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SEMINAR REPORT ON NUCLEAR POWER PLANTS IN INDIA (Heavy Water)


SEMINAR REPORT ON
NUCLEAR POWER PLANTS IN INDIA (Heavy Water)
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Index

  1. Introduction        

  1. Moderator System

  1. Heat Transport System

  1. Shutdown System

  1. Steam Turbine

  1. Turbo Generators

  1. Cooling Tower

  1. Primary Heat Transport System

  1. Auxiliaries of PHT system

  1. Turbine

  1. Turbine Oil System

  1. Moisture Separator and Reheater System

  1. Feed Water System

  1. Main Components of Compressors

  1. Conclusion

  1. References

















List of Figures


Fig.1. Nuclear Power in India                                                                5

Fig.2. Schematic Diagram of a Nuclear Power Plant                           6

Fig.3. Nuclear Fission of Uranium                                                        7

Fig. 4. H2O and D2O                                                                              8

Fig. 5. Control Rods in Reactor Building                                              9

Fig.6. A Turbo Generator                                                                      11

Fig.7. Natural Draught Cooling Tower                                               12

Fig.8. Induced Draught Cooling Tower                                               13

Fig.9. Turbine                                                                                                  14

Fig.10. Moisture Separator and Reheater                                                 15

Fig.11. Feed Water System                                                                          17

Fig.12. Compressor                                                                                        19

Fig.13. Intercooler                                                                                         20




Fig. 1. Nuclear Power in India



Introduction



Fig.2. Schematic Diagram Of a Nuclear Power Plant




   Nuclear Power Plants generate electricity from nuclear heat. A Thermal Power Plant burns coal to produce heat. The heat converts water to steam, which in turn drives the turbine and the generator to produce electricity. The main difference between the Nuclear Power Plant and the Thermal Power Plant is that Thermal Power Plant gets heat energy by burning coal whereas the Nuclear Power Plant produces heat by the process of fission of Uranium nuclei. The heat produced is used to convert water into steam. The steam runs the turbine and generator and the electricity is generated by the generator which is then transmitted to grid.
   The fission energy is transported from the fuel bundle with the help of high-pressure heavy water called Primary Heat Transport (PHT). The PHT System takes away the heat from the fuel bundles and exchanges this heat to the Dematerialized water (Feed Water) into the steam generator and the steam at a pressure of 40-48 Kg/cm2 is generated in the steam generator.      This steam is then transported through pipes to turbine hall where the turbine hall where the turbine and the generator are installed. This steam is then admitted to the turbine, which runs at a speed of 3000 rpm. The generator also runs at the same speed as both are coupled together.

   The generator rotor is excited here with the DC voltage and EMF is produced at the stator of generator at 16.5 KV to 220 KV by a three phase transformer and this stepped up electricity is then sent to the switch yard for further transmission to the Northern Grid. This steam after working in the turbine is condensed into a surface type of condenser and the condensed steam are again fed back to the steam generator through a chain of six re-heaters.

   The probability that a high energy neutron emitted in a fission reaction can directly cause fission is quite low. In order to build a system in which fast neutrons sustain the chain reaction, highly enriched uranium is needed, which is very expensive. A more feasible way is to use materials that slow down the neutrons to such lower energies at which the probability of causing fission is significantly higher. These neutrons slowing down materials are the so called moderators. With the aid of some adequate moderator one might as well achieve chain reaction using natural (0.7% U235 content) Uranium.

   If we simplify things a little, there are two major requirements that a moderator material should meet; it should have a low atomic mass number as possible as possible and its neutron absorbing ability should be as low as possible. In practice, four elements meet these requirements: water, heavy water, graphite and Beryllium. Among these water is the most widespread moderator. The properties of heavy-water are actually better but its disadvantage is the very high price.

   Obviously, the number of neutrons present in the reactor must be regulated or controlled since this determines the rate of fission and thus the energy released per second. In order to control the chain reaction one should use materials which tend to capture neutrons at a high probability. The most widespread neutron absorbers are Cadmium and Boron. In India, at Narora Atomic Power Plant, Cadmium sandwiched steel rods have been used, because due to low strength Cadmium alone cannot be used.

