Over the past couple of days I've read several news stories, diaries, and comments concerning the nuclear plant emergency in Japan. In responding to a comment recently it occurred to me that most people have a very weak understanding of the term nuclear meltdown. This has led to a great many people experiencing fear and anxiety regarding the events in Japan. My hope is that some clarification of what this term means will help folks have a more accurate perception of the news reports coming in.
When many people - especially those of my generation, I'm 54 - hear the term meltdown they immediately think of The China Syndrome and Jack Lemmon breathlessly telling Jane Fonda and Michael Douglas that in a certain type of accident the reactor core could all melt down into a super dense molten glob that would then be unstoppable as it bored its way through the earth to China. Man! Talk about some scary shit!
I suspect among the majority of those who think they know what a meltdown is already, this is what they think it is. And while there is a kernel of truth to this Hollywood magic, there is much that is overstated to the point of fantasy.
The kernel of truth is that core materials can melt and also can form a molten glob, or multiple molten globs, in the bottom of the reactor vessel. The part about being unstoppable and boring to China is bogus.
During normal operations a nuclear plant, even a small one such as Fukushima 1.1, generates a massive amount of heat energy. This is converted by the steam turbines into electrical energy. As long as the heat is being continuously drawn away from the core and not allowed to build up everything is hunk-dory. And during operations, even a momentary local heat build up is not a concern because hotter water is less efficient at slowing neutrons sufficiently that they can interact and create new free neutrons. This causes local deviations to quickly settle out.
Even when a plant is shut down there is a relatively small amount of heat generated by the natural decay of the elements in the fuel. There is no reaction heat because the control rods absorb the neutrons being generated by the natural decay and prevent them from being able to interact/react. Just as in operations mode, this heat needs to be removed from the reactor to prevent the temperature from increasing This is the problem in Japan, the ability to remove heat disappeared when the diesel generator crapped out on them and the backups were rendered inoperable by the tsunami.
As the heat continues to be generated and the temperature rises in the reactor it causes water to boil off into steam, slowly uncovering the fuel. Not being underwater is not a good thing for light water reactor fuel. Being in the steam-laden atmosphere above the water line, those portions of the fuel rods do not cool as efficiently as the parts underwater. (Fuel rods run vertically as do control rods.) And after temperature rises further a point is reached where the steam begins to interact with the metal cladding that surrounds the fuel. This process creates hydrogen gas as a byproduct. This was the source of the infamous hydrogen bubble at Three Mile Island, and also of the hydrogen which caused the explosion of the secondary containment building at Fukushima. (More on that later.)
BTW - Did I mention that we are now in the process of melting down? This is where a lot of the confusion comes in. The term meltdown is being used from the point where fuel cladding starts to become unstable due to heat buildup. When you read in a report that a plant is melting down, this is all that can be inferred from that statement, that this process has reached the point where fuel cladding failure has begun. It does not mean that a Chernobyl style nuclear disaster is imminent. It simply means that fuel is melting. Not that this is a good thing, but it is also not the end of the world.
As temperatures continue to increase above about 3300 F the actual cladding melts, releasing the uranium oxide and various reaction by-products such as iodine and cesium into the reactor water/steam environment. (Some fuel pellets will have failed prior to this from plastic deformation due to pressure effects on the softened metal.) Molten materials fall to the bottom of the vessel and form a complex lava containing uranium, zirconium, steel, boron, and other elements present in the reactor. In theory, if this lava is allowed to reach a significant mass and cooling is not returned to the reactor it can result in the lava causing a failure of the reactor vessel. I say in theory because this has not actually been observed. Chernobyl had significant amounts of hardened lava that collected under the reactor vessel but I found no reports that the underside of the vessel had failed in any way. Given the catastrophic failure of the top of the reactor it is likely that it merely bubbled up over the sides and collected underneath.
It is also theorized that if this molten slag of lava drops from the reactor vessel, that it can interact with the steel reinforced concrete of the primary containment floor and eat its way through to be in direct contact with the environment. Even in the extreme accident in Chernobyl this did not occur. There is no credible theory that claims this molten slag could bore its way through the earth - at least outside of Hollywood.
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This has been a very frustrating situation to monitor. As many of you well know, I am a proponent of nuclear power. I once worked in the industry, though not for the past 26 years, and believe that it is a crucial part of a sane energy strategy to replace fossil fuels and prevent certain global catastrophe. I am also a human being and fully understand the fears and legitimate concerns that a circumstance such as this presents. I wish to in no way minimize the potential danger represented by Fukushima or tell people that their fears are unfounded. I do, however, wish to convey accurate information that enables people to put the current situation in a realistic perspective concerning the likelihood of various outcomes.
Given that the problems were mostly related to the lack of offsite power and not structural damage, the prospects were always good that this would not end up being a catastrophe. Even so, when I awoke this morning and checked the latest, I was seriously questioning my support of nuclear power. The reason for this was that I had read that the primary containment had been blown up in an explosion and that radiation was spewing into the environment. This went against everything I thought I knew about reactor operations and the realm of possible events. I could not conceive of what created enough of an impulse to destroy the containment. I have a very pragmatic view of reality. When reality contradicts what I believed to be true, I adjust my beliefs. And my beliefs regarding nuclear power were on the chopping block this morning.
Then I read an update from koNko which made it clear that it was a hydrogen explosion in the secondary containment. This made perfect sense and relieved me of my pressing need to recant my pro-nuclear position. It did, however, wake me up to the need to help keep misinformation tamped down. One of the key lessons from Three Mile Island was that the poor flow of accurate information provided a vacuum into which every crazy theory flooded. And over the past couple of days I have read some exceptionally sensationalistic material.
Even the so-called expert on Rachel Maddow, Edwin Lyman, was engaging in fearmongering and paranoid conspiracy theorizing, saying that the Japanese weren't being forthcoming and that it appeared that things there were spiraling out of control. He also warned that the reactor contents might drop through the reactor vessel and through the containment and into the environment, not qualifying that by explaining how unlikely this was given that the reactor was manned and that remediation efforts were ongoing.
And in my own efforts to correct obvious misinformation and provide some perspective, I have been called a shill, a troll, an HB Gary plant and a Chamber of Commerce hack. Why is it that hard core anti-nukes believe that being against nuclear power is a mandatory position for progressives? Trust me, if I were a C of C or HB Gary plant I certainly wouldn't have been hanging around Daily Kos since 2004 or posting diaries such as this: Time to Defund the Right Boy, my corporate masters must be PISSED at me :)