ThorEA - Towards an Alternative Nuclear Future.pdf

thorium oxide mixed fuels is accessible. Additionally there
is a need for a large-scale thorium-fuel irradiation study,
particularly as thorium fuel elements will spend substantially
longer in the high fast neutron fields of the ADSR core (years)
than uranium based fuel spend in the core of a thermal reactor
(months). The materials research associated with these issues
could be performed by collaborative programmes between UK
HEIs and nuclear fuel manufacturers; it will require specialists
in metallurgy, chemistry, physics and mechanical engineering.
Collaboration with the National Nuclear Laboratory and
exploitation of their extensive experience and world class
facilities would be of particular value.
Coolant selection
Much of the research hitherto undertaken to date ADSR systems
has assumed the use of a either lead or lead-bismuth coolant.
The use of such a coolant has many potential advantages;
however corrosion of sub and superstructures remains a concern.
In Russia Pb-Bi coolants have been successfully deployed in the
reactors of nuclear submarines.
The resolution issues associated with molten metal coolants
will require expert metallurgical input from HEIs and other
research organizations; it will require an understanding of the
mechanisms leading to the cause of corrosion and proposals to
overcome them. The eventual solution should be demonstrated
in near operational conditions. The R&D necessary to overcome
these issues could in part be carried out in collaboration and
participation with, in the European Spallation Source (ESS)
project, for which the search for new liquid metal target systems,
as an alternative to mercury, is already underway.
It is considered possible that the molten metal coolant and
the molten metal spallation target could be contiguous,
simplifying the circulation system and the core design. Such
a system would be particularly advantageous if the multiple
spallation target option proved optimal. Complex fluid
dynamics calculations and simulations will be required to
demonstrate the efficacy and feasibility of such a system.
Reactor core principles
The construction of the world’s first thorium fuelled ADSR
will set new standards in nuclear reactor technology, not
least because of the sub-critical operation, the coupling of
a monolithic external device to the core (i.e. the accelerator
complex) and the deployment of thorium fuel in what
is essentially a fast reactor mode. Considerable work on
calculation, simulation and modelling of the configuration
and performance of the reactor core will therefore be required
ahead of deployment in order to qualify and quantify both
performance and safety, and to establish licensing protocols.
Much of the basis upon which future performance is predicted
has been undertaken by Prof Carlo Rubbia at CERN. Further
work will be needed to confirm that the initial principles and
suppositions are valid, and also to investigate and select the
optimum fuel configurations and coolants etc to optimise fuel
burn-up and minimise waste. Members of ThorEA are already
undertaking such studies at The University of Cambridge.
Spent fuel management
ADSRs afford the potential of using alternative fuel
configurations with the benefit of providing reduced
production of long-lived, high level waste and extended
fuel burn-ups without the need for fuel movements during
operation. However it is in the long-term operation of the
reactor that the real benefits of waste reduction become
compelling. To fully exploit these advantages repeated
re-processing of the fuel will be necessary. Re-processing
technologies for uranium/plutonium fuel already exist
however, technologies to deal with thorium fuel mixtures
will need to be developed.
For both open and closed fuel cycles and as opposed to
uranium, the back-end of the thorium fuel cycle presents
several unique challenges that need to be resolved at
a commercial, rather than demonstrational, level. For
example, as part of this process the isotopic inventory
of spent thorium fuel needs further investigation. The
challenges in the chemistry of the back-end of the thorium
fuel cycle offer several R&D opportunities for the UK:
development of alternative extraction;
development of two stream (for mixed Th-U fuel) or
three stream (for mixed Th-U-Pu fuel) reprocessing
routes for recovery of Th, U & Pu from irradiated fuels;
management of transuranic waste and handling and
conditioning high level liquid wastes;
development of non-aqueous reprocessing techniques.
Recognising that the utilisation of thorium requires an initial
conversion of fertile Th-232 into fissile U-233 (the so-called
breeding phase), there are two conceivable strategies to
manage the spent fuel:
an open fuel cycle, whereby spent fuel is disposed of,
without or with separation;
a closed fuel cycle effected by reprocessing the spent
fuel, separating elements and re-fabricating new fuel
to be returned to the reactor.
The open fuel cycle is arguably the most proliferation-resistant
strategy to deal with nuclear fuel (both fresh and spent),
and avoids the complications associated with partitioning
and re-fabrication of highly radiotoxic actinide fuels. The
thorium-fuel-cycle spent fuel is particularly radiotoxic due
to the presence of U-232 (a strong gamma emitter), thus
hindering the handling of waste fuel but also creating a natural
proliferation-barrier.
For thorium-fuelled ADSRs, an open fuel cycle could be realised
either by seeding the fuel with waste plutonium to create an
active fuel rod prior to its insertion into the reactor; alternatively
a pure thorium fuel rod may be irradiated for 6 to 12 months to
breed the fissile material and make the road active.
A report prepared by: the thorium energy amplifier association 59