The FuTure oF nuclear Fuel cycle - MIT Energy Initiative

advanced technology opportunities
The united States has not made significant investments in
understanding fuel cycle options for several decades. In this
time new options (appendix B) with potentially better characteristics
but major uncertainties have been partly developed.
Several of these are described here.
LWRs with modified cores. The appeal of liquid-metal
cooled fast reactors (lMFrs) is that they enable a closed fuel
cycle that extracts 50 times as much energy from uranium
as does the once-through (open) fuel cycle of existing lWrs.
But demonstration lMFrs to-date have been more expensive
compared to existing light water reactors (lWrs) and
so have never been commercialized. The sodium fast reactor
was chosen in the 1970s as the preferred sustainable reactor
because of its high conversion ratio. our analysis indicates
that a lower conversion ratio near unity is preferable for a
sustainable reactor. advances in the design now indicate
that a hard-spectrum lWr could have a conversion ratio
near unity and be a sustainable reactor. Such modified-core
lWrs are likely to be less costly to develop than lMFrs, because
only the reactor core needs to change, and may be
more economic to operate. Moreover, this approach may
provide the option of using some of the existing reactor
fleet for a closed, sustainable fuel cycle that would greatly
extend uranium resources. There has been significant work
in several countries on such modified-core lWrs, but additional
research and demonstration would be required to
determine commercial viability.
Advanced High-Temperature Reactors. In the last decade,
a new reactor concept has been proposed that uses liquid
fluoride salts as coolants and graphite-matrix coated-particle
fuel. The reactor combines the coolant developed for
molten-salt reactors and the fuel developed for gas-cooled
high-temperature reactors. one variant uses pebble-bed
graphite-matrix coated-particle fuel. With this option, the
fuel pebbles would not be fixed in place as with a conventional
fuel assembly, but would slowly move through the
reactor core. This allows continuous re-fueling and three
dimensional optimization of the reactor over time, enabling
novel fuel cycles. For example, such a reactor may
be operated in a combined uranium-thorium fuel cycle in
a once-through mode or may have a high conversion ratio
(near unity) if operated with a closed fuel cycle. The reactor
would operate at low pressures and with high coolant temperatures
resulting in increased efficiency of electric power
generation. Thus such reactors could have lower capital
costs and enhanced safety and nonproliferation characteristics
relative to lWrs. rd&d would be needed to determine
long-term commercial viability. This reactor is also called the
fluoride salt high-temperature reactor.
Uranium from Seawater. Seawater contains about four
billion tons of uranium, enough to support thousands of
reactors for thousands of years. recent Japanese research
suggests that the cost of obtaining uranium from seawater
may ultimately be low enough to be commercially feasible,
which would enable once-through fuel cycles for centuries.
The economic viability of this option depends upon the
long-term durability in seawater of the ion exchanger that is
used to separate uranium. r&d is required to determine the
potential for seawater uranium.
Nuclear renewable futures. historically, nuclear energy
has been considered as a source of base-load electricity,
which constitutes a quarter to a third of the world’s total
energy needs. however, there are additional candidate markets,
such as meeting peak and other variable electricity
demands by coupling base-load nuclear reactors to huge
energy storage systems, or production of renewable liquid
fuels in nuclear-powered biorefineries. Viability depends
upon both the economics of nuclear power and successful
development and commercialization of technologies such
as gigawatt-year heat storage, high-temperature electrolysis
for hydrogen production, and hydrocracking of lignin.
developments in this area could significantly expand lowcarbon
energy options for the united States and may drive
market requirements (temperature of delivered heat, reactor
size, etc.) that, in turn, would drive reactor and fuel cycle
decisions.
chapter 10. recommended analysis, research, development, and demonstration Programs 141