Effective delayed neutron fraction for fluid-fuel systems

The problem of the effective delayed neutron fraction for subcritical multiplying systems in which the fuel is fluid and recirculated is considered. A consistent definition requires the introduction of a physically meaningful adjoint problem. One-dimensional diffusion calculations allow to study the role of the neutronic and hydraulic parameters characterizing the system and of the external source in determining the value of the delayed neutron fraction. A significant reduction of the role of delayed emissions is experienced in all cases considered.