Fire PRA and the NFPA 805 Transition Process

To date approximately 50 of the 104 licensees in the US have committed to transitioning their fire protection licensing basis to the NFPA 805 standard. NFPA 805 is intended to be an alternate method to meet the intent of existing fire protection requirements in 10 CFR 50.48, Appendix R, and General Design Criterion (GDC) 3. While earlier methods relied solely upon a deterministic (rule based) approach, the NFPA standard provides flexibility to a nuclear power plant licensee through implementation of a mix of both deterministic and performance-based methods to analyse and manage fires.

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Managing fire risk at a nuclear power plant is not simple 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 results 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.

The Fire PRA underpins the performance based method of satisfying the new standard. It effectively overlays the potential likelihood and impact of fire events onto a plant logic model (the PRA) 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 PRA must be performed according to an acceptable methodology which meets the ASME-ANS-RA-S 2009 PRA standard. The preferred methodology is embodied within NUREG/CR 6850, although certain aspects have been refined principally for the purpose of removing modelling conservatisms. Once completed, the fire PRAs must undergo an intensive peer review process and any findings must be resolved prior to transition.

The fire PRA is divided into 15 separate tasks which require the expertise of a multi -disciplined team including, electrical and I&C engineers, fire modelling experts, human factor and PRA specialists and operations personnel. This fire PRA approach is fundamentally different from the deterministic approach. The deterministic approach, with prescribed exceptions, assumes 'full area burn-up' and requires the licensee to demonstrate a shutdown path which remains operable under such conditions. The fire PRA considers the actual physical characteristics of each plant fire area including component/cable location, the potential frequency and severity of fires. This approach provides a more detailed understanding of the actual risk significance of fires in each area of the plant as well as addressing the risk associated with propagation of fires between areas.

The role of the fire PRA in the NFPA 805 transition process is to facilitate the evaluation of the risk significance of identified variations from the deterministic requirements (VFDRS), as compared with the risk from a hypothetical compliant plant. The fire PRA can then be used to identify improvement opportunities and assess the risk significance of VFDRs so as to support their disposition. For example some plants use local field actions, so called 'recovery actions', to ensure an operable safe shutdown path. In the 'ideal' deterministically compliant plant such recovery actions would not be needed. Instead automatic features or control room actions would replace the function of the recovery action, or other design features (such as additional protection to cabling) would be included to negate the need for a recovery action. Other examples of non-compliance include potential fire scenarios where a multiple spurious actuation could make a safe shutdown path unavailable, credible fires where damage to redundant trains is possible, and credible fires where the time for operator response has limited time margin. Disposition of VFDRs includes either design/operational change to eliminate the VFDR, or demonstration that the VFDR is acceptable from a risk perspective.

Typical resolutions have included major plant modifications to reduce fire risk (e.g. Reactor Coolant Pump) seal modifications to reduce the likelihood of Seal LOCA following loss of all cooling.

Enhancements such as establishing transient free zones or installing incipient detection (to reduce the frequency of a fire), changing detection and/or suppression (so as to reduce the potential to damage risk significant equipment), providing additional protection to equipment, moving cables, changing the design or changing procedures and training so as to improve mitigation capability.

Since 2006 Jacobsen Analytics has been providing technical leadership and support to numerous US utilities for the implementation of fire PRAs to substantiate compliance with the NFPA 805. To date we have provided such services at Diablo Canyon, Beaver Valley, DC Cook, Callaway, Cooper and Davis Besse.

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