The boiling water reactor is the second most common type of nuclear reactor. The reactor is where the controlled fission of nuclear fuel nuclei takes place, releasing nuclear energy that, as heat, is used to generate electricity. Ordinary water is used as both the moderator and the coolant. The water boils directly inside the reactor.

Water must constantly flow through the reactor core. This is ensured by a recirculation circuit. Water is taken from the reactor and pumped through a set of jet pumps to the bottom of the reactor vessel, from where it flows through fuel channels.

The steam from the reactor is led to the turbine.

After condensed in a condenser, feedwater runs back to the reactor. Because it is in direct contact with nuclear fuel, it has to be very pure to prevent corrosion. The content of radioactive elements in the feedwater is regularly controlled. Higher radioactivity of feedwater could signal damage of the nuclear fuel.

Spent fuel withdrawn from the nuclear reactor is stored under water. The water acts as a radioprotective shielding and as a coolant.

The fuel loading machine is a piece of equipment which handles nuclear fuel and is designed for loading and unloading operations with nuclear fuel.

The steam turbine is used to drive an electric generator. A steam turbine is a rotary heat engine that converts the kinetic and thermal energy of flowing steam into mechanical rotary motion transmitted by a mechanical shaft. The turbine consists of one or several sections. Propeller wheels, a part of the machine stator, are called distribution wheels. The others, which are connected to the rotating shaft of the machine, are called impellers and, together with the shaft, form the machine rotor. Large steam turbines are usually divided into several sections– high-pressure and low-pressure, or even medium-pressure. Steam pressure and temperature in a BWR power plant are low compared to a modern coal-fired power plant and the steam turbine is generally very large. Because the steam is exposed to the core, there is some radioactive contamination of the turbines but this is short-lived and turbines can normally be accessed soon after shutdown.

An electric generator transfers the rotational movement of the turbine rotor to electric energy using magnetic induction. Electric generators consist of a rotor and a stator. The rotor usually generates a rotating magnetic field, and coils are located in the stator, where electric voltage is induced.

The steam from the turbine’s output is condensed inside the condenser. The condenser is a kind of heat-exchanger. The steam gives its heat to cooling water from a secondary cooling circuit. This cooling water can run through cooling towers or can be taken from a river or sea, if such a kind of large water area is available. Cooled water from the primary circuit is then pumped as feedwater back to the reactor.

In order to condensate water in the condenser, the heat has to be removed from the steam. This is done by cooling water usually taken from a river, lake or sea. This is why the power plant has to be located on the shore of such a large body of water. Having been used as cooling-water, the water is warmed up. The warmer the water, the less oxygen it contains, which is harmful for fish and other fauna in the river. If the temperature is too high, the cooling water must be additionally cooled down in the cooling towers before its release into the river.

A large pool of water located below the reactor’s pressure vessel. BWRs are equipped with a set of safety valves that vent the excess steam pressure and collect it after condensation a in suppression pool (also called a ‘wetwell’, or ‘wet torus’ in those reactor designs where the suppression pool has a toroidal shape).

The reactor’s containment made from reinforced concrete is usually smaller than for a PWR nuclear power plant, because a BWR works with lower temperatures and lower pressures. Due to the presence of steam inside a reactor, drywell is connected to a suppression pool, where in case of accident the steam can be diverted, cooled and condensed.

Net electrical power output is a measure of how much electricity is supplied to the grid. Part of the generated electricity is consumed by the power plant itself for production purposes. Since the power output differs between winter and summer, due to seasonal variation in the cooling efficiency, an average power output or the lowest summer output is used.

A set of nuclear fuel assemblies located in the lower part of the vessel. The fission reaction is held in the uranium fuel and produced heat boils the water flowing through the reactor core. The resulting mixture of water and steam is then separated in the upper parts of the vessel, steam is dried and passed to the turbine. The reactor core consist of 370 - 800 assemblies according to design. Light water serves as a coolant and as a moderator (to slow down neutrons). Its usual operating pressure is around 7 MPa and temperature 285°C. Control rods are inserted from the bottom between fuel assemblies to control the course of fission reaction.

