic (29%) in extrusive type fuel elements. Moreover, the impregnated type fuel
elements possess some other advantages as compared with extrusive type fuel elements
with the Al cladding:
• under the impregnating the mechanical interaction between the fuel particles, which is
inevitable during the extrusion, is absent; moreover, the integrity of the protective
metallic barrier on the particulates is preserved;
157 of 455