   The so called control rods are important tools to control reactor. These are made of neutron absorbing materials and can be moved between the fuel assemblies. For example, if one wishes to decrease the power of the reactor it is sufficient to push a control rod a little inner. The control rods are particularly useful for the short term control and stopping the reactor chain reaction. For long term regulation usually Boric acid dissolved in the moderator is used.




Fig. 3. Nuclear Fission of Uranium

Moderator System




Fig. 4. H2O and D2O



   Moderation is a process of the reduction of the initial high kinetic energy of the free neutron.
   Since energy is conserved, this reduction of the neutron kinetic energy takes place by transfer of energy to a material known as a moderator. It is also known as neutron slowing down, since along with the reduction of energy comes a reduction in speed.


Heavy Water as a Moderator

   Heavy water is used both as a moderator and coolant. The moderator system comprises of Heavy water and Helium system. Callandria is always full of moderator up to 96% and remaining volume is covered by helium gas, which acts as a cover gas. Moderator (D2O) system circulating pump take suction from bottom of callandria and discharge back to the callandria through moderator heat exchanger for maintaining moderator temperature. Working pressure and temperature of moderator system are 8kg/cm2 and 630C respectively.



Heat Transport System

   The fuel coolant system is called Primary Heat Transport System. It is a high pressure and high temperature circuit. The heat generated in the fuel bundles, as a result of uranium fission, is removed by the heavy water serving as a primary coolant. High pressure high temperature heavy water areas have been separated from the high pressure high temperature light water for recovery of high isotopic purity of heavy water.


Shutdown System




Fig. 5. Control Rods in Reactor Building


   They have two diverse and independent shut down system, one of them is fast and other is slow.

Primary Shutdown System

   The system is meant to shut down the reactor whenever any operating parameter crosses a set limit. The system operates automatically and can also be operated manually. The system has 14 rods of Cadmium sandwiched in stainless steel as neutron absorbing element. Any trip signal actuates the mechanical drum assembly and the criticality is reduced to sub criticality in a span of 2.3 seconds.

Secondary Shutdown System

   The Secondary Shutdown system comes into action when the primary shutdown system fails to operate. It is provided as a backup protective system. It consists of 12 liquid poison tubes which remain empty during normal course of operation. But during operation the system enables the filling of tubes with a neutron absorbing liquid. The principle is such that when liquid filled tanks are pressurized then the liquid rises up in liquid tubes located inside reactor. It makes the reactor subcritical in 1.4 seconds.

Automatic Liquid Poison Addition System (ALPAS)

   The primary and secondary shut down system are unable to maintain the state of sub criticality for long enough therefore an additional system known as Automatic Liquid Poison Addition System is employed. Liquid poison is added in the moderator. This poison will absorb the neutrons and thus will interrupt chain reaction. Poison can be added either manually or automatically.



Steam Turbines

   The turbine is a impulse reaction type, designed for saturated steam. After expansion in a single flow high-pressure turbine, moisture is extracted in separator-cum-reheater units where steam is reheated to 2330C before it enters the double flow low-pressure turbine. Steam is extracted from suitable stages of the turbine to provide for 6 stage regenerative feed heating, with a final feed water temperature of 1700C.


Turbo-Generator

   The total quantity of steam flow to the turbine is around 1333.58 tonnes/hr. Stator conductors are water-cooled. The stator core is cooled by dematerialized water and rotor are cooled by Hydrogen, since excitation current requirement is large with a maximum of 5000 Amps, inductor-type high frequency exciter is chosen to avoid commutation problems which are experienced in DC exciters.



Fig.6. A Turbo Generator






Cooling Towers




Fig. 7. Natural Draught Cooling Tower



   Each unit has two 128m high natural draught cooling towers, the most imposing civil structures at site. These are used for dissipation of heat from the condensers and non-active auxiliary coolers, in addition, two induced draught cooling towers are provided to remove heat from the heat exchanger of the active process water system. To ensure containment of potentially active cooling water with the provision of cooling towers. Small amount of water has been taken for the makeup water losses from the Ganga.





