TRS435_web

chemical properties, which can be managed in a safer way for long term intermediate
storage or disposal (e.g. americium and curium).
A proper conditioning, resulting in a reduction of the solubility, is the
most obvious approach. Incorporation into a stable matrix reduces the
migration risk of accidental dispersion during the storage period and migration
in the repository.
However, major changes in the safety situation will arise in a P&T fuel
cycle, due to the transition to new fuels, which are generally much more
radioactive both before and after irradiation, with higher levels of residual heat
and helium production resulting from intense alpha activity.
Transmutation requires new fuel fabrication plants and irradiation
technologies, which must be developed and implemented on an industrial scale.
Existing nuclear power plants could in principle be used for transmutation, but
many practical obstacles may arise (e.g. interference with the daily operation of
the plants). New irradiation facilities such as dedicated FRs, accelerator driven
transmutation devices and even fusion reactors have been proposed for transmutation–incineration
purposes.
One of the main objectives of P&T has always been to reduce the long
term hazard of spent fuel or HLW, this hazard being associated with the
radioactive source term itself. In contrast, in the management of waste the long
term radiological risk is stressed; this long term radiological risk is a
combination of the potential hazard and the confining properties of the
geologic media. The measures that have to be taken for hazard reduction are
very different and much more fundamental than those for risk reduction.
For a desired reduction factor in total radiotoxicity of 1000, a target value
of 99.9% has to be achieved for the recovery of each of the individual TRUs
during the reprocessing–partitioning operations. With a 100-fold reduction in
the TRU content the reduction in radiotoxicity could theoretically be reached
after less than 1000 years.
It is obvious that a 100-fold reduction of the TRU mass in the waste
compared with the OTC cannot be achieved in a single pass through a reactor.
Multiple recycling will hence be necessary. In fact, the ideal P&T system should
have a fuel cycle that is fully closed for the TRUs. Only the fission products
would enter the waste stream, together with a 0.1% fraction of the cumulated
and/or recycled TRU fraction. Given the limitations of irradiation facilities,
such a system must be operated for many decades before equilibrium is
reached in the core composition and in the radiotoxic output of the TRU losses.
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