Is the kind of massive radiation release that occurred with Chernobyl possible at the Fukushima plant?
No, it can’t have that kind of massive release. It simply can’t do that. The question is to what extent the zirconium alloy, which clads the fuel pellets, is damaged in the core, and how much of the fuel has failed. And I don’t necessarily mean melted, I mean failed. There’s been an ambiguous use of the word ‘melting’ applied to the core. But when people talk about meltdown, they should be very specific about what they mean by the word.
At Fukushima, there are four primary barriers to releases: the fuel zircalloy cladding, a pressure vessel, an inner containment structure, and a confinement building. To a large extent, the core material seems to be contained. Apart from, of course—and this is where the speculation runs wild—there’s the question of the source of the radiation they’re detecting in certain areas where water has accumulated. Indications today are that it isn’t the cores. They’ve been dumping or spraying tremendous amounts of water onto and into the damaged buildings, so surely someone is considering this water as a possible source.
But until they go in and see, we have little more than speculation to go on, because they don’t know to what extent—if any—the cores are damaged, and they don’t know to what extent the pressure vessels are damaged, although that’s unlikely. They also don’t know to what extent the pipes are damaged, and they don’t know to what extent the lower portion of the containment building is damaged. So, on the one hand, I can’t speculate on what is going on inside. But even so, and given what nuclear engineers know in terms of the plant layout, it’s just not true that it’s a Chernobyl situation.
So, you’d say it was unfair to draw parallels between Fukushima and Chernobyl?
They are very, very different and it’s very unfair to draw that parallel. There are two parts to this. One is the myths that currently surround Chernobyl. The other is the sheer difference between the incidents—the causes of the accidents and the structural, engineering and physics differences.
For a start, there are three or four primary and important differences between the two reactor designs. The first is the difference between the Western Light Water Reactors—pressurized water reactors and (like those at Fukushima) boiling water reactors—and the plant at Chernobyl.
Western Light Water Reactors are water cooled and water moderated. The first one is simple—water is used to cool the fuel, to take away the heat, to eventually create steam and then after that make electricity. It’s the water moderation that’s the very important difference. What moderation means is that the water is used to slow down neutrons in the core and make them accessible for the reaction to take place. In the Chernobyl type reactor, water is a coolant, but it’s not a moderator—the moderator is graphite, and that points to one important design and structural difference.
In the Light Water Reactor core, apart from the fuel itself, it’s virtually all metal. You have the fuel contained in a special kind of zirconium alloy, there’s the stainless steel vessel, and the super structure is metal. In the Boiling Water Reactor (BWR) that you find at Fukushima, you have a reactor pressure vessel that’s approximately six inches thick steel—it’s basically a big kettle that contains the core. In the Chernobyl reactor, there was no pressure vessel. So right there, there are two very big differences—the BWR is contained in a very robust pressure vessel, the Chernobyl reactor was not. The BWR reactor is a singular metallic vessel, while the Chernobyl reactor is approximately 1700 individual pressure tubes piercing about 2,000 tonnes of graphite. Western LWRs contain essentially no graphite. Those are very big differences.