Negative coolant void reactivity in large fast breeder reactors with hydrogenous moderator layer

Placing a thin zirconium-hydride (ZrHu) layer between seed and blanket is very effective in reducing the coolant-void reactivity of water-cooled and liquid-metal-cooled fast reactors. The fast neutrons which are generated in the seeds at voiding are moderated through the layer. In the blanket the neutron absorption increases and fast fission decreases. The total neutron balance becomes negative at voiding. It is effective even when the layer thickness is 1 or 2 cm. Compared with the conventional homogeneous introduction of the moderator in a seed, the breeding ratio is not much reduced due to the locality of the moderation. The negative void reactivity is proved for the non-fiat, large-sized radial heterogeneous cores where the layers are placed between seeds and blankets.