RRFM 2009 Transactions - European Nuclear Society

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Figure 2 – Microstructure of U-9% Mo / Nb / Al-12% Si fuel core
3. Conclusions
1. Based on the analysis of possible ways of the U-9%Mo/Al fuel composition radiation
resistance increasing, results of post-irradiation tests and calculations, the fuel elements of
impregnated type are suggested for using as fuel elements for Russian research reactors
instead of the 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 matrix material impregnated into the fuel element is the Al-12% Si alloy.
The main advantage of impregnated type fuel elements is the increased charge of fuel
particulates (up to 65% of volume), this value is more than two times larger than analogous
characteristic (~30%) 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;
• strength properties of Zr-1% Nb alloy exceed considerably (~4 times) the
corresponding parameters of Al, that, under other equal conditions, possibly will
enable to avoid the bulging of the cladding observed in the fuel elements of extrusive
type with the Al cladding at large burn up.
2. The estimations carried out for particle sizes and protective barrier thicknesses which we
have chosen show that ~95% of fission fragments flying out the fuel particle stop in the Nb
layer which mean thickness is 3.5 μm; otherwise speaking, only ~25% of the whole number
formed fission fragments get to the Al-12% Si matrix.
Thus, the protective metallic layer around the fuel particles not only prevent the diffusion
interaction at the fuel-matrix boundary, but also delay the main part of fission fragments flying
out the fuel particles and decrease the degree of matrix radiation damage.
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