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Pressurize Water Reactor (PWR): Generic Design Overview. Contents. Introduction to Pressurize Water Reactor Reactor core and associated features Reactor coolant system Containment structures and systems Emergency Core Cooling System Supporting and auxiliary systems.
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Contents • Introduction to Pressurize Water Reactor • Reactor core and associated features • Reactor coolant system • Containment structures and systems • Emergency Core Cooling System • Supporting and auxiliary systems
Fundamental safety functions Control of reactivity Confinement of radioactive materials Cooling of the core • Steam generators • RHR • Safety injection • … • Control rods • Boron concentration • Fuel cladding • Primary cooling system • Containment
Pressurize Water Reactor • Pressurized water reactor units use a dual cycle concept, in which the closed primary cycle is separate from the secondary cycle. • The point of heat transfer between the two cycles is the steam generator(s). • The RCS is the primary cycle and is located inside the containment building. • The secondary cycle includes the steam supply system, the turbine generator (where steam energy is used to generate electric power), and the condensate and feed water systems. • Systems, components, structures, and buildings which have safety functions or are required to maintain the integrity of the RCS or the reactor core, under accident conditions, must be built to Seismic standards and other safety standards as appropriate.
Reactor Vessel - Inlet and outlet nozzles are located in a horizontal plane below the reactor vessel flange but above the top of the fuel assemblies. Coolant enters the reactor vessel through the inlet nozzles and flows down the annulus between the vessel wall and the core barrel of the core support structure, then turns at the bottom and flows up through the core to the outlet nozzles. All reactor vessel internals are supported from the internals support ledge, which is machined into the reactor vessel flange.
Reactor Vessel Cutaway RPV Segments Core flow paths
Reactor Core Components The reactor core components consist of the fuel assemblies and all components which can be inserted into fuel assemblies to affect reactor power, power distribution, or flow distribution. Fuel Assembly All fuel assemblies are mechanically identical, open cage assemblies. Each fuel assembly consists of 264 fuel rods, 24 guide thimble tubes, and 1 instrumentation guide thimble supported and aligned by grid assemblies and top and bottom nozzles in a 17 x 17 fuel rod array. The instrumentation guide thimble is located in the centre position and provides a channel for the insertion of an in core neutron detector. The absorber rod guide thimbles provide channels for insertion of an RCCA, a neutron source assembly, a burnable poison assembly, or a thimble plug assembly, depending on the position of the particular fuel assembly in the core.
Reactor Core 17x17 fuel assembly Spacer Grid assembly Fuel assembly
Reactor Core Fuel Pin In Core Instrumentation
Reactor Coolant System (RCS) • The RCS consists of four similar heat transfer loops connected in parallel to the reactor vessel. Each loop contains; - a steam generator, - a reactor coolant pump, - penetrations for connection with auxiliary and emergency systems, and appropriate instrumentation, • In addition - a pressurizer is connected by a surge line to one loop for pressure control. A pressurizer relief tank is provided to collect any release from the pressurizer relief and code safety valves. • All RCS components are located inside the containment
Steam Generators The steam generators are vertical, shell-and-U-tube heat exchangers which; - Transfer energy from the radioactive reactor coolant to the nonradioactive steam system. - The dry, saturated steam produced is used to operate the main turbine and auxiliaries to produce electricity. - The steam generator U-tubes provide the boundary between the primary and secondary systems. Steam generator level, steam flow, feed flow and steam pressure instrumentation is provided for indication, control, and protection.
Reactor Coolant Pumps • The reactor coolant pumps provide sufficient flow to ensure adequate heat transfer from the core to the steam generators. • A flywheel extends flow coast down after a loss of power to maintain flow through the core and help establish natural circulation. • An antireverse rotation device prevents the pump from turning backwards • Shaft sealing is accomplished by a film-riding, controlled-leakage seal with a backup, rubbing-face seal.
Pressurizer The pressurizer, build with heaters, sprays, and relief and safety valves, The pressurizer is used to: • pressurize the RCS during plant start-up, • to maintain normal RCS pressure during steady-state operation, • to limit pressure changes during RCS transients, and • To prevent RCS pressure from exceeding design values. Pressurizer pressure, level, and temperature instrumentation is provided for indication, control, and protection.
Containment The purposes of the containment are as follows: 1. Provides a barrier to prevent the escape of radioactivity during normal and accident conditions 2. Provides protection against internally and/or externally generated missiles 3. Provides biological shielding during normal and accident conditions 4. Provides Seismic Category I supports for the reactor coolant system (RCS) and its associated support systems.
Auxiliary Feed water (AFW) System The purposes of the AFW system are as follows: 1. To provide feed water to the steam generators to maintain a heat sink for the following conditions. a. Loss of main feed water (MFW), b. Unit trip and loss of offsite power, and c. Small-break loss-of-coolant accident (LOCA). 2. To provide a source of feed water to the steam generators during plant start-up and shutdown.
Auxiliary Feed water (AFW) System The AFW system is activated automatically. It has: Motor-Driven Auxiliary Feed water Pumps and Turbine-Driven Auxiliary Feed water Pump The following conditions will automatically start AFW pumps: 1. Low-low water level in any single steam generator, 2. Loss of one main feed pump (MFP) if power is greater than 80 percent, 3. Loss of both MFPs at any power level, 4. Safety injection actuation signal, and 5. Loss of power to the Class IE power distribution system. 6. Low-low level in any single steam generator
Main Steam System The purposes of the main steam system are: 1. To transfer steam from the steam generators to the turbine-generator and other secondary system components, 2. To provide overpressure protection for the steam generators, and 3. To provide diverse methods of decay heat removal.
Main Steam System • During normal operation the four steam generators deliver saturated steam through four separate steam lines to the main turbine. • These lines are cross-connected by a bypass header to ensure that the steam generators are loaded equally. The bypass header also supplies steam to a number of secondary plant components. • Each main steam line is provided with an isolation valve and check valve just outside containment. The main steam piping from the steam generators to these valves is Seismic Category I and contains several safety-related features. • The main steam isolation valves automatically close on either of the following signals: High steam line flow coincident with low steam pressure or low-low Tavg, or High-high containment pressure.
Turbine & Auxiliaries - The nuclear steam supply system supplies steam to the main turbine. The main turbine consists of one high pressure turbine and three low pressure turbines coupled to the main generator. - Hydraulically controlled governor valves regulate the flow of steam to the turbine, thereby maintaining a desired speed or electric load for the grid.
Turbine & Auxiliaries Steps : - The thermal energy of the steam is converted to mechanical energy in the high pressure turbine, and the steam is exhausted to the moisture separator reheaters. - In each moisture separator reheaters (MSR) the steam is dried, reheated, and superheated prior to its entry into the low pressure turbines. The MSRs improve secondary cycle efficiency and minimize low pressure turbine blade erosion - Energy conversion occurs again in the low pressure turbines as the steam expands into the vacuum of the main condenser.