Fig.8 . Induced Draught Cooling Tower









Primary Heat Transport System 

General 
   The main function of primary heat transport system is to transport the heat produced in the fuel to steam generator, in which the steam is generated from ordinary water (i.e. Demineralised Water) to run the turbine. 
   The heat transport medium is pressurized heavy water and is circulated through the main circuit by Primary Circulating pumps (PCP’s). PHT system is pressurized to 87 kg/cm2 to carry more heat without boiling.

Principal Features
   The following features are incorporated in PHT system:
(1). Continuous circulation of coolant through the reactor at all times by various modes as listed below:
    (a). In normal operations circulation of coolant is by primary circulating Pumps (PCP’s).
    (b). Flywheel is provided in the PCP motors to extend the pump rundown time so mat in case of loss of power the flow reduction is gradual.
    (c). After loss of power to pumps, adequate simulate would be maintained through thermosyphoning.
    (d). When main circulating pumps are not operating, arrangement for cooling the fuel is done by means of shutdown cooling pump below 1500C.
    (e). in case of major leakage of coolant, there is provision for emergency injection of heavy water from D2O accumulator (a large tank contain15 ton D2O) when the pressure falls down to 55 kg/cm2 and it continues to fall by light water injection at 32 kg/cm2.


AUXILARIES OF THE PHT SYSTEM 

Purification
   Impurities like corrosion products, crud and fission products (if comes to PHT water) are removed from the system by purification. The circuit also helps to achieve a pH value between 9.5 to 10.5 and maintain the conductivity of D2O to 30 ┬Ásiemens/cm to avoid corrosion as PHT system pipe lines, made of carbon steel. 

   For reactor core cooling when the system temperature is below 150°C and for holding the system at low temperature during plant maintenance (shut down) an auxiliary cooling system is provided which is known as standby cooling system or shutdown cooling system. The system is connected between the reactor outlet and inlet header at each end of the reactor. 

PRIMARY RELIEF 
   Once the reactor has been operated at significant power the fuel will continuously supply heat. it normal heat removal fails and normal pressure control fails or their capacities are exceeded, the increase in coolant volume caused by the reactor heat, would be passed out through primary system relief valves (RVs), one relief line connects me pressurized end of south shutdown booting hoop to the bleed condenser through 3 instrumented relief valves in parallel. 


TURBINE 

   The turbine is a tandem compound horizontal impulse reaction type, which when running at 3000 rpm will drive a 235 MW generator. It consists of a single high-pressure cylinder and one double to low pressure cylinder. The high-pressure section has hve sections (stages) in one cylinder. Steam is bled for feed heating. The 0.26% wet steam at pressure of 40 kglcmz and 250°C is supplied to CIES valves (combined isolating and emergency stop valves). The turbine is pressure compounded to expand the steam in two cylinders, HP and LP. After expansion in HP cylinder, the steam is exhausted into two separators cum reheaters where moisture is separated and steam is reheated to 233°C before it enters the double flow low-pressure turbine.


 


Fig.9. Turbine



   The exhaust steam from HP. cylinder is led to the two moisture separator cum reheaters units (Separator - Reheaters). Steam from separator-reheater is taken to the LP cylinders. One L.P. interceptor emergency and governor valve is installed in each reheat line. Bellow typo steam expansion joints are provided on HP. cylinder exhausts, separator-reheater inlet and outlet side and at LP. interceptor valves to take care of thermal expansion of the pipes. LP cylinder has five stages in each flow. Steam is from various stages of L.P. cylinder for feed heating. 

   The exhausts from the L.P. cylinder are led through the diffuser to the condenser. Steam extracted from suitable stages of HP and LP turbine to provide for 6 stage regenerative feed heating with a final water temperature to 1700C.

   The rotating element of the turbine consists of the H.P. rotor and L.P. rotor solidly coupled together to form a single shaft. This shaft is rigidly coupled to the rotating element of the generator on the L.P. out board bearing side. The thrust bearing is provided in the pedestal between the H. P. and L P. outer cylinder.

   Cooling water sprays are provided at either end of the L. P. cylinder, which automatically come into action in case of excessive rise in exhaust hood temperature.
   
