ASTRID operation procedures to comply with grid regulation and a plant lifetime up to 60 years
Résumé
The GEN4 type ASTRID reactor acronym stands for Advanced Sodium Technological Reactor for Industrial Demonstration. Amongst its functional specifications there are two main items related to plant operation considerations The ASTRID capability to accommodate the requirements of the French grid regulator and a reactor designed for a lifetime up to 60 years.This paper presents the advances made on these two items which are innovative and mandatory for a fast reactor fleet development.The French grid regulator requires that reactors not only have to participate to the grid frequency adjustment (which varies due to the whole grid production to consumption ratio balance) but also be able to switch to the house load operation whenever the regulator orders it. The frequency adjustment of the grid requires a storage capacity corresponding to 7% of the maximal reactor power. Therefore two options could be envisioned for the plant control The first option deals with a nominal reactor power operation corresponding to 93 % of the full reactor capacity and the power to the grid would be regulated by some of the reactor control rods. The second option would be a steady full 100% reactor power operation and the electric power modulation delivered to the grid would be mastered by the tertiary circuit. For both options this paper presents the consequences expected on the plant control and safety criteria.The second main item deals with a 60 years lifetime. This implies a design optimization for internal components which are definitely not replaceable (primary and internal vessel, core support structure) and those designed to be exceptionally replaced (above core structure, primary pumps and intermediate heat exchangers). The mechanical stresses on these structures are due to thermal loads accumulated during all transients expected during the whole life of ASTRID. The relevant transients to take into account are the two main shutdown procedures The emergency shutdown (SCRAM) and the rapid shutdown. The emergency shutdown is related to safety purposes and consequently it has been decided to maintain a 100% primary flowrate during the sequence which implies an increase of the thermal stresses on the above core structure compared to a sequence during which the primary flowrate is reduced. Therefore, the paper presents a study which has been performed to optimize the thermal stresses on main components due to these two types of reactor shutdowns. Amongst the parameters considered, the kinetics of the secondary flowrate decrease and control rods insertion are essential.
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