7 Types of Nuclear Reactors Explained

Types of nuclear reactors are; Boiling Water Reactor (BWR), Pressurized Water Reactor (PWR), Light Water Graphite-Moderated Reactor (LWGR), Advanced Gas-Cooled Reactor (AGR), Fast Neutron Reactor (FNR), Small Nuclear Reactor (SMR), Molten Salt Reactor (MSR).

This article discusses the types of nuclear reactors, as follows;

1). Boiling Water Reactor (BWR) (as one of the Types of Nuclear Reactors)

A boiling water reactor works through a closed continuous water-steam cycle, whereby water in the reactor vessel is heated to at least 287.8°C or 550°F by nuclear fission in the core, converted to steam that is used to drive a turbine for electricity generation, and subsequently converted back to water by a circulating water-coolant [8].

The nuclear fuel used in a boiling water reactor is usually enriched uranium due to its relatively-high energy density that makes the continuous supply of large amounts of heat possible.

In boiling water reactors, water serves as the coolant in the reactor core, as well as the heat-circulating fluid.

The amount of energy a boiling water reactor produces depends on its capacity and scale, which could far exceed 1,000 MW.

The main disadvantage of a boiling water reactor is its efficiency, which hovers around 30% under practical conditions. Boiling water reactors are also associated with high cost and high maintenance requirements.

2). Pressurized Water Reactor (PWR)

A pressurized water reactor is a type of nuclear reactor in which pure water is heated by nuclear fission, and delivered to a steam generator under high pressure, in order to generate electricity [2].

Pressurized water reactor works by maintaining high pressure within the vessel that prevents heated water from vaporizing, so that the water is vaporized in a steam generator by action of a primary coolant loop, and the steam is then channeled to a turbine for electricity generation.

The steam generator itself works by a continuous cycling of heat-transferring and heat-absorbing fluids; where the heat-transferring fluid flows in the primary coolant loop and helps vaporize the pressurized water which is the heat-absorbing fluid in the secondary coolant loop.

Pressurized water reactor is the most commonly used type of nuclear reactor, and this is as a result of its relatively simple and effective design. In PWRs, water usually serves as the coolant fluid that transfers heat.

The difference between pressurized water reactor and boiling water reactor is that the water in a pressurized water reactor is not vaporized through boiling, but rather by the action of a steam generator. This generally improves the energy efficiency of PWRs, so that they are likely to perform better than BWRs.

Differences in their modes of operation also imply that PWRs tend to have a greater operational durability and longer service life than BWRs.

Pressurized water reactors are commonly used in naval vessels and submarines to provide propulsion power [1].

Advantages of pressurized water reactor include operational efficiency, durability and long service life; while disadvantages include the need for maintenance of high pressure.

3). Light Water Graphite-Moderated Reactor (LWGR) (as one of the Types of Nuclear Reactors)

The light water graphite-moderated reactor (LWGR or RBMK) is a Soviet-designed nuclear reactor in which light water is circulated as a coolant while graphite serves as a moderator for fast-neutron reflection and conversion [5].

Light water graphite-moderated reactors use enriched nuclear fuel.

Water or graphite is used as a moderator in nuclear reactors because these materials are capable of reflecting neutrons released from the enriched natural uranium fuel during fission, thereby preventing these neutrons from being absorbed by other components of the reactor, which could lead to inefficiency.

It must be noted that light water graphite reactor is different from light-water reactor (LWR), which uses ordinary water as a moderator to reflect neutrons coming from the core.

Light water cannot be used as a moderator where enriched natural uranium is serving as nuclear fuel, because it tends to absorb neutrons and can increase cost while reducing efficiency.

Light water graphite reactors also essentially differ from fast breeder reactors and fast neutron reactors in terms of type of fuel and mode of operation.

4). Advanced Gas-Cooled Reactor (AGR)

Advanced Gas-cooled Reactors (AGRs) were developed in the United Kingdom, as part of measures to boost the performance of nuclear power plants and energy usage in general.

An advanced gas-cooled reactor works by circulating pressurized gas through the core and steam generator, such that the gas transfers heat effectively from the fission process to the electricity generation process.

The gas in AGCR type reactor is usually carbon dioxide, although helium may also be used. High conductivity is a major criteria for selecting coolant fluid in this type of reactor.

There are not many AGR reactors in the world, with most of them still being operated in the UK.

Aside the gaseous coolant another important component of an AGR reactor is the moderator, which is usually comprised of graphite and surrounds the core [6].

This component is useful to prevent neutron absorption, and preserve the efficiency of the system.

5). Fast Neutron Reactor (FNR) (as one of the Types of Nuclear Reactors)

Fast neutron reactors are nuclear reactors that are equipped with a neutron spectrum at their core which enables them maintain high-energy, rapid fission reactions and produce large amounts of energy from uranium fuel [7].

A fast neutron reactor works by using a neutron-rich component in its core zone to initiate and sustain fission without delay, and with a continuous supply of neutrons that increases the efficiency of the overall process, so that several times the amount of energy produced from a given quantity of fuel in a conventional thermal reactor can be produced in fast reactors.

Fast neutron reactors are fairly efficient, especially in their ability to extract energy from fuel. They are also capable of using various nuclear fuels, including Pu-239 and U-235.

What differentiates fast reactors from other types is their ability to commence fission without any delay or build-up process.

Fast neutron reactors are used in Russia, and represent the product of multiple phases of nuclear technology modification.

6). Small Nuclear Reactor (SMR)

Small nuclear reactors, or small modular reactors (SMRs), are nuclear reactors that deliver an electricity output of less then or equal to 300 MWe [4].

A small reactor can power thousands of houses on a yearly basis, based on its output capacity.

However, small nuclear reactors are not used for such purposes because they are not very economical and can have worse environmental impacts than large conventional reactors.

7). Molten Salt Reactor (MSR) (as one of the Types of Nuclear Reactors)

Molten salt reactor is a type of reactor in which a liquid or semi-solid salt plays a crucial role as the fuel or coolant, or both.

A molten salt reactor works by sending a continuous stream of liquid radioactive fluoride or chloride through the reactor core to undergo fission, while more of the liquid salt may also be circulated as a coolant at low pressure.

A molten salt reactor can produce up to 300 MW of power, although it is not used in practical scenarios due to the corrosive nature of salts and the difficulty of finding suitable materials to produce a sustainable version of this type of reactor.

Molten salt reactor is a generation IV reactor, alongside sodium-cooled, gas-cooled, supercritical-water-cooled, high-temperature gas-cooled, and very-high-temperature reactors.

Generation IV reactors are a set of six thermal and fast reactor models designed to improve sustainability, safety, affordability and efficiency of nuclear power generation, based on assessment by the Generation IV International Forum on nuclear energy [3].

The six models of generation IV reactors are as follows;

1). Sodium-cooled fast reactor (SFR)

2). Gas-cooled fast reactor (GFR)

3). Supercritical-water-cooled reactor (SCWR)

4). High-temperature gas-cooled reactor (HTGR)

5). Very-high-temperature reactor (VHTR)

6). Molten-salt reactor (MSR)

Conclusion

Types of nuclear reactors are;

1. Boiling Water Reactor (BWR)

2. Pressurized Water Reactor (PWR)

3. Light Water Graphite-Moderated Reactor (LWGR)

4. Advanced Gas-Cooled Reactor (AGR)

5. Fast Neutron Reactor (FNR)

6. Small Nuclear Reactor (SMR)

7. Molten Salt Reactor (MSR)

References

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