document: An increase of neutron leakage from the core region can be achieved through modifications in the core geometry (usually by adopting a pan-cake geometry of the active core region at the expense of the general neutron economy). Extensive studies determined a set of core design modifications that optimised the total sodium void reactivity (becoming less positive). Among the most efficient design solutions identified is an enlarged sodium plenum above the active core region in combination with an absorber layer above the sodium plenum (to reduce neutron backscattering from the reflector region above the plenum). Fig. 19 shows the combined effect of different upper plenum thicknesses of the absorber and boron layers. It can be observed that the sequential increase of the layer's thickness converge to an asymptotic value of reactivity reduction slightly over 800pcm. The pair of values selected was 60cm for the sodium plenum and 30cm for the boron layer. These modifications implied a considerable increase in the sub-assembly length that was compensated by reducing the upper axial reflector width (Sun et al., 2013).
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