Different Types of Nuclear Power Plants

Different types of nuclear power plants

a- Pressurized water reactor

In the figure shown above simplified pressurized water reactor that comprised of three loops:

The primary loop … contains the heat source consisting of a nuclear fuel core positioned within a reactor vessel where the energy resulting from the controlled fission reaction is transformed into sensible heat in the coolant moderator generator where the heat is transferred to secondary loop through a number of U-type tubes. The reactor coolant then returns back to the reactor vessel to continue the process. An electrically heated pressurizer connected to the loop maintains a pressure above the saturation pressure so that bulk boiling does not occur

The secondary loop … dry steam produced in the steam generator flows to a turbine-generator where it is expanded to convert thermal energy into mechanical energy and hence electrical energy. The expanded steam exhausts to a condenser where the latent heat of vaporization is transferred to the cooling system and is condensed. The condensate is pumped back to the steam generator to continue the cycle.

The tertiary loop … the heat rejection loop where the latent heat of vaporization is rejected to theenvironment through the condenser cooling water. Depending on the specific site, this heat is released to a river, lake, ocean, or cooling tower system.

The pressurized water power plant can considered to compromise of two basic islands:

A- NUCLEAR ISLAND (NSSS)1-core reactor 2-pressurizer 3-coolant pump

4-steam generator5-auxillary fluid system 6-control system

B- TURBINE ISLAND 1-steam turbine 2- generator 3-cooling tower

NSSS

Reactor core

Reactor vessel is a heavy walled which houses the nuclear core and its mechanical control rods as well as necessary support and alignment structures. The vessel is cylindrical in shape with a hemispherical bottom head and a flanged and gasket upper head for access. It is fabricated of carbon steel, and is cladded with stainless steel to limit corrosion. The internal core support and alignment structures are removable to facilitate inspection and maintenance as is the alignment structure for the top mounted control rod drive mechanisms. Vessel inlet and outlet nozzles for the primary loops are located at a level well above the top of the fuel core.

Number of fuel assemblies arranged in three regions to optimize fuel performance. While all fuel assemblies are mechanically identical, enrichment of the uranium dioxide fuel differs from

assembly to assembly. In a typical initial core loading, three fuel enrichments are used. Fuel assemblies with the highest enrichments are placed in the core periphery, or outer region, and the groups of lower enrichment fuel assemblies are arranged in a selected pattern in the central region. In subsequent refueling, one third of the fuel is discharged and fresh fuel is loaded into the outer region of the core. The remaining fuel is re-arranged in the central two thirds of the core in such a manner as to achieve optimum power distribution and fuel utilization.

Rod cluster control assemblies used for reactor control consists of absorber rods attached to a spider connector which in turn is connected to a drive shaft. The absorber (control) rods are loaded with an absorber material that has a high affinity for neutrons. The rod cluster control assemblies are raised and lowered by a drive mechanism on the reactor vessel head. The drive mechanism allows the rod cluster control assemblies to be released instantly when necessary for rapid reactor shutdown Insertion of the assemblies during a trip is by gravity.

Pressurizer

Pressurizer connected to one of the reactor coolant hot legs maintains reactor coolant system pressure during normal operation, limits pressure variations during plant load transients, and keeps system pressure within design limits during abnormal conditions. For example, a transient that could decrease system pressure is counteracted by flashing water within the pressurizer

which is kept at saturation temperature by the automatic heaters. An increasing pressure transient is limited by spraying cooler water from the primary loop into the pressurizer steam space to partially collapse the steam bubble or by automatic operation of relief and safety valves

Coolant pump

Heavy flywheel on the pump motor shaft provides long coast down times to prevent rapid decreases in core cooling flow during pump trips. Interlocks and automatic reactor trips ensure that forced cooling water flow is present whenever the reactor is at power. Additionally, two separate power supplies are available to the pump motor when the plant is at power.

Steam generator

Steam generator is a vertical u-tube design with upper moisture separator that produce steam with quality of 99.75 percent. Preheated feed water enters the top of the unit, mixes with effluent from the moisture separators and then flows downward on the outside of the tube bundle. The other feed water comes from the downside of the generator which there is a heat transfer in the tubes and feed water converts to steam with water droplets for which separated by moisture separator at the top of the generator .

Auxiliary fluid systems

Normal and emergency operation of the reactor coolant system requires a number of support functions to: maintain water inventory, purify and treat primary coolant, remove residual heat following a plant shutdown, provide cooling water to pumps and motors, supply ventilation air, and provide emergency supplies of core cooling water. These functions are provided by auxiliary systems.

Control

Power control of a PWR is based on balancing reactivity through the use of mechanical and chemical neutron absorbers and proper allowance for physical phenomena which influence reactivity. The principal natural phenomena which influence transient operation are the temperature coefficients of the moderator and fuel and the buildup or depletion of certain fission products. Reactivity balancing may occur through the effects of natural phenomena or the operation of the reactor control system using either the rod cluster controls or chemical shim. A change in the temperature of either moderator or fuel, such as might occur due to an increase or decrease in steam demand, will add or remove reactivity and cause the power level to change until the reactivity change is balanced out.

Rod cluster control assemblies are used to follow fairly rapid load transients and for startup and shutdown. The chemical shim system uses a soluble neutron absorber, boron in the form of boric acid, which is inserted in the reactor coolant during cold shutdown, partially removed at startup, and adjusted in concentration during core lifetime to compensate for such effects as fuel consumption and accumulation of fission products which tend to slow the nuclear chain reaction.

