Preliminary design and study of an innovative option for gas fast reactors

Overview & Synopsis

The Gas cooled Fast Reactor (GFR) is one of the six reactor concepts selected in the framework of the Generation IV forum. The main characteristics of the CEA GFR concept are a 2400MW core based on a ceramic pin type fuel as a reference, with an inlet temperature of 400°C and an outlet temperature of 780°C. The power conversion system is based on an indirect cycle with helium on the primary circuit, a Brayton cycle with a mixture of nitrogen and helium on the secondary circuit and a steam cycle on the tertiary circuit. In accidental situations, the use of the gas coolant circulation as the main way to remove the decay heat has been selected. A specific system (DHR system) has been designed: it consists of three loops (3*100% redundancy) in extension of the pressure vessel, equipped with heat exchangers and blowers. In the current preliminary viability studies, GFR primary compression system relies on three axial blowers (operating in parallel) driven by 3 electrical motors. The present study concerns an alternative design of the primary compression systems, assuming that the 3 primary compressors are driven by the 3 turbogenerators of the secondary circuits. This new system requires that the 3 shafts connecting the turbines and the compressors of the secondary circuits are also connected to their corresponding primary blowers, via longer shafts crossing the primary circuit vessel. This new cycle is the only new element of complexity in this alternative design. This fact should be put in regards of the advantage of no requirement for external energy for driving the compression system (excepting for start-up) and of the safety advantages: suppression of the Loss of Flow Accident due to a primary motor failure and possibility to use this new cycle to improve the grace delay of the reactor using the turbomachinery to drive the primary blowers during the beginning of any accidental situation. This paper first presents the main differences of design compare to the reference cycle. Then, transient behaviour in an accidental situation is computed with the CATHARE 2 code and analyzed. Finally the main advantages of this new cycle at a safety pointy of view are discussed.