Hazards PSA for New Build Reactor Designs

There are currently several different new build reactor designs in various stages of the licencing process within the UK each of which must satisfy the relevant assessment criteria set out by the Office for Nuclear Regulation (ONR). A fundamental part of the assessment criteria relates to the need for, and use of, probabilistic safety assessment (PSA) and specifically internal and external hazards PSA. The next generation of reactor designs offer improved safety performance through multiple, redundant and diverse methods for performing the critical safety functions meaning that combinations of random failures are rendered highly improbable. However, hazards pose a unique threat to plant safety and operability meaning that developing a full understanding of the nature and magnitude of this threat is paramount in achieving a well-balanced and robust design.

Jacobsen Analytics Ltd. (JA) is an unparalleled provider of PSA services acting for both licensees and the UK regulator in developing and reviewing hazards PSAs for new build reactor designs according to the latest international industry standards and guidance. Our capability and experience has been forged over many years from working in high profile projects both on the domestic and international stage ensuring that hazards PSA provide meaningful insights to the designs currently undergoing regulatory assessment as part of the UK's new nuclear build programme. JA can provide the full lifecycle of hazards PSA development from the initial identification, screening and prioritisation of all potential internal and external hazards through to development of a detailed, all modes hazards PSA for selected hazards likely to contribute significantly to overall plant risk. Three of the hazards usually subject to full quantitative assessment are discussed in more detail in the following sections.

Internal Fire

Managing fire risk at a nuclear power plant is complex for a variety of reasons. Electric generating plants, by nature, have many fire hazards that must be addressed. Nuclear plants have additional challenges unique to nuclear power. The plants themselves are complex and the designs diverse. In order to operate the plants efficiently and economically, most plant functions are designed to be remotely controlled from a central control room. This can result in a very high concentration of cables in relatively small areas of the plant. When exposed to a fire, these cables can fail, resulting in either the loss of a desired function or the occurrence of an undesired function.

A Fire PSA effectively overlays the potential likelihood and impact of fire events onto a plant logic model which is built to reflect the mechanisms by which safety equipment failures interact, leading to an inability to control and cool the reactor. The Fire PSA must be performed according to an acceptable methodology which meets the relevant ONR TAG assessment expectations. The preferred methodology is embodied within NUREG/CR 6850, JA have unrivalled experience in application of this comprehensive approach to Fire PSA for new build designs and have obtained several best practice commendations from our most recent application within the Generic Design Assessment (GDA) process.

The Fire PSA is divided into several tasks which require the expertise of a multi-disciplined team including electrical and I&C engineers, fire modelling experts, human factors and PSA specialists and operations personnel. Although the process and approach is well-defined, it was designed to be applied to an operating plant rather than a plant in design. This can lead to both benefits and challenges. The principal benefits of this detailed approach are the insights that can be gained into the design and operation at a stage where modification is not cost-prohibitive. It also provides assurance that the risk from internal fires is well understood and controlled to an appropriate level, both of which are key requirements within the UK licencing process. The challenges of this approach lie principally with the (lack of) availability of the necessary plant design and operational data required for the different steps of the analysis.

Key inputs relate to the location of PSA components, cable routing for those components and the location and fire rating of fire barriers between fire compartments including any penetrations in those barriers. It is of critical importance to understand the availability of this information early on such that the methodology and approach can take account of these factors without requiring overly conservative assumptions which can act to distort the overall risk profile and reduce the validity of any risk insights.

Internal Flooding

Next generation reactor designs rely on the presence of large volumes of water to absorb heat from the reactor and provide an injection capability in case of a fault condition. These sources of water and their associated piping act as internal flooding sources which can have a significant and damaging effect on the plant unless effective engineered and procedural precautions are considered; ideally at the design stage. The modern standard approach to internal flooding PSA consists of several tasks that initially require a comprehensive review of all potential flood sources which can affect components that can cause or are required to mitigate initiating events. Flooding scenarios are then developed and iteratively refined to model the impact of floods on PSA components and relevant operator actions using a plant logic model (the PSA). Results are then obtained, analysed and insights gained on the risk-significant flooding sources such that appropriate defences can be established to mitigate the internal flooding risk.

JA has extensive capability and experience in performing internal flooding PSA according to the current international best practice approach with recent experience of performing a full scope, all modes internal flooding PSA for a next-generation reactor design currently undergoing review by the ONR. Similar to our record in conducting internal fire PSA, our analysis received several best practice commendations from the peer review team.

The study included a full quantitative assessment of high energy line breaks and involved appropriate use of hydraulic analysis to model flooding in risk significant plant areas. Based on our analysis, recommendations were made to improve the design and integrity of critical components against damage caused by internal flooding and high energy impacts.

Similar to the Internal Fire PSA, the approach to internal flooding PSA relies heavily on the availability of plant data to support the various tasks. Gaps or omissions in key input data such as the location of flooding sources, information on system pressures and temperatures and locations of PSA components will require the use of assumptions which may impair the risk insights to be gained from the analysis.

Seismic PSA

Seismic hazards have the capacity to cause significant plant damage and, even with robust seismic design criteria being applied to next generation reactor designs, seismic hazards still feature on the spectrum of hazards that can contribute significantly to overall plant risk.

A full seismic PSA consists of the following four principal activities:

  1. Seismic Hazard Analysis - this activity develops a series of seismic hazard curves which present the annual frequency of exceedance for different magnitudes of seismic event.

  2. Seismic Fragility Evaluation - this activity provides seismic fragilities usually for structures or equipment that is contained within the PSA model.

  3. Seismic PSA Initiating Events - this activity defines the various seismically induced initiating events and determines the development of the logic model (the PSA).

  4. Seismic PSA Model Development and Quantification - these activities develop the logic model which is usually based on the existing (internal events) plant logic model and prepares it for quantification. Quantification then provides the risk results and associated insights.

JA has extensive experience in performing all aspects of seismic PSA (except fragility evaluation, for which we have associates who specialise in this activity) and have applied this experience in producing and reviewing Seismic PSAs on behalf of clients in the UK and internationally.

Hazard PSA Tools

The production of hazards PSAs invariably involves gathering and manipulation of large amounts of plant data relating to plant locations, ignition sources, PSA component locations etc. Using our innovative approach and desire to achieve cost-effective solutions for our clients, we have developed a database tool that allow us to efficiently collate and manipulate this data for the purposes of conducting hazards PSAs. The tools allow automatic interfacing with the PSA quantification software to provide seamless and error-free development of fire and flooding scenarios for quantification. This approach is of particular value in relation to the often-iterative approach in conducting hazards PSAs where initial simplified and conservative approaches are refined to focus analytical effort on risk-significant scenarios or plant locations that warrant further analysis. Each tool is provided with appropriate verification and validation (V&V) evidence and client training and user guides are available as required.