pwr is the abbreviation for the pressurized water reactor

8/8/2019 PWR is the Abbreviation for the Pressurized Water Reactor

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PWR is the abbreviation for the Pressurized Water Reactor. These reactors were originally designed

by Westinghouse Bettis Atomic Power Laboratory for military ship applications, then by the

Westinghouse Nuclear Power Division for commercial applications. The first commercial PWR plantin the United States was Shippingport, which operated for Duquesne Light until 1982.

In addition to Westinghouse, Asea Brown Boveri-Combustion Engineering (ABB-CE), Framatome,Kraftwerk Union, Siemens, and Mitsubishi have typically built this type of reactor throughout the

world. Babcock & Wilcox (B&W) built a PWR design power plant but used vertical once-throughsteam generators, rather than the U-tube design used by the rest of the suppliers. Industry

consolidation has occurred so that Framatome-ANP and Westinghouseare two key remaining

manufacturers. Refuelings are done with the plant shutdown.

Reproduced by permission

The Pressurized Water Reactor (PWR) has 3 separate cooling systems. Only 1 is expected to have

radioactivity - the Reactor Coolant System.

The Reactor Coolant System, shown inside the Containment, consists of 2, 3, or 4 Cooling "Loops"

connected to the Reactor, each containing a Reactor Coolant Pump, and Steam Generator. TheReactor heats the water that passes upward past the fuel assemblies from a temperature of about 530F

to a temperature of about 590F. Boiling, other than minor bubbles called nucleate boiling, is not

allowed to occur. Pressure is maintained by a Pressurizer (not shown) connected to the ReactorCoolant System. Pressure is maintained at approximately 2250 pounds per square inch through a

heater and spray system in the pressurizer. The water from the Reactor is pumped to the steam

generator and passes through tubes. The Reactor Cooling System is expected to be the only one with

radioactive materials in it. Typically PWRs have 2, 3, or 4 reactor cooling system loops inside thecontainment.

In a Secondary Cooling System (which include the Main Steam System andthe Condensate-Feedwater Systems), cooler water is pumped from the Feedwater System and passes on the outside
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of those steam generator tubes, is heated and converted to steam. The steam then passes through the aMain Steam Line to the Turbine, which is connected to and turns the Generator. The steam from the

Turbine condenses in a Condenser. The condensed water is then pumped by Condensate Pumps

through Low Pressure Feedwater Heaters, then to the Feedwater Pumps, then to High Pressure

Feedwater Heaters, then to the Steam Generators.The diagram above simplifies the process by only

showing the condenser, a pump, and the steam generator.

The condenser is maintained at a vacuum using either vacuum pumps or air ejectors. Cooling of thesteam is provided by Condenser Cooling Water pumped through the condenser by Circulating

Water Pumps, which take a suction from water supplied from the ocean, sea, lake, river, orCoolingTower (shown). A discussion of cooling towers is provided in the photo section.

PWRs of varying sizes have been built since the late 60's.

PWR Cycle

The Pressurized Water Reactor designs are similar for the units provided by the various manufacturers. Differences are

illustrated by the table below:

Manufacturer MWt MWe Loops Pressurizer

ReactorCoolantPumps

perLoop

SteamGeneratorsper Loop

Westinghouse450-3000

167-1000

1-4 1 1 1

Framatome2700-3600

900-1300

3-4 1 1 1

Babcock & Wilcox 2400-3000

800-1000

2 1 2 1

Combustion Engineering2400-3600

800-1300

2 1 2 1

ABB 3000 1000 4 1 1 1

Mitsubishi 3000 1000 4 1 1 1

A number of simplified diagrams (courtesyWestinghouse) illustrate the design of the pressurized water reactor facilities.

The following are colored perspective and arrangement drawings of

Reactor Coolant System

Reactor Vessel

Reactor Coolant Pump
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Steam Generator

Pressurizer

The flow paths are illustrated by acolored graphic flow diagram and ablack and white line drawing. The following

describes the flow path and the linked drawings illustrate, in detail, the composition and major parts of the components.

