The history of human use of nuclear energy is not an encouraging one. I know I myself grew up dreaming of George Jetson and the unlimited energy future that we’d have when Fusion finally came online. I remember having a comic book that depicted road building equipment composed of a hot nuclear reactor melting everything in its path to make a highway of Glass. I’ve always been a supporter of Nuclear energy because I know that if left up the engineers nearly any issue can be solved and it could have been a safe technology.
We were told that reactors based on graphite control rods were dangerous because graphite Burns. We were told that the xirconium fuel rods favored for the past 50 years would be safer and that with our superior engineering we could not possibly have a meltdown in our “superior” design.
I guess I was wrong.
Why Fukushima is a mess — Zirconium.
A New York Times article from June 9 1995 writes:
“Zirconium is used in nuclear reactors to provide the cladding, or outer covering, for the cylindrical fuel rods that power a nuclear reaction. Packed inside the zirconium cladding are pellets of uranium oxide or other fissionable materials.
Zirconium isn’t used for safety. On the contrary it’s purpose was described in the next paragraph of that article:
“Zirconium is the metal of choice in this application because it absorbs relatively few of the neutrons produced in a fission reaction and because the metal is highly resistant to both heat and chemical corrosion.”
If you want to store nuclear rods safely, why not put each rod in a lead sleeve. But then the problem here is that we were never supposed to accumulate 10 years of fuel rods, much ten, twenty, thirty years and tons of them. Of course the engineers cover all the posibilities:
Low neutron absorption is vital to any structural material used in a nuclear reactor because large numbers of neutrons produced by the reaction must be free to interact simultaneously with all the nuclear fuel confined inside hundreds of fuel rods. This interaction sustains the necessary chain reaction throughout the reactor’s core.”1
the trouble is that Zirconium in fuel rods is kept cool by water, and if there is a loss of coolant the immediate environment is in a whole lot of trouble. Wikipedia states “Whereas there is no clear threshold of oxidation, it becomes noticeable at macroscopic scales at temperatures of several hundred °C.” And the article continues:
“One disadvantage of metallic zirconium is that in the case of a loss of coolant accidents (LOCA) in a nuclear reactor, the Zr cladding rapidly reacts with water steam at high temperature. Oxidation of zirconium by water is accompanied by release of hydrogen gas. This oxidation is accelerated at high temperatures, e.g. inside a reactor core if the fuel assemblies are no longer completely covered by liquid water and insufficiently cooled. Metallic zirconium is then oxidized by the protons of water to form hydrogen gas according to the following redox reaction:
Zr + 2 H2O → ZrO2 + 2 H2 “2
What happens next is a horror show. My friend summarizes as follows, talking about Fukushima 3:
And then… now pay attention, this is tricky. First you need to understand how the water in SFPs does more than keep the rods cool, and shield the radiation. If there was no water, decay neutrons leave the vicinity too fast to interact with other rods enough to support a criticality. Not enough neutron-reaction feedback, so no chain reaction criticality. (Never mind that the rods would be overheating and catching fire in air, that’s not the point here.)
In other words it is melt and burn not just melt.
On the other hand, with water present the neutrons are slowed and absorbed by the water fast enough to prevent them interacting with other rods, so… not enough feedback, and no chain reaction criticality.
No water no reaction — except that the fuel rods are heating up.
Got that? No water, neutrons leave too fast. With water, neutrons slow down too much and get absorbed. Both states mean no nuclear chain reaction, no criticality.
Water, no reaction
Now for the cold, logical and horribly inevitable truth. It follows from the above that for some degree of neutron slowing/absorption in between the ‘water’ and ‘no-water’ states, there is a condition in which neutron-fission positive feedback within the stacked rods is just right, and prompt criticality will occur within milliseconds.
Now Zirconium catalyzes water into steam gas and burns water.
What might this intermediate state between water and no-water be? Why, it’s simple. It would be water in the process of flash boiling into steam. At some point, the expanding froth of water/steam will hit the right average density between the fuel rods, and there’ll be a nuclear chain reaction.
In other words, Fukushima Daichi had a mini nuclear reaction.
