The Pebble Bed Modular Reactor Layout and Neutronics Design of the Equilibrium Cycle

The Pebble Bed Modular Reactor (PBMR) is a high-temperature helium-cooled, graphite moderated pebble bed reactor with a multi-pass fuelling scheme. The design, that displays the characteristics of Generation IV reactors, has reached a high level of maturity. Excess reactivity is limited by continuous refueling while adequate passive heat removal ensures an inherent safe design with no event with significant fission product release being possible. The core neutronic design for the 400 MWth PBMR is performed by the VSOP99 code system. The design is for an annular core with an outer diameter of 3.7 m and an inner diameter of 2 m shaped by the fixed central reflector. The effective cylindrical core height is 11 m. For the equilibrium core VSOP results show that the fuel sphere powers (maximum 2.7 kW) and operational temperatures (<1100 C) fulfil the design criteria. Adequate reactivity control and long-term cold shutdown are provided by two separate and diverse systems while the overall negative reactivity temperature coefficient is illustrated over the total operational range.