Target Selection for Main Control Room Fire Modelling

The Main Control Room (MCR) is the eyes and ears of a nuclear power plant. From this central location all parameters are monitored and operations are coordinated. Being the nerve centre of the plant, the MCR is usually one of the most risk significant compartments in all the plant. Fires in MCR can therefore lead to major disruptions in plant operation, but due to the high level of control and indication instruments in the room, it is a challenging room to evaluate for fire risk and perform detailed fire modelling. Apart from the high concentration of instruments and controls which could be lost, you also have to consider false or spurious signal generation, hot gas layer (HGL) development and deterioration in environmental conditions which has implications on both equipment and operator occupancy/habitation.

A fundamental step in performing MCR fire modelling is to determine the equipment / vulnerable targets in the MCR. Varying types of information at different levels of detail can be made available to the analyst. When information is not readily available the analyst is required to pull together information from all available sources in order to create a representative fire model.

Jacobsen Analytics was tasked with performing the MCR detailed fire modelling for a multi-unit NPP. For the various units, the MCR cable routing was not available. This was a challenging situation as usually some, if not all the component to cable routing is available for MCR. The primary input available with regards to target location was Main Control Board (MCB) and cabinet fascia layout drawings. To illustrate the target selection process, the flow chart in Figure 1 outlines the typical target selection process when cable routing data is available. The first 3 blocks are associated with cable routing data which was not available in this particular analysis. For comparison, the flow chart in Figure 2 shows the adapted target selection process applied to overcome the gap in cable routing. This revised process is discussed further below.

To overcome the challenge of identifying targets and their location, the approach applied was to review the layout drawings and identify each controlled component found on the MCB. This was done methodically, taking each MCB section in turn. Passive components and components for which only indication is able (as opposed to a control switch or dial) were identified. The output of this step was to identify the active components found on each MCB section and cabinet.

Detailed photos of the MCB and other cabinets were used to support and substantiate the drawings. Due to the strict access control measures of typical main control rooms, standard walkdowns were not possible. Where control room simulator facilities were available, these were utilised to validate the information gathered. In addition higher level information such as system and train identifiers were used to validate the component to MCB section mapping was accurate. The active controls identified on each MCB section were then mapped to components found in the PSA model. In order to model a delay in fire spread to the adjacent cabinet section, it was requested internal MCB and cabinet photos be provided. The photos in conjunction with structural drawings were used to confirm the presence of any physical separation between cabinet sections.

Once the MCB fascia control to component mapping was completed, this information had to be incorporated into the PSA model. This was achieved by creating new cable routing entries to represent the MCB fascia control component to PSA component link. It was important to ensure all PSA component failure modes were captured during this step.

The NUREG 6850 Appendix L methodology was applied to estimate the probability of component damage in each MCB section. As there was uncertainty with regards to how the cables from each on the MCB Image: NIC File Photo components was routed from its fixing on the MCB fascia to the penetration out of the MCB section (bundled together or physically separated), a conservative target set separation distance of zero (d=0) was selected.

In summary, detailed MCR fire modelling was performed starting from the challenging position of having no cable to component mapping. The actively controlled components on each cabinet were identified through a review of drawings and detailed photos, which allowed the PSA model cable mapping for the MCR to be generated.