Tepco has recently released measurements that provide convincing evidence that virtually all of the corium in Fukushima Daiichi unit #1 remains safely stored inside an intact reactor pressure vessel. Despite all claims to the contrary, no substantial quantities of that material have melted through the pressure vessel to fall onto the concrete floor of the surrounding containment structure.
It has always seemed far fetched to me to think that material from a nuclear reactor that melted several hours after fission has stopped contains the power density necessary to melt through carbon steel pressure vessels that are 6-12 inches thick. My basis for making that statement comes from having spent several sleepless nights in a drydock watching people with specially designed torches cutting into submarine hulls to provide maintenance access. I also had the opportunity during at least one repair period to be the guy responsible for signing the requisition chits for the pallets full of gases used to power those torches.
Melting thick steel is not a job for a mass of metal that is only being heated by radioactive decay whose heat production is falling rapidly.
A couple of days ago, Tepco drilled a hole into the thick concrete foundation that supports the reactor pressure vessel. The hole was approximately 8.7 meters above the containment floor. The workers then inserted a radiation probe. Based on the readings from the probe, it is not possible to truthfully assert that the corium left the reactor pressure vessel and made it to the floor of the containment under the vessel.
The probe registered its highest reading (11 Sv/hr) at the level where it was inserted – 8.7 meters above the containment floor. I do not know the details of the apparatus, but it is reasonable to assume that the workers were using gravity to lower the probe and had no mechanism that would enable the probe to be raised above the entry point.
The measured radiation dose rate was lower at all points below the entry point until it reached a minimum at the surface of a pool of water at the bottom of the space. The other sensors inserted with the radiation measuring probe determined that the water was fresh – with a chloride concentration of about 19 parts per million – and that it was about 2.8 meters deep. The radiation reading at the surface of the pool of water was only 0.5 Sv/hr.
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