A boiling water reactor or BWR is a type of nuclear reactor. It is the second type of reactor most used in nuclear power plants in the world. Approximately 22% of the nuclear reactors installed in the different nuclear power plants use the boiling water reactor.
The main function of this type of reactors is their installation in power plants of power for the generation of electricity.
The most important feature of the boiling water reactor (BWR) is the use of pressurized water as a neutron moderator and as a core coolant. Unlike the pressurized water reactor (PWR), it does not have a steam generator.
By not having to withstand such high pressures, this type of reactor does not need such a robust housing.
Boiling water reactor (BWR) working
The boiling water reactor uses a single light water cooling circuit (it is tap water, in nuclear engineering the tap water is called light water). The light water circulates through the core of the reactor, enters boiling and a part of it turns into vapor.
The steam generated in the reactor core comes out from the top and goes directly to the turbines. The steam leaving the reactor is treated by steam driers and water separators before entering the turbines. The turbines will be responsible for operating the electric generator and generating electricity.
Next, the steam passes through a condenser to turn it into liquid water again and start the cycle again.
Characteristics of boiling water nuclear reactor (BWR)
It uses a single cooling circuit, so that the steam that moves the turbine is formed by water that has passed through the interior of the reactor. For this reason, the turbine building must be protected to avoid radioactive emissions.
On the other hand, the need for more space for steam dryers and separators in the reactor vessel forces the control rods to enter the lower part of the reactor, so auxiliary energy is needed to raise them and stop the reactor in case of emergency.
Distinctive features of the BWR
In nuclear power plants with reactors that are not boiling, the temperature of the water in the primary circuit is below the boiling point. At temperatures necessary to obtain an acceptable efficiency (above 300°C), this is possible only at high pressures, which requires the creation of a high strength housing.
In the secondary circuit, saturated steam is produced at a pressure of 12 to 60 atm at temperatures of up to 330°C. In the boiling reactors, a mixture of steam and water is produced in the core. The water pressure in the primary circuit is approximately 70 atm. At this pressure, water boils in the core volume at a temperature of 280°C. Boiling water reactors have a number of advantages compared to non-boilers. In boiling reactors, the casing operates at a lower pressure, in the nuclear center circuit there is no steam generator.
The peculiarity of the boiling reactors is that they do not have boric control, the compensation for the slow changes in the reactivity (for example, the burning of the fuel) is carried out only by intercased absorbers made in the shape of a cross. The boric regulation is not feasible due to the good solubility of boron in a pair (most will be taken to the turbine). Boron is injected only at the time of fuel overload to create a deep subcriticality.
In most boiling reactors, the absorption bars of the control and protection system are located in the lower part. Therefore, its efficiency increases significantly, since the maximum thermal neutron flux moves in reactors of this type in the lower part of the nucleus. Such a scheme is also more convenient during fuel accelerations and releases the upper part of the control rod drives of the reactor, thus allowing more convenient disposal for water vapor.
Advantages and disadvantages of the boiling water reactor
Advantages of this type of nuclear reactor
The boiling water reactor does not use steam generators or pressure compensators.
Lower operating temperatures, even in the fuel rods.
Due to the rejection of neutron absorption in boron and slightly weaker moderation of neutrons (due to vapor), the operating time of plutonium in a reactor of this type will be longer than in the PWR and the proportion of uranium -238 used will also be greater.
Disadvantages of this type of reactor
Impossibility of recharging nuclear fuel without stopping the nuclear reactor.
More complicated management, the presence of prohibited modes in the power / flow capacity of the heat carrier, the need for a greater number of feedback sensors.
A reactor vessel about 2 times more in volume than a comparable power PWR is needed.
Although designed for lower pressure, it is more difficult to manufacture and transport.
Turbine contamination with water activation products: N-17 with short life and traces of tritium. This complicates maintenance work a lot. In addition, the traps must be configured to extract radioactive corrosion products from the steam loops.
Radiolysis cavitation and corrosion in the fuel rods with the elimination of radioactivity in the turbine and the condenser, as well as with the elimination of hydrogen and oxygen from AZ (real cases oxyhydrogen gas explosions to the damage system nuclear power plant Hamaoka- 1 and Brunsbuttel nuclear power plant)