The movable boron-carbide (B₄C) control rods (also called regulating rods) are sufficient to provide reactivity control from a cold shutdown condition to a full load condition. The absorber concentration is decreased by pulling the rods out of the reactor core, causing the reactor power to increase. Inserting more control rods causes the reaction to be inhibited and the power output decreases. Control rods are inserted from the bottom of the reactor vessel, because the reactors upper part is filled with steam with lower moderating power than liquid water. The control rods have a cruciform cross section and move in the gaps between every group of four fuel assemblies. In addition to the use of control rods, it is possible to control the reactor output via the rate of coolant circulation.

A BWR fuel bundle typically contains 100 rods, which are spaced in a 10x10 square array supported by lower and upper tie plates. Three types of rods are used in a fuel bundle: tie rods, water rods, and standard fuel rods. Rods are encapsulated in a square-shaped tube called a fuel channel - a fuel bundle and channel together forms a fuel assembly. Between the fuel assemblies, the control rods in the shape of cruciform blades move. In order to obtain the best performance and avoid local power peaking, BWR fuel rods have several axial segments with different enrichments and a BWR fuel assembly has several different rods with different enrichments.

The reactor vessel is a pressure vessel with a single full-diameter removable head. The base material of the vessel is low alloy steel, which is clad on the interior except for the nozzles which have a stainless steel weld overlay to provide the necessary resistance to corrosion. In the lower part of the vessel is situated a reactor core with control rods inserted from the bottom of the vessel, in the upper part the steam is formed and dried before it is allowed to flow to the turbine. Since the vessel head is exposed to a saturated steam environment throughout its operating lifetime, stainless steel cladding is not used in its interior surfaces.

The coolant is highly pure light water free of dissolved absorbers. The water is brought into the reactor above the core and uniformly distributed around the entire circumference of the vessel. It runs down along the vessel walls and only then passes through the fuel assemblies.

The main function of jet pumps is to force a flow through the nuclear core. It increases the power of the reactor compared to that obtained by natural circulation. 16 - 24 jet pumps are located in the cylindrical annular zone of the reactor and contain no moving parts. Cooling water is supplied into the jet pumps inlet by a recirculation pump located outside the reactor vessel. The water in the jet pump nozzle is accelerated to a high velocity because of the constriction at the nozzle outlet. This fast flow creates a reduced pressure that sucks water from between the core shroud and the vessel’s inner wall. Both streams mix beyond the nozzle aperture resulting in a flow to the base of the core where it is directed to each of the fuel channels.

The function of the reactor water recirculation system is to circulate the required coolant through the reactor core. Recirculation pumps are located outside the reactor vessel.

Feeds water intake into jet pumps. The function of the reactor water recirculation system is to circulate the required coolant through the reactor core. Recirculation pumps are located outside the reactor vessel.

The steam dryer is mounted in the reactor vessel above the steam separator. Steam from the separators flows upward and outward through the drying vanes. It removes all remaining water droplets from the steam before letting it into the turbine.

The steam separator assembly consists of a domed base on top of which is welded an array of standpipes with a three-stage steam separator located at the top of each standpipe. These fixed axial flow-type steam separators have no moving parts and are made of stainless steel. In each separator, the steam–water mixture rises through the standpipe impinging on the vanes, giving the mixture a spin to establish a vortex wherein the centrifugal forces separate the water from the steam in each of the three stages. The separated water exits the lower end of each stage of the separator and flows back to the reactor.

Dried steam exits the reactor vessel and flows into the turbine.

The reactor stands on a cylindrical concrete pedestal to which it is mounted via the vessel support skirt. The middle part of vessel’s bottom is used for the control rod entrance.

