Six Years After Fukushima, Robots Finally Find Reactors’ Melted Uranium Fuel

Earlier robots had failed, getting caught on debris or suffering circuit malfunctions from excess radiation. But the newer version, called the Mini-Manbo, or “little sunfish,” was made of radiation-hardened materials with a sensor to help it avoid dangerous hot spots in the plant’s flooded reactor buildings.

After three days of carefully navigating through a shattered reactor building, the Manbo finally reached the heavily damaged Unit 3 reactor. There, the robot beamed back video of a gaping hole at the bottom of the reactor and, on the floor beneath it, clumps of what looked like solidified lava: the first images ever taken of the plant’s melted uranium fuel.

No one knew for sure exactly how far those molten fuel cores had traveled before desperate plant workers — later celebrated as the “Fukushima Fifty” ...able to cool them again by pumping water into the reactor buildings...

As officials became more confident about managing the disaster, they began a search for the missing fuel. Scientists and engineers built radiation-resistant robots like the Manbo and a device like a huge X-ray machine that uses exotic space particles called muons to see the reactors’ innards.

“Until now, we didn’t know exactly where the fuel was, or what it looked like,” said Takahiro Kimoto, a general manager in the nuclear power division of the plant’s operator, Tokyo Electric Power Co., or Tepco. “Now that we have seen it, we can make plans to retrieve it.”

Tepco is keen to portray the plant as one big industrial cleanup site. About 7,000 people work here, building new water storage tanks, moving radioactive debris to a new disposal site, and erecting enormous scaffoldings over reactor buildings torn apart by the huge hydrogen explosions that occurred during the accident.

For the nuclear ignorant, such as myself, now that they know where the reactor core is could they not distribute Boron tubes over the active fuel to dampen the nuclear flux and therefore the heat?
I know there are other “control rod” elements capable of absorbing the protons and thereby restricting further splitting of the Uranium. Is it possible to just flood the zone with Silver or another neutron absorber?

No. Boron is a good neutron absorber, and - if dumped as a powder, metal pellets, or liquid over the melted core mass - could absorb neutrons.

But.

A reactor can ONLY go critical if the fuel is pure enough and enriched enough to distribute the enriched U235, U233, or Pu 239 in exactly the right very closely calculated geometry across the width, height, and length of the core with exactly the right amount of moderator material around the fuel to intercept the first free neutron, slow it down by multiple collisions inside the moderator without being absorbed by anything else (previous decay products, previous fission daughter products, oxides, supports, piping or steel or chromium or nickel or concrete or fuel module zirconium and old control rods (hafnium) or reactor internals of any kind) and then survive to be absorbed by an enriched U or Pu nuclei. That nuclei then has to fission, and release more neutrons.

Biggest problem is the core geometry - Wrong shape and it cannot go critical regardless of purity or enrichment or presence of moderator.
Moderators boil off (reason for the melting!) and so - if it is melted fuel, then it can have no moderator (water) around it. If the original core material melted and then was covered by water later - which is likely in many areas - then the melted fuel is the wrong shape to re-absorb enough neutrons before they leave the area or get absorbed.
The fuel concentration and presence of poisons by earlier fission daughter products and control rods melted into the mix at the bottom of the core and down the pipes around the core is the final nail: The heat and decay energy in the radiation comes from the continued decay of the fission daughter products. NO fission daughter product can fission again - it's impossible.

ONLY regions of “clean” enriched fuel can fission hit by slow moderated neutrons - and the melted fuel core are contaminated by too much junk metal to go critical.

Silver is not a usable neutron absorber. Hafnium, boron, and large number of daughter product isotopes do absorb neutrons.

And someone can maybe tell everyone the number of deaths that have directly occurred NOT from the Tsunami but from the failure of the Tsunami flooded nuclear plant. How many?

https://en.wikipedia.org/wiki/List_of_nuclear_and_radiation_accidents_by_death_toll

https://en.wikipedia.org/wiki/Fukushima_Daiichi_nuclear_disaster_casualties

Yet, irrationally, Japan stopped 54 of its nuclear power plants, and only recently has put five of them back online.

If anything, what Fukushima represents, to Japanese nuclear power plants, is that Japan needs to review just where each plant is located with consideration for the history for Japan of both earthquakes and Tsunamis, and it needs to look at all the measures being taken at each plant for mitigating problems that might occur in the case of an earthquake or Tsunami. It is that last part where the Fukushima plant failed, initially, not from the general nuclear power functions themselves. Had the “flood safety” measures at the site been better, the plant’s operations would not have been disrupted by the Tsunami. That was a failure of site design and not of the design of the plant generally.

Japan has lessons it learned and can still learn from Fukushima. Ending its nuclear power program is just throwing out the baby with the bathwater.

I should have paid more attention in geometry.

True, true. 8<)

Geometry helps. Trigonometry. Add a few integral and differential calculus courses, all of the other math and computer classes up through Bessel's functions, reactor kinematics and materials classes, reactor controls classes, core theory, relativity and all of the general nuclear physics nuclear core curricula you need before that, engineering mechanics and statics classes, materials design, couple of heat transfer, thermodynamics and fluid flow theory, throw in several semesters of chemistry and other general background type stuff like electrical engineering... 8<)

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