COMPLETE DOCUMENT (1862 kb) - OECD Nuclear Energy Agency

This is confirmed by the radiotoxicity balance. In any case, incineration of americium generates a large
amount of curium that must be processed to diminish the overall radiotoxicity of the waste. Moreover,
the SUPERFACT experiments in the Phénix fast reactor revealed considerable helium production in
targets containing americium, which could limit the permissible concentration.
Transmutation of pure curium in reactors is a highly problematic operation. Curium is the
most intensely radioactive of the actinides for both neutron emission and α-activity which interfere with
handling operations in fuel and target fabrication. In addition, the most abundant of its isotopes is
244 Cm, which decays with a half-life of 18 years to form 240 Pu and has a low fission cross-section which
makes it relatively unsuitable for transmutation in a reactor. Separating the curium after removal from
the reactor and placing it in interim storage for a sufficiently long period to allow significant radioactive
decay (only 2% of the initial 244 Cm remains after a century) should be considered as one among the
several options for handling curium. The daughter nuclides, mainly 240 Pu, could then be recycled. This
solution, however, involves the interim storage of large quantities of a highly radioactive element, and
will require further assessment from a safety standpoint in particular.
In addition to the minor actinides, three long-lived fission products were considered;
technetium ( 99 Tc), iodine ( 129 I) and caesium ( 135 Cs). The 135 Cs is found only in small amounts. Caesium
irradiation in reactors would be inefficient, as more 135 Cs would initially be formed by neutron capture
from lower isotopes than would be eliminated. Isotopic separation would be necessary for transmutation
of this element to be of any interest.
The destruction rate of 99 Tc and 129 I by neutron capture is low because of their low capture
cross-sections and particular resonances, which make it necessary to recycle these elements in a high
flux of appropriate spectrum.
Transmutation studies on long-lived radioactive wastes have been performed:
• to define acceptable recycling conditions, considering the effects of recycling on the core
properties (effects on reactivity and safety parameters) and on the fuel cycle (radioactivity
levels, neutron sources, and residual power);
• to assess the potential for radiotoxicity and mass reduction prior to disposal of long-lived
radioactive waste from nuclear reactors;
• to identify the data required for fuel cycle studies (isotopic composition, mass flux).
In this systems study, the recycling of the minor actinides is considered in thermal reactors
(standard PWRs loaded with UO 2 and MOX, High Moderating PWRs, etc.), in fast neutron reactors
(oxide, metal and nitride fuels), and in dedicated systems (accelerator-driven systems, and MA burner
reactors). The transmutation of fission products is also considered in thermal reactors, notably the
heavy water reactors (CANDU), in thermal high flux reactors and in fast neutron reactors.
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