Fire PSA Task 11: Detailed Fire Modelling Case Study

In 2016, Jacobsen Analytics Limited (JA) was approached and contracted to support a project to perform fire PSAs, for five units, across two nuclear power plants with an ambitious schedule. Working in an international collaboration, JA’s specific task was to provide expert support and to lead the Task 11 detailed fire modelling analyses, specifically for the Main Control Rooms (MCR), Main Relay Rooms and back-up Control and Main Relay Rooms. The previously conducted analyses for these rooms had yielded higher than expected risk-estimates when compared to similar power plants. Therefore, the task set for Jacobsen was to use their knowledge and experience in this field to refine the analyses to acquire more realistic results.

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 the potential loss of mitigating systems and an inability to control and cool the reactor. The Fire PSA is comprised of a series of discrete and well defined tasks which is usually performed according to an acceptable methodology. For this project, the client wanted to follow the industry recognised and well documented NUREG/CR 6850 and JA have extensive experience in the application of this comprehensive approach to Fire PSA, for new build and site-specific designs.

Figure 1: Image of NPP from NRC Photo Library

In the Fire PSA Task 11, a detailed fire modelling analysis is conducted for compartments which have been found to be potentially risk significant. It considers the growth and propagation of a fire and the possibility of it being detected and suppressed before a specific target set is damaged. Three categories of fire scenarios are normally analysed, general single compartment, MCR and multicompartment scenarios. The ultimate output of Task 11 is a set of fire scenarios, frequency of occurrence of those scenarios, and a list of target sets (in terms of fire PRA components) associated with the scenarios. For scenarios involving the MCR, the possibility of forced abandonment is also evaluated.

Initially, a thorough investigation of the previous analyses was conducted to determine the reason for the abnormally high risk estimates. It was concluded that the cause was overly simplified modelling, which was the result of a number of conservative assumptions and inaccurate application of the guidance. In short, two of the main issues were that a fire propagation analysis was not fully being developed (taking credit for solid barriers, crediting distances to targets, applying accurate severity factors, taking credit for detection and suppression) and conservative cable routing assumptions were leading to whole room damage. Ultimately, with an accurate implementation of the latest guidance and a robust methodology, it was hoped that more realistic results could be obtained.

Due to the challenging schedule and the development of 5 unit analyses in parallel, an approach was devised, for each room type, that incorporated the relevant methods from the latest FAQs, the aforementioned NUREG/CR 6850 and the more recently released NUREG 2178 guidance. These generic methodologies, by room type, needed to be comprehensive, but also be defined with a degree of flexibility to cope with the unique features found in each unit. Examples of which were ignition source type, ignition source configuration and location, the number of sources and the dimensions of rooms. Furthermore, generic fire scenarios representing the fire propagation of an ignition source plus any resultant secondary fires to certain target sets, were developed and applied in conjunction. The CFAST modelling code was used to determine the resultant environmental conditions for a range of different room dimensions so as to determine the time to damage for each type of target and the time for operator abandonment; this ensured that bounding and representative cases could be defined.

Developing the analyses in the generic manner was essential, due to the large number of rooms and ignition sources that required evaluation, but it was also necessary due to way previous tasks in the Fire PRA had been conducted. One of the largest issues was missing, or incorrectly defined input information, an example of which was the location of ignition sources and their NUREG CR/6850 Task 6 classification; erroneous binning leads to a dilution of the fire ignition frequencies. While the ignition source information was easily amended, the severe lack of cable routing information, such as the location and the cable to component mapping was not easily determinable within the timeframe of the strict schedule. This ultimately meant that targets could not be correctly captured, which would likely lead to an underestimation of the risk.

Usually, target sets would be defined by a combination of analysing the geometric location of each target with respect to the ignition source’s zone of influence and the hot gas layer temperature/elevation. With a lack of, or in some instances no cable routing data, it was extremely difficult to determine how complete this data was and it was therefore necessary to define conservative target sets to ensure that the resultant CDF was not being under-estimated. For the MCR, attempts were made to fill any gaps in the cable routing by using panel pictures and drawings to map components to panels. Where there were no pictures at all, the system associated with each panel was determined and failed. For the MRRs, in some cases it was necessary to define entire banks of cabinets damaged, or even whole room damage for certain fire scenarios.

Further issues which required consideration included:

• Concerns over the robustness of the internal events modelling
• Treatment/consideration of spurious actuations possibly non-conservative

Jacobsen Analytics Ltd has built a reputation worldwide and gained wide experience in implementing the NUREG/CR 6850 Task 11 methodology. Our clients include:

• Beaver Valley US (FENOC)
• DC Cook US (Indiana Michigan Power)
• Callaway US (Ameren Corp.)
• Tihange Belgium (Tractebel)
• Doel Belgium (Tractebel)
• Hatch US (Southern Electric)
• Barakah UAE (KEPCO E&C)
• Wylfa UK (HGE)

Figure 2: Image of NPP from NRC Photo Library

For any inquiries as to how Jacobsen Analytics Ltd can meet your detailed fire modelling requirements, please contact us by telephone, email, or via LinkedIn. Contact details can be found on our website: www.jacobsen-analytics.com.