Reactor- this diagram shows how the reactor is constructed with its major components. Water at 530F enters the

reactor from the nozzles at about mid-height. The water flows downward on the outside of the core barrel to the

bottom of the reactor. Then the flow turns upward past the fuel assemblies, removing heat from the assemblies

and increasing in temperature to 590-600F. After leaving the core area, the water mixes in the upper plenum andleaves the reactor through nozzles. Flow then goes to the ...

Steam Generatorwhere the radioactive Reactor Cooling System water enters at the bottom, flows through small

(1/2-inch) inverted U-tubes. That water loses its heat as it passes through the tube being cooled by the non-

radioactive water outside the tube. Non-radioactive feedwater enters through the nozzles at the mid-height of the

steam generator at a temperature of about 425F. The water flows downward outside a wrapper sheet to the area

just above the tubesheet where the water turns and flows upward past the U-tubes. The water increases intemperature and turns to steam. A moist steam at about 510-547F with pressure of 720-1005 pounds per square

inch is produced. The moist steam travels upward to steam separators (chevron separators and swirl vanes) which

allow 99.75% purity steam to pass of the steam generator and the remaining water is directed back to the lowerpart of the steam generator. The reactor cooling system water enters the steam generator at ~590F and leaves at ~

530F. That water then flows to the ...

Reactor Coolant Pump which pumps the water back to the reactor.

Pressurizeris used to control the pressure in the reactor cooling system so that boiling does not occur in thereactor. The pressurizer also is used to act as a surge tank for the system taking up the level variations in the

system. Heaters are installed at the bottom of the pressurizer for heating the water to about 652F and 2250 pounds

per square inch. Automated pressure control valves (called power operated relief valves) and safety valves,

connected to the top of the pressurizer, can open to control and maintain pressure.

Secondary Systems line drawings illustrate the major sub-components in the non-radioactive part of the system

where steam flows to the turbine, condenses in the condenser, then is pumped back to the steam generator first by

condensate pumps, then by feedwater pumps. The feedwater heaters improve the efficiency of the cycle by

recovering and reusing energy that would otherwise be lost. By doing so efficiency of the cycle is raised to 33%.

Boiling Water Reactor (BWR)

BWR is the abbreviation for the Boiling Water Reactor. These reactors were originally designed byAllis-Chalmers and General Electric (GE). The General Electric design has survived, whereas all
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Allis-Chalmers units are now shutdown. The first GE US commercial plant was at Humboldt Bay

(near Eureka) in California. Other suppliers of the BWR design world-wide have included - ASEA-

Atom, Kraftwerk Union, Hitachi. Commercial BWR reactors may be found in Finland, Germany,India, Japan, Mexico, Netherlands, Spain, Sweden, Switzerland, and Taiwan. Japan and Taiwan have

the newest BWR units.

The BWR reactor typically allows bulk boiling of the water in the reactor. The operating temperature

of the reactor is approximately 570F producing steam at a pressure of about 1000 pounds per squareinch. Current BWR reactors have electrical outputs of 570 to 1300 MWe. As this the PWR designs,

the units are about 33% efficient.

Reproduced by permission

In the figure above, water is circulated through the Reactor Core picking up heat as the water moves

past the fuel assemblies. The water eventually is heated enough to convert to steam. Steam separators

in the upper part of the reactor remove water from the steam.

The steam then passes through the Main Steam Lines to the Turbine-Generators. The steamtypically goes first to a smallerHigh Pressure (HP) Turbine, then passes to Moisture Separators

(not shown), then to the 2 or 3 largerLow Pressure (LP) Turbines. In the sketch above there are 3

low pressure turbines, as is common for 1000 MWe plant. The turbines are connected to each otherand to the Generator by a long shaft (not one piece).
http://www.nucleartourist.com/images/bwr-cycle.gif


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The Generatorproduces the electricity, typically at about 20,000 volts AC. This electrical power is

then distributed to a Generator Transformer, which steps up the voltage to either 230,000 or 345,000

volts. Then the power is distributed to a switchyard or substation where the power is then sent offsite.

