Nuclear experts caution – rightly – that the Fukushima Daiichi accident is ongoing, and that no one has enough information from that chaotic situation to draw any final conclusions about the “lessons learned.”
But there are a few lessons that are already obvious – things that nuclear operators and regulators worldwide will need to confront and resolve if nuclear is to continue to be the major source of carbon-free electricity that its advocates foresee.
Lesson #1, look site-wide for what nuclear engineers call common cause failure modes. For Fukushima Daiichi, that turned out to be a five-story-high wall of water. That single event failed backup electric power the same way at every one of the six reactors strung along the seacoast. Engineers had combed through plant designs to root out single failure points that could fail multiple systems within an individual reactor unit, or fail a key system and its backup. Nobody had looked at that entire site and said, “Is there one external event that can fail the same safety system on every reactor?”
That question hasn’t been considered even a credible one to ask at most multi-reactor nuclear plants worldwide. In fact, some U.S. sites with two units were designed with “swing” emergency diesel generators, which could switch from one unit to another. The assumption was made that only one reactor would be in trouble at any time. That can’t continue to be a safe assumption.
Lesson #2, expand the backup electrical supply to the best available. The Fukushima reactors had eight hours of battery power, which was only designed to power limited amounts of machinery and instrumentation while operators fixed whatever ailed the unit. It wasn’t enough. Many US plants have only four hours’ supply. Nuclear plants should invest in the maximum available. Lots of research is being done on better batteries, for everything from electric cars to megawatt-scale backup for wind farms. Nuclear needs to join the better battery research effort – it could be the best money the industry ever spent.
Lesson #3, revisit what’s “safety related.” Nuclear operators and regulators tend to go round endlessly on this seemingly arcane subject. If a component is part of a safety related system, it’s more costly to buy, install and maintain. It has to meet initial quality standards, and be tested regularly to ensure it will work if needed. Emergency diesel generators were the second line of defense at Fukushima, but their fuel tanks were not. The tanks were outside the reactor building – and we know because they were washed away by the tsunami, disabling the generators and leaving the nuclear units dependent on batteries. Regulators and plant owners alike need to make sure that all parts of an emergency system are protected.
Lesson #4, hydrogen. It blows up. Free hydrogen is created when irradiated nuclear fuel heats up enough for its supporting metal structures to burn – which they do by scavenging oxygen from water or steam. Take away the oxygen from H2O, and you have free hydrogen. Hydrogen blew up at Three Mile Island, but inside a reinforced concrete containment that withstood the blast. Hydrogen blew out the less robust panels at the top of the Fukushima Daiichi reactors (which are a substantially different design than TMI), and may have damaged the primary containment in at least one unit as well. In the 1980s and 1990s, nuclear operators and the Nuclear Regulatory Commission wrangled about what was needed to protect different types of plants from hydrogen explosions. Fukushima ought to get everyone focused on ensuring hydrogen can’t reach detonable concentrations, in any plant design.
Lesson #5, move that spent fuel to casks. The NRC considered some years ago requiring utilities to move older spent fuel out of spent fuel pools and into casks. Spent fuel assemblies are stored in the casks a few at a time, which are parked on a concrete pad somewhere on the plant grounds. Operators fought hard, and successfully, against that requirement, saying spent fuel pools were just as safe and the cost couldn’t be justified.
Fukushima has upended that calculation. The alternative to casks is packing the fuel ever more tightly into the pools, using special storage racks that prevent criticality, which is cheaper. But packed spent fuel pools are believed to be the source of most of the radioactive material leaking out of Fukushima. It's not a total solution -- spent fuel assemblies can't go into casks until they've cooled for at least five years. But moving them out would reduce the radioactive inventory sitting in the pools. Regulators and operators need to take a whole new look at the risks of spent fuel pools.
And there’s a sixth lesson emerging from Fukushima, but smart nuclear operators already know it: Your people will make the difference. Fukushima operators stuck with their plant in the direst moments, refusing to panic and run. Dedicated workers from utility Tokyo Electric Power Co. and its contractors are risking their lives and health to get the wrecked reactors under control, and render them harmless to the rest of Japan. Without them, things would be so much worse. Engineer, analyze, plan for everything you can – but it’s your people who’ll count when the chips are down.