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Nuclear Technologies and National Security

Materials for Microreactor Applications

Providing materials information for microreactor developers.
Figure 1: Materials used in a typical microreactor. A heat-pipe microreactor (HP-MR) is used as an example. Note that every microreactor design is unique.

Crucial Material Performance Criteria

A nuclear microreactor is a complex engineering system that involves a series of harsh environment. Hence, materials in nuclear microreactors are required to deliver required performance under such challenging conditions, including elevated temperature, corrosive coolant, intense radiation effects, etc. In this section, a number of key performance criteria are briefly discussed to provide ideas about what make materials qualified for microreactors applications and what the challenges are.

Range of Operating Temperatures

Elevated temperatures is probably the most characteristic condition found in a microreactor. As the efficiency of a thermal system is directly determined by the its operating temperature, reactor designers alway pursue higher operating temperatures. The maximum operating temperature of a microreactor, however, is mainly determined by the limits of the materials within. Temperatures affects almost all the physical phenomena involved in a nuclear materials.

Melting, Eutectics, and Interdiffusion

First of all, materials in microreactors must be able to maintain their integrity. For majority of the materials, their solid status must be maintained. Therefore, the melting temperature of a material used in a microreactor must be lower than the operating temperature with a certain safety margin. Usually, multiple materials are used together. Hence, eutectics effect must be considered for melting assessment. Moreover, even if the eutectic melting point is not reached, interdiffusion occurring on the interfaces of different materials may still consumes the materials, compromising their integrity. As diffusion is a Arrhenius process controlled by temperature, the operating temperature needs to be low enough to avert the full consumption due to interdiffusion.

Mechanical Properties

Many materials are required to take load in microreactors, especially for those structural materials. In that case, these materials must retain sufficient mechanical strength in addition to integrity. Generally speaking, mechanical strength, both elastic and plastic, of materials degrades as temperature increases. Therefore, in a microreactor, all the materials need to have their yield strength higher than the load as well as to have limited creep strain throughout the lifetime.

On the other hand, structural materials are preferred to have ductility so that they could yield instead of direct fracture. In that case, monolithic ceramics are usually not favored as structural materials in microreactors. However, ceramic matrix composites, which has some ductility, may be used in such cases.

Radiation Tolerance

Materials experience complex modifications at microstructural level under irradiation, leading to a series of shifts in macroscopic properties and thus causing degradation.

Cavity Evolution and Radiation Swelling

Radiation displaces atoms in materials and thus forms point defects. These point defects may lead to evolution of more complex defect structures such as voids. Meanwhile, nuclear reactions such as (\alphaα, n) creates helium atoms, which can stablize these voids to form bubbles. In nuclear fuels, gaseous fission products such as Xe and Kr also participate in the bubble formation. All these cavities lead to macroscopic volumetric swelling, changing the dimension of materials. Such dimension changes may lead to stress concentration or even blockage of coolant channels and thus must be assessment as a performance criterion. Additionally, the formation of cavities may also lead to degradation of other properties such as thermal conductivity and mechanical properties, which also need to be taken into consideration. It is worth mentioning that other radiation effects, including radiation-induced amorphization, may also contribute to swelling or other degradation.

Radiation-Enhanced Diffusion and Radiation-Induced Segregation

The point defects caused by radiation would facilitate the diffusion processes within the materials. As a result, the diffusion rate becomes higher than sole intrinsically activated diffusion, accelerating microstructural evolution in materials in microreactors. More importantly, such radiation-enhanced diffusion is also the origin of radiation creep process, which affects the mechanical properties.

The point defects caused by radiation may also have biases for different species, leading to preferential diffusion of some elements. As a results, radiation effects would lead to segregation of some elements that are not thermodynamically favored. Such radiation-induced segregation may lead to formation of secondary phases and thus shift the bulk properties of the original materials.

Environment Compatibility

Fluids are used in reactor mainly as coolant. For example, liquid alkali metals are used as coolant in sodium-cooled microreactor (e.g., MARVEL) and working fluid in heat pipe microreactor (HP-MR); molten salts are used as coolant in molten salt microreactor (MS-MR); and helium is used as coolant in gas cooled microreactors. These fluids are either corrosive themselves, or may contain impurities that may facilitate corrosion. The materials in microreactors that are directly exposed to these fluids are required to have sufficient corrosion resistance.

Neutronic Compatibility

The materials selected to be used for a microreactor should have minimal impact on microreactor’s neutronics performance when used within the core region, which is the case of moderator or heat removal technologies.

Neutronics impact can be estimated in terms of reduced k-eff and reduced core lifetime and burnup, or as increased need for fissile enrichment to meet similar irradiation performance. Consequently, material selection will impact primarily the economics performance discussed in the following section . In addition, reactivity coefficients such as the Fuel and Moderator Temperature Coefficients are affected by material decisions. For instance, increased neutron absorption in thermal energy spectrum will lead to reduced Doppler effect which needs to be accounted for in core design analyses.

Economics Considerations

Economics considerations are instrumental in selecting advanced material solutions for nuclear reactors. Here are a few key points that need assessment by reactor vendors on a case-by-case basis:

Cost of material technology This includes the cost of the raw materials considered , the manufacturing of the different sub-components, and the assembling of the technology. Those should be considered for the first of a kind prototypes, but also on the nth of a kind, after considering costs reduction through learning and factory manufacturing.

Economic benefit of material technology

The cost of the new material technology, if higher than the conventional technology, must be compared with the benefit provided in terms of:

  • Increased thermal efficiency, enabling higher electrical output and fuel utilization
  • Reduced fissile enrichment of fuel inventory
  • Increased core lifetime and ultimate burnup
  • Increased reactor compactness
  • Potential for re-using the material technology several cycles or core lifetimes

Most of these benefits can be quantified with an economic value, while other benefits may not be easily quantifiable but enable meeting design requirements, for instance considering core compactness for space applications.

Cost of material technology qualification

New materials that affect safety of a nuclear reactor need careful qualification in nuclear environment, which is typically a long and costly process. Some of the technology developed at ANL would combine already qualified material together, reducing the requirement for qualification of the designed technology.