Operation and safety

Playing with fire

9 March 2010

Two US nuclear power plants have been implementing a new risk-based fire safety analysis that aims to spell out more clearly than before, where, and how, a fire might break out, and what its consequences might be for the plant. By Will Dalrymple

The two plants, the single-PWR Shearon Harris plant operated by Progress Energy in North Carolina and the three-PWR Oconee Nuclear Station operated by Duke Energy in South Carolina, are working with the US Nuclear Regulatory Commission to implement a new fire safety programme. It is based on recent standard NFPA 805, and nuclear power plants across the USA are now volunteering to take it on board.

In a May 2009 letter to a Senate subcommittee, the NRC explains why it is allowing US plants to switch from the 1981 deterministic fire safety model to the performance-based model first promulgated in law in 2004. “Fire models simulate fire behaviour using sophisticated calculations. Advances in computer technology have allowed these models to more accurately and precisely simulate fire behaviour and fire conditions in nuclear power plant environments. The NRC has verified and validated the applicability of several fire models for use in nuclear power plant applications. Fire probabilistic risk assessment (PRA) is a tool that quantitatively evaluates the frequency of fires, the effectiveness of fire detection and suppression capabilities, and the impact of fires on nuclear safety.”

Harris engineer Josee MacIntyre explains the effect of the work on Harris. “The plant now understands the risk involved with an actual possible fire in each area of the plant instead of the older understanding of each fire going to flashover or consuming the entire area, and causing significant damage within the whole area. We can now evaluate the postulated damage caused by a plausible fire and protect against that risk.”

MacIntyre gives an example of how the work has benefitted Harris: “A non-intervening combustible zone was expanded close to the chillers because of the risk associated with a transient fire in that area. However, in other areas, it was determined that some of the suppression and detection systems installed in the plant were not required under the NFPA 805 programme. This will allow the plant to change the compensatory actions when one of those systems is not operational to be commensurate with the hazard.”

The old rules treated fires in a simplistic way: any fire, no matter how small, would be considered catastrophic. Although this assumption may be the safest, it requires a much higher level of protection than may actually be needed. David Goforth, Duke Energy NFPA 805 technical manager says: “Under the old rules, if you had a fire in an area, you had to burn everything. That could encompass a lot of equipment and cables, and it may be harder to meet the requirements to protect the core.” He adds: “The previous system was strict, and required a lot of modifications that really don’t need doing from a risk point of view. Now we can apply effort where risk is the greatest.”

Goforth adds that newer plants tend to be more in compliance with the previous deterministic rules; but the older plants particularly benefit from the analysis. “A lot of plants used operator actions to compensate for not meeting part of the deterministic rules. Now we can evaluate the risks of operator actions; now we can measure the risk, and that is a huge step up.”

The new understanding is based partly on a new computer model, Goforth says, that is built up partly from extensive data taken from the plant itself. “The system is very complex and time-consuming. Part of what we have to do now is go back in, and look at all the electrical cables – there are literally thousands and thousands of them – and they all now have to be routed, put into a database, inserted by particular areas, and that way we can use the PRA. We start a [virtual] fire and see what damage it does to equipment, and see if we have enough protection to put the fire out and protect the core.”

He continues: “Part of what the analysis did was identify all the cables, whether they were a single conductor, or a multiple conductor with up to 30 strands. It would look to see what electrical short-circuits would be caused if they were to melt together, or what happens when it goes to ground, and how long the shorts would last.” Then engineers insert that data into their computer risk models, and predict what might equipment might be damaged by fires that start and grow in different hypothetical cases, Goforth explains.

David Miskiewicz, a Progress Energy engineer, says that the Harris effort used NRC’s NUREG 5850 to develop a fire PRA. It was reviewed by an industry panel for compliance with ASME standards. The programme includes classical fire protection methods as well as a safe shutdown analysis (SSA) that is similar to US regulation 10CFR50.48 Appendix R (the 2004 US regulation that incorporated NFPA 805). The five-year project is expected to finish in 2010. It has included:

• More than 50 model logic changes to incorporate components and impacts of SSA and multiple spurious operations caused by multiple circuit faults

• More than 400 PRA components added to the safe shutdown cable routing database

• More than 1900 ignition sources and over 21,000 targets identified primarily by walkdowns

• Detailed circuit analysis performed on over 2000 cables

• Fire modelling insights applied to over 70 sources

• More than 2400 scenarios remain in current analysis

All of this paperwork has lead to at least five modifications at the station. In addition to additional cable separation, the plant has tested extensively a new type of three-hour fire-rated cable specifically for power and control purposes in safe shutdown circuits, says Kevin Riley, Progress Energy’s supervisor of major projects. “The cable looks like instrument tubing and is installed in a similar manner. The use of this cable allows us more flexibility in the routing of the cable as it can be routed through potential fire areas,” Riley says. The station has also installed a manual transfer switch that allows easier transfer of power on its swing charging pump, which completes a job that used to take hours in minutes, he says. “In addition, we are currently installing a new backup diesel generator and pump which can supply our reactor coolant pump seals and emergency power to our safety-related battery chargers should we suffer a complete loss of AC power. We have also installed an incipient detection system which is designed to detect the very early stages of a fire to allow operators the time to locate the potential fire and take mitigating actions. This system is used in other industries such as the telecom and IT industries, but this is the first large scale usage at a commercial nuclear power plant.”

Progress and Duke staff have conducted more than 30 pilot meetings with the NRC since the project began in 2006, says Progress’s supervisor of corporate fire protection Jeff Ertman.

Progress Energy volunteered for the programme partly because it improves safety, and partly because it has helped stabilise the regulation of its stations, Ertman says: “Both the utility and regulator are seeking a stable regulatory environment for fire protection and see the transition to NFPA 805 as the most direct and cost effective method for achieving same, while allowing for increased plant understanding and identification of risk significant aspects of design and operation.”

According to the NRC, the transition to NFPA 805 is voluntary. Once a utility decides to switch, it sends the NRC a letter of intent that lays out a schedule. After the letter, it has a period of self-assessment up to three years before submitting a licence amendment request (LAR) to the NRC requesting a change to NFPA 805. During this period the NRC is prepared to give some enforcement leeway to stations that are in the midst of learning from their own analyses and from the pilot programmes during the transition, as long as they compensate for the risks in other ways. Then the NRC reviews the LAR, and writes a safety report approving or disapproving of the change. Both Harris and Oconee plants submitted their LARs in May 2008. The NRC expects to publish its safety evaluation of Harris in March 2010, and Oconee a few months later. A total of 17 utilities are planning to migrate 51 reactors at 33 US nuclear power plants to NFPA 805 standards. The exact date of the Oconee evaluation is significant for the first US plants who are moving over to the new system. The first plants to follow the pilot plants have a six-month window of enforcement discretion from the date of the Oconee evaluation. The NRC is expecting to receive most applications in late 2010.

Author Info:

Will Dalrymple is editor of NEI

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