A fuel rod is a long and narrow hollow Zircaloy tube hermetically sealed on both ends. Inside the tube fuel pellets are stacked, fixed by plenum spring. Its length varies from 1.8 to 4.5 m and the diameter is about 12 mm.

A fuel rod is a long and narrow hollow Zircaloy tube hermetically sealed on both ends by welding Zircaloy end plugs. Its length varies from 1.8 to 4.5 m and the diameter is about 12 mm. Inside the tube fuel pellets are stacked, fixed by plenum spring on both ends. The narrow gap between the pellet and the casing is usually filled with helium for better heat removal. This gap also provides space for possible pellet expansion caused by heat and gases produced during fission. A Fuel rod also holds radioactive fission products released by the fuel pellets.

A cylindrical form of nuclear fuel made of sintered uranium oxide UO₂. Uranium is enriched with uranium ²³⁵U isotope to various degrees from natural uranium to approx. 2,2%. Because a BWR cannot use boron in cooling water to absorb neutrons, burnable neutron poison like gadolinium is intimately mixed with some of the UO₂ fuel. Fuel pellets are stacked in a zircaloy tube forming a fuel rod.

Two rods in the fuel assembly are water rods, hollow tubes of Zircaloy 2 cladding without uranium fuel. Small holes are located at the lower and upper ends, allowing water to be driven through the rod. The water serves as a moderator (slows down neutrons) and change in its flow inside fuel bundle helps to control the fission reaction course.

A fuel rod shorter than other fuel rods situated in the lower part of reactor core. Together with the use of different enrichments in different parts of the core, it allows the optimization of thermal and hydraulic performance. The multiple lengths improve the ratio of fuel to moderator to compensate for steam generation.

A square-shaped tube fabricated from Zircaloy 4 encapsulating a fuel bundle. The fuel channels direct the core coolant flow through each fuel bundle and also serve to guide the control rod assemblies - cruciform blades moving in the gaps between every group of four fuel channels.

A zircaloy grid that holds fuel rods at proper locations.

Steam enters the turbine with a certain internal heat energy, which gradually changes in the individual sections of the turbine by expansion to the kinetic energy of the rotating turbine shaft.

The impeller diameters of the last low-pressure sections are limited by the maximum centrifugal force acting on the blades and thus their length. If there is more steam in the low-pressure area of the turbine, it is necessary to divide it into two, sometimes three, low-pressure parts. For multi-section steam turbines, the impeller diameter of each subsequent section is larger, because the gradual expansion of the steam also increases its volume.

The rotor of a steam turbine consists of a central spindle and several runners mounted on the spindle. The energy of the steam in the turbine causes the rotor to rotate, transferring mechanical energy to an electrical generator.

The runner blades have a complex shape, and their manufacture is subject to stringent requirements. They are usually either cast or precision milled. Due to the changing parameters of steam, the blades of each runner are larger then the previous ones. In the case of low-pressure sections their length would exceed strain limits, hence the steam is split into several smaller parallel low-pressure sections.

Wheels with immobile guide vanes are attached to the turbine casing. These vanes direct the steam to the runner blades. Every runner on the rotor has its own stationary wheel with guide vanes.

The entire long turbine rotor is supported by several radial sliding bearings. To eliminate axial forces, a thrust bearing is installed on the spindle. Turbine bearings have their own circulatory system for cooling and lubricating oil.

A turbine's stator casing is usually cast from steel, and for low-pressure parts can also be a welded structure. It is sectioned horizontally and its shape follows the shape of the rotor. A sectioned casing permits convenient installation of guide vanes and precise rotor placement.

The electric generator works on the principle of electromagnetic induction - the rotating magnetic field formed by the rotor coils generates an alternating electric voltage in the fixed stator coils.

In current power systems, a three-phase power line is used, so all power electric generators usually have at least three pairs of stator coils, one for each phase.

Power electric generators usually have at least three pairs of stator coils, one for each phase. The resulting electrical current is then led via three encapsulated conductors to a transformer.