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Nuclear Power Plants and Earthquake Risks: Lessons Learned from 50 years of Seismic Analysis

Extreme Loads and Structural Risk

Nuclear Power Plants and Earthquakes

The nuclear industry provides 10% of the world's power and as the second highest non-fossil fuel product after Hydro power, is a key component in the shift to a cleaner, more secure energy future. One of the critical aspects of a Nuclear Safety Case is Seismic Safety. Seismic analyses are conducted to evaluate the potential impact of earthquakes on a nuclear power plant's safety systems, structures and components. These analyses consider site-specific seismic hazards, including ground motion characteristics, fault lines and historical earthquake data. Design requirements are based on conservative estimates of potential seismic events to ensure adequate safety margins. Developing the methodologies that are still used today, ABS Group has been supporting the Nuclear Power sector to achieve seismic safety excellence for over 50 years.

However, there are several common misconceptions about managing seismic hazards in the nuclear industry that can lead to misunderstandings or misinformation. Let's explore some of these misconceptions.

Myth #1 - All Nuclear Power Plants are at Equal Risk from Seismic Hazards

The first misconception is that all nuclear power plants are at equal risk from seismic hazards. In reality, the seismic hazard level varies depending on the location of the nuclear power plant. Seismic hazard assessments are site-specific and take into account factors such as the proximity to active fault lines, ground motion characteristics and historical earthquake data. Even in the UK, where seismicity is relatively low compared to other parts of the world, each power station has unique guidelines based on its specific location.

Myth 1 - All Nuclear Power Plants are at Equal Risk from Seismic Hazards

Myth #2 - Analysis and Calculations are the Only Way to Prove Seismic Resilience

Myth 2 - Analysis and Calculations are the Only Way to Prove Seismic Resilience

Advanced analysis and calculations are commonplace in engineering design and independent assessments. However, when it comes to seismic qualification of plant and equipment, experience can go a long way in deciding whether something will withstand the loads of an earthquake. For over 30 years, ABS Group has been using the Seismic Walkdown methodology, which we supported the development of, to assess whether mechanical plant and equipment can withstand demands from earthquake loading.

The seismic walkdown methodology was first developed for chemical and nuclear facilities in the early 1980s to create a practical, reliable and cost-effective alternative to the more rigorous seismic qualification of equipment achieved via testing or analysis. A critical component of the seismic walkdown review is its consideration of the plant and equipment in its current condition and location. This is important as it takes into account any variations from concept, design or installation drawings or references and assesses the true condition, including material degradation and damage. Additionally, the seismic walkdown review enables potential seismic interaction concerns from adjacent plants and equipment to be identified, which cannot be considered satisfied by a desktop-based assessment.

While the walkdown methodology can be a relatively simple process for mechanical plant and equipment, it can have a slightly different function for buildings and critical structures. When understanding the load impacts on buildings, walkdowns allow engineers to evaluate the in-situ condition of a building, which can be subsequently used to provide the basis for advanced analysis and calculations.

Myth #3 - Rigidity Will Stop a Structure From Falling Down

Earthquakes generate complex and dynamic ground motions that can cause significant shaking and deformation of buildings and structures. Rigid structural anchoring can restrict the ability of structural components to move and deform independently, leading to increased stress concentrations at the connections. This lack of flexibility can result in higher forces and moments being transmitted to the structural elements, potentially leading to structural damage or failure.

On the other hand, introducing flexibility allows for the energy created as part of the event to be dissipated throughout the structure, reducing the forces imposed. So, while rigidity does allow for stronger structures, they may not be as efficient as those designed to displace during a seismic event.

Myth 3 - Rigidity Will Stop a Structure From Falling Down

Myth #4 - Qualifying the Safety Critical Equipment is Enough

Myth 4 - Qualifying the Safety Critical Equipment is Enough

A more holistic evaluation of seismic safety includes consideration of the environment and secondary effects. During an earthquake, equipment and surrounding objects may experience dynamic interactions due to ground motion, resulting in relative movements, impacts and collisions. These interactions can affect the performance and integrity of the equipment as well as the surrounding objects. For example, if an object in close proximity to the equipment experiences excessive movement or impact, it may cause damage to the equipment or hinder its ability to function properly.

Considering the objects around the equipment can also help identify potential risks and vulnerabilities associated with seismic events, this information can be used to develop appropriate mitigation measures, such as repositioning or reinforcing nearby objects to reduce the potential for damage or failure of the equipment. By taking into account the objects around the equipment, the overall seismic risk to the facility can be better understood and managed effectively.

Myth #5 - Removing a Load Will Not Impact Seismic Qualification

Seismic qualification is typically based on the dynamic response of a building or equipment during an earthquake. The dynamic response is influenced by the mass, stiffness and damping characteristics of the building and its components. When a load is moved within a building, it can change the distribution of mass and stiffness, altering the dynamic response of the building. This can potentially affect the performance and integrity of the building and its components during an earthquake, as the load may introduce additional forces, stresses and strains that were not considered in the original seismic qualification. Therefore, any changes in load distribution within a building may impact the seismic qualification and need to be carefully evaluated.

Myth 5 - Removing a Load Will Not Impact Seismic Qualification

Moving or removing a load within a building can also affect the interactions with structural components, such as walls, floors and columns. Structural components are designed to withstand the anticipated loads based on the original configuration and arrangement of loads in the building. When a load is moved or removed, it can impose different loads on the structural components, potentially exceeding their design limits. This can result in increased stresses and strains on the structural components, leading to structural damage or failure, and compromising the seismic performance of the building and its components.

ABS Group: Your Seismic Experts

Managing seismic hazards in the nuclear industry is complex and difficult to navigate as it is a highly regulated process. Nuclear power plants are designed and constructed to withstand a wide range of hazards, including earthquakes, and multiple safety layers are in place to prevent the release of radioactive material. Seismic hazards are thoroughly considered in the design process of nuclear power plants, and site-specific assessments are conducted to ensure safety.

Seismic risk is different for every facility, even in low-risk countries like the UK, each facility has a different risk level that needs to be considered. This risk can be analyzed through advanced technical analysis and calculations, but also through experienced personnel following the guidelines set out in the SQUG GIP guidance.  Our experience in this field is invaluable, often teaching our clients that a more flexible design can provide a better overall result when mitigating against this risk, as well as the importance the surrounding environment can play in the outcome of the assessment. It is important to take a holistic view when assessing seismic risk, and to remember that it is an ongoing process that must be revised whenever changes are made to a building's function and if equipment is moved/removed/added.

Over the decades, ABS Group has seismically qualified hundreds of structures, plants and equipment for the nuclear power industry and our team of highly skilled civil and structural engineers are available to provide engineering support no matter how big or small the problem.

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