The steam, after passing through the turbines, then condenses in the Condenser, which is at a vacuum

and is cooled by ocean, sea, lake, or river water. The condensed steam then is pumped to LowPressure Feedwater Heaters (shown but not identified). The water then passes to the Feedwater

Pumps which in turn, pump the water to the reactor and start the cycle all over again.

The BWR is unique in that the Control Rods, used to shutdown the reactor and maintain an uniform

power distribution across the reactor, are inserted from the bottom by a high pressure hydraulically

operated system. The BWR also has a Torus (shown above) or a Suppression Pool. The torus or

suppression pool is used to remove heat released if an event occurs in which large quantities of steamare released from the reactor or the Reactor Recirculation System, used to circulate water through

the reactor.

The General Electric BWR designs are designated BWR-1 through BWR-6. Typical examples of eachof these classes of BWR are:

ABSTRACT

Some four hundred Boiling Water Reactors (BWR) and Pressurized Water Reactors (PWR) have been inoperation for several decades. The present concept, the High Pressure Boiling Water Reactor (HP-BWR), makes use of this operating experience. The best parts of the two reactor types are used andthe troublesome components are left out. This means improved safety. The increased thermalefficiency is beneficial to the environment as less cooling water is released per produced kWh. Withsome modifications the used components can be used to make this design cost effective and possible toachieve in the currently not too distant future.

1. Introduction

Since the 1950s several hundred Boiling Water and Pressurized Water Reactors (BWRs and PWRs) inuse. There is a wealth of operating experience. During this have been time many difficulties occurredwith a number of important components. This concept, the High Pressure Boiling Water Reactor (HP-BWR) offers a solution to use the best parts from each type (BWR and PWR) and leave out thetroublesome components. This means an important increase of safety. As an extra benefit, alsoincreased efficiency attained beneficial for the environment as less cooling water is released perproduced kWh. The HP-BWR is using with some modifications- currently manufactured parts makingthis a cost effective, realistic concept.

2. The High Pressure Boiling Water Reactor HP-BWR

The High Pressure Boiling Water Reactor (HP-BWR) offers improved nuclear safety and less damage tothe environment. The HP-BWR is an environmentally friendly, effective alternative.

The High Pressure- BWR


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The HP-BWR uses a modified PWR reactor vessel and BWR type fuel and control rods. However, herethe cruciform control rods are gravity operated with ample space between the crosses and the fuelboxes. The control roads are manoeuvred electromagnetically, which means that they will drop into thecore when there is a loss of electrical power as in the PWRs. The traditional PWR control rods are fingershaped and are surrounded by a tube with a minimum of clearance. The traditional BWR control rodsare operated from below with hydraulic pressure. Therefore, at the bottom of the traditional BWRreactor vessel there are a great number of penetration points for the control rods. Directly below thereactor vessel there is an elaborate system of numerous high pressure hydraulic pipes to actuate thecontrol rods. Taking the best fro and leaving out the drawbacks of both the traditional BWR and PWRsystems is a substantial safety improvement.

All the pipe connections to the reactor vessel are well above the reactor core. This allows the omissionof core spray. The moisture separators and steam dryers are outside the reactor vessel, leaving freespace for the control rods.

Internal circulation pumps. These allow the use of orifices at the inlet of the fuel boxes so that the one-phase pressure drop will predominate over the two-phase pressure drop. This reduces the risk ofhydrodynamic oscillations. However, if suitable methods are found to facilitate natural circulation eventhe circulation pumps can be left out.

The use of the HP-BWR means improved Carnot cycle thermal efficiency up to about ~40% instead ofabout ~30%. The reason is that the HP-BWR steam temperature corresponds to 15MPa while thetraditional BWRs steam temperature corresponds to 7MPa and the traditional PWRs steamtemperature corresponds to 6MPa. The HP-BWR is lenient to the environment as less damaging coolingwater is released per produced kWh to the recipient, sea or river or to the air via a cooling tower.

