I just stumbled into a slightly later (2017) article on
the difficulties being faced by Tepco in the Fukushima Daiichi reactor cleanup - last we heard from them they were attempting to survey the level of damage in reactor 2, but their hardware was struggling because of the
very high levels of radiation being encountered. Possibly as much a 1000 sieverts.
Video images being sent back are noisy and resemble old analog television from the flight decks of aircraft carriers in the 60s, where powerful radars were sweeping the TV reporters equipment and causing interference.
its too radioactive for most of the hardware they have attempted to use. It turns out much more radioactivity is being encountered inside the reactor than had been expected before. Additionally they have verified that as suspected, during the original incident, molten nuclear fuel did indeed "melt down" burning a hole underneath the reactor core and leaving an area where there is both this hole and a great deal of once molten uranium metal. But, don't worry,
there is no danger of a critical mass and a hugely dangerous nuclear fission reaction contaminating one of the mostly densely populated parts of the world for thousands of years. According to TEPCO.
They are monitoring the situation very closely.
" “Unshielded spent nuclear fuel can produce over 1 million Rem per hour on contact (10000 Sv) the year after its removed from the reactor. This decays over time naturally.”
(But at Chernobyl, the rate its been decaying is now proving to be slower than their math had predicted, and they don't know why.)
“The hole isn’t new,” he continued. “This is the first time they’ve gotten into the undervessel area. That hole has been there and these are expected dose rates for unshielded fuel. There is no evidence of a reaction for several reasons. One of which is Tepco has been monitoring the gaseous fission product levels and there has been no change indicative of a critical reaction.”
That said, there is still a significant problem here. A significant amount of slag may have melted through, even if the bulk of the material remains within the RPV. Previously, Tepco was expecting to find a 73 sievert problem. The sheer amount of radioactivity in this area will complicate cleanup and debris disposal, particularly if we can’t use robots to help with the most dangerous parts (humans are an even worse idea, given our unfortunate tendency to total biological failure). Despite how it’s being reported, this isn’t some brand-new crisis or failure, but it’s going to make life more difficult and expensive for those attempting to clean up the mess."
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