This shutdown system passively injects poison into the moderator by using the increased system steam pressure in the case of a low probability event of failure of the wired (sensors, signal carriers and actuators) shutdown systems. The AHWR has two independent shutdown systems, one comprising mechanical shut off rods (SDS-1) and the other employing the injection of a liquid poison into the low pressure moderator (SDS-2). Both these shutdown systems require the actuation of active signals for a reactor shutdown to occur. The proposed scheme of a passive shutdown is actuated by high steam pressure due to the unavailability of a heat sink, following a failure of the SDS-1 and the SDS-2. The schematics of a passive shutdown by MHT high pressure are shown in Fig. VI-7.

In the event of a pressure rise, high steam pressure opens a rupture disc and steam pressure is transmitted to open a passive valve connected to the pressurized poison tank; the reactor is shutdown by passive poison injection into the moderator. Following a reactor shutdown, the system reaches a hot shutdown condition due to effective passive decay heat removal by the ICs. Inadvertent poison injection is avoided by keeping the margin on a rupture disc with burst pressure above the expected pressure gradient after a reactor shutdown by the SDS — 1 or the SDS-2.