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Figure II.14 MA-loading methods in fast reactor
Homogeneous (a)
Heterogeneous (b)
Mixed (c)
For an initial minor actinide content of 1%, the combination of these two effects leads to a
reduction of about 2% of the initial enrichment in plutonium, in order to obtain the same reactivity at
end of cycle as in the reference case without minor actinides:
• a reduction of the absorber negative reactivity due to the spectrum hardening;
• a reduction of the effective β value due to the lower delayed neutron yields for 237 Np and
241 Am than for 238 U (factor of approximately 5), a very significant drop in the Doppler
effect (approximately 10% for an initial minor actinide content of 1%) due to the spectrum
hardening and to the depopulation of the energy range corresponding to 238 U resonance
caused through a strong absorption by 237 Np and 241 Am;
• a significant increase in the reactivity effect resulting from 237 Np and 241 Am tends to
amplify the variation of absorption during voiding.
However, as regards sodium voiding, the most restrictive situation corresponds to the
end-of-life configuration, since the void effect coefficient increases throughout the cycle due to the
gradual accumulation of fission products. Conversely, the penalty due to the presence of minor actinides
is highest at the beginning of the cycle since their concentration decreases afterwards.
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