VI — 1. Introduction

General Electric (GE) has developed a new passive safe boiling water reactor called the economic simplified boiling water reactor (ESBWR), which is based on the previous simplified boiling water reactor (SBWR) design with some modifications of safety systems and the containment size relative to the reactor power [1, 2]. Major differences between the current boiling water reactors (BWR) and the ESBWR are in the simplification of the coolant circulation system and the implementation of a passive emergency cooling system. The ESBWR reactor core has a rated thermal output of 4500 MW*th.

The ESBWR relies on natural circulation and proven passive systems to improve safety, economics, and performance. In ESBWR concepts, the safety is accomplished by eliminating the recirculation pump, thus relying on natural circulation cooling. The coolant is circulated by natural circulation as a result of the density difference between the high void, two-phase fluid in the chimney and the exterior single-phase liquid in the downcomer. The tall chimney not only enhances the natural circulation flow, but also ensures the ample time for core uncovery before the emergency core cooling system (ECCS) comes in play. The emergency core cooling and containment cooling systems do not have an active pump injecting flows and the cooling flows are driven by pressure differences. Large volumes of suppression pool (SP) functions not only as a primary heat sink during the initial blow down, but also as coolant inventory to prevent the core uncovery through the gravity equalization lines.

By relying on natural circulation at operating pressures (7.2 MPa) and increased chimney height, the ESBWR has enhanced natural circulation flow inside the vessel. The schematic of natural circulation inside the reactor pressure vessel (RPV) is shown in Figure VI-1. The driving head of core flow is proportional to the core and chimney height and void fraction inside the downcomer shroud. In ESBWR, the differential water level is increased by approximately 8.2 m compared to the conventional BWRs. The greatly increased driving head enhances the natural circulation flow in the ESBWR compared to the conventional BWRs. Aforementioned ESBWR design features results in an average core flow per bundle over three times greater than that of a conventional BWR under natural circulation at similar bundle power. The use of natural circulation eliminates pumps, motors, controls, piping and many other components that could possibly fail.