|Fig. 1: Illustration of a light-water small modular reactor.  (Courtesy of the GAO. Source: Wikimedia Commons)|
Nuclear power is currently the largest non-greenhouse-gas-emitting electric power source in the United States. Over the past two decades, it has contributed almost 20% of electric generation capacity. Due to its low dispatchability and high reliability, it has been a major provider of baseload power.  This report aims to briefly discuss reactor technologies that are commonly used today (light water reactors) as well as technologies that are not yet commercialized but have promising outlook to soon become commercialized.
The US Nuclear Regulatory Commission defines small modular reactors as those that generate 300 MWe or less. Other than their size, another distinguishing trait of SMRs is their compact size which allows them to be assembled in a factory and moved to a site where they will be operated.  The attractiveness of SMRs mainly stems from their lower initial capital cost, relative to conventional nuclear plants, which makes them more accessible to developing countries as well as faster construction times due to their modular designs.  Fig. 1 below shows a simplified diagram of a small modular reactor based on light water.
|Fig. 2: Simplified Layout of Pressurized Water Reactor (Courtesy of the NRC. Source: Wikimedia Commons)|
In order to prevent nuclear meltdowns, nuclear reactors require some means of cooling as well as a neutron moderator which absorbs energy from neutrons. In the case of light water reactors, normal water, or water that contains the hydrogen-1 isotope is used to achieve both objectives. This is in contrast to the case of heavy water reactors which are less common but use heavy water or deuterium oxide as a neutron moderator and coolant. The reactor core is typically made of fuel rods that are approximately 3.7 m in length and contain cylindrical pellets, each approximately the width of a pencil. Groups of hundreds of rods form fuel assemblies. During nuclear reactions, fissile uranium within the fuel assemblies is struck by neutrons resulting in products of nuclear fission and heat. Since the fuel assemblies are submerged and enclosed in a steel pressure vessel, heat is transferred directly to the surrounding light water and is sufficient to convert the water into steam which can then drive turbines, then be condensed back to a liquid.  LWRs can be further classified as pressurized water reactors or as boiling water reactors.
|Fig. 3: Simplified Layout of Boiling Water Reactor (Courtesy of the NRC. Source: Wikimedia Commons)|
Pressurized water reactors are the most common type of reactor, making up 65% of US nuclear reactors. The PWR has three separate fluid systems. The reactor coolant system is a closed system and is the only one that is expected to be highly radioactive and transfers heat through a heat exchanger to other fluid systems. The working fluid system contains the water that is used to drive the turbine and obtains its heat via heat exchange with the reactor coolant system. The condenser coolant system is used to condense the outgoing steam from the turbine back into liquid water.  These reactors are usually operated at high temperature and pressure of approximately 600F and 2200 psi, respectively. [4,5] An overview of the pressurized water reactor is shown in Fig. 2.
Boiling water reactors account for 35% of reactors in the USA. A BWR has two water loops. The reactor coolant system is a closed system and is the only one expected to be radioactive. The condenser coolant loop is used to cool the outgoing steam from the power- generating turbine. The main difference between the PWR and the BWR is that, in the BWR, the reactor coolant liquid is used directly as the working fluid and so passes through the turbine directly. For this reason, there is a higher risk of containment breach in BWRs.  The typical layout of a BWR is shown in Fig. 3.
© Gregory Tuayev-Deane. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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