核反应堆工程核反应堆工程 (4).ppt

Nuclear Engineering ReactorIntroduction of Boiling Water Reactor Overview1Boiling Water Reactor System Boiling Water Reactor Cycles 23BWR Containment Systems 4Boiling Water Reactor ECCS1 BWR systemBWR system1 BWR systemnBWR design subsequently underwent a series of evolutionary changes with one purpose in mind:simplicity.The BWR design has been simplified in two key areas:reactor systems and containment design.nIt has no steam generator but adopt direct circulation,low system pressure and simple circulation circuit.Water is circulated through the reactor core,producing saturated steam,dried in the top of the vessel,and directed to the steam turbine-generator.nThe steam produced by the nuclear core is radioactive.The radioactivity is a very short-lived isotope(half-life of 7 seconds)so that the radioactivity of the steam exists from the reactor vessel only during power generation and there is no long-lived radioactive particles.2 BWR cyclesnThe BWR is characterized by the fact that bulk boiling takes place in the core.However,BWRs can be further characterized by whether the steam generated in the core passes directly to the turbine or is used to make steam for the turbine in a primary/secondary heat exchanger.nWhen the steam generated in the core passes directly to the turbine,the plant is classified as a direct-cycle BWR.When the steam/water mixture generated in the core passes through a steam generator,such that the steam going to the turbine is produced on the secondary side of the steam generator,the plant is classified as an indirect-cycle BWR.nThe final cycle of interest is the so-called dual-cycle BWR,in which part of the steam going through the turbine is produced directly in the core and part is produced in a steam generator.2 BWR cyclesVarious boiling water reactor cycles2 Dual-cycleDresden-1 circulation systemDual-cycle BWRThe dual-cycle design was chosen as the basis for the first commercial BWR since it combined reliability,stability,and fairly high power density with a well-controlled reactor response to load changes.2 BWR cyclesnThe unique features of a dual-cycle BWR can best be described with reference to some operational transients.If the load on the turbine increases,it tends to slow down;however,in a dual-cycle plant,a signal is sent to the secondary steam generators control valve,which causes sufficient steam to be admitted to a lower stage of the turbine to carry the increased load.nThus,in most cases the load change is taken up by the secondary steam generator such that the primary pressure and steaming rate from the reactor are essentially unchanged.nThe dual-cycle plant could handle many operational transients with minimum perturbation to the primary system and a minimum of control rod action.2 BWR cyclesnHowever,indirect-cycle BWR plants required the additional capital cost and maintenance expense associated with large steam generators and large containment buildings and requires relatively high operating pressures on the primary side.nSignificantly,the inherent simplicity and thermodynamic potential of the BWR concept can be realized fully only in a direct-cycle plant.nThus,starting in 1963,all subsequent plant offerings were exclusively of the direct-cycle type.3 BWR Containment Dry containment The dry containment is a large dry shell structure,surrounding the primary system,which could withstand the pressure increase resulting if all reactor fluid mass and energy were released through a rupture of the largest pipe.Pressure suppression containment designs have substantially smaller volumes than dry containments because of the high energy absorption capacity of the water pool.3 BWR Containment nThe drywell surrounds the reactor and provides a primary barrier to steam/water release during a LOCA and to subsequent fission product releases.nPressure suppression in the wetwell region then takes the steam and rapidly condenses it in the water pool.The suppression pool is a large,passive heat sink,which can absorb all the steam released during a LOCA plus an additional amount that is generated by decay heat in the core.nThe pressure quickly reaches a maximum value,which decreases rapidly after the vent water is expelled and steam/air discharge to the pool begins.3 BWR Containment It is a typical containment pressure transients for the drywell and wetwell resulting from a large break LOCA and complete discharge of the reactor coolant.3 BWR Containment GE pressure suppression system designs3 BWR Containment 4 BWR ECCSnThe main purpose of the emergency core cooling system(ECCS)is to provide sufficient cooling of the core to prevent gross core meltdown and/or fuel cladding fragmentation,thereby limiting release of radioactive materials and ensuring that the core maintains a coolable geometry.nThe typical BWR ECCS is composed of four separate subsystems:the high pressure core spray(HPCS)system,the automatic depressurization system(ADS),the low-pressure core spray(LPCS)system,and the low pressure core water injection(LPCI)system.nIn addition,the reactor core isolation cooling(RCIC)system and the residual heat removal(RHR)system can be used for emergency core cooling.4 BWR ECCS