   Gland seals are provided to prevent the leakage of steam to atmosphere from the high
pressure ends of the turbine and ingress of air into the turbine at the low-pressure ends
between rotary and stationary components. Live steam is tapped from the steam line and supplied to the L.P. and HP. turbine glands through pressure regulating valves.



Turbine Oil System

   A single oil system serves both the turbine hydraulic control system and lubrication system. During normal operation of the turbine, the main oil pump, the impeller shaft of which is directly coupled with turbine shaft, takes suction from the oil tank, provides the oil to the oil system. A portion of the main oil pump discharge drives the oil turbine driving the booster pump and discharge into lower pressure header for bearing lubrication oil. 
   This oil system provides oil for bearing lubrication and high-pressure oil for the turbine control gear and generator shaft sealing system. 

Moisture Separator Reheater System 

Separator Drain System

   The exhaust steam from the high pressure turbine contains a large quantity of moisture so before admitting the steam to low pressure turbine. It is passed through combined moisture separator and reheater unit, which remove the moisture from the HP exhaust steam. 




Fig.10. Moisture Separator and Reheater




Reheater Drain System

   The exhaust steam from the high pressure turbine is initially passed through the moisture 
separators of combined separator and reheater units which remove the moisture from the HP. exhaust steam. The outlet steam from the moisture separators is routed inside the shell foe reheating in two stages first in bleed steam reheaters and subsequently in the live steam 
reheaters. The bleed steam and live steam give their latent heat to the HP. exhaust steam and in turned get condensed.

Feed Water System
   A feed water heater is a power plant component used to preheat water delivered to a steam generating boiler Preheating the feed water reduces the irreversibility’s involved in steam generation and therefore improves the thermodynamic efficiency of the system. This reduces plant operating costs and also helps to avoid thermal shock to the boiler metal when the feed water is introduced back into the steam cycle. 




Fig.11. Feed Water System



   In a steam power plant (usually modelled as a modified Rankine cycle), feed water heaters allows the feed water to be brought up to the saturation temperature very gradually. This minimizes the inevitable irreversibility's associated with heat transfer to the working fluid (water). 

   The Feed water Systems have two major functions; 

-Supply adequate high quality water to the steam generator.
-Heat the water from about 49 C to about 170 C.
Main Components Of Compressor


Fig.12. Compressor


(1) Suction Filter
   Suction valves are provide in the inlet of the suction filter which sucks the air from the 
atmosphere and sent it to suction filter, which filters the dust particles in the air and 
passes it to LP cylinder.
(2) L.P. Stage
   From suction filter the air enters into the LP cylinder, where the compression of the air takes place and it is up to 2.8 Kg/cm2. 

(3)  Intercooler
   Now the low-pressure compressed air passes through the delivery valve to the water
cooled inter cooler. There air is cooled very near to atmospheric temperature. 









Fig.13. Intercooler


(4) H.P. Stage

   Now air is sucked by the 2nd stage i.e. HP stage through the suction valves and 
compressed to 8.5 Kg/crn2

(5) After Cooler
   After Hp cylinder, air is again cooled in after cooler and finally passes to air receiver 
through a non-return valve. High-pressure water is used for cooling. 


(6) Receiver

   With the help of discharge valve, which gets opened because of pressure gradient, the compressed air is discharged to receiver. The capacity of air receiver is 4.5 m3. Air receiver is provided to dampen the pulsation from the discharge line and also serve as a reservoir to take care of sudden unusually heavy demand or in emergency. 

(7) Air Dryer
   Air-drying plant is provided to supply dry air (Dew point= -40°C) for instruments to avoid the malfunctioning of instrument by moisture and dirt. This is achieved by passing air through Pre-filters, Absorbers (filled with silica gel), after filters. There are three air dryer units one for each unit and one common to both (can be connected to any unit). Two prefilters and two afterfilters are connected to every dryer out of them one remain in service and other on standby.


Conclusion



   Nuclear power is the fourth largest source of electricity in India after Thermal, Hydro electric and Renewable source of electricity. As of 2013, India has 21 nuclear reactors in operation in 7 nuclear power plants, having an installed capacity of 5780 MW and producing a total of 30292.91 GWh of electricity, while six small reactors are under construction and are expected to generate an additional 4300 MW.