PLENTIFUL ENERGY

sized so the fission gas pressures on the cladding are kept to reasonable levels. Very
long burnups become possible.
The improved EBR-II fuel design did not differ greatly from the earlier design.
The fuel composition was the same. Schematically, the design itself looks the same.
The important additional clearance and the lengthened plenum were simple
changes, but they made the difference. A schematic of the EBR-II metal fuel is
shown in Figure 6-1. The fuel slug, the gap filled with sodium, and the plenum can
be seen. The fuel slug can be full length or it can be in segmented in pieces one on
top of the other; the performance is the same. Since they are easiest to fabricate,
most fuel has been in segmented pieces.
Figure 6-1. Schematic of metal fuel
Ease of fabrication generally may not seem to be that big a factor, but it is
hugely important where plutonium, particularly recycled plutonium, is to be
involved in the fabrication. Recycled plutonium builds up considerable amounts of
the higher plutonium isotopes. Highly radioactive, hands-on fabrication is unwise if
not impossible, yet hands-on fabrication is almost mandatory when fabrication is
complicated. The high power densities in the fuel of a fast reactor demand excellent
heat transfer, and in the absence of another heat transfer medium like sodium, very
tight fits of fuel to cladding are necessary. In the FFTF, the individual oxide fuel
pellets were ground to size to give the fit necessary for adequate heat transfer, and
the higher plutonium isotopic content of the fabricated fuel was limited by the
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