Navigation and service

Boiling water reactors

Schematic diagram of a boiling water reactor (BWR) - German only Boiling water reactorSchematic diagram of a boiling water reactor (BWR) - German only Source: Deutsches Atomforum e. V.

Like pressurised water reactors, boiling water reactors (BWR) belong to the design of light-water-reactor.

Compared with the pressurised water reactor (PWR), there is a relatively low pressure in the reactor pressure vessel of the boiling water reactor (about 70 bar, thus about half as high as in the PWR).

The coolant water flows through the reactor core from bottom to top, discharging the heat produced in the fuel elements. Part of it evaporates above the reactor core at approximately 290°C (steam dome). The steam emerging is directly led to the turbine, driving it. This is done via steam dryers which separate the humidity contained in the steam.

Cooling water system

Info: Light-water reactors

The difference between the various reactor types is in the coolant used (water, gas or liquid metal) and the moderator used (a substance that slows down fast neutrons, thus enabling and maintaining the chain reaction – thermal fission). Water or carbon in the form of graphite can be used as moderators.

Light-water reactors

Today, light-water reactors are used in Germany, which are the most common types of reactors used world-wide. Among light-water reactors are pressurised water reactors and boiling water reactors. In light-water reactors, normal water (light water) is used as coolant. At the same time the water serves as moderator.

One molecule of water (H2O) consists of two hydrogen atoms and one oxygen atom. If both hydrogen atoms (H) do have only one proton (positively charged module) in the core but no neutron (uncharged module of the nucleus), the combination with oxygen is termed "light water".

In the case of "heavy water", on the other hand, both hydrogen atoms in the core have one proton and one neutron each. These hydrogen atoms are also termed deuterium – an isotope of hydrogen.

The number of protons and neutrons in the core determine the mass number of a nucleus. The hydrogen atoms of heavy water show a larger mass (u≈2) than the hydrogen atoms in light water (u≈1).

The "spent" steam that has transferred a major part of its energy to the turbine, is cooled in the condenser with the help of another circuit (cooling water system), condenses to water again and is fed back to the reactor core through pumps.

Radioactive materials reach turbine

The pipelines (main steam lines and feed water lines) lead from the containment into the power house. Since the water steam may contain radioactive materials, the main stream lines, the turbine, the condenser and the feed water lines may contain radioactive depositions. That is why, in the case of the BWR, the power house is also part of the plant's control area and is correspondingly protected (e.g. shielding of the turbine).

A number of safety devices have been installed to immediately separate the reactor from the power house in the event of an accident (so-called penetration isolation).

Control of nuclear fission in the BWR

Circulation pumps integrated in the reactor pressure vessel mix the feed water pumped from the condenser with the water in the reactor pressure vessel that has not evaporated. Depending on the volume circulated, the temperature of the coolant flowing through the fuel elements changes. This also influences the share of steam in the area of the reactor core.

Steam has a lower moderation effect than water. The more steam there is in the area of the reactor core, the fewer nuclear fissions take place. Thus, the reactor power decreases (negative steam bubble coefficient). By changing the speed of the circulation pumps, the reactor power can thus be influenced via the share of steam bubbles in the coolant water. A lower coolant flow rate reduces the reactor power by increasing the share of steam bubbles, and vice versa.

The reactor control rods containing neutron-absorbing material (so-called neutron poisons) are loaded into the reactor core from below and regulate the reactor. In the event of a reactor trip the control rods are pneumatically "shot" into the reactor core, thus terminating the chain reaction.

Futher Information

State of 2018.02.09

© Federal Office for the Safety of Nuclear Waste Management