The viable way to harness nuclear energy today is the deployment of fission reactors. Although the physics of the work required to use fusion reactors technically have been demonstrated, it takes time to adapt fusion systems to commercially viable systems. At the same time, it takes a lot of effort to bring it into the competitive energy engineering field.
However, it is necessary to develop materials that aim to perform under extreme conditions. This concerns both ways that provide options for both fission and fusion technologies. Materials play a key role in the feasibility of nuclear power system, such that they are among the key classification factors and metrics for nuclear systems. The nuclear system type (fission or fusion) is as follows:
• Purpose and functionality,
• moderator type,
• Cooler type,
• Fuel type,
• Structural material options,
• Neutron-energy classification,
• Core design options (homogeneous or heterogeneous, etc.),
• Energy conversion process type and application options,
• Peripheral interfaces,
With a focus on nuclear fission systems as a near-term and currently available commercial option, there are several well-researched aspects besides emerging new technologies. Materials play an enabling role, creating possibilities for extending the life of modern nuclear reactors. Materials are also the key factor that makes emerging new technologies viable. Notable examples are the very important developments that offer crash-proof fuels and robust structural materials that perform under extreme conditions due to high temperatures and high energy radiation effects.
Performance Conditions and Material Options
There are many options for material selection, modern nuclear reactors mostly use light water (light water reactors) that is either prevented from boiling (in pressurized water reactors) or allowed to boil (in boiling water reactors). All nuclear reactors using water pressurize their primary system to achieve the required performance characteristics. Pressure levels range from about 6 MPa in boiling water reactors to 15 MPa in pressurized water reactors. A number of contemporary commercial nuclear reactors use heavy water (CANDU reactors). Alternative options for primary coolant options are liquid metals such as molten sodium and gases such as helium. The reactor types are called liquid metal cooled reactors and gas cooled reactors, respectively. Depending on the liquid metal choice, an expensive intermediate loop may be required to isolate the working fluid energy conversion loop from the high levels of radioactivity induced in the primary loop.
A significantly different option is offered by the liquid salt configurations found in molten salt reactors. In all of these advanced reactor options, performance in their ability to support and withstand internal conditions while offering material interactions, compatibility and essential properties is vital to successful developments to increase commercial feasibility levels in competition with contemporary options. Moderators and structural materials include both solid and liquid options. Graphite, beryllium, steels and composites are among the solid form options. Light, heavy water or liquid salts are among the liquid form options. The choice of moderators and structural materials is guided by internal conditions and performance characteristics. Often, materials are expected to remain compatible and perform under extreme conditions for extended periods of time.
Some material choices offer unique features and opportunities not found in others. For example, due to the properties of liquid metals such as high boiling points and very low vapor pressures, pressurization is not necessary or very low. This offers a significant distinction and advantage over light water and gases that require pressurization to achieve the required performance characteristics in a system. As noted, typical pressures in light water systems are between 6 and 15 MPa, while typical pressures in gas systems are between 4 and 7 MPa. The need to support these pressure levels places requirements on structural materials for primary systems, including containers for components and connecting pipes. The absence of the need for pressurization is a significant relaxation of demand for materials. In particular, liquid salts take advanced nuclear systems further by eliminating the requirements for solid structural materials to withstand prolonged direct proximity to nuclear fuel. This offers significant advantages in terms of system life as well as system security. Molten salt reactors require unique technologies that support salt environments not found in other nuclear reactor configurations.
Material Selection and System Design
In nuclear power systems, material selection determines the neutron energies available in reactor cores. Consequently, he defines these systems as thermal, fast nuclear reactors or systems. The contemporary nuclear fleet consists predominantly of thermal reactors, but a number of fast reactors have been built and operated over the years, and several fast reactors are in operation today. Thermal reactors use water, heavy water, helium, carbon dioxide and graphite as material choices, fast reactors sodium, lead and steel. Considering the importance of fast reactors for future sustainable nuclear energy routes, liquid metals had advantages over other core material options that take into account their heat removal capabilities. Some of the advantages of these liquid metals are as follows:
• It has excellent heat transfer properties,
• They are characterized by wide temperature ranges where they remain in liquid state and can offer the high temperature performance matching characteristics of gas cooled reactors,
• Helium and heavy salts are coolant types for fast reactors,
• It has excellent resistance to nuclear radiation damage,
• It has high thermal conductivity and low specific heat due to low temperature gradients in the cooling system. Combined with high boiling temperatures, local hot spots are naturally minimized in fast reactor configurations,
In terms of challenges, liquid metals are chemically active and corrosive, requiring the use of special and often costly structural materials and processing technologies, while oxygen present even in small quantities oxidizes sodium to Na 2. It then settles on the cold walls, causing clogging problems. The relatively high freezing point of sodium requires the use of electric or other heaters to prevent the cooler from freezing during low power operation or extended shutdown. Additionally, liquid metals are not universally available and costly, so all these challenges are engineering challenges. It has been overcome and solved by the use of advanced materials designed to operate in a liquid metal environment. Gas coolers are another option for advanced nuclear reactors. Thermal reactors require separate moderators such as graphite or heavy water, as they have very little moderation capabilities at reactor pressures.
Although there has never been a gas-cooled fast reactor in operation, there are significant interests in industry to develop and apply a range of gas-cooled fast reactor technologies. Gaseous refrigerants are generally available, inexpensive, safe and manageable. They provide high plant thermal efficiencies by allowing operating modes with high reactor exit temperatures. Furthermore, gaseous chillers allow highly efficient direct thermodynamic cycles and do not pose a serious activation problem when purified. However, gases have poor heat transfer properties and low volumetric heat capacities. It also requires more pumping power and larger channels than liquid coolers, and pressurization to reduce pumping requirements. Especially for low molecular mass gases such as He, sealed systems are needed.
Because of poor heat transfer, high fuel temperatures are required if high heat removal rates from the reactor are to be achieved. Therefore, it can be argued that material selection and material availability are key factors determining nuclear reactor feasibility, both from a technology standpoint and commercial application. Fortunately, advances in materials make a wide variety of nuclear reactors possible today.
Writer: Ozlem Guvenc Agaoglu