1000 + tons of nuclear fuel debris inside stricken Fukushima reactors

Fukushima No. 1 Nuclear Power Plant operator TEPCO says it estimates that each of the three melted reactors at the plant contains around 364 tons of nuclear fuel debris and that the utility may need to reassess how that fuel is removed in its troubled quest to decommission the plant.
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The change in approach is a result of images that have been recorded by the Little Sunfish underwater robot that last month was sent into reactor 3, one of the reactors that experienced explosions and meltdowns following the megaquake and tsunami that struck northern Japan in March 2011.

Contrary to the popular belief that the reactor core had melted and fallen to the bottom of the reactor vessel, TEPCO spokesman Takahiro Kimoto said the images taken by the Little Sunfish in fact indicate the pressure vessel probably withstood the heat of the molten fuel, which appeared instead to have seeped through holes involved in insertion of the reactor’s control rods. 

“We do not presume that the vessel, which is 14 cm thick, melted and collapsed together with the fuel, but that part of the fuel instead made its way down through holes,” Kimoto said.

Experts say that the control rods, which are used to moderate the nuclear chain reaction and are made of zirconium alloy, would have melted as the reactors overheated and are almost certainly a component of the clumps of debris that were captured on camera by the robot between July 19 and July 21. 
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Also captured were pictures of rubble around the fuel debris, such as maintenance work scaffolding and apparatus for holding the rods in place, which could further complicate the fuel removal process. 

It is estimated that the removal of the highly radioactive debris, which is a mixture of melted nuclear fuel and reactor debris often referred to as corium, could start in 2021 and would be a crucial step in the 40-year decommissioning process. It is estimated to cost as much as US$72 billion.

First, however, a concrete plan of how exactly that rubble will be removed will need to be thrashed out, with the recent findings inside reactor 3 probably leading to a new approach.

One new method already proposed was brought to public attention as early as June and confirmed by the Nuclear Damage Compensation and Decommissioning Facilitation Corporation (NDF) on July 31. 

The NDF, which is a corporation that has been providing technical support in the three-part decommissioning road map, has reportedly come up with a plan to remove the melted fuel from the side of a partially submerged primary containment vessel (PCV) by keeping air in the upper part.

As reported in my book, Yoshida’s Dilemma, according to one senior official of the International Research Institute for commissioning (IRID), the accepted and safest method to remove such debris would be to completely fill the vessel with water, this reducing the radiation risk.

However, with the huge number of holes and cracks in the reactors would mean highly toxic water would simply leak out, meaning management of radioactive water would also complicate the proceedings. Furthermore, unless some kind of specialist sealant could be applied first, repairing such damage would be a laborious, and dangerous process the IRID official told me.

Indeed, plans to undertake such repairs in order to prevent leakage of the radioactive water were jettisoned due to there being "too many issues" involved. 

At a meeting held in Iwaki, Fukushima Prefecture to discuss countermeasures for the decommissioning and handling of the contaminated water, NDF chief Hajimu Yamana explained the organization's fuel debris removal method, which would employ robotic arms and other remote devices while flushing water over the debris to reduce radiation risk. 

However, effective ways to solve issues such as how to block radiation and prevent the scattering of airborne radioactive dust have yet to be found. 

“Special tools and techniques will have to be developed to undertake such a task that has never been attempted before anywhere in the world,” former U.S. Nuclear Regulatory Commission chairman and TEPCO advisor Dale Klein was reported as saying in a Bloomberg report on NDF’s proposed fuel removal method. “Once Tepco has identified the characteristics of this material, then they can develop a plan to remove this material in a safe manner.”

The Bloomberg report also mentions that the "defueling" process at the Three Mile Island nuclear plant in the U.S., where a nuclear accident took place in 1979, took six years to complete and involved the removal of the partially melted fuel core from inside the pressure vessel of the No. 2 reactor, which remained intact. "Fukushima offers a more complex challenge since three reactors suffered total meltdowns, with melted fuel rupturing pressure vessels and falling to the bottom of the units," the report says.

Sources: TEPCO, Mainichi, NHK, Bloomberg, Japan Times.