Description Components Safety Radiological Safety Construction, Commissioning & Operation Summary Reactor Vessel Internal Inspection History of FBTR


    The reactor is protected against Transient Over-power Accident by feedback through negative temperature and power coefficients. Interlocks are provided to ensure that only one control rod can be operated at a time.  Insertion of two control rods is sufficient to shut down the reactor from the most reactive configuration.

    The Double Envelope of the primary sodium loops safeguards against Loss of Coolant Accident. Leak-before-break enables early detection before any leak escalation. In the event of any leak, sodium level in the reactor stabilises above the outlet pipes and circulation by pumps is maintained. Engineered safety is also provided for the very improbable incident of failure of the main coolant boundary and Double Envelope. If the leak is outside the reactor, complete draining of the reactor is avoided by the Siphon Break Pipe. Sodium level in the reactor in this incident stabilizes above the core but below the outlet pipe. Under these stagnant conditions, the reactor is cooled by circulating nitrogen in the annular space between the Reactor Vessel and its Double Envelope by circulating nitrogen between the Reactor Vessel and its Double Envelope. If failure of the main coolant boundary and Double Envelope occurs inside the Reactor Assembly, the leaked sodium collects in the steel vessel. There is a provision to flood the Reactor Vessel with sufficient sodium from a dedicated sodium flooding system to ensure submergence of the core. Biological shield cooling system removes heat from the steel vessel.

        Absence of any valve in the loops and operation of two pumps in parallel safeguard against Loss of Flow Accident.  During any offsite power failure, the reactor trips and primary pumps coast down due to the inertia of the flywheels, providing at least 30% of the rated flow, 40 s after power failure. The pumps are meanwhile taken over by the emergency diesel power. If a station blackout occurs, dedicated battery banks take over and run the primary sodium  pumps. Decay heat removal beyond this is possible by natural convection by opening the trap doors of the steam generator casing.

    Subassemblies are flow tested in nitrogen before loading in the reactor. Plugging of any subassembly in the reactor is avoided by providing radial entry sleeves in the Grid Plate. Plugging during operation is detected by a Plugging Detection Subroutine in the Central Data Processing System, which initiates alarm, Lowering of Rods and scram when any fuel subassembly outlet temperature exceeds its set limits. Void coefficient of reactivity being negative at all positions of the core, sodium boiling due to flow starvation of any subassembly adds negative reactivity. In the event of clad failure, cover gas activity alarm annunciates, followed by trip from Delayed Neutron Detection System.

    Provision of ramps at entry points of FBTR to prevent flood water entry, retrofitting of Unreinforced Masonry walls, installation of underground FRP diesel storage tanks, commissioning of 140kVA portable DG sets, etc. have been completed.