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– deterioration of the effectiveness of absorbents (control rods, soluble boron);
– an increase in the reactivity effect associated with the coolant void reactivity
coefficient.
For thermal reactors, it is also necessary to over-enrich the fuel, which is a heavy economic
penalty for a UO 2 fuel.
For thermal reactors, the influence of the minor actinides on the safety parameters depends
also on the moderation ratio. The variations are less pronounced in the case of the HMR (high
moderation reactor) so that a slightly higher minor actinide content can be accepted.
In heterogeneous mode, if the targets are placed in the core, the consequences are similar to
those in homogeneous mode. However if the targets are placed at the periphery of the core, the impact
on the physical properties of the core remains slight if the actinide concentration has been fixed so as to
limit the power release during irradiation to a value compatible with the cooling possibilities in order to
avoid local power peaking.
The effectiveness of transmutation is characterised by three values which are used in the
Tables II.5 to II.9.
• depletion/consumption:
(initial mass – final mass) of MA
initial mass of MA
• fissioned fraction rate:
mass of fissioned MA
initial mass of MA
• specific consumption:
(initial mass – final mass) of MA
energy produced by the reactor
2.3.2 Transmutation of minor actinides in thermal reactors
2.3.2.1 Present day PWR reactor [77-79]
Homogeneous recycling of minor actinides in UO 2 -fuelled PWRs
The Np and Am recycling was studied in a N4 type PWR, rated at 1 470 MWe, with UO 2 fuel
enriched at 4% 235 U, average burn-up of 47.5 GWd/tHM and fuel management of 1/5.
The irradiated fuel is cooled for 5 years before reprocessing. The minor actinides ( 237 Np or
Am) are mixed with the UO 2 fuel at a content between 0.5% and 5%. Then, the manufactured
assemblies are stored for 2 years before use.
The introduction of minor actinides, homogeneously mixed in the fuel, induces a reduction
both of the initial reactivity, caused by neutron absorption capacity, and of the loss of reactivity over the
cycle, due to the generation of more reactive isotopes resulting from their transmutation. To keep the
cycle management unchanged, these effects are compensated by over-enriching the fuel and modifying
the boron content.
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