ORNL-5388 - the Molten Salt Energy Technologies Web Site

4-5%
4.6.2. Carbide- and Metal-Fueled LMFBRS
D. L. Selby
P. M. Haas H. E. Knee
Oak Ridge National Laboratory
Another method that is being considered for improving the breeding ratios of LMFBRs
and is currently under development1 is one that uses carbide- or metal-based fuels. The
major advantages of the metal- and carbide-based fuels are that they will require lower
initial fissile inventories than comparable oxide-based fuels and will result in shorter
doubling times. This is especially true for metal-based fuels, for which doubling times
as low as 6 years have been calculated.2 Since for fast reactors the denatured fuel cycle
would have an inherently lower breeding gain than the reference plutonium-uranium cycle,
these advantages would be especially important; however, as discussed below, before either
carbide- or metal-based fuels can be fully evaluated, many additional studies are needed.
Carbi de-Based Fuels
Carbide-based fuels have been considered for use as advanced fuels in conventional Pu/U
LMFBRs. Burnup levels as high as 120,000 MWD/T appear feasible, and the fission gas release
is less than that for mixed oxide fuels.3 Carbide fuels also have a higher thermal conductivity,
which allows higher linear power rates with a lower center-line temperature. In
general, the breeding ratio for carbide fuels is higher than the breeding ratio for oxide
fuels but lower than that for metal fuels.
Both helium and sodium bonds are being considered for carbide pins. At present 247
carbide pins with both types of bonds are being irradiated in EBR-11. Other differences in
the pins include fuel density, cladding type, cladding thickness, type of shroud for the
sodium-bonded pin, and various power and temperature conditions. The lead pins have already
achieved a burnup level of 10 at.%, and interim examinations have revealed no major problems.
Thus there appears to be no reason why the goal of 12 at.% burnup cannot be achieved.
In terms of safety, irradiated carbide fuel releases greater quantities of fission gas
upon melting than does oxide fuel.
either an advantage or a disadvantage.
Depending upon the accident scenario, this could be
be the potential for large-scale thermal interaction between the fuel and the coolant [see
discussion of potential FCIs (eel-Coolant Interactions) below].
Metal-Based Fuels
Another problem associated with carbide fuels may
Reactors with metal-based fuels have been operating in this country since 1951
(Fermi-I, EBR-I, and EBR-11). Relative to oxide- and carbide-fueled systems, the metal-
fueled systems are characterized by higher breeding ratios, lower doubling times, higher
heat conductivity, and lower fissile mass. These advantages are somewhat offset, however,
by several disadvantages, including fuel swelling problems that necessitate operation at
lower fuel temperatures.