2.6 The Most Used Nuclear Reactors: PWR and BWR

PWR – the most widespread type of nuclear reactor

The most widespread type of nuclear reactor in the world is the pressurized water reactor known by the abbreviation PWR and its Russian modification known as VVER. The reactor uses ordinary water circulating within the primary circuit to remove heat from the core. The water in the reactor is heated up to 320 °C and is under such high pressure that it remains liquid and does not boil under any circumstances. The heated water flows from the reactor through pipes to the steam generator where it transfers thermal energy to the secondary circuit.

The basis of a pressurized water reactor is a thick-walled cylindrical pressure vessel welded from rings of low-alloy carbon steel, withstanding approximately 150 times atmospheric pressure. Due to higher corrosion resistance, the inner surface of the vessel is provided with a layer of stainless-steel lining of several millimetres. The upper part of the pressure vessel forms a removable cover, allowing access to all fuel assemblies located in the core during a fuel change shutdown. During operation, the lid is hermetically connected to the container body using bolts. This is the most complex part of the pressure vessel, because all the mechanisms of the regulatory and emergency elements and the outputs of the internal reactor measurements pass through it.

Fuel for PWR reactors is most often in the form of small ceramic pellets made from uranium oxide enriched to 3.5—5%. Pellets stacked in closed zirconium tubes form fuel rods from which square or hexagonal fuel assemblies are then assembled. The core of the reactor, composed of 4-metre-long fuel assemblies, has a diameter of 3 m. Control rods containing boron or cadmium are used for immediate control of reactor power and boric acid dissolved in coolant is used for slow control and compensation of excess reactivity.

The coolant in PWR reactors is ordinary light water. It is safe, cheap and its properties are well known. When passing through the core, the water flows between the fuel cells and is heated by approximately 30 °C from them. In the upper part of the reactor, above the core, the hot water is evenly divided into individual circulation loops. It flows through the primary pipe to the steam generators where it transfers the heat generated in the fuel to the steam-water mixture of the secondary circuit. The cooled water of the primary circuit is returned from the steam generator to the reactor using the main circulation pump.

Water, which removes heat from the reactor, is also a moderator of neutrons. Slowing down neutrons is needed to increase the probability of fuel fission and contributes to the successful course of the fission reaction during reactor operation. Compared to other moderators such as graphite or heavy water, ordinary water has a slightly lower moderating ability, so the fuel in PWR reactors must be enriched with the fissile isotope uranium 235 to a minimum of 2%.

For safety reasons, the reactor and all parts of the primary circuit are placed in a protective cover, which fulfils the function of mechanical protection of the reactor against external influences and at the same time functions as a closed hermetic space to protect against the leakage of dangerous substances into the external environment. During normal operation and in the event of an accident, the massive containment acts as high-quality radiation shielding. 

BWR - reactor with steam generator function

The second most common type of nuclear reactor is the boiling water reactor, known by the abbreviation BWR. Like the PWR, it is cooled and moderated by light water, but it is converted into steam in the reactor, which flows directly into the turbine. Power plants with BWR reactors are in principle a single-circuit. This means that the same water and then steam that removes the heat from the reactor propels the turbogenerator and returns to the reactor after condensation behind the turbine. Although such a connection is simpler and cheaper, on the other hand, it brings the possibility that radioactivity will reach the turbine. The controlled area must therefore also include the entire auxiliary building.

The reactor is a steel pressure vessel but in the case of a BWR it is proportioned for about half the pressure of a PWR. The structures inside the pressure vessel allow not only the safe storage of nuclear fuel in the core but also ensure the functions of the steam generator, especially regarding the separation and drying of the generated steam. Due to the presence of the hot steam at the outlet of the core and the occupation of the upper space of the pressure vessel by the separator and dryer steam systems, the control and emergency rods are inserted into the reactor from below. The performance of BWR reactors can be regulated mainly by changing the position of these control rods and by changing the flow rate of the coolant with the content of the steam-water mixture through the core.

The fuel is ceramic pellets made of slightly enriched uranium oxide. Square fuel assemblies of approximately 4 metres in length are assembled from pellets stacked into 14 mm diameter fuel rods. Four fuel assemblies, together with an absorption rod with a cross-section in a shape of cross moves, form the basic fuel module of BWR reactors. Fuel is refuelled to the reactor once a year during shutdown, when the reactor is opened, the inside of the reactor structure is removed, and a quarter to a third of the fuel assemblies are replaced using a loading machine.

Ordinary demineralized water with a pressure of 7 MPa fulfils the function of a coolant for heat removal and at the same time, a moderator that slows down fast neutrons. At this pressure, the saturation temperature is around 285 °C, so the generated steam leaving the reactor is at this temperature. A specific feature of boiling reactors is the presence of a certain amount of steam bubbles in the core. A larger amount of bubbles in boiling water reduces its moderating abilities and thus dampens the ongoing fission reaction. The mentioned property is used for the regulation of the reactor performance.

The construction of nuclear power plants with BWR reactors has a number of advantages. Apart from the simplicity and compactness, there is the need for lower uranium fuel enrichment, better fuel utilization efficiency due to higher plutonium formation, longer equipment life resulting from lower operating pressure and lower power density compared to PWRs and the last but not least, the large negative temperature coefficient of reactivity that has a significant effect on the safety of BWR reactors.

In power plants with BWR reactors, all operational systems, including the fuel exchange system, are hermetically sealed within a protective cover consisting of a steel jacket and a concrete building. In case of an accident, the protective cover fulfils an important safety function — it prevents the release of fission products into the environment.