What would cause a volume of water in a SFP to flash boil? Why… a strong nuclear chain reaction nearby! The huge flux of neutrons from a zone of the rods that went critical, will be absorbed by the water nearby… as in, deeper in the SFP. The neutrons heat the water enormously, and the water suddenly wants to boil. Nothing stopping expansion but its own inertia, and so it begins to expand…
And here is the horror:
It turns out that SFPs [Fuel Rods stored in "Swimming pools" or in reactors] are actually much like strings of firecrackers tied together.
My scientist friends says that Zirconium when hot enough burns like a sparkler to be more accurate.
If the tops of the rods are exposed for a while, they burn. In the worst case there’s a prompt criticality in the burning section, either spontaneously as pellets fall into clumps, or as a result of shock movement of pellets due to hydrogen explosion.
The criticality causes a huge neutron pulse. Which in turn causes a volume of water lower down in the pool to flash boil.
As that water changes state into steam (which takes a little while, say, oh, half a second) a moment will come when a new area of the fuel rods in the volume of expanding water/steam froth suddenly reaches criticality. Causing another huge pulse of neutrons.
In other words a mini nuclear explosion.
These in turn heat another shell of water, still deeper in the SFP. And the cycle repeats until it runs out of fuel rod length, or water, or some other limit is reached. In the case of Fukushima #3 SFP, the cycling stopped after the third criticality pulse. Why not two pulses, or five? It doesn’t matter. The point is it kept going after the first.
Note that each prompt criticality will only be brief, since it will stop as soon as the flash boiling water expands further and becomes sufficiently like ‘no water’. It’s likely that the rods going briefly critical would still be sitting there afterwards, rather than actually blowing up. They just produced a brief, huge thermal and neutron power pulse.
Like a huge flourescent lamp.
Then afterwards, exposed to air, they’ll suffer a zirconium fire and burn. But that’s much later compared to the fast process of the ‘SFP firecracker string’ effect. The newly exposed rod casings burning may be why unit #3 had so much white smoke in the day or so after the main explosion.
Smoke and tons of downwind fallout.
The fuel pellets thrown for kilometers around SFP #3 would be from the initially burnt top sections of the rods, before the nuclear-flash boiled water below turned the SFP into a giant steam-powered shotgun. That fired three times. Three bangs.
And this is why Fukushima is probably still in meltdown. He continues:
…The original hydrogen explosion produced a powerful distributed overpressure pulse, that blew out the concrete side wall panels and set the concrete roof slab moving rapidly upwards. The steel girder structure below the roof slab would have been relatively unaffected by the hydrogen explosion overpressure due to the low surface area of the beams, and their strength and ductility. But the high velocity jet of material ejected upwards from the SFP by the steam explosions then completely obliterated the roof structural girders above the SFP. Basically, the jet cut them to pieces.
[were] only invisible pressure waves coming in the distinct pulses that we heard as three bangs.
And his conclusion is:
Some generations of nuclear plants may have insane features like fuel pools up on top of tall buildings, in earthquake zones. Emergency generators in basements, that get flooded. Backup diesel fuel tanks on seaside docks, that get washed away. There are endless such stupidities, but they don’t tend to be universal to all designs. Fukushima #3 demonstrates there can be unseen fatal traps across all nuclear plants. Deep conceptual flaws, unaccounted for in designs. Spent fuel pools are suddenly and unexpectedly shown to be fundamentally unsafe.3
The point is that it appears that Daiichi 3 exploded, and Daiichi 4 with it’s huge load of spent fuel may be still on fire and in danger of collapse. This is a calamity for the whole world.
- Fukushima is still dangerous:
- A film in German on the Fukushima Lie:
- Nuclear Crimes (Free book):
- Tepco hiding information:
- 1. http://www.nytimes.com/1995/06/09/nyregion/zirconium-covering-for-fuel-rods.html
- 2. Wikipedia on Zirconium: http://en.wikipedia.org/wiki/Zircaloy
- 3. http://everist.org/archives/Fukushima/20120430_Message_of_Fuku3.htm