RRFM 2009 Transactions - European Nuclear Society

changes can be caused by void swelling in stainless steel cladding, irradiation
shrinkage, and/or growth in materials with non-symmetric crystal structure).
Corrosion or attack of the cladding by the coolant remains an issue for most of the reactor
fuels considered here, particularly for LFR fuels, where the oxygen content of the lead or
lead–bismuth coolant must be controlled to prevent corrosion of stainless steel cladding and
core components. Addressing these phenomena has a significant impact on burn-up limits
for fuel utilization.
4.2 Fuel candidates for fast reactors
Four fuel systems have been investigated as candidates for fast reactor fuels [5,6]:
• mixed-oxide (MOX) fuel, historically (U,Pu)O2 or (U,TRU)Ox, where TRU represents
transuranic elements, similar to the uranium oxide fuel used in commercial light water
reactors (LWRs) and characterized by irradiation stability and relatively high melting
temperature;
• metal alloy fuel, typically characterized by ease of fabrication, high thermal conductivity,
and high uranium and plutonium densities;
• mixed carbides (MC), typically (U,Pu)C; and
• mixed nitrides (MN), typically (U,Pu)N.
For mixed carbides and mixed nitrides, the uranium and plutonium densities and thermal
conductivities are closer to those of metal alloy fuels than to those of MOX fuels, and they
exhibit good irradiation stability and relatively high melting temperatures (Table 1).
Table 1: Basic properties of oxide, nitride, carbide and metallic fuel
Pu/(U+Pu)=0.2
Carbide
(U,Pu)C
Nitride
(U,Pu)N
Oxide
(U,Pu)O 2
Metallic fuel
(U-Pu-Zr)
Heavy atom density (g/cm 3 ) 12.9 13.5 9.7 14.1
Melting point (°C) 2305 2720 2730 1070
Thermal conductivity (W/m.K) 12.8
(at 1000°C)
13.5
(at 1000°C)
2.1
(at 1000°C)
17.5
(at 500°C)
In the early days of the development of fast reactors, the key issue was to demonstrate the
feasibility of breeder reactors (producing more fissile materials than they consume). In that
respect, and among the various possible fuel candidates (Table 1), it was therefore natural
to opt for a very dense fuel in heavy nuclides with metal fuel as a first choice. This is the
reason why most of the very first liquid-metal cooled breeder reactors made use of metallic
alloys.
However, the performances of these first metallic fuels were very limited in burn-up (a few
GWd/tHM) because of important swelling under irradiation.
In the 50’s, CEA also launched a R&D program on metallic fuels (basic physical properties,
fabrication process, in-pile behaviour) looking for ternary compounds (such as U-Pu-Mo) to
further improve the fuel performances. Again, swelling was much too high and, as a
consequence, frequent spent fuel reprocessing was required to recover the fissile materials,
thus inducing prohibitive fuel cycle costs.
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