Basic Components of PWR (part 5)

Reactor Coolant Pumps 

Main function:

Each contains a vertical, signal- stage, shaft pump designed to pump large volumes of coolant at high pressures and high temperatures.

An impeller attached to the bottom of the rotor shaft pumps reactor coolant. The coolant is drawn up through the bottom of the casing, up through the impeller, and discharged through the diffuser and an exit nozzle in the side of the casing. The impeller and casing represent proven conventional designs. The diffuser converts velocity head from the impeller to pressure head, and the circular casing collects the flow and discharged it to the single discharge nozzle. All parts of the pump in contact with the reactor coolant are of stainless steel, except for the bearing and certain special components.

Steam Generator

Function:

Generating the steam outside the PWR, steam generators are heat exchangers with pressurized water on the hot side (hot leg).

In large PWR, using of 4 steam generators are necessary to produce steam at about 293 C and, 6 MPA with an over all efficiency in the range of 32-33%.

Steam generator consists of three integral sections:

1.    An Evaporator section consists of: U- tube heat exchanger where heat from the reactor is transferred through the tube walls to convert pure secondary- side feed water into steam. The welding of the tubes must ensure to be with zero leakage across the tube joints. An emergency feed water connection is provided at the top of the bundle for feed water addition in the event of black out, or, other conditions where main feed water pumps are not available.

2.    A moisture Separation Section: A moisture separator recirculates the flow on the hot leg side of the tubes, the recirculated water flows through the space under preheated section, which provided to isolate the tube sheet from the colder feed water.

3.    The steam Drum: It has two bolted and gas kited access openings for inspection and maintenance.

Turbine          

It is a simple machine that consists of a series of bladed wheels fixed to an axial, which rotates at high speed as steam at high temperature and pressure strikes the turbine blades.

The inlet to the turbine is always dray steam and its temperature would be reduced and some of the steam is condensed.

 Types of Turbines:

1.    High Pressure Turbines: it is a double flow element with an impulse control stage followed by reaction blading in each end of the element. The steam enters the high-pressure element through two steam chests, one, located on each side of the high-pressure casing through 4 inlet pipes ( 2 in the base and 2 in the cover) and the other through the cross under piping to the moisture separator-reheaters. The rotors in high-pressure turbine are machined from an alloy steel forging.

2.    Low pressure Turbines: It is a double flow element employing reaction blading. Steam enters at the center of the bald path; flows through the blading to an exhaust opening at each end, and then down word to the condenser. Rotors in low pressure turbines are consisting of a series of alloy steel discs shrunk on a shaft and keyed in position are also machined from alloy still forgings. There is an intermediate pressure turbine that feed to it the steam with intermediate pressure.  

Basic Components of PWR ( part 4 )

External Components

Pressurizer

According to the criteria the pressure of the system must be higher than the saturation pressure of liquid to avoid bulk boiling of the coolant.

The main function of pressurizer :

1-   Maintains the coolant pressure during steady state operation.

2-   Limits pressure change caused by thermal expansion and contraction during normal load transients.

3-   Prevents coolant pressure from exceeding limits.

Pressurizer Level

Under steady state operation, pressurizer level represents a balance of water flow into the reactor coolant system and water letdown flow into the chemical and volume control system. Therefore, when water level begins to change we have an imbalance of in and out water flow conditions. This imbalance represents a change in water inventory in the reactor coolant system. Transient load condition will also produce a change in pressurizer water level. This is brought about by the fact that in the pressurized system, water volume changes as a function of reactor coolant average temperature, when coming down in load the decrease in reactor coolant average temperature causes the total water volume is not change in water inventory. However, the level does change with temperature. By programming the pressurizer level set point as a function of reactor coolant average temperature, the level control system will not respond from external load disturbances. Under steady state conditions, the level controller will only take control action to maintain level at the programmed set point. This will serve to correct for changes in water inventory in the reactor coolant system then when the level does deviate from the set point, the level control system will re-establish the in and out water flow conditions, and maintain the required water inventory by restoring the level back to the set point.  

Pressurizer Pressure Control System   

The pressurizer pressure control system limits pressure excursions, which might induce reactor trip, changes in reactivity, and actuation of the relief valves. The pressurizer pressure is maintained at a fixed set point the pressurizer pressure controller is provided with proportional, rate, and reset adjustments. Deviation of pressurizer pressure from its set point is the pressure compensation signal sent to the average temperature control system for rod speed control. The out put of the pressurizer pressure controller is compared with fixed set points of the proportional spray controllers and the proportional heaters controls. The control system will perform as follows some of the heater is energized to provide heat input to compensate for pressurizer heat losses and achieve equilibrium between water and steam.

 For small pressurizer pressure variations, the proportional heaters will increase or decrease heat to restore system pressure back to set point.

For large pressurizer pressure variation caused by larger in surges, steam is compressed and pressure is raised above the set point.                 

Basic Components of PWR (part3)

The coolant

Light water used as steam in PWR, where it is maintain at high pressure, approximately 15Mpa. At this pressure the water will not boil, at least not to any great extent. Since the water does not boil in the reactor. This is done in steam generators, which are heat exchangers with pressurized water on the hot side, high pressure, heated coolant water from the reactor enters at the bottom and passes upward and then downward through several thousand tubes each in the shape of an inverted U, the outer surfaces of these tubes are in contact with lower pressure and cooler feed water returning from the turbine condenser. Heat transferred from the hot water inside the tubes causes the feed water to boil and produce steam. The lower section of a steam generator where this boiling occurs is called, the evaporator section.

The wet steam produced in the evaporator passes upward into a portion of the steam generator known as the, steam drum section.

Here the steam is dried in various moisture separators before exiting to the turbines, steam generators are also manufactured with straight tubes rather than U tubes.

Because water is essentially incompressible, even small changes in coolant volume could lead to large changes in pressure which could have deleterious effect on the system. To prevent this from happening, one coolant loop of “PWR” is equipped with a pressure maintaining surge tank known as a pressurizer, which used to maintain the coolant pressure during steady state operation limits pressure changes caused by thermal expansion and contraction during normal load transients and prevents coolant pressure from exceeding limits.

The Moderator

The moderator used to moderate, that is, to slow down the neutrons from fission to thermal energies, nuclei with low mass number are most effective for this purpose, so that the moderator is always a low mass number material light water used as a moderator in PWR.

Control Rods

Control rod is used for reactor startup, to follow load changes and to control small transient changes in reactivity. The control elements of a rod cluster control (RCC) assembly consists of cylindrical neutron absorber rods having approximately the same dimensions as a fuel rod and connected at the top by spider –like bracket to form rod clusters.

Two types of rod cluster are employed:

1.    Fully length, the fully length type in corporate rods of silver-indium-cadmium absorber material extending the fully length of the core. Stainless steel tubes encapsulate the absorber material isolating it from the reactor coolant. Fully length rod cluster controls provide operational reactivity control and can shut the reactor down at all times, even with the most reactive rod stuck out of the core.

2.    Part length, the part length rod cluster controls, although identical in external appearance and design, incorporate rods with absorber material only in the bottom quarter of the tube. The remainder of the tube is filled with aluminum oxide. The absorber region of the part length rod is positioned at various elevations in the core to shape the axial power distribution. A control axial xenon redistribution and accompanying power level changes. Each rod cluster control is coupled to its drive shift, which is actuated by a separate drive machines mounted on the reactor vessel head, reactivity of the core is changed by raising or lowering the cluster in the core.

Basic Components of PWR (part2)

We talk about some of internal components of Pressurized water reactor such as fuel pellets, fuel rods and fuel assemblies in the previous article.

We will continuo describing the internal components for PWR.

Cladding

 Function of the cladding:

1.    To maintain a barrier between the fuel and the coolant in order to maintain the primary circuit as clean as possible.

2.    To maintain a predetermined-geometry of the fuel element.

3.    The cladding is generally the only feature that stops the fuel and fission products from getting into the primary circuit.

4. be transparent to neutrons, so that it doesn’t absorb neutrons that could be used to induce further fission.

5. have a high thermal conductivity, and not have a high thermal expansion coefficient.

Requirements of cladding material:

1.    The cladding and duct materials should have the lowest possible neutron absorption cross section.

2.    The cladding is subjected to steadily rinsing stresses due to fuel swelling and fission gas release. To withstand these stresses the claddings must posses and retain good multi axial rupture strength, creep strength, and ductility.

3.    The wall thickness of the cladding should be as thin as possible for reasons of good neutron economy and minimum thermal stresses.

4.    The cladding material should be chosen to have melting point or phase transition point away from maximum temperature.

5.     The fuel and coolant may react slowly with the cladding so that a “wastage” allowance must be made. These reactions may also affect the mechanical properties and will have to be taken into consideration in safety analysis.

Note, In PWR we use zirconium has very attractive nuclear properties once it is separated from hafnium with which it generally occur in nature. The resistance of zirconium to corrosion is increased by addition of tin, iron, chromium and nickel.  Also stainless steels was used but has been totally superseded by zircalloy because of the high absorption cross section.

Basic Components of PWR

The Pressurized Water Reactor consists of internal and external components.

The internal component consists from fuel pellets, fuel rods, fuel assemblies, cladding, the coolant, moderator and control rods.

 The external components consists from pressurizer, reactor coolant pumps, steam generator and turbine.

We will discuss some of internal components in this article and follow in future articles.

1. fuel pellets

The basic component of the core is the cylindrical fuel pellet. It is composed of slightly enriched uranium dioxide powered that is compacted by cold pressing & then sintered to attain the required density the sintered uranium dioxide is chemically inert at reactor temperatures and pressures with respect to the cladding and enclosed gases. The slightly dished ends of each pellet permit axial expansion at the center of the pellets. The consequences of any accidental branch of the cladding are minimized by the ability of the uranium dioxide lattice to retain fission products and to resist deterioration caused by high temperature water.

2. Fuel rods   

Uranium dioxide pellets are inserted into zircalloy-4 tube, and each end of the tube is sealed by welding an end plug to form a fuel rod. The pellets are prevented from shifting during handling and shipment by compression spring between the top end plug of the fuel pellet stack.

3. Fuel assemblies

A square array of fuel rods structurally bound together constitutes a fuel assembly. Control rod guide thimbles replace fuel rods at selected spaces in the array and fastened to the top and the bottom nozzles of the assembly spring clip grid assemblies are fastened to the guide thimbles along the height of the fuel assembly to provide support for the fuel rods. The fuel rods are contained and supported, and the rod to rod center line spacing is maintained within this skeletal frame work.

The bottom nozzle of the fuel assembly controls the coolant flow distribution and also serves as the bottom structural element.

The top nozzle of the fuel assembly function as the fuel assembly upper structural element and forms a plenum space where the heated reactor coolant is mixed and directed toward the flow holes in the upper core plate. The spring clip grids provide support for the fuel rods in two perpendicular directions.