Now that we understand what a nuclear reactor is, why it is used, and how it works (covered in part 1), let’s take a deeper dive into the different types of nuclear reactors, their benefits and limitations, and strategies to design and model nuclear reactor cycles using AxSTREAM System Simulation.
Types of most commonly used nuclear reactors:
There are several types of nuclear power reactors available worldwide. Based on their design, they use uranium with different concentrations as fuel, moderators to delay the fission process, and coolants for heat transfer. However, the most commonly used nuclear reactors are pressurized water reactors (PWRs) and boiling water reactors (BWRs). PWRs dominate the global nuclear fleet with 301 units comprising 66% of all nuclear power plants in operation , followed by BWRs at 16% with 72 units, and all other types of reactors accounting for the remaining 18%.
Pressurized water reactors (PWRs):
PWRs were designed and implemented commercially sooner than BWRs due to the earlier notion that pressurized liquid water would be much safer to handle than steam in the reactor core and would add to the stability of the core during operation. That is why the first commercial reactor in Shippingport was a PWR. Figure 4 shows a schematic design of a typical PWR plant.A PWR plant consists of two separate light water (coolant) loops, primary (nuclear part) and secondary (conventional portion) as shown in Figure 4. PWRs use ordinary water as both coolant and moderator. The PWR primary loop works at an average pressure of 15 to 16 MPa, with the help of a set of pressurizers, so the water does not boil even at a temperature of 320 to 350 ℃ in the reactor. The heat from the primary water (nuclear part) transfers to the secondary water (conventional part) in the steam generator (Figure 4). There, secondary water converts into steam which drives the turbine to generate electricity. The core water cycles back to the reactor to reheat, repeating the process.
The main advantage of PWRs is that the intermediate secondary system effectively separates the primary radioactive coolant from the environment. On the other hand, this intermediate system means more components, more possible failures, and a higher cost. There have been several problems with steam generator vibrations and leaks in the past. Nevertheless, PWR technology proved to be generally reliable and cost-effective.
Boiling water reactors (BWRs):
BWR design embodies a direct cycle system of cooling: only one water loop and hence no steam generator (Figure 5). In contrast to a PWR, in a boiling water reactor, water boils in the reactor itself. The steam produced with a temperature of around 290 ℃ is directly led to the turbine producing electricity, as shown in Figure 5. The unused steam is then condensed back to water and reused in the heating process. The pressure in the BWR reactor vessel is about half of a PWR’s force, so its walls are thinner (around 15 cm). Because the systems for steam separation are inside the reactor vessel, it is large (diameter around 6 m and height around 22 m). BWRs have containment, but their design can vary considerably. Because the turbine is driven by primary steam, which becomes radioactive when passing through the reactor core, an additional biological shield is required for the whole steam system.The advantage of the boiling water reactor is its relatively simple design. Its disadvantage is that the turbine, the condenser, and other steam system parts are contaminated with radioactive substances. Despite there being no steam generators and the cost of some other components being higher, the result is that the total investment and operating costs are very much comparable with those of a PWR.
Benefits and limitations of nuclear reactors:
Nuclear reactors are advantageous as they do not contribute to global warming. The fuel cost of nuclear reactors is relatively low because a small amount of nuclear fuel can generate a large amount of electrical energy. Also, nuclear reactors’ low quantity fuel requirement reduces the mining and transportation charges. Hence, it is relatively inexpensive to operate compared to other sources of electricity generation. A nuclear reactor’s long lifespan is yet another advantage of atomic reactors. The energy generation process used by nuclear reactors does not lead to the emission of toxic gases or contaminants into the environment, hence zero greenhouse emissions exist.
The limitations of using nuclear reactors or nuclear power plants for energy production include a high risk of radioactive explosions. The waste produced by nuclear reactors is difficult to destroy and remains radioactive for a relatively long time. It is generally discharged into water bodies, which further affects marine life. The radioactive nature of the waste produced by nuclear reactors implies that the waste tends to remain radioactive for a long time, which is harmful to the health of living beings and the environment. The set-up or installation cost of nuclear reactors is significantly high. A nuclear reactor does not support the quick production of energy, hence cannot be used in applications that require instant energy generation.
Nuclear power plants modeling capabilities in AxSTREAM System Simulation:
The thermodynamic cycle of nuclear power plants can be accurately modeled and analyzed through AxSTREAM System Simulation, using an integrated 0D-1D reduced-order modeling approach. Figure 6 shows an AxSTREAM System Simulation project modeling a 0D model of the nuclear power plant’s conventional part (nuclear steam turbine cycle with multiple extractions).
A 220 MW nuclear steam turbine cycle with multiple extractions has been modeled here. This cycle includes numerous components such as a steam generator/boiler, multiple turbines with extractions, feed water heaters, pumps, valves, condenser with cooling loop, liquid separator, generator, etc. By modeling this cycle, designers can calculate related thermodynamic parameters in each component, such as flow rate, pressure, temperature, enthalpy, etc., and the overall cycle performance, such as thermal efficiency, power production, etc. Additionally, designers can utilize “Multi-run” tools available in AxSTREAM System Simulation, such as “Map” and “Case” for off-design performance calculations of nuclear turbine cycles, and “Plan” and “Monte Carlo” for optimization tasks of nuclear turbine cycles. With the help of these multi-run tools, designers can study how nuclear power plants will behave in the real world and improve the overall performance of the power plants. AxSTREAM System Simulation can do a lot more with its unique key features and capabilities.
Nuclear reactors are a vital source of energy that has been providing electricity to millions of people worldwide for over half a century. While they pose some risks and challenges, the advantages of nuclear power, such as reliability, low carbon emissions, and low operating costs, make it a compelling option for countries looking to meet their energy needs while reducing their carbon footprint. With proper safety protocols and waste disposal systems in place, nuclear power can continue to be a reliable and efficient energy source for years to come. SoftInWay offers consulting and support services to help engineers make the right decision on designing/modeling energy conservation systems, including power generation using nuclear power plants. Our technical team has extensive experience and a thorough understanding of the most advanced nuclear reactors/nuclear power plants—along with their pros and cons.
- World Nuclear Industry Status Report [Internet]. 2018 Nov 1. Available from: https://www.worldnuclearreport.org/
- Power Reactor Information System, IAEA [Internet]. 2018 Nov 1. Available from: https://pris.iaea.org/PRIS/home.aspx