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pursued. Since 137 Cs and 90 Sr are the principal sources of decay heat in vitrified HLW, separation from
HLLW merits further attention.
However transmutation cross-sections of 137 Cs and 90 Sr in LWR and FR neutron spectra are
so small, and accelerator driven transmutation systems without accompanying fission reactions are so
expensive, that it is doubtful whether R&D efforts in transmutation of these nuclides are justified. The
development of specific nuclear reactions by for example “Adiabatic Resonance Crossing” might be a
new route which ought to be explored.
Other fission products with half-lives shorter or equal to about 30 years need not be
considered in a comprehensive P&T strategy.
For TRU nuclides the situation is different, since most of the “heavy metals” are linked to
each other in the radioactive series (4n, 4n+1, 4n+2, 4n+3). Important nuclides are 241 Pu, 243 Cm, and
244 Cm which are mother isotopes respectively of 241 Am, 239 Pu and 240 Pu, and these are very important
contributors to the total radiotoxic inventory. Despite their relatively short half-lives, these nuclides may
not be disregarded in a P&T scenario. Incineration or transmutation of these nuclides can greatly
modify the long-term radiotoxic inventory.
At the other extreme of the time scale lie the very long-lived fission products ( 135 Cs, 99 Tc, 129 I),
plus 237 Np and the natural actinides (U series) with extremely long half-lives. The partitioning of the
long-lived fission products is difficult, while some are accompanied by other isotopes of the same
elements, and isotopic separation of fission products cannot presently be considered.
In the case of 135 Cs the presence of natural 133 Cs might interfere with transmutation-depletion
reactions. A similar interference from natural 127 I should be investigated in case of 129 I transmutation.
Transmutation of 99 Tc can proceed without interferences since it is the only long-lived isotope without
any natural equivalent.
Most TRUs have long half-lives which are significant compared with the span of a human life,
a civilisation or even a geological period. In principle every P&T action which might decrease the
long-term impact of man-made actinides is beneficial, but there are limitations beyond the grasp of
mankind.
Three time periods can be considered as cut-off limits for further hazard assessment:
• 10 000 years, as administrative limit for a spent fuel repository (USA);
• 100 000 years as the time when uranium daughter products, 226 Ra and others, start to
dominate the radiotoxicity of the spent fuel;
• 1 000 000 years; the period significant in the evolution of the earth crust.
Ten thousand years is perhaps short from both scientific and technical points of view, but one
million years is definitely beyond any anthropological horizon. A cut-off period of 100 000 years seems
a suitable “logarithmic” compromise for the assessment of radiological effects.
Except for 237 Np, there are no significant TRU inventories which will survive the million-year
term, and beyond that period the natural radioactivity of uranium and its decay products become the
dominant radiotoxic inventory [188]. Not only natural uranium but also depleted and reprocessed
uranium need to be considered when comparing the long-term radiotoxicity of the different man-made
TRUs.
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