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2. TRANSMUTATION
2.1 Introduction
Nuclear power generation is inevitably accompanied by the formation of neptunium,
plutonium and higher actinides from uranium (see Annex E). The long half-lives of some isotopes of
these elements, and of a few fission products, give rise to concern about possible long-term radiological
effects.
When plutonium is multi-recycled, the minor actinides will dominate the long-term
radiotoxicity of the wastes. The reprocessing and separation processes give rise to a mixture of
Am+Cm+lanthanides (or rare earths) which is difficult to further separate, because of the similarity of
these elements’ chemical properties. The impact of the separation performance on the americium
transmutation should be investigated. Since reprocessing losses of plutonium are low (about 0.1%)
compared to those expected for minor actinides, the latter will account for the major part of the
long-term radiotoxicity of the wastes. In these conditions, the complete recycling of plutonium offers no
advantage from the standpoint of reducing potential radiotoxicity, unless the minor actinides are also
reduced with a view to minimise the radiotoxic inventory of the wastes to be stored.
Three minor actinide elements to be transmuted in reactors are considered: neptunium,
americium and curium.
The activity of neptunium and americium is low enough to consider them for recycling in
reactors without prior interim decay storage. Two options are available for transmutation: in the
homogeneous mode, the element is mixed in a suitable chemical form with the standard reactor fuel; in
the heterogeneous mode, the element is placed in the reactor separately from the fuel in a device known
as a “target”. The choice between these options depends on the behaviour of the particular nuclide in the
reactor and in the fuel cycle.
Two other aspects of the minor actinides must be taken into account: the effect of their
presence on reactor operation – primarily from a safety standpoint – and their transmutation yield. The
principal core characteristics liable to be affected by the presence of actinides are the reactivity and the
safety parameters (transient over-power and loss-of-coolant incidents).
The initial reactivity value is modified, as is the rate at which it diminishes. A positive value
must be maintained throughout the reactor cycle. The initial fuel enrichment in fissionable isotopes ( 235 U
or 239 Pu) or the absorber content of the core may be modified to compensate for the variations compared
with the standard core resulting from the presence of minor actinides for incineration.
Recycling of the minor actinides (neptunium and americium) is possible in thermal reactors
and in fast neutron reactors, either in homogeneous or heterogeneous mode. The mass balance shows the
advantage of a fast neutron spectrum over thermal spectrum in allowing a higher burn-up to be reached.
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