RRFM 2009 Transactions - European Nuclear Society

Figure 2: Cellular fuel sub-assembly with composite cladding material (SiC-SiC f )
Significant progress has been made recently about the selection of constitutive materials
(clad structure and liner) to ensure leak-tightness to fission products and comply with
requirements of thermo-mechanical integrity and adequate chemical compatibility between
materials. For the clad, candidate composites are Tyranno SA3 and Hi-Nicalon Type S
fibers, both existing commercial-grade carbon composites. The internal liner is made of
refractory metallic materials based on Mo, W, Nb, or Si based intermetallics. These metals
are known to be more or less neutrons absorbers. The current design is a 50 µm layer of W-
14Re.
4. Survey of fuel options for fast neutron reactors: potential and challenges
4.1 General fuel requirements for fast neutron reactors
Although the SFR, LFR, GFR fuel designs are quite different, their basic functional
requirements are the same, summarized as follows [6]:
• to retain hazardous radionuclides in all but the most unlikely postulated conditions,
• to maintain a geometry that can be cooled,
• to maintain fissionable material in a controllled and predictable geometry, and
• to provide a convenient form for fuel handling.
Other mission-specific or system-specific requirements include reliable operation at high
temperatures; compatibility with post-irradiation disposal or recycling technology; and
technology-specific requirements for physical properties such as actinide element density,
thermal conductivity, and melting temperature.
Neutron irradiation, high temperatures, and accumulation of fission products all work to
degrade and stress the fuel’s ability to meet these requirements. These factors limit the inservice
lifetime or utilization of the fuel. In general terms, the degradation mechanisms that
operate in current fuel designs include:
• chemical attack of the fuel cladding or fuel particle layers by fission products or fuel
constituents, which weakens the barrier properties;
• stress of the cladding or fuel particle layers caused by increasing fission gas
pressure and/or by volumetric swelling of the fuel material due to accumulating
gaseous and solid fission products retained in the fuel material; and
• irradiation effects in the cladding or fuel particle layers, which can lead to
embrittlement, enhanced creep damage, or dimensional changes. (Such dimensional
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