Application of Method of Characteristics to Simulate Water Hammer in Emergency Boron Inyection System of CNAI

Abstract

Within the Life Extension Program (PEV) of the Atucha I Nuclear Power Plant, the layout of the Boron Injection System (TB) has been redesigned to reduce personnel exposure during routine maintenance. This layout redesign needs the calculation of the forces developed in the pipes and their joints, which will later be used to determine the support requirements for the system. The system includes a rupture membrane, which breaks upon reaching a certain pressure difference, allowing the flow of heavy water with boric acid into the moderator tank. The transient event of membrane rupture and the injection of the first drop of boron occurs in less than 270 ms, resulting in very rapid pressure changes and pressure waves propagating through the piping. To calculate these forces, the internal PWC software, based on the Method of Characteristics, was re-drawn, the modeling of the system’s rupture disk was revised, and simulations were performed for two possible triggering scenarios: a LOCA 2A accident and a spurious shot. Velocity and pressure fields were obtained for the simulated cases, and from these, the forces developed in the pipes and their joints were calculated as a function of time.