Comments by T. Fronte
|Cooling corium at Fukushima|
|Cooling the "corium"||The Fukusmima case|
In many normal operating cases, should the coolant stop flowing through
a reactor , the fuel rods become insufficiently cooled and the rise of
their temperature may within minutes initiate a meltdown. Even if the
coolant flow is fully restored , some paths may have been restricted by
molten or distorted parts ; the spots which can’t any more be reached
by the needed rate of coolant will suffer more overheating and the disorder
may expand in the reactor core.
A molten core, also known as “corium”, is an heterogeneous
jumble of radioactive fuel , metallic structures and casings ; depending
on the shape and the location of this disparate conglomerate, it may prove
very difficult or impossible to cool or to remove .
The cost and the time frame of the deactivation are largely unpredictable . For instance, the “official end of clean up” at the unit 2 of Three Mile Island occurred 14 years after the 1979 accident . At Chernobyl , the case is still on going , although the accident occurred tens of years ago, in 1982.
Most likely , the cores of the reactors
started to meltdown in the early stages of the accident, in march 2011
; this is consistent with the releases of products of radioactive fission
- iodine-131 and caesium - which suggest that the radioactive fuel was
escaping from overheated and cracked rods. In addition, the hydrogen responsible
for several explosions is the product of water interacting with overheated
zirconium fuel assemblies.
Two months after the incident, officials have confirmed the meltdowns
of the reactors 1 to 3; in addition they have reported that some reactor
vessels and containments are breached.
The efforts are first of all aimed at preventing the coolant from flowing
through many unidentified leaks; pending this goal, part of the coolant
which is sprayed or injected in the molten reactors reaches the environment
; an other part is stored , waiting for later decontamination process.
The amount of coolant released in the environment can hardly be evaluated.
The highly contaminated coolant which has been collected and stored for
more than two mounts could be in the scale of 100.000 tons. It’s
about the capacity of 40 average Olympic swimming pools, 2500 m3 each
This is a facet of the disaster.