Development of a secondary shutdown system for the 500 MW(e) PHWR

Pressurised Heavy Water Reactors (PHWRs) of advanced design are supposed to have two independent fast acting shut down mechanisms in order to cater to the present day safety requirement of defense in depth. The 500 MW(e) Indian PHWR is one such reactor which is being designed to have two independent shut down systems functioning on diverse principles. Of these, the second system referred to as the secondary shut down system introduces poison into to the moderator in the form of jets. This system is under development in Reactor Engineering Division (R.E.D.). The system conceived consists of a vessal containing pressurised helium connected to poison tanks through quick acting solenoid valves. The tanks are connected to horizontal injection tubes in the calandria. On actuation, gadolinium nitrate (GdNO3) solution from the tanks passes to the injection tubes which have a number of holes through which poison enters the moderator. An experimental facility was set up to measure the poison jet growth rate and the jet spread. A formalism to estimate the reactivity worth introduced by these jets has also been developed by R.E.D. The description of the system designed, the experimental studies and the calculation based on these experiments is presented in this paper. (author)