TURBINE ISLAND

Steam turbine

The usual design of the steam turbine for an 1100 MWe PWR is an in-line combination of a single high-pressure turbine and three low pressure turbines Steam admission to the double flow high pressure turbine is controlled by four sets of governor valves with quick acting stop valves located ahead of them for rapid isolation. Four separate pipes convey the steam from the governor valves to the nozzle chambers. Thermal energy is converted to mechanical energy by expansion through a control stage (first stage) and a number of reaction stages. Steam pressure after the single control stage is measured and used as a load index for the reactor control system.

Upon leaving the last row of high-pressure blades, the steam has a significantly high moisture content which must be reduced before it enters the low-pressure turbines. High moisture content in steam adversely impacts efficiency and turbine maintenance. To accomplish moisture reduction, exhaust steam from the high-pressure turbine is passed through a stage of moisture separation and reheat. Low pressure turbines are provided with two or more moisture separator- reheated (MSR) units in parallel.

Generator

The generator and exciter are directly driven by the steam turbine. The generator consists of a water cooled stationary stator with a hydrogen cooled rotor. The stator is mounted within the generator frame on a series of spring supports to minimize vibration noise and damage. The rotor, which provides the rotating electrical field, is cooled by blower-driven hydrogen gas at a pressure of several atmospheres. The hydrogen in turn is cooled by water. A subsystem maintains hydrogen purity and provides nitrogen purging when access to the generator is necessary.

b- Boiling water reactor

Boiling water reactors (BWRs) are nuclear power reactors generating electricity by directly boiling the light water in a reactor pressure vessel to make steam that is delivered to a turbine

generator. After driving a turbine, the steam is converted into water with a condenser (cooled by sea water in Japan), and pumped into the reactor vessel with feed water pumps. A part of the water is sent into the reactor vessel after being pressurized with recirculation pumps installed outside of the vessel and fed into the reactor core from the bottom part of the reactor vessel with jet pumps.

Inside of a BWR reactor pressure vessel (RPV), feed water enters through nozzles high on the vessel, well above the top of the nuclear fuel assemblies (these nuclear fuel assemblies constitute the "core") but below the water level. The feed water is pumped into the RPV from the condensers located underneath the low pressure turbines and after going through feed water heaters that raise its temperature using extraction steam from various turbine stages.

Now let us discuss the BWR structure Reactor core

Reactor core and internal structures of class BWR reactor vessel are shown in Figure In a reactor vessel, there are a reactor core that mainly consists of fuel assemblies and control rods in the center, equipment for generating steam for a turbine, such as a steam-water separator and a steam dryer in the upper part of the vessel, equipment for 6 reactor-power control, such as control rod guide tubes and control rod drive housings in the lower part of the vessel, and jet pumps etc. that surrounds the reactor core and composes the coolant flow path in the periphery of reactor core.

Fuel rods are structured to contain uranium-dioxide pellets, a plenum spring etc. in a zircaloy cladding tube, of which both ends are weld-sealed with end plugs after pressurized with helium gas. The plenum is a space provided so that the fission gas discharged from fuel pellets

accompanying fuel burnup is accommodated and the fuel rod internal pressure does not become excessive

Steam turbine

The main steam system transports “dry” steam from the reactor pressure vessel to the turbine via four pipes. The amount of steam produced is proportional to the reactor core’s power level. As core power level increases, more steam is produced.

The turbine has one high pressure and three low pressure turbines all connected by a common shaft. The steam initially enters the high pressure turbine. The steam spins the turbines. The turbines spins at a constant rate of 1,800 or 3,600 revolutions per minute.

The steam exits the high pressure turbine and enters the moisture separator / reheater (MSR). The MSR removes any water droplets formed in the steam as it lost energy passing through the high pressure turbine. The steam exiting the MSE splits into three paths to flow in parallel through the low pressure turbines.

When the reactor power level increases to produce more steam, the turbine control valves open to admit more steam while maintaining a constant pressure at the turbine inlet when the turbine is manually or automatically tripped, the turbine stop valves close within seconds

Generator The generator rotor is connected to the turbine shaft. The electricity produced by the generator flows to an electrical transformer that increases the voltage level to that of the transmission system. Power lines radiate from the site to transport electricity to commercial and residential customers.

Condenser The circulating water system takes water from a nearby lake, river, or ocean and pumps it through the condenser. The cool water converts the steam back into water. The condensation of the steam creates a vacuum in the hot well, helping to “pull” steam through the low pressure turbines. The condensate pumps, powered by electric motors, take water from the hot well. The condensate filter/demineralizers remove particles and dissolved ions from the water being returned to the reactor pressure vessel.

c- Advanced gas cooled reactor

A gas-cooled reactor (GCR) is a nuclear reactor that uses graphite as a neutron moderator and carbon dioxide (helium can also be used) as coolant. Although there are many other types of reactor cooled by gas, the terms GCR and to a lesser extent gas cooled reactor are particularly used to refer to this type of reactor

The GCR was able to use natural uranium as fuel, enabling the countries that developed them to fabricate their own fuel without relying on other countries for supplies of enriched uranium, which was at the time of their development only available from the United States or Soviet Union.