Safety systems, built on state-of-the-art tech-nology, are essential for achieving acceptable levels of plant safety to minimize hazards to the reactor and the general public. The second shutdown system (SSS) as an engineered safety feature and a part of the reactor protec-tion system (RPS) is a means for rapidly shutting down a nuclear reactor,keeping it in a subcritical state and serving as a backup to the first shutdown system (FSS). In this research, one SSS with two types of optimum chamber designs is proposed that take into account the main current characteristic features of the Tehran research reactor with improvements over earlier designs. They are based on a liquid neutron absorber injection that is preferably differ-ent, diverse, and independent from the FSS based on the rod drop mechanism. The major design characteristics of this SSS with two different chambers were investigated using MCNPX 2.6.0 code. The performed calculations showed that the designed SSS is a reliable shutdown sys-tem, assuring an appropriate shutdown margin and injec-tion time, with no significant effects on the effective delayed neutron fraction while causing minimal variations to the core structure.Further,the reasonable financial cost and the prolongation of the operation cycle are additional advantages of this design.