PLENTIFUL ENERGY

enlargement. In another test, metal fuel operated for 223 days after the cladding
breach, including many start-up and shut-down transients. The breach site remained
small. It was concluded that although metal fuel is very reliable, if a fuel failure
does occur the failed fuel pins can be left in the core until their normal end of life
without raising any particular operational or safety concerns. This is true of no other
fuel, in fast reactors or in LWRs.
The eutectic formation (melting) temperature between the fuel and the cladding
is a critical parameter for metal fuel. The onset of fuel/cladding eutectic formation
starts at 700-725 o C range, depending on the fuel alloy and cladding types. At onset,
however, not much occurs. In fact, even at a hundred degrees above the eutectic
temperature the eutectic penetration into the cladding is minimal in one hour. Only
at much higher temperatures, approaching the melting point of the fuel material
itself on the absence of cladding, does the eutectic penetration into cladding become
rapid. Eutectic formation therefore, is not a primary safety concern even during
transient overpower conditions.
It does have a practical effect. Reactor designers tend to place limits on the
coolant outlet temperature to assure the eutectic temperature is not reached at the
fuel/cladding interface. A coolant outlet temperature limit of 500-510 o C is very
conservative, and up to 550 o C provides ample margins to the onset of eutectic
formation. The conservative design approach will probably be relaxed with more
experience.
6.9 Minor Actinide Containing Fuel
Equilibrium recycled IFR fuel will contain transuranic elements near to
plutonium in atomic weight, so the U-Pu-Zr fuel test matrix included fuels
containing these minor actinides. The typical composition of the actinides in LWR
spent fuel is about 90% plutonium and 5% each for neptunium and americium. IFR
ternary fuel, U-20Pu-10Zr, will contain about 1% Np and 1% Am. For an upper
bound case, a 1.3% Np and 2% Am addition was selected for the irradiation tests.
Americium has high volatility and fuel fabrication at elevated temperatures with
Americium will be a challenge for any fuel material. For the IFR metal fuel, the
standard injection casting technique was altered slightly to have the feedstock of
80Pu-20Am inserted into the molten fuel late in the melt cycle to minimize the time
at temperature. When the Pu-Am stock was added, ―sparks‖ were observed. The
sparks were probably induced by boiling of contaminants in the Pu-Am feedstock,
which contained about 3% calcium and 2000 ppm magnesium. Both of these
elements boil below the casting temperature. The resulting agitation of the molten
pool probably contributed to the higher-than-expected evaporation of americium.
Three slugs were cast, and about 40% of americium was evaporated in the casting.
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