A startup procedure for natural circulation boiling Water Reactors

A start-up procedure for natural-circulation Boiling Water Reactors is illustrated. The procedure, developed in order to avoid flashing-induced instabilities at low pressures and low powers, does not require any additional system. The reactor vessel is pressurized while steam is produced outside the core and riser sections. In this way, the firsts stage of the pressurization takes place with the reactor operating in single-phase natural-circulation conditions, thus without occurrence of flashing-induced flow oscillations. At the same time, a smooth steam production and pressurization of the system is achieved. A simple analytical model is applied to the ESBWR of General Electric to have a first impression on the order of magnitude of the parameters involved and to verify whether the start-up of the reactor can be achieved in reasonable times. The feasibility of the procedure is then studied in details by means of the ATHLET code (Lerchi and Austregesilo, 2003), developed by GRS (Germany). This code has been successfully qualified against low-pressure flashing-induced instabilities within the framework of the EU project NACUSP