PLENTIFUL ENERGY

6.1 What Makes a Good Fast Reactor Fuel?
The ability to withstand the combination of high temperature and intense
radiation is the characteristic of most importance in a fast reactor fuel. It means
long life, and that, in turn, eases the load on the rest of the fuel cycle. The ability to
generate high power without excessively high internal fuel temperatures is
important too. Fast reactors generate their power with less tonnage of fuel than their
thermal reactor counterpart. Paradoxically, the higher the enrichment, the less fuel
you need. Instead of the three or four percent in the thermal reactor, enrichments in
a fast reactor are in the range of twenty to twenty-five percent. The basic cross
sections of all materials are small for fast neutrons, so the fraction of the fuel
material that is fissile must make up for its smaller cross sections. With a lot of
fissile material in each pin the power produced per pin must then be
correspondingly high to minimize the total amount of fissile material required for a
given power production. So the fuel must withstand high power densities as well as
withstanding long irradiation times. The two are related, but they are not identical.
The most basic feature is time in the reactor. The longer the fuel can remain in
the reactor, the smaller the amount of fresh fuel needed. And for a system that
recycles its fuel, like the IFR, this means the flows to spent fuel processing are
smaller, which means smaller and cheaper processing and new fuel fabrication.
Breeding new plutonium in the core means that the fissile content of the fuel stays
fairly constant as the fuel burnup proceeds, so the flow of fissile material can be
minimized only by long fuel residence times in the reactor.
In theory, processing could be avoided completely by designing for very long
fuel residence times indeed, up to the entire lifetime of the reactor, and periodically
such systems are proposed. Generally the power densities allowed must be low. The
phenomenon that normally limits fuel lifetime in a fast reactor is irradiation damage
to the fuel cladding, and this directly limits the power density. Even with high
breeding capability so adequate reactivity is maintained, the fuel lifetime is limited
by the cladding. The intensity of the neutron flux and the length of the exposure to
the flux—the ―fluence‖ as it is called—on the fuel cladding is what is important in
limiting fuel life, not the fuel material itself. The result is that the power produced
by such a reactor will be correspondingly low, or alternatively, a very much larger
core, and thus a larger reactor vessel and components, will be needed. There isn‘t
any way around this tradeoff. The construction cost of nuclear systems combined
with low power production is deadly to plant economics.
At the other extreme—and more realistic economically—was the scheme
implemented by the designers of Experimental Breeder Reactor-II in the early
1960s. As the metal fuel of those days couldn‘t withstand long residence times,
their idea was to keep fuel in the reactor as high a percentage of the time as possible
by rapidly processing it at low burnup and immediately returning it to the reactor.
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