Exploring Nuclear Reactor Types | AGRs, PWRs, BWRs, and PHWRs Unveiled

June 1, 2024

Understanding the Mechanics, Advantages, and Distinctions of Advanced Nuclear Reactors

Read more into the world of nuclear reactor technology with our comprehensive guide on:

  • Advanced Gas-cooled Reactors (AGRs),
  • Pressurized Water Reactors (PWRs),
  • Boiling Water Reactors (BWRs),
  • Pressurized Heavy Water Reactors (PHWRs).

Each reactor type utilizes unique mechanisms for cooling and moderation, directly impacting their operational efficiency, fuel usage, and safety measures. From AGRs achieving remarkable thermal efficiencies through high-temperature operations to the simplicity and compactness of BWRs that boil water directly in the reactor core, we uncover the intricacies that set these reactors apart. Additionally, we explore the economical and continuous operation of PHWRs, favored for their use of unenriched uranium and online refueling capabilities. Join us as we delve into the details of these nuclear reactors, shedding light on their construction, working principles, and how they contribute to the global energy landscape.

What Are Advanced Gas-cooled Reactors (AGRs)? 

AGRs use carbon dioxide as a coolant and graphite as a moderator , this allows them to operate at higher temperatures thereby achieving greater thermal efficiency usually around 40%. This choice of coolants and moderator also allows a wider range of uranium fuels to be used. However the construction and maintenance of the said reactors is far expensive due to the materials required for high-temperature operations. 

What Are Pressurized Water Reactors (PWRs) And How Do They Work?

PWRs are also a type of nuclear reactors characterised by use of water as coolant and moderator , at the core uranium fuel atoms split and release energy in the form of heat at water circulating around the core, a pressuriser is used to keep the water in its liquid state despite the high temperatures it’s at. The heated and pressurised water then flows into a steam generator where it heats a separate water line to produce steam that moves turbines generating electricity. 

What Is A BWR and How Does It Work

Boiling Water Reactors (BWRs) are a type of nuclear reactor that generates electricity by boiling water to produce steam directly within the reactor core. In a BWR, nuclear fission occurs when uranium fuel atoms split, releasing heat and neutrons. This heat raises the temperature of water circulating around the uranium fuel in the reactor core, causing it to boil and produce steam.

How Is a BWR Different From a PWR Reactor?

Unlike Pressurized Water Reactors (PWRs) that use a secondary loop, the steam generated in the BWR core is directly used to drive a turbine connected to a generator, producing electricity. After passing through the turbine, the steam is condensed back into water in a condenser and then recirculated back to the reactor core to be heated again. Control rods, made from materials that absorb neutrons, are inserted into or withdrawn from the reactor core to control the rate of fission and, consequently, the amount of steam produced. This direct cycle of heating water to produce steam within the reactor simplifies the design and operation of BWRs, eliminating the need for a separate steam generator and allowing for a more compact system. Safety systems are in place to cool the reactor and contain radioactive materials in the event of an emergency, ensuring the safe operation of the reactor.

What Are PHWRs And How Do They Work ?

Pressurized Heavy Water Reactors (PHWRs) are characterized by their use of heavy water (D2O) for both cooling and moderating neutrons, enabling the efficient use of natural uranium as fuel. This design allows for significant moderation of neutrons, increasing the probability of nuclear fission reactions without necessitating the enrichment of uranium. The heavy water circulates through the reactor core, absorbing the heat produced by fission, and then transfers this heat to a secondary water circuit via a heat exchanger, generating steam without any direct contact between the two systems. This steam is used to drive turbines that produce electricity, with the steam subsequently condensed and recycled. PHWRs feature natural uranium dioxide pellets as fuel, contained within rods that are assembled into bundles, and utilize control rods to regulate the fission rate and power output. A distinctive advantage of PHWRs, particularly those of the CANDU design, is their capability for online refueling, which allows for continuous operation without shutdowns for fuel replacement. This operational flexibility, combined with the ability to use unenriched uranium, makes PHWRs economically attractive despite the higher cost and complexity associated with heavy water usage.

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