boiling water reactor-intro

8/13/2019 boiling water reactor-intro

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The core of a typical boiling water reactor (BWR)contains about 100 tons of nuclear fuel.

The fuel consists of uranium dioxide pellets loaded inmetal fuel rods placed in a square array called a fuel abundle. BWRs like Browns Ferry and Peach Bottom have

764 fuel bundles in their cores.

BWRs operate for 18 to 24 months between refuelingoutages. During refueling, roughly one-third of the fuel

bundles are removed from the core to the spent fuel pooland replaced with fresh fuel bundles.


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The core is housed within the reactor pressure vessel(RPV). The RPV is forged of steel that is approximately

seven inches thick. Steel plates are welded together toform a hollow cylinder. A domed lower head is weldedto the bottom side of the cylinder. A domed upper headis bolted to a flange on the top side of the cylinder.

During refueling, the upper head is unbolted andremoved to provide access to the core and RPVinternals.


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Control rods govern the power level of the core. Control rodscontain material like boron that absorb the neutrons emitted by

splitting atoms. Control rods are cross-shaped. A control rod slipsbetween within four fuel bundles when inserted in the core.

Control rods enter the reactor core from below. When fully

withdrawn, the control rods remain within the reactor pressurevessel in the lower plenum region. The control rods positioned byhydraulic pistons. Water pressure applied beneath the pistonmoves a control rod into the reactor core. Water pressure applied

above the piston moves a control rod out of the reactor core.

During normal operation, control rods can beincrementally inserted and withdrawn in 6-inch notches.

During a reactor trip, control rods are fullyinserted into the core within a handful ofseconds to interrupt the nuclear chain

reaction and shut down the core.


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The reactor recirculation system consists of two loops. Each loopcontains a recirculation pump and motor along with isolation and

control valves.

The recirculation system simply takes water from the reactorvessel and returns it to the reactor vessel where it flows through jet

pumps into the lower dome beneath the reactor core.

The operators use the recirculation system to control the reactorpower level. By increasing the recirculation pump flow rates, more

water enters the reactor core region to slow down more neutronsand allow more fissions to increase the reactor core output.

Conversely, decreasing the recirculationpump flow rates pushes less water through the

reactor core, fewer neutrons are slowed down

and the reactor core output decreases.


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The main steam system transportsdry steam from the reactorpressure vessel to the turbine viafour pipes.

The amount of steam produced isproportional to the reactor corespower level. As core power level

increases, more steam isproduced.


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The turbine has one high pressureand three low pressure turbines allconnected by a common shaft. Thesteam initially enters the highpressure turbine.

The steam spins the turbines. Theturbines spins at a constant rate of1,800 or 3,600 revolutions perminute.


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The steam exits the high pressureturbine and enters the moistureseparator / reheater (MSR). TheMSR removes any water dropletsformed in the steam as it lostenergy passing through the highpressure turbine. The steamexiting the MSE splits into threepaths to flow in parallel through

the low pressure turbines.


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The generator rotor is connected tothe turbine shaft. The electricityproduced by the generator flows toan electrical transformer thatincreases the voltage level to that ofthe transmission system. Powerlines radiate from the site totransport electricity to commercialand residential customers.


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When the reactor power levelincreases to produce more steam,the turbine control valves open toadmit more steam whilemaintaining a constant pressure atthe turbine inlet.

When the turbine is manually orautomatically tripped, the turbinestop valves close within seconds.


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The turbine and generator are located within theturbine building outside of the primarycontainment housing the reactor pressure vessel.

Each of the four steam pipes passing through theprimary containment wall is equipped with twoisolation valves that close in event of an accidentto minimize the reactor coolant inventory loss andthe amount of radioactivity reaching theenvironment.


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To protect the steam pipes from the pressureincrease when the main steam isolation valves orturbine stop valves rapidly close, safety reliefvalves (SRVs) automatically open. The openingsetpoints for the SRVs are tiered so that only a fewSRVs open for small pressure rises and many ifnot all SRVs open for larger pressure rises.


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The SRVs discharge to the suppression pool, alarge body of water that is an essential part of theBWR pressure suppression containment design.

More than one million gallons of water in thesuppression pool absorbs the energy in steamflowing from open SRVs or broken piping insidecontainment. As a result, the pressure (andtemperature) rise inside containment is muchlower than it would be without the suppressionpool.


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The steam exiting the three low pressureturbines enters the condensor, also calledthe hotwell.


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The circulating water system takes waterfrom a nearby lake, river, or ocean andpumps it through the condensor. The cool

water converts the steam back into water.

The condensation of the steam creates avacuum in the hotwell, helping to pull

steam through the low pressure turbines.


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The condensate pumps, powered byelectric motors, take water from thehotwell.


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The condensate filter/demineralizersremove particles and dissolved ions fromthe water being returned to the reactorpressure vessel.


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The condensor is at a vacuum pressure andthe reactor pressure vessel is at a pressureover 1,000 pounds per square inch. Thecondensate booster pumps help increasethe water pressure between these

extremes.


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The steam-driven feedwater pumps send water into the primarycontainment and the reactor pressure vessel. Each feedwater pipe isequipped with a check valve, not an isolation valve, to enable flow to

enter but not leave the primary containment and reactor vessel.


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The residual heat removal (RHR) system performs many functionsincluding cooling when the reactor is shut down, cooling thesuppression pool water during normal operation and accidents, and

providing makeup water to the reactor pressure vessel duringaccidents.


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The core spray system has a single function: in event of an accident,the core spray pumps transfer water from the suppression pool (orstorage tank) to the reactor pressure vessel to compensate for

inventory lost through a large broken pipe.


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In event of a small broken pipe, the high pressure coolant injection(HPCI) system uses a steam-driven pump to transfer water from astorage tank (or suppression pool) to the reactor pressure vessel.


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Several stages of low pressure heaters use steam from the turbine towarm up the water being returned to the reactor vessel. Thisimproves the efficiency of the plant as less reactor power is wasted

warming water and more goes to boiling water and making steam.


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The high pressure heaters use turbine steam to warm the water up to375 to 400 degrees Fahrenhite. Water leaving the condensor has itspressure increased by the condensate, condensate booster, and

feedwater pumps. The increasing pressure allows its temperature torise from around 100F to nearly 400F before entering the reactor vessel.


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