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Core characteristics
above:
The following conditions are assumed for neutronic assessment in the two modes defined
• the proportions of minor actinides, 237 Np, 241 Am, 243 Am and 244 Cm in the material
recovered from the LWR spent fuel, are 54, 23, 17 and 6 wt%, respectively;
• a quantitative Pu recovery and 98% of minor actinides to be recovered in reprocessing.
• the core is composed of inner and outer regions with a combined thermal output of
1 600 MWt;
• the refuelling interval is 15 months with 3 batches, and the refuelling time is 60 days;
• the average discharge burn-up is ~90 GWd/tHM.
The performance parameters [83] of the FBR core at equilibrium recycle in the two modes
described above are summarised in Table II.10 with the non-recycle core included for comparison. Here,
the feed plutonium comes only from LWR and is used once-through. In the self-recycle core, the minor
actinides and rare earths amount to 0.6 and 0.3 wt%, respectively, at equilibrium recycle.
In the minor actinide-enriched mode, the following significant characteristics appear:
• the burn-up reactivity swing becomes smaller with the increase in minor actinide
enrichment;
• the minor actinide composition shifts to higher masses with recycling;
• the fast fission contribution causes an increase in void reactivity;
• the minor actinide-enriched core has a smaller Doppler constant due to the reduced
uranium content;
• at full-power, the power coefficients which relate to fuel, cladding, structural components
and coolant, and to the Doppler effect are reduced.
Taking the decreased β eff of minor actinide-enriched core into account, it is quite difficult to
use metal fuel with minor actinides of higher than a few percent.
Transmutation performance
In the self-recycle core, the minor actinides amount to 0.6 wt% at equilibrium. In an FR with
2 wt% minor actinide-enriched fuel, the transmutation rate is 31% at each refuelling the reactor can
consume the minor actinides and plutonium recovered from 2.5 LWRs per year.
Effect of lanthanide nuclides
In nuclear reactors, lanthanide nuclides (i.e. rare earth isotopes) are created by fission of
actinides. But transmutation rates in fast reactors are not changed substantially by their presence
[86,87]. Rare earth fission products have capture cross-sections that cause them to act as a poison.
With low distribution factors there will be large burn-up reactivity loss. The weight ratio of the total
rare earth fission products to minor actinides in pyrochemical process is ~1. The reduction of reactivity
due to rare earth fission product must be compensated by increased plutonium concentration [86].
Therefore, the increase of rare earth fission products content causes the decrease of Doppler coefficient
and β eff [87]. But the transmutation rate is not changed markedly by the content of rare earth fission
products.
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