A nuclear reactor is a key device of nuclear power plants, nuclear research facilities or nuclear propelled ships. Main purpose of the nuclear reactor is to initiate and control a sustained nuclear chain reaction. Nuclear reactors are used:

  • at nuclear power plants for electricity generation
  • at nuclear research facilities as a neutron source
  • as a propulsion of nuclear propelled ships.
From the physics point of view, the main differences among reactor types arise from differences in their neutron energy spectra. In fact, the basic classification of nuclear reactors is based upon the average energy of the neutrons which cause the bulk of the fissions in the reactor core. From this point of view nuclear reactors are divided into two categories:

  • Thermal Reactors. Almost all of the current reactors which have been built to date use thermal neutrons to sustain the chain reaction. These reactors contain neutron moderator that slows neutrons from fission until their kinetic energy is more or less in thermal equilibrium with the atoms (E < 1 eV) in the system.
  • Fast Neutron Reactors. Fast reactors contains no neutron moderator and use less-moderating primary coolants, because they use fast neutrons (E > 1 keV), to cause fission in their fuel.
thermal vs. fast reactor neutron spectrum

The spectrum of neutron energies produced by fission vary significantly with certain reactor design. thermal vs. fast reactor neutron spectrum

Most common nuclear reactors are light water reactors (LWR), where light water is used as a moderator. LWR’s are divided into two categories:

  • Pressurized water reactors (PWR) – are characterized by high pressure primary circuit (to keep the water in liquid state)
  • Boiling water reactors (BWR) – are characterized by controlled boiling in the primary circuit

Pressurized water reactor – PWR

Pressurized water reactors use a reactor pressure vessel (RPV) to contain the nuclear fuel, moderator, control rods and coolant. They are cooled and moderated by high-pressure liquid water (e.g. 16MPa). At this pressure water boils at approximately 350°C (662°F).  Inlet temperature of the water is about 290°C (554°F). The water (coolant) is heated in the reactor core to approximately 325°C (617°F) as the water flows through the core. As it can be seen, the reactor has approximately 25°C subcooled coolant (distance from the saturation).

The hot water that leaves the pressure vessel through hot leg nozzle and is looped through a steam generator, which in turn heats a secondary loop of water to steam that can run turbines and generator. Secondary water in the steam generator boils at pressure approximately 6-7 MPa, what equals to 260°C (500°F) saturated steam. Typical reactor nominal thermal power is about 3400MW, thus corresponds to the net electric output 1100MW. Therefor the typical efficiency of the Rankine cykle is about 33%.

Nuclear reactor - WWER 1200

Nuclear reactor and primary coolant system of WWER-1200.
Source: http://www.bellona.ru/

Boiling water reactor – BWR

A boiling water reactor is cooled and moderated by water like a PWR, but at a lower pressure (7MPa), which allows the water to boil inside the pressure vessel producing the steam that runs the turbines. A BWR is like a PWR but with many differents.  The BWRs don’t have any steam generator. Unlike a PWR, there is no primary and secondary loop. The thermal efficiency of these reactors can be higher, and they can be simpler, and even potentially more stable and safe. But the disadvantage of this concept is that any fuel leak can make the water radioactive and that radioactivity can reach the turbine and the rest of the loop.

See also: Boiling water reactor

ABWR boiling water reactor

A boiling water reactor (BWR) is cooled and moderated by water. It takes place at a lower pressure as in PWR, what allows the water to boil inside the pressure vessel producing the steam that runs the turbines.
Source: www.nuclearstreet.com

Manufacturing of the Reactor Vessel – Youtube

Nuclear Reactor - Description
Description of VVER-1000 reactor.

  1. Control Element Drive Mechanism
  2. Reactor vessel head assembly
  3. Reactor pressure vessel
  4. Coolant inlet – outlet nozzles
  5. Downcomer for coolant
  6. Neutron reflector
  7. Fuel assemblies

Source: www.wikipedia.org

Fuel Consumption – Summary

Consumption of a 3000MWth (~1000MWe) reactor (12-months fuel cycle)

It is an illustrative example, following data do not correspond to any reactor design.

