THERE’S A NEED FOR increased diligence on safety,” says Paul Orzeske, chief of risk engineering firm Jensen Hughes. He was marking the opening of an office in London, with the goal of enhancing the science of safety in the nuclear and other sectors.

Jensen Hughes, which specialises in fire protection analysis and probabilistic risk assessment (PRA), has been working actively in the UK nuclear sector for the last decade. It is supporting the Office for Nuclear Regulation in the safety reviews of the advanced nuclear plants and in the generic design assessment process, including for the Horizon project.

Dr Richard Stahl, managing director of UK operations, and now heading the London office, told NEI that there is a lot of learning from other sectors, including infrastructure, mining and tunnelling, that can be used to benefit the nuclear industry.

Fire hazard assessment

Nuclear power plant fire regulations have similar themes around the world. However, each country has implemented them differently. Doug True, Jensen Hughes executive vice president says the general philosophy is to separate equipment systems so that one event cannot damage all of the equipment essential to respond to the fire. He notes that a trend has been the move from deterministic towards probabilistic risk assessments.

A traditional deterministic approach is to postulate a bounding fire, and then look at what could be affected, and what is remaining in the plant to mitigate the effects of that fire. That would include equipment, but also the operators and their response to the fire.

True says a probabilistic assessment is a much more granular analysis that looks in a probabilistic manner at ignition sources, the types of fire that could come from those ignition sources, the damage it could cause and the mitigation capability of the plant. It combines the likelihood of the fire with the probability of the available equipment failing, to assess the most significant fires in a plant.

Every existing plant has been licensed on a deterministic basis, but in the last ten years it has become clear that fire is an important contributor to nuclear plant safety.

“The probabilistic methods provide a much greater level of insight into the facility,” notes True. “Over the last eight to ten years, many US plants have operationalised PRA so that operators have a clear understanding of the fire risks.” But other countries are moving at different paces.

Nuclear factor

There are no specific fire protection challenges for nuclear plants. But as in any complex facility, it is vital to understand the relationships and configuration control, to ensure that safety-related cables are partitioned, says True.

Fire protection needs also vary depending on the age of the facility and whether the plant is in operation or outage. Generally, there are limits placed on combustible materials that can be taken into reactors during outages, and it is important to adhere to them. Fire protection systems must be maintained to match the configuration of the facility. Cable ageing management programmes, in particular, are becoming more important as operating life is extended, notes True.

Looking ahead to new technologies, True suggests that fires may be a lesser concern for new advanced reactor designs such as small modular reactors (SMRs) due to their inherent safety characteristics. “We will still have fire protection requirements, but from a nuclear perspective fire protection concerns will be a nuclear safety issue,” notes True.  

Lessons from the past

There have been some significant fire events in the history of nuclear power, from Windscale in 1957 to Browns Ferry in 1975. Since that era, the industry’s approach to fire protection has changed “dramatically” says Doug True.

“The level of detail that we look for in fire safety [analyses] has significantly increased, as has an awareness of operating staff to fire issues.”

True adds, “If you look at the operating experience of facilities, we have not had any fires that are anywhere near as significant as those. The industry has come a long way.