The FuTure oF nuclear Fuel cycle - MIT Energy Initiative

p Once-through fast reactor fuel. There has been a significant interest in and initial development
of a once-through sustainable fast reactor that after the initial core loading uses
depleted uranium or natural uranium fuel (Appendix B). The viability of such advanced
once-through fuel cycles is dependent upon successful development and demonstration
of better fuel cladding materials.
CloSed Fuel CyCle teChnoloGy
There are many closed fuel cycles with different goals, reactors, and fissile materials. The
common characteristics of closed fuel cycles are a set of backend fuel-cycle operations
where (1) SNF is physically and/or chemically separated into different product streams,
(2) selected products are converted into new fuel assemblies or reactor targets, and (3) the
wastes are converted into chemical and physical forms acceptable for disposal. Closed fuel
cycles can accomplish four functions that can’t be accomplished by open fuel cycles.
p Purification and fissile concentration. In a reactor fissile fuel is fissioned and fertile materials
are converted to fissile fuel. The changing composition of the fuel (primarily buildup
of fission products) may shut down the nuclear reactor. In reactors with conversion
ratios less than 1, the fissile concentration decreases with time. Recycle separates fissile
material to enable its recycle into new fuel and thus bypass reactor neutronics limits.
p Fuel assembly replacement. Radiation damages the fuel over time. A closed fuel cycle
enables replacement of the clad and other components of the fuel. In many fast reactors,
this is the primary purpose of a closed fuel cycle. Fissile material is produced as fast as it
is consumed and the buildup of fission products does not shut down the reactor. In such
closed fuel cycles the amount of SNF that is recycled (or even the need to recycle) is determined
by clad materials properties; thus, better materials reduce the need for recycle.
p Convert form of fuel. If fuel is moved from one reactor type to another, the physical form
of the fuel must be changed.
p Waste management. Some types of SNF are unacceptable for direct disposal and must be
converted into acceptable waste forms. The need for SNF processing for waste management
purposes is driven by SNF storage, transport, and disposal requirements.
In the LWR closed fuel cycle where plutonium is recycled to produce MOX fuel, the primary
purposes of the closed fuel cycle are purification (removal of fission products) and
increasing the concentration of the fissile material (plutonium) to enable fully utilizing the
fuel and secondarily fuel assembly replacement. In many fast reactor systems with metallic
fuel, the primary purpose of the closed fuel cycle is fuel assembly replacement and only
secondarily purification—radiation damage to the fuel clad limits fuel lifetime. In a closed
fuel cycle where LWR SNF is used to startup fast reactors, purification and conversion of
the fuel form is required. Fast reactors require higher concentrations of fissile material than
do LWRs and have different fuel forms. There are several cases where reprocessing is required
for waste management purposes. In the 1950s the British built Magnox reactors for
electricity and production of plutonium for weapons. The fuel is a uranium metal fuel in
a magnesium-alloy clad that is chemically unstable in most environments. The SNF was
originally processed to recover plutonium for weapons purposes but today is reprocessed
to produce an acceptable waste form.
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