  • Typical reactor may contain about 165 tonnes of fuel (including structural material)
  • Typical reactor may contain about 100 tonnes of enriched uranium (i.e. about 113 tonnes of uranium dioxide).
  • This fuel is loaded within, for example, 157 fuel assemblies composed of over 45,000 fuel rods.
  • A common fuel assembly contain energy for approximately 4 years of operation at full power.
  • Therefore about one quarter of the core is yearly removed to spent fuel pool (i.e. about 40 fuel assemblies), while the remainder is rearranged to a location in the core better suited to its remaining level of enrichment (see Power Distribution).
  • The removed fuel (spent nuclear fuel) still contains about 96% of reusable material (it must be removed due to decreasing kinf of an assembly).
  • Annual natural uranium consumption of this reactor is about 250 tonnes of natural uranium (to produce of about 25 tonnes of enriched uranium).
  • Annual enriched uranium consumption of this reactor is about 25 tonnes of enriched uranium.
  • Annual fissile material consumption of this reactor is about 1 005 kg.
  • Annual matter consumption of this reactor is about 1.051 kg.
  • But it corresponds to about 3 200 000 tons of coal burned in coal-fired power plant per year.

See also: Fuel Consumption

Components of a nuclear reactor

The key components common to most PWR types of nuclear reactors are:

Reactor Pressure Vessel

VVER-1000 reactor

VVER – Reactor Pressure Vessel with Reactor Internals.
Source: gidropress.podolsk.ru
used with permission of АО ОКБ “ГИДРОПРЕСС”

The reactor pressure vessel is the pressure vessel containing the reactor core and other key reactor internals.

It is a cylindrical vessel with a hemispherical bottom head and a flanged and gasketed upper head. The bottom head is welded to the cylindrical shell while the top head is bolted to the cylindrical shell via the flanges. The top head is removable to allow for the refueling of the reactor during planned outages.

There is one inlet (or cold leg) nozzle and one outlet (or hot leg) nozzle for each reactor coolant system loop. The reactor coolant enters the reactor vessel at the inlet nozzle and exits the reactor at the upper internals region, where it is routed out the outlet nozzle into the hot leg of primary circuit and goes on to the steam generator. The primary circuit of typical PWR is divided into 4 independent loops (piping diameter ~ 800mm), each loop comprises a steam generator and one main coolant pump, but can differ according to certain reactor design. Therefore numerous inlet and outlet nozzles, as well as control rod drive tubes (in case of BWRs) and instrumentation and safety injection nozzles penetrate the cylindrical shell. This number of inlet and outlet nozzles is a function of the number of loops.

The body of the reactor vessel is constructed of a high-quality low-alloy carbon steel, and all surfaces that come into contact with reactor coolant are clad with a minimum of about 3 to 10 mm of austenitic stainless steel in order to minimize corrosion.

The reactor pressure vessels are the highest priority key components in nuclear power plants. The reactor pressure vessel houses the reactor core and because of its function it has direct safety significance. During the operation of a nuclear power plant, the material of the reactor pressure vessel is exposed to neutron radiation (especially to fast neutrons), which results in localized embrittlement of the steel and welds in the area of the reactor core. In order to minimize such material degradation radial neutron reflectors are installed around the reactor core. There are two basic types of neutron reflectors, the core baffle and the heavy reflector. Due to higher atomic number density heavy reflectors reduce neutron leakage (especially of fast neutrons) from the core more efficiently than core baffle. Since the reactor pressure vessel is considered irreplaceable, these ageing effects of the RPV have the potential to be life-limiting conditions for a nuclear power plant.

Nuclear reactor - WWER 1200

Nuclear reactor and primary coolant system of WWER-1200.
Source: gidropress.podolsk.ru
used with permission of АО ОКБ “ГИДРОПРЕСС”

In typical modern pressurized water reactors (PWRs), the Reactor Coolant System (RCS), shown in the figure, consists of:

All RCS components are located inside the containment building.

At normal operation, there is a compressed liquid water inside the reactor vessel, loops and steam generators.  The pressure is maintained at approximately 16MPa. At this pressure water boils at approximately 350°C (662°F).  Inlet temperature of the water is about 290°C (554°F). The water (coolant) is heated in the reactor core to approximately 325°C (617°F) as the water flows through the core. As it can be seen, the reactor contains approximately 25°C subcooled coolant (distance from the saturation).

___________________________________________

Volumes of typical PWR are in the following table.

It is an illustrative example, following data do not correspond to any reactor design.

volume-of-reactor-coolant-systemThis high pressure is maintained by the pressurizer, a separate vessel that is connected to the primary circuit (hot leg) and partially filled with water (partially with saturated steam) which is heated to the saturation temperature (boiling point) for the desired pressure by submerged electrical heaters. Temperature in the pressurizer can be maintained at 350 °C. At normal conditions, about 60% of volume of pressurizer occupies the compressed water and about 40% of volume occupies the saturated steam.

