RRFM 2009 Transactions - European Nuclear Society

2. Advanced nuclear fuel cycles
The rationale for the research and technology development programs in the field of ADS must
be seen in the context of the efforts to establish innovative nuclear fuel cycles including
partitioning and transmutation (P&T).
Presently, industrial maturity can be claimed for two fuel cycle strategies, i.e. the "Once
Through Fuel Cycle" (OTC), and the "Reprocessing Fuel Cycle" (RFC) in which plutonium and
very limited uranium quantities are being recycled. It is helpful to recall some key data that set
the stage for any fuel cycle (waste management) discussion: worldwide, the annual spent fuel
discharge is in the range of 10’500 – 11’000 t heavy-metal (HM), while the industrial
reprocessing capacity amounts to ~5’000 t HM [3]. Hence, less than ½ of the discharged spent
fuel can be processed. Worldwide, the cumulative inventory of stored spent fuel is estimated to
be ~130’000 t HM, and the amount of reprocessed spent fuel is estimated to be ~70’000 t HM.
The latter inventory has been transformed into high-level waste (HLW) and spent light water
reactor (LWR) mixed uranium-plutonium oxide (MOX) fuel. Considering the relatively low
uranium ore prices, this situation is expected to continue over the next few decades (the
cumulative inventory of stored spent fuel could reach 192’000 t HM by 2010). Therefore, it is
likely that the need for repository space will increase accordingly. Taking Yucca Mountain
(63’000 t HM capacity) as reference repository, the present worldwide inventories would require
two repositories for the spent fuel, and one for the HLW. For the USA alone (OTC strategy),
assuming a life time extension of the present nuclear reactors to 60 years, and no new
reactors, the capacity of Yucca Mountain will be exceeded by ~2050. Given the strong public
opposition to the construction of geologic repositories, it is understandable that over the last
decade or so, in various countries and at an international level, more and more studies have
been carried out on advanced and innovative waste management strategies aiming at reducing
the amount of long-lived radioactive waste through transmutation in fission reactors or ADS. In
several IAEA Member States, P&T is being revisited with the goal of reassessing its merits and
investigating new approaches that could be followed in implementing this innovative fuel cycle
and waste management option.
3. Status of ADS research and technology development
P&T is a complex technology that implies the availability of advanced reprocessing plants,
facilities for fuel fabrication of transuranics (TRUs), and irradiation facilities beyond the
presently existing nuclear reactors. For the major part, partitioning consists in extending the
current reprocessing techniques: in addition to uranium, plutonium and 129 I, also minor actinides
(neptunium, americium and curium), and, possibly, also long-lived fission products ( 99 Tc, 93 Zr,
135 Cs, 107 Pd and 79 Se) would be extracted from the liquid high level waste. This technology,
could, to a certain extend, be extrapolated on the basis of decades-long industrial experience in
Europe and Japan. Transmutation, on the other hand, requires fully new fuel fabrication plants
and irradiation technologies and their implementation on an industrial scale. The use of existing
nuclear reactors as transmutation devices results in modest incineration and transmutation
yields, and, more importantly, is limited by both safety and operational considerations.
Therefore, new concepts, i.e. dedicated fast reactors, and sub-critical systems (ADS and even
fusion/fission hybrids) have been proposed as incineration/transmutation devices.
ADS rely on the availability of a hard neutron source produced by the spallation process
induced by a high-energy proton beam impinging on a heavy nuclide target. While ADS present
an attractive potential, various specific economics and technological issues remain to be solved
(e.g. capital costs, additional energy consumption, accelerator reliability, various physics and
material science issues linked, e.g. to nuclear data, fuel performance, heavy liquid thermal
hydraulics, etc).
The assessment of the potential of the ADS technology must consider the fact that the
spallation neutron source is much less effective than the fission one. Basic physics
considerations comparing spallation and fission neutron sources lead to the conclusion that
transforming the energy of one fission event (which yields roughly 200 MeV of energy and 3
hard neutrons) into spallation results in approximately 20 MeV of energy and, in an optimum
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