Coping with fire

Brush and forest fires have marked the last 12 months as being the highest profile fire and safety problem that the nuclear industry has had to face. This was somewhat unusual, as on-site fires had previously been the main cause for concern.

At the 4th Fire and Safety Conference, which was hosted by NEI in London in February, this issue was well covered. Two speakers at the conference, Gary Swearingen of Pacific Northwest National Laboratory (PNNL) and Tony Stanford of Los Alamos National Laboratory gave talks on the two very different events, both speaking from personal experience.

The main difference between the two fires was that the Hanford fire was a fast-moving brush fire, while that at Los Alamos affected close-packed trees rather than brush, resulting in a slower moving, but hotter and more intense fire. However, both establishments found that they had common problems that were difficult to deal with. One such problem was that posed by staff involved in protecting the establishments being well aware that their own homes were under threat. This was a great worry, and posed a problem for the Human Resources departments to ensure that everyone was kept informed and reassured.

Other new problems faced by both sites included that of evacuating the complex of non-essential personnel. This proved to be a problem as the fires made several of the roads leaving the sites impassable, and thus making the evacuation procedure significantly harder.

On matters more commonly covered at the conference, the philosophical debate between deterministic and probablistic safety analysis was as lively as ever.

A number of speakers described their experiences at various plants around the world, either in the steps taken to prevent fire incidents, or in the experience of dealing with an incident.

Of particular note were two talks on the statistical base of fires at nuclear facilities. These talks were by Steven Nowlen of Sandia National Laboratories and Wayne Sohlman of Nuclear Electric Insurance. It was interesting to learn what the most common types of fire were. Figures on the duration, nature and cause of fires, as well as how the fire was extinguished, were presented.

Sohlman indicated that the majority of fires were extinguished in less than 15 minutes, and that the majority of fires were extinguished using extinguishers immediately to hand.

Sohlman highlighted one factor that was a contributory factor to many fires. This was the common habit of placing skips of rubbish close to walls. While this is tidy, and a good thing for general safety reasons, it does mean that any fire that breaks out in the rubbish will have access to the wall, and that therefore the potential fuel content of the fire is increased by the fuel content of the wall. In addition, if the fire spreads to the wall, then the fire can easily spread to the far side of the wall.

In general terms, the database provided useful information on reported incidents. This lead to a lively discussion as to how much value to place on these results, because planning for fire protection requires planning for protection against unusual events.

However, Mark Snell of Darchem Flare gave a presentation about the large-scale modelling of fires. This was of considerable interest, and he indicated a number of tests that they had carried out indicated some surprises in how quickly and easily fires grew and spread. He described how, even watching with care, it was sometimes difficult to control and contain fires.

This practical experience proved to be valuable. Cases of extensive fires are, thankfully, rare. Rarity, however, means that there is a lack of information about how fires develop. The examples of how fires were observed to spread during the incidents in the Channel Tunnel or potentially at Hong Kong airport were used as models, and examined. The modelling of the Channel Tunnel fire indicated that the drivers’ safety zone was anything but safe. This enabled modifications to be made to resolve this problem.

Snell emphasised that there were many advantages to sharing information about major fire issues between different industries. Information was hard to come by, and there was a great deal that could be learnt. Many issues were common, and he felt that more could be done to improve cross-industry communication.

At the end of the conference, there was a workshop on cabling, which resulted in a discussion on the fire safety aspects of cabling. This was of special interest, as cabling is presents a variety of issues; it is a potential source of a fire start, it is a means by which fire can be spread through barriers, and fire damaged cables can produce spurious signals, activating or deactivating equipment elsewhere in the complex. This last point is a complex and complicated issue, as a fire in one location may end up having a significant impact in a totally separate location. The conference discussed the best method of protecting a plant from spurious operations of equipment; whether by minimising the likelihood of cables issuing spurious commands, or by ensuring that the random operation of a piece of equipment doesn’t have a serious impact on the overall operation of the plant. According to the deterministic philosophy, the key is to ensure that the reactor can be safely shutdown, assuming that one of the pre-defined fire-zones has been completely affected by a serious fire. If this can be achieved, then the plant is deemed to be safe.

However, there was still considerable interest in methods of improving the fire resistance of cables. In addition, there was a lot of interest in methods of reducing the likelihood of spurious signals being sent through cables.

One method is to ensure that multi-line cables are designed such that live elements that are separated from other live elements as much as possible. By this method, it was possible to minimise spurious signals.

Overall, the Conference was considered to be a success. As one delegate said, there was a special feeling in giving a presentation in the same hall that had seen presentations from some of the famous presidents of the Institute of Mechanical Engineers, as their portraits looked down on the conference from the walls.