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3.1 Core design constraints
In practice, the design of a TRU or MA burner core, like that of any reactor core, is not only
constrained by the above-mentioned basic neutronic criterion, but also by performance and safety
parameters, such as the reactivity swing during burn-up, coolant void reactivity effect, Doppler
coefficient, effective delayed-neutron fraction, etc. In particular, for a sodium-cooled fast reactor core,
the substitution of normal MOX fuel by TRU- or MA-dominated fuel has an unfavourable influence
on several of these parameters, and this is one of the reasons for the recent revival of various fast and
thermal reactor concepts which were studied in the past, but have not been introduced as commercial
systems. For example, the (positive) coolant void effect in sodium-cooled fast reactors, which
deteriorates in a minor actinide burning regime, can be mitigated by substituting the sodium by lead,
or even eliminated by substituting the liquid metal by a gas coolant.
To ensure that a critical burner core performs satisfactorily and has acceptable safety parameters,
it is usually necessary to blend the TRU or minor actinides with the fertile materials uranium or
thorium. However, blending reduces the transmutation effectiveness of the system. In this context,
accelerator-driven systems offer interesting additional parameters of freedom by removing the
criticality constraint and increasing the safety margin to prompt criticality. The latter feature is
particulary important for MA burners, which are difficult to control as critical systems because the
effective delayed-neutron fraction is only about half of that of a normal fast reactor.
In response to the new core design issues raised by P&T and the increased interest in advanced
reactor technology in general, government and industry funded design teams in Europe, the Far East
and the USA are currently spending a considerable effort on the optimisation of a broad range of
advanced reactor designs featuring both critical and accelerator-driven cores.
3.2 Transmutation effectiveness
Various, sometimes conflicting definitions for transmutation effectiveness, usually based on the
minor actinide balance of a particular core, can be found in the literature (see, for instance, [6]). For a
system with fully closed fuel cycle, the difficulty of defining a core-specific transmutation effectiveness
is circumvented by defining an “overall transmutation effectiveness” as the relative content of the top-up
fuel in transuranic and minor actinides, M B /M T , i.e. the already discussed quantity τ.
It is interesting to notice that the overall transmutation effectiveness does not depend directly on
the choice of the neutron spectrum, the fuel type and the coolant, but is governed by the abovementioned
performance and safety constraints. For a critical TRU burner based on liquid metal
technology, τ is smaller than about 0.5, and in the case of homogeneous MA recycling in a EFR-type
fast reactor τ is less than 0.1. The possibility to operate sub-critical MA and TRU burner cores with a
fertile-free top-up fuel and hence 100% overall transmutation effectiveness, i.e. τ = 1, is probably the
most important advantage of accelerator-driven systems.
3.3 Neutronic transmutation effect
Using the same notation as before, the radiotoxicity reduction relative to the top-up fuel, R T (t), is:
R T (t) = R N (t) M T / M W
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