A second shutdown system (SSS) is designed for the Tehran Research Reactor (TRR) completely independent and diverse from the existing First Shutdown System (FSS). Given limitations, specifications, and requirements of the reactor, the design of SSS is based on the injection of liquid neutron absorber. The plan has the ability to satisfy the major criterion of required negative reactivity worth, to transfer the reactor to subcritical state in needed time, with necessary shutdown margin and for the required duration. Design calculations are performed using the stochastic code MCNPX2.6.0, deterministic code PARET and Pipe Flow Expert software. The ORIGEN2 code and HotSpot health physics code are also used for simulation of environmental pollution release. The SSS chambers cause a decrease of about 5% and 15% in total and thermal neutron flux, respectively. To demonstrate the SSS role in enhancing reactor safety, the probable accident of core meltdown is investigated. As a consequence of this accident, the radioactive pollution in and out of reactor containment is released. Without existing the SSS and in case of failure of FSS, the residents within 58000 m2 of the reactor perimeter would receive about 1 mSv which is more than the annual limit of absorbed dose for the community.
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