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


Reactor core

The core consists of 745 closely packed locations, with fuel at the centre, surrounded by nickel reflectors, thoria blankets and steel reflectors.The core is vertical and freestanding, with the subassemblies supported at the bottom by the Grid Plate and held on to the latter by collapsible hold-down springs. The subassemblies are hexagonally shaped. As per the initial design, the core was rated for 40 MW t and had 65 fuel subassemblies, three test steel subassemblies, six control rods, 143 nickel reflectors, 342 thoria blankets and 163 steel reflector subassemblies. In addition, there are 23 storage locations in the outermost steel reflector row.

Reactor Assembly

The Reactor Vessel houses the core and serves as a conduit for the primary sodium coolant flow through the core. Charging and discharging of subassemblies are done from the reactor top with the reactor in shutdown state. The reactor assembly consists of a cylindrical stainless steel vessel enclosed by its stainless steel double envelope. The interspace between main vessel and its double envelope is filled with inert gas nitrogen. The sodium inlet pipe joins the Reactor Vessel at the bottom and two sodium outlet pipes radially branch out of the vessel above the core. The reactor is closed at the top by Large and Small Rotatable Plugs, which serve as top shields and also enable access to the core locations for fuel handling. The Rotatable Plugs are cooled by nitrogen. Liquid Metal Seals of Tin-Bismuth alloy, backed with Inflatable Seals, isolate the reactor cover gas from the Reactor Containment Building atmosphere. The Liquid Metal Seals are frozen during reactor operation and melted during rotation of the plugs. The space between the Liquid Metal Seals and the Inflatable Seals, called the Interseal Space, is maintained in argon at a pressure higher than the reactor cover gas to prevent the leakage of active cover gas into the reactor building.

The Small Rotatable Plug houses the Control Plug which carries six Control Rod Drive Mechanisms and Core Cover Plate with thermocouples for monitoring the outlet temperatures of the fuel subassemblies. Ten neutron shields, each 24 mm thick, surround the core and minimize the incident flux on the Reactor Vessel. A set of Thermal shields are provided inside the Reactor Vessel to minimize the thermal stresses due to cold and hot shocks. The radial and axial shifts of the Grid Plate are monitored by two displacement measuring devices. A steel vessel with thermal insulation surrounds the Reactor Vessel. Radial shielding is provided by borated concrete and structural concrete. The borated concrete is cooled by water pipes embedded close to its inner periphery. The entire Reactor Assembly is suspended from the top, with the load taken by structural concrete. The reactor is closed at the top by the Anti-Explosion Floor, which is bolted to tie-rods anchored on the structural concrete the reactor building.

Sodium systems

FBTR uses liquid sodium as coolant, which is reactive with air or water. Maintenance of nuclear grade purity of the coolant is very important to minimize corrosion of structural materials and also avoid plugging of narrow flow passages in the reactor core and coolant circuits. This is achieved by controlling the routes for impurity ingress mainly through the inert argon cover gas and sodium pump seals and also by on-line purification and monitoring system to limit oxygen (<10 ppm), hydrogen (<2 ppm) and carbon (<30 ppm) impurities in sodium.

        Primary sodium is pumped into the reactor by primary sodium pumps and flows by gravity to the intermediate heat exchangers and then back to the pump suction. The intermediate heat exchangers are vertical, counter-flow heat exchangers and transfer heat from the active primary sodium to the inactive secondary sodium. Primary sodium flows on the shell side and secondary sodium on the tube side. The shell is fixed and the tube bundles are removable.

        Secondary sodium is pumped into the intermediate heat exchangers by secondary sodium pumps. After removing heat from primary sodium, the secondary sodium enters the steam generators. A surge tank is interposed between the intermediate heat exchangers and steam generators as a buffer against pressure wave transmission to intermediate heat exchangers during sodium-water reaction in steam generators due to any water leaks. The four sodium pumps are vertical, single stage centrifugal pumps with axial suction and radial discharge. Each pump has a fixed shell and a removable assembly comprising the impeller, diffuser and shaft. The shaft is supported by taper roller bearings at the top and guided by hydrostatic bearings at the bottom. The four sodium pumps have logged cumulative operation of 8,46,277 h without any major intervention.

For safety reasons, there are no valves in the primary sodium main loop. Flow control is by controlling the speeds of the pumps. The pumps are driven by dc motors and powered by Ward Leonard drives. Flywheels mounted on the Ward Leonard drives provide sufficient inertia to run the pumps to ensure that fuel clad hot spot temperature is within limits in the event of power failure. The pump drives are provided with emergency power supply from the station diesel generators and battery backup is provided for the primary pump drives to provide adequate flow for safe removal of decay heat.

    All the sodium capacities are provided with an inert cover of argon above the free sodium levels. Argon purity is maintained through NaK bubblers. Both primary and secondary sodium systems are provided with cold traps for sodium purification. Plugging indicators monitor sodium purity.

The steam generator modules are of once-through, counter-flow type, with sodium entering the shell side from top and water entering the tube side from bottom. The modules have a serpentine configuration, with evaporation and superheating occurring in a single pass. Due to the absence of blow-down, feed water chemistry is maintained within very stringent limits. The steam generator modules are housed inside an insulated casing. By opening the trap doors of the casing, it is possible to remove decay heat from the reactor by natural convection.

The entire primary sodium circuit is provided with a nitrogen-filled envelope called Double Envelope, designed to minimize the sodium level drop in the reactor in the event of any sodium leak. The annular gap between the Reactor Vessel and its Double Envelope is used for emergency cooling of the core during the very-low-probability incident of simultaneous rupture of coolant boundary and its Double Envelope outside the reactor. Nitrogen is also used for sodium fire fighting in the cells housing the primary sodium system in the unlikely event of failure of the main coolant boundary and its Double Envelope

Generating plant

The steam-water circuit consists of all the equipment in a conventional power plant. An on-line condensate polishing unit meets the stringent feed water chemistry requirements of the once-through steam generators. A cooling tower cooled by induced draft fans serves as terminal heat sink. The turbine is a single cylinder, 16 stage, non-reheat condensing turbine and is designed to produce 16.4 MWe with 72.5 t/h flow of superheated steam at 120 kg/cm2 and 480 ?C. The generator is rated for 19.3 MV A, 6.6 kV, 3φ, 50 Hz, 0.85 pf, 3000 rpm. The rotor is air-cooled with a closed circuit. The generator field is powered by a shaft driven exciter rated for 110 kW and 220 V (DC).

Instrumentation and control

    The reactor power control and shutdown are by six control rods. For shutdown, the rods are inserted into the core in two modes, i.e., lowering of rods, wherein all the rods are driven down by the respective drive mechanisms, and scram, wherein  all the rods drop down by gravity.

    A Central Data Processing System  processes  the core outlet temperatures of individual fuel subassemblies and generates reactor trip signals to limit fuel and clad hotspot temperatures.

    Sodium levels in the capacities are monitored by continuous and discontinuous level probes. Flows are monitored by permanent magnet type electro-magnetic flow meters. Sodium leak is detected by spark plugs, wire type and ionization types of detectors.

    Water leak into sodium in the steam generator at the incipient stage is detected by Steam Generator Leak Detection System to measure hydrogen in sodium in ppb levels.  Medium leaks are detected by monitoring the expansion tank cover gas pressure. Quick closing valves isolate the steam generators on the sodium and water sides in the event of large leaks  and rupture discs provided in the sodium headers relieve pressure build up.

Component handling

Fuel handling is done off-line using Charging and Discharging Flasks. There are special flasks for handling of control rod drives, irradiation devices, pumps and intermediate heat exchangers. For moving the active components between Reactor Containment Building and the adjacent Active Building, there is a Secondary Flask, which moves on rail carriages. Spent fuel is stored in air-cooled cast iron blocks

Auxiliary systems

Station auxiliaries comprise the Service and Circulating Water System for heat removal from main and dump condensers and other heat exchangers, fire water system, chilled water system, compressed air system to provide service, instrument and mask air, service argon system, service nitrogen system with a Pressure Swing Adsorption nitrogen plant and Horton sphere, Air-conditioning and Ventilation System, make-up Demineralised water plant and auxiliary boiler. New DM water plant was commissioned.