When the Shiitake hits the fan


Unless you’ve been in a coma, you’ll know about the serious turn of events in Japan (and elsewhere). Earthquake, Tsunami and nuclear incident. People say things come in threes but you’re bound to say that if you can only count up to three. There will always be those who criticise that nuclear power is too risky or not enough precautions were taken. Against this backdrop, our appetite for disasters means that some news coverage appears almost to be willing it to escalate because they are already bored. What often surprises me is despite the coming onslaught, people have the presence of mind (or maybe it’s just habit) to film the events on mobile phones. This is very valuable for investigators but I think I might be tempted to just get out of the way.

As it happens, the chain of events (earthquake, tsunami, nuclear incident) is routinely planned for in the region. But the scale of the earthquake (now upgraded to magnitude 9) and its subsequent tsunami, was unexpectedly large.

Planning for low frequency, high consequence events is never easy and there’s plenty of people who will tell you you should have done more. But you’re never dealing with certainties, only estimates. There’se a good deal of error involved (yes, I know the figures produced seem very precise, but this belies the gross assumptions, not least the issue of foreseeability).

There is clearly no point in even thinking about a devastating event where absolutely nothing can be done. For example, a large asteroid hitting the planet would destroy all life. Hiding under the kitchen table or some lame attempt to shoot it down (like in the Hollywood movies) would have no effect – we would be like ants under a blowtorch (don’t try that at home, it’s generally considered cruel). The trouble is, what can be done keeps changing. No doubt in 200 years time, we’ll have the technology to persuade the asteroid to move on (“move along, this is not the planet you’re looking for”).

Planning for these complex, infrequent, difficult events is given special treatment. It comes under various names, but quantitative or quantified risk assessment (QRA) is what we’ll use here. There’s a common erroneous perception that QRA is just risk assessment with numbers instead of words. Smug satisfaction ensues when office-dwelling risk assessment neophytes think they’ve made the transition to QRA when they’ve swapped risk descriptions like high, medium, low for arbitrary numbers 1, 2, 3. The smugness deepens if they have a 5×5 matrix, giving 25 possible combinations to analyse an office trip hazard over which you have long ago fallen over but can do nothing about until you’ve figured out in which category to put it. The risk matrix is a convenience to aid prioritisation, not a magic wand. With QRA, the numbers have real meaning and are based on real measurements (or at least best estimates).

QRA begins with developing a firm idea of all likely accident initiating events that may occur at an installation. It’s then possible to estimate their occurrence frequency (e.g. from historical accident data, component reliability data and sometimes from computer scenario modelling). Various consequence scenarios are also modelled and, combined with the frequency data, help quantify the risk.
QRA is fraught with the potential for error. Conclusions can change dramatically if you’re unaware of a potential eventuality – that is, you have simply missed a hazard because you’re unaware of it or erroneously considered it insignificant. Clearly, this is especially the case for events that don’t happen very often because you know very little about them. This is why it is only one piece of the jigsaw and has to be used with expert experience and judgement too.

For complex systems, it’s important to consider chains of events and not only internal to the plant but external events too. This makes it pretty technical at the start, but the real judgement comes in deciding whether a risk is tolerable and, if not, what more needs reasonably to be done. For something like offshore and nuclear industries, this will need to take a count of the huge potential benefits to society, as well as the real threats. It’s not an easy decision to make and, in the case of nuclear, is very contentious. Once the politicians wade in there is no hope….