Combined Space-Time Effects of Xenon Transients and Reactor Control During Load Following in a Pebble-Bed High-Temperature Reactor

Extending the previously presented steady-state characteristics of the pebble-bed high-temperature reactor with the new  “once through then out” (OTTO) fuel concept, we have investigated its load-following properties, i.e., the slow core transients in connection with adjustments of the reactor output to meet the power demand. We consider neutronic and thermodynamic characteristics of this strongly asymmetric core design during the related xenon transients where further novel features of this reactor type emerge. Our two-dimensional analysis considers the extremely space-dependent core conditions of a medium sized OTTO pebble-bed reactor during the transient, where the inhomogeneous xenon redistribution and a rod control acting only in the top reflector compete with each other to influence the axial power-density profile. The resulting variations in the maximum fuel temperatures are remarkably small, which reemphasizes the favorable thermodynamic properties of this new reactor concept.