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whereas the proportion of rare earths is low, and therefore has little influence, hence the increase in the
initial reactivity.
The lanthanides have a low average absorption cross section (about 0.21 barn), which
explains their limited impact on initial reactivity. However, as they are not fertile, their introduction
results in an increase in the loss of reactivity during irradiation owing to competition with 239 Pu
formation.
In order to maintain the same fuel management as in the reference case, it would be necessary
to increase the plutonium enrichment if the decontamination factor between Am and lanthanides falls
below 15. If DF>15, the effect of lanthanides is compensated by the influence of the minor actinides.
The increase in the void effect (∆ρ void ) is +2.4% per vol.% of minor actinides and +0.89%
per vol.% of RE in the core. The reduction of the Doppler effect (∆ρ dopp ) is 5.3% per vol.% of minor
actinides and 1.3% per vol.% of RE in the core. Lower DFs are permissible for recycling in fast
reactors than in PWRs.
Heterogeneous recycling of minor actinides in fast reactor [77]
Mass balances for heterogeneous recycling
Table II.8 shows the calculated transmutation performance in heterogeneous recycling mode
with target positioned in the first row of the radial blanket of an FR (see Figure II.14 (b)).
Specific consumption values range from 4 to 15 kg/TWhe, which is encouragingly higher than
in standard PWRs. On the other hand, the fission rates are very limited. Any improvement in these rates
requires either a lengthening of the irradiation time, which may be limited by the behaviour of the
targets, or a multi-recycling mode.
Americium recycling
FRs.
Table II.9 shows a comparison between the different alternative strategies for recycling Am in
The three cases correspond to the irradiation of targets placed in the first row of the radial
blanket of the EFR-type core. The targets have identical dimensions to the fertile UO 2 assemblies. UO 2
is replaced by americium mixed with a matrix, as required. The cases differ in the initial mass of
americium loaded. All the available space is occupied by americium in case 1. In cases 2 and 3, the
targets are loaded with mixtures of americium and inert matrix, Al 2 O 3 in this case. The common limiting
criterion is the radiation damage to the cladding materials which is set at 200 dpa NRT for very
radiation resistant steel types.
Depletion and fission percentages increase sharply with irradiation time when the initial
content is reduced. In contrast, the absolute value of specific consumption decreases sharply, depending
on the initial mass loaded.
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