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Fukushima’s Reactor #2 is far more radioactive than previously realized

Earlier this week, Tepco, the Japanese company in charge of cleanup operations at Fukushima Daiichi, seemed to be on the verge of announcing rare good news. Specifically, the company had found some of the fuel debris associated with the failure of Reactor #2 at the bottom of the reactor’s containment vessel. Locating the damaged reactor’s fuel (or what’s left of it) is critical to the clean-up effort.

What the company ultimately discovered, however, is that the amount of radiation pouring off the damaged reactor below the reactor pressure vessel is 530 sieverts per hour, vastly higher than had previously been communicated. Previous measurements of the radiation inside the containment vessel had pointed to an exposure level of 73 sieverts per hour.

One thing I want to note at this point is these findings are being misreported across the internet as evidence Fukushima’s radiation output is “spiking” or “soaring.” This is not the case. The radiation levels within Reactor #2’s containment vessel have been largely unknown for years, due to the difficulty of getting proper readings from remote equipment in the first place. The entire reason Tepco used muon tomography to examine the interior of both Reactor #1 and Reactor #2 was because they didn’t have another means of identifying where fuel was inside the reactors. The 73 sievert metric is mentioned in reports dating back to 2012 — I’m not sure when or how Tepco measured it, but implying this is a sudden leap in radiation output is simply inaccurate.

That said, 530 sieverts is a great deal of radiation. One sievert is the maximum amount of radiation exposure NASA allows for astronauts over their entire lifetime. And it’s clear Tepco didn’t expect this problem — the company has been working on a robot it could use to directly examine the containment vessel in much greater depth, proclaiming the radiation-hardened robot would withstand a dose of 1,000 sieverts. If you assume a 73 sievert per hour dose, that gives the device nearly 14 hours of useful life. (A Japan Times report from 2012 indicates that even this much radiation is difficult to harden against).

A composite photo of the metal grate and the hole in it, directly below the Reactor Pressure Vessel.

A composite photo of the metal grate and the hole in it, directly below the Reactor Pressure Vessel.

At 530 sieverts, that same robot wouldn’t even last two hours, and the distortions to its cameras and capabilities would be severe. The 530 sievert dose isn’t a direct measurement, it’s an analysis based on how much distortion was visible over a remote camera feed during the investigation this week. There’s also a 6.5-foot hole in a metal grate underneath the reactor pressure vessel.

The condition of Reactor #2

Over at Reddit, nuclear engineer Hiddencamper has written about what happened at Fukushima Daiichi both immediately