Using direct cycle the system is simplified. Still, the usual PWR steam lines can be used through thecontainment wall to the turbine. A great advantage is that the complicated and costly steam generatorsare left out.

The moisture separators and the steam dryers are outside the reactor vessel in the containmentinstead of the huge troublesome steam generators.

Simple dry containment is used instead of the complicated, inert, pressure suppression wetcontainment which requires a great deal of surveillance.


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3. The Traditional Boiling Water Reactor, BWR

The basic principles of the traditional Boiling Water reactor are well known

Traditional BWR

As there are pipe connections to the reactor vessel below the reactor core, a pipe break can empty thevessel leaving the core uncovered, without the cooling water. Therefore, a core spray is required. Thisis a common feature for the BWRs with external circulation pumps or jet pumps. However, this drawback is eliminated at a later design stage with the Advanced Boiling Water Reactor, ABWR. All BWRcontrol rods are inserted to the core using hydraulic power; some with electric motors too. This makesthe lower part of the rector both inside and outside the bottom of the reactor vessel extremelyelaborate. To make things worse, in the past, cracks, corrosion and leakage occurred at thepenetrations at the lower part of the reactor vessel.

Structural sketch of reactor pressure vessel


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Traditional Advanced Boiling Water Reactor (HitachiABWR)

The huge reactor vessel would require an enormous dry containment building. Therefore, a pressuresuppression containment system is used instead. The containment is separated into two parts, theupper dry well and the lower wet well with the suppression pool. If the separation is not perfectly leak-tight the wet well cannot fulfil its function to suppress the pressure in the dry well in case of a pipe

break. Further complication is that the traditional BWR containment operates inertly, making difficultthe entrance into it.

The nice thing about the BWR is that it operates in direct cycle mode without the troublesome steamgenerators.

4. The Traditional Pressurized Water Reactor, PWR

Most of the worlds operating reactors are traditional PWRs.

Traditional BWR


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The control rods are operated from above. Undoubtedly some leakages were observed at thepenetrations which in a few cases led to the need to replace the reactor pressure vessel head.

Reactor Concept Manual - Pressurized Water Reactor Systems

The simple electromagnetic devices which manoeuvre the rods worked reliably. This assures a highdegree of safety. A basically continuous, uninterrupted bottom of the reactor vessel avoids anysuspicions of its integrity.

Cutaway View of Reactor Vessel


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A four-loop Westighouse plant has four steam generators, four reactor coolant pumps, and a pressurizer. The four-loopunits in the United States are Braidwood 1 and 2, Byron 1 and 2, Callaway, Catawba 1 and 2, Comanche Peak 1 and 2, D.C.Cook 1 and 2, Diablo Canyon 1 and 2, Indian Point 2 and 3, McGuire 1 and 2, Millestone 3, Salem 1 and 2, Seabrook,Sequoyah 1 and 2, South Texas Project 1 and 2, Vogtle 1 and 2, Watts Bar 1, and Wolf Creek. Each of these plants has 193fuel assemblies arranged inside a reactor vessel that has an internal diameter of 173 inches (except South Texas has aninternal diameter of 167 inches). The fuel assemblies are arranged in 17 x 17 array except for Cook and Indian Point, whichhave 15 x 15 fuel. The electrical output of these plants ranges from 950 to 1250 megawatts.

The curse of the traditional PWRs is their steam generators. These complicated and costly huge piecesof equipment are disappointingly short lived because of the corrosion of the internal tubes, which cancause leaks. The plant owners used to change them after some fifteen years. An extremely expensiveand troublesome and also time consuming operation.

Steam dryer in a SG


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In the upper part of the steam generators there is the moisture separator and the steam dryer. TheHP-BWR is borrowing this equipment which can be used without the troublesome steam generators.

5. References

All university text books written for nuclear engineering students contain detailed descriptions of both

Boiling Water Reactors and Pressurized Water Reactors. Also manufacturers in Europe, Asia andAmerica publish data about their designs. There is also a wealth of information about BWRs and PWRson the internet.

The Indian Advanced Heavy Water Reactor (AHWR) is being designed and

developed to achieve large-scale use of thorium for the generation of commercial

nuclear power. This reactor will produce most of its power from thorium, with no

significant external input of uranium-233, in the equilibrium cycle.


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AHWR is a 300 MWe, vertical, pressure tube type, boiling light water cooled, and

heavy water moderated reactor. The reactor incorporates a number of passive safety

features, and it is associated with a fuel cycle having reduced environmental impact. At

the same time, the reactor possesses several features, which are likely to reduce its

capital and operating costs.

The AHWR fuel contains three rows of fuel pins surrounding a central displacer rod.

The inner two rows contain thirty (Th-U233)O2 fuel pins and the outer row contains

twenty four (Th-Pu)O2 fuel pins. The central rod contains dysprosia in zirconia matrix.The central rod of fuel also incorporates a water tube for the injection of Emergency

Core Coolant System (ECCS) water directly on fuel pins during a postulated Loss of

Coolant Accident (LOCA). AHWR fuel is currently designed for an average burn-up of

24 GWd/Te. Its design makes it amenable for reconstitution, if desired to facilitate a

further extension of burn-up in the (Th-U233)O2 fuel pins in future.


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AHWR employs natural circulation for cooling of the reactor core under all conditions.

All event scenarios initiating from non-availability of main pumps are therefore

excluded. During incidents leading to increase in void, the negative void coefficient of

reactivity brings down the reactor power without necessitating any external control or

operator action. The ECCS is designed to remove the core heat by passive means in case

of a postulated LOCA. In the event of a rupture in the primary coolant pressure

boundary, the cooling is initially achieved by a large flow of borated water from

advanced accumulators, and later cooling of the core is achieved by the injection of cold

water from a large Gravity Driven Water Pool (GDWP) located near the top of the

reactor building and later submerging the core.


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In AHWR, subsequent to energy absorption by Gravity Driven Water Pool (GDWP), the

Passive Containment Cooling System (PCCS) provides long term containment cooling

following a postulated LOCA.

The principle of double containment has been adopted in designing the containment for

AHWR. For containment isolation, a passive system has been provided in the AHWR.

The reactor building air supply and exhaust ducts are shaped in the form of U bends of

sufficient height. In the event of LOCA, the containment pressure acts on the water pool

surface and pours water by swift establishment of syphon into the U-bends of the duct.

Water in the U-bends acts as a seal between the containment and the external

environment, providing necessary isolation between the two.

The AHWR fuel cycle will be selfsufficient in U233 after initial loading. The spent fuelstreams will be reprocessed and thorium and U233 will then be recycled and reused.

There are also plans to recycle the actinides back into the reactor.

Incidentally, the thorium fuel cycle also presents low proliferation risks, a factor

considered significant by several nuclear supplier nations for export of nuclear

technology. A quantitative analysis of the AHWR fuel cycle substantiates this feature.

Important Design Parameters of

AHWR

Reactor power 920 MWTh

Core configuration Vertical, pressure tube type

design

Coolant Boiling light water

Number of coolant

channels

452

Pressure tube ID 120 mm

Lattice pitch 270 mm (square pitch)


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No. of pins in fuel cluster 54

(Th-Pu)O2 - 24 pins

(Th-U)O2 - 30 pins

Pin OD 11.2 mm

Heated fuel length 3.5 m

Total core flow rate 2175 kg/s

Coolant inlet temperature 259oC (nominal)

Feed water temperature 130 oC

Average steam quality 19 %

Steam generation rate 414 kg/s

Steam drum pressure 70 bar

PHT loop height 39 m

Calandria diameter 8000 mm

Calandria height 5000 mm


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pwr is the abbreviation for the pressurized water reactor

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