Safety, from a technical perspective:
Safety, from an engineering perspective, is always about trade-offs. You can make a perfectly safe car, but it’ll be so expensive that no-one will buy it. So we have cars that are fairly safe, as well as fairly affordable, fairly comfortable, and fairly reliable, and globally kill over a million people per year. Still, the thing about cars is, the driver benefit from driving around in them, and the risk of dying in an accident falls (mostly) upon them.
For a car, we can accept that there are going to be accidents, we design to keep drivers and passengers as safe as we can in those accidents, but at some point, if the accident is harsh enough, then we accept that a crash will be unsurvivable, and up to four people will die in what’s left of that car. That sucks, but at an overall society level, we accept that this known risk is justified is bearable.
For nuclear power, there’s enough radioactivity inside a nuclear power station to kill an astoundingly huge number of people. So we’re not talking about four people. We’re talking about an unbearable number. That fact alone dominates the trade-offs about safety design. There are possible accidents that simply must never happen. Anyone designing a nuclear power station must be able to say “here are all the bad things that could possibly happen in this design and here are how we will stop them from happening”. Something as dangerous as a nuclear power station must never do anything surprising. Sadly, nuclear plants keep surprising us.
The long history of nasty nuclear surprises:
There are well over five hundred million cars in the world. We must have crashed getting on for a million cars in potentially fatal accidents. This is a huge pile of experience to learn from.
There are less than five hundred nuclear power stations in the world. We’ve crashed three of them. This is not a lot of experience to learn from.
We know how cars crash. We know what cars are made out of – steel. It’s crushingly boring stuff, we’ve had hundreds of years of experience making things out of steel, it’s entirely well understood, we can design a car, crash it on a computer before it’s ever been built, and predict how survivable a crash is down to a tiny error.
Nuclear power plants are not well understood. They operate under weird conditions of high radiation, high temperatures, high pressures, and strange mixtures of materials. They have a history of producing surprises, expensive and dangerous surprises.
One example – what happens when you turn the power knob down:
Here’s a really basic surprise – what happens when you turn a nuclear power station down? You’d think that turning the power knob down would turn the power down, right? Wrong. The basic feature that makes them safe is called the power co-efficient of reactivity – basically what happens when you take your foot off the go pedal. For a car, you take your foot off the accelerator, it slows down – a negative co-efficient. In the Chernobyl accident, the reactor happened to have a positive co-efficient at low power levels, resulting in the reactor power increasing from 5% of design level to kaboom in a few seconds. Positive co-efficients are bad and should be designed out. So, Canada produces radioactive isotopes for medical uses, the reactors they use for this are ancient. The replacement MAPLE reactors were designed to be utterly safe, to the point of boring tedium, which is what you want in a nuclear reactor. Hence they were designed to be negative, but when built they turned out to be positive. That’s a pretty massive fuckup on a fundamental safety feature in a modern design. They were canceled at a cost of about US$300 million. Whoops.
Another example – breakaway corrosion in Magnox reactors:
Even when we’re using normal materials, the weird conditions catch us out. We all know that steel rusts, it oxidises due to oxygen in the air. So when the Brits were building the Magnox reactors in the 1950s, they wanted to make them maintenance free and affordable. And hey, they used carbon dioxide as the cooling gas, no oxygen around, so they could use mild steel, not stainless steel, and it wouldn’t rust. Sweet! Yeah, unfortunately, they discovered an entirely new form of corrosion called breakaway corrosion, which is just as bad as it sounds. They discovered this after building and running the Magnox reactors for several years and, being English, they were rather put off their tea. The steel components were deep inside the reactor cores and, because the reactor had been designed to be maintenance-free, they hadn’t designed in a way to replace the parts that were rusting. You couldn’t take the whole thing apart coz it had been running for several years and was thus horribly radioactive. Hell, it was a bugger even getting inside the reactor core to have at look at just how rusty the parts were. In the end, the only real solution was to turn the reactors down so they ran less hot, rusted less rapidly, and produced less power, making them even less economically viable. Whoops.
So nuclear power stations keep on surprising engineers. We can’t test them to destruction, coz each one costs silly money and contains enough radioactivity to render large areas uninhabitable. We’re taking a massive punt each time we try a new design and, if we’re lucky, then every surprise will just cost huge amounts of money. If we’re unlucky, then that’s another part of the planet rendered uninhabitable for decades.
The point of these examples is not that engineering is hard. Engineering is always hard. You make something new, it sucks, you learn, you do it better next time. Engineering proceeds by making mistakes. However, the reality of nuclear safety is that mistakes are incredibly costly, so learning is slow. More on that tomorrow.