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So, the initial content of minor actinides is limited by their influence on the Doppler effect and
on the sodium void effect which can create the difficulties in reactivity control on coolant voiding,
specially in a large core. In large cores however, penalties resulting from these effects can be limited by
a suitable design:
Mass balances
• preferential disposition of actinides in the outer part of the core in order to limit the effect
of sodium voiding;
• moderator introduction in the core in order to reduce the voiding effect and simultaneously
increase the Doppler effect.
Table II.7 gives the performance obtained by homogeneous recycling of the americium and
neptunium in an EFR type fast breeder reactor.
The incineration ratio of 237 Np does not vary either with its initial content nor with the size of
the reactor.
On the other hand, the incineration ratio of 241 Am is closely related to the initial content as this
isotope is produced in the core by radioactive decay of 241 Pu. The ratio is similar to that of 237 Np
(approximately 50%) for a 5% initial content, but decreases significantly with a reduced initial content.
The overall incineration ratios for all the minor actinides are lower, and the difference between 237 Np
and 241 Am is no longer significant.
The 238 Pu fraction in the total plutonium is approximately 4.5% for the EFR, with a 2% initial
content of minor actinide ( 237 Np or 241 Am). As initial minor actinide contents increase, the proportion of
238 Pu also increases. Solvent radiolysis problems may occur at reprocessing if the 238 Pu content of the
irradiated fuel exceeds 5%. The initial content of minor actinides must, therefore, be limited to 2.5% in
an EFR type core.
In the case of a 1 000 MWe-class FR core with mixed oxide fuel, MA transmutation has no
serious drawbacks in terms of core performance, provided that the homogeneous loading method can be
employed with a small ratio of MAs to fuel (~5 wt%) [81]. Since a 1 000 MWe-class LWR produces
about 26 kg of MAs per year, a MOX fuel fast reactor with 5 wt% MA loading can take up to the
output from six LWRs.
These values represent acceptable limits for the core safety parameters, in particular in
relation to the Doppler and sodium void effects. The acceptable content is thus higher than in a
UO 2 -fuelled N4 PWR.
Multiple homogeneous recycling of Pu+MAs present in spent LWR-MOX fuel after advanced
reprocessing was assessed [81] with respect to mass balance. The TRU components were assumed to be
incorporated into fresh fast reactor fuel in a ratio 66% depleted-U and 33% TRU (Pu+MAs). This was
submitted to multiple recycling in a sequence of 5 years irradiation and 12 years cooling. The results
show a steady increase of the 238 Pu and 244 Cm concentration in the fuel discharged after having reached
a burn-up of 150 GWd/tHM. In order to fully incinerate 1 tHM Pu+MAs from LWR-MOX, 15 FBuR
sequences are necessary. The overall Pu+MAs destruction during the multiple recycling operations
amounts to 88.4% which leads to an actinide reduction factor of about 12 in a period of 255 years.
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