The ironic challenge of nuclear power safety
In studying the history of Chernobyl, Three Mile Island and the ongoing events at Fukushima, a subtle but important connection appears. The problems at Fukushima today share a fundamental similarity with the cause of Chernobyl’s disaster. Moreover, within that similarity lies a path to making nuclear power safer.
Obviously there are huge differences. Chernobyl was a massive disaster that killed thousands of people, the only accident to ever reach level 7 on the International Nuclear Event Scale (INES). When I started writing this article, Fukushima was classified as level 4, although that was before the containment building at reactor 3 exploded, and trouble really started in reactor 2. I had written that it was likely to be re-classified as level 5, and now lots of people are saying they think it might end up as level 6. I had written that I think it’s extremely unlikely to reach level 7 where thousands of people die from radiation poisoning, but the way things are going, I’m not so confident of that any more. :(
For a decent explanation of the defense-in-depth strategies of the Fukushima reactors, read this overly-optimistic article. This article has been widely distributed and republished because its “you’re all over-reacting” message is a nice one to hear and it comes from a seemingly credible source, a scientist at MIT. But the article has an interesting past, originally including a major technical confusion, mixing up moderators which speed up nuclear reactions with control rods which slow them down. This mistake was fixed fairly quickly, and then article moved to a new location hosted by MIT, along the way shedding its re-assurances that nobody would get any more radiation than from “a long distance flight”. Clearly things are worse than that. Nonetheless, Fukushima was built with many layers of protection, making a Chernobyl-scale disaster much less likely. But things just keep getting worse there.
Fukushima faces the same problem Chernobyl was trying to fix
As we’ve all probably heard, the Chernobyl reactor exploded while performing an experiment. The causes of the disaster are many, but most fundamentally the reactor design was unstable. Relying on cooling water as a nuclear damping material gave the RBMK-style reactors a positive void coefficient meaning that as the water boiled from liquid to gaseous state, the nuclear reaction accelerated. This is fundamentally unstable since it can create a positive feedback cycle, as it did during their fateful experiment. The reactor heats up, which boils water, and since steam is less dense than liquid water there is now less nuclear damping material to slow the reaction, so it goes faster. (Modern reactors don’t do this.) In fact just 36 seconds after operators started the experiment, somebody hit the “Oh Shit” button (which unfortunately due to even worse design actually exacerbated the problem), and seconds later the reactor core tragically exploded. Chernobyl’s core didn’t have time to melt — it just exploded. Then large amounts of radioactive graphite burned in a hot fire which carried toxic ash high into the atmosphere. Thousands got sick and died.
Despite what the Soviets wanted everybody to think afterwards (and even convinced the IAEA for 7 years), the motivation for the experiment at Chernobyl was wise and well-intentioned. The operators were not insane, stupid, nor psychotic. They knew that their reactor relied on the external power grid to run its cooling systems. Of course they had backup diesel generators on site in case the power grid failed, but they also knew these generators could take up to a full minute to kick in. That seemed like too long of a gap, so they were trying something creative — using the momentum in the plant’s own steam turbine to power the cooling pumps as the turbine was coasting down, unpowered. They were thinking to themselves “hey, we’ve got this great power source, why don’t we use it to run the cooling pumps instead of relying on the external grid.” Great idea. They’d tried the experiment a couple times before. It hadn’t worked. This time it really didn’t work. But because the reactor was so unstable when the experiment started that a slight decrease in cooling caused it to explode, not because the idea was flawed.
The heart of Fukushima’s problems are the same — the electrical grid around them was taken out by the earthquake. They shut down their own reactions almost instantly after the quake, and thus were no longer producing their own electricity. So to power the cooling pumps they needed to switch to backup power. Unfortunately the backup generators failed, most agree due to the tsunami.
So Fukushima has this ironic problem. They have an incredibly hot thing. Even 48 hours after stopping the fission reaction, the core is still producing megawatts of decay heat. Enough heat to boil 20 tons of water each hour. They need electricity to run the pumps to cool down this incredibly hot thing. But they don’t have any electricity. There’s an electrical power plant (a device to turn heat into electricity) with tons of heat coming off of it, but they don’t have any power to run the cooling pumps, so it overheats. Ironic, no? This irony was at the core of the experiment that Chernobyl was attempting — use the energy of the offline plant to run the cooling systems.
Safer designs are possible
In principal it seems you should be able to design a reactor that uses this vast quantity of heat (which is power — heat equals power) to run the systems needed to cool the thing off. Fundamentally this is just an engineering problem. Shouldn’t we be able to design something that can keep itself cool using its own energy even when disconnected from the grid? Happily the answer is yes. But sadly the answer was not yes in the 1970’s when these plants were built. Not quite at least.
In fact, these old GE Mark I reactors do have emergency core cooling systems designed to help with this, but were never meant to be a complete solution, and clearly didn’t work. New experimental designs achieve cooling completely passively without any need for active pumping. But AFAIK these designs have never made it to commercial scale.
A major lesson of Fukushima is clear: extremely unlikely disaster events are highly correlated with each other. So safety systems should not have external dependencies. I believe nuclear power has an important place in our path away from fossil fuels towards renewables, but to get there, we need safer designs.