Introduction
An interactive 3D model of a boiling water reactor (BWR) often used in nuclear power generation plants.
How Boiling Water Reactors Work
This video is part of our Introduction to Steam, Boilers and Thermodynamics Video Course
Boiling Water Reactor (BWR) Components
Overview
The reactor vessels utilised for a particular product line are dependent on the vintage of the plant, core cooling regulations, type of recirculation system, and technology used during its period of design. The reactor vessel houses the reactor core, serves as part of the reactor coolant boundary, supports and aligns the fuel and control rods, provides a flow path for the circulation of coolant past the fuel, removes moisture from the steam exiting the reactor vessel, limits the downward control rod motion following a postulated failure of a control rod drive housing, and in all cases except the BWR/2 product line provides an internal refloodable volume following a loss of coolant accident.
BWR Reactor Vessel
This BWR vessel is an insulated pressure vessel mounted vertically within the drywell and is comprised of a cylindrical shell with an integral hemispherical bottom head; the top head is also hemispherical but is removable to facilitate refuelling operations. The base material of the vessel is high strength alloy carbon steel. All internal surfaces including the shell, heads, flanges, and attachments are clad with Type 304 stainless steel to a thickness of 0.25 inches. Small nozzles which are not practicable to clad internally with stainless overlay are solid nickel-chromium-iron alloy.
Vessel Head
The vessel head is attached to the vessel shell by six-inch diameter studs that are threaded into bushings in the vessel flange. Spherical washers and closure nuts are match marked in sets of two and are used in sets. To secure the head to the vessel shell, the studs are elongated by hydraulic stud tensioners, which permit the nuts to be turned while the stud is under tension.
Leakage of radioactive coolant and steam to atmosphere, between the mating surfaces of the vessel and closure head flanges, is contained by two self-energising O-ring gaskets. These silver-plated and polished Ni-Cr-Fe- (Inconel) O-rings are -approximately 0.50 inches in diameter. The O-rings are designed to have no detectable leakage through the inner or outer member during any reactor operating condition.
Vessel Internals
The major reactor vessel internal components included in this boiling water reactor are the core support assembly, core shroud, diffuser, core plate, upper core grid, core spray system sparger, feedwater sparger, steam separators and dryers.
Core Support Assembly
The core support assembly consists of a stainless-steel forged ring that is welded to an Inconel segment. The Inconel segment is welded to the lower shell of the vessel. The core support assembly supports the core shroud and separates the recirculation system suction from its discharge.
Core Shroud
The core shroud is supported by the core support assembly. The core shroud along with the core support assembly forms a 17-inch water annulus inside the reactor vessel wall. In addition, a flow barrier is provided by the lower portion of the shroud and the support assembly. This conical skirt, welded to the reactor vessel wall, effectively separates the recirculation inlet core flow from the downcomer annulus flow.
Diffuser
The vessel diffuser is a cylindrical shell hanging downward from a shelf provided by a ring girder. The diffuser contains hundreds of 1.25-inch diameter holes and is approximately eight feet in height. The diffuser serves a twofold purpose; it prevents direct contact of the recirculation flow to the control rod guide tubes, and provides a uniform flow of coolant below the fuel orifice region.
Core Plate
The core plate is provided to laterally guide and align the control rod guide tube and fuel support castings. Peripheral fuel assemblies located outside the control rod pattern are supported vertically by the core plate. These peripheral fuel assemblies rest in a fuel support piece that is welded to the core plate. The core plate prevents recirculation flow from bypassing the fuel assemblies by directing the flow into the control rod guide tube.
Upper Core Grid
The upper core grid or top guide is mounted and supported by brackets inside the shroud. Bolts are provided to laterally position and level the top guide. Half of the hold down bolts are used to attach the top guide to the ledge of the core shroud.
Core Spray Sparger
Independent core spray loops are installed in the vessel above the upper core grid (top guide) and within the core shroud. The loops are connected to the core spray system which is used for core cooling under loss of coolant accident conditions.
Feedwater Sparger
The feedwater spargers are mounted to the reactor vessel wall in the upper part of the downcomer or annulus region. The spargers, each supplied by one of the two feedwater nozzles, complete a half circle of the vessel interior and discharge water radially inward. A number of 1-inch holes in each sparger permits the cooler feedwater to mix with downcomer recirculation flow before coming in contact with the vessel.
Steam Separator
The steam separator assembly consists of the shroud head and an array of standpipes with steam separators located above each standpipe. The shroud head mates with the core shroud and is bolted to it. The shroud head is a dished unit and forms the cover of the core discharge plenum region. A metal-to-metal contact seals the separator assembly and the core shroud flange.
Steam Dryer
Steam dryers are required to dry the wet steam. The dryer assembly is supported by internal vessel pads. Vertical guides inside the vessel provide alignment during installation; hold down bolts maintain the unit in position.
The dryer assembly is mounted in the vessel above the steam separator assembly and forms the top and sides of the wet steam plenum. Steam that has passed through the separators enters the chevron-type dryer units. A series of troughs and tubes remove the remaining moisture which flows into the downcomer annulus.
Additional Resources
https://en.wikipedia.org/wiki/Boiling_water_reactor