It must be noted the volume of coolant significantly changes with the temperature of the coolant. The total mass of the coolant remains always the same, a change in water volume is not a change in water inventory. The reactor coolant volume changes with temperature because of changes in density. Most substances expand when heated and contract when cooled. However, the amount of expansion or contraction varies, depending on the material. This phenomenon is known as thermal expansion. The change in volume of a material which undergoes a temperature change is given by following relation:

thermal-expansion

where ∆T is the change in temperature, V is the original volume, ∆V is the change in volume, and αV is the coefficient of volume expansion.

Chart - density - water - temperature

Density of liquid (compressed) water as a function of temperature of water

The volumetric thermal expansion coefficient for water is not constant over the temperature range and increases with the temperature (especially at 300°C), therefore the change in density is not linear with temperature (as indicated in the figure).

See also: Steam Tables

At normal conditions the total volume of coolant in the reactor coolant system is almost constant. On the other hand, during transient load conditions the volume can significantly change. These changes are naturally reflected in a change in pressurizer water level. When the average temperature of reactor coolant goes gradually down, the total water volume is also decreasing, which lowers the pressurizer level. On a gradual load pick-up, the increase in reactor coolant average temperature causes the total water volume to expand, which raises the pressurizer level. These effects must be controlled by pressurizer level control system.

Core Barrel

core barrel

The core barrel inside a reactor pressure vessel of LWR. It is only an illustrative example.

In general, reactor internals  are divided into three structural units:

  • the lower core support structure
  • the upper core support structure
  • the in-core instrumentation

The core barrel belongs to the lower core support structure, because it houses a reactor core. Other lower core support structures (lower core plate, core baffle or heavy reflector) are attached to the core barrel, which transmits the weight of the core to the reactor vessel. The barrel is a long, cylindrical, one-piece welded structure. Like most components of the internals, the core barrel is made of low carbon, chromium-nickel stainless steel, because it is situated in a corrosive environment (primary coolant comprises boric acid), and the material should not get oxidized.

The lower internals and also the core barrel remain in place during refueling, but may be removed for reactor pressure vessel in-service inspections.

Neutron Reflector

It is well known that each reactor core is surrounded by a neutron reflector or reactor core baffle. The reflector reduces the non-uniformity of the power distribution in the peripheral fuel assemblies, reduces neutron leakage and reduces a coolant flow bypass of the core. The neutron reflector is a non-multiplying medium, whereas the reactor core is a multiplying medium.

Neutron Reflector

Neutron reflector inside a reactor core of LWR. It is only an illustrative example.

The neutron reflector scatters back (or reflects) into the core many neutrons that would otherwise escape (i.e. reduces the neutron leakage). By reducing neutron leakage, the reflector increases keff and reduces the amount of fuel necessary to maintain the reactor critical for a long period. In LWRs the neutron reflector is installed for following purposes:

  • The neutron flux distribution is “flattened“, i.e., the ratio of the average flux to the maximum flux is increased. Therefore reflectors reduce the non-uniformity of the power distribution.
  • Because of the higher flux at the edge of the core, there is much better utilization in the peripheral fuel assemblies. This fuel, in the outer regions of the core, now contributes much more to the total power production.
  • The neutron reflector scatters back (or reflects) into the core many neutrons that would otherwise escape. The neutrons reflected back into the core are available for chain reaction. This means that the minimum critical size of the reactor is reduced. Alternatively, if the core size is maintained, the reflector makes additional reactivity available for higher fuel burnup. The decrease in the critical size of core required is known as the reflector savings.
  • Neutron reflectors reduce neutron leakage i.e. to reduce the neutron fluence on a reactor pressure vessel.
  • Neutron reflectors reduce a coolant flow bypass of a core.
  • Neutron reflectors serve as a thermal and radiation shield of a reactor core.

Components of a nuclear reactor core

The key components common to most PWR types of nuclear reactor cores are:

Did you know?

The world’s first nuclear reactor operated about two billion years ago. The natural nuclear reactor formed at Oklo in Gabon, Africa, when a uranium-rich mineral deposit became flooded with groundwater that acted as a neutron moderator, and a nuclear chain reaction started.  These fission reactions were sustained for hundreds of thousands of years, until a chain reaction could no longer be supported. This was confirmed by existence of isotopes of the fission-product gas xenon and by different ratio of U235/U238 (enrichment of natural uranium).

The existence of this phenomenon was discovered in 1972 at Oklo in Gabon, Africa.

Bang Goes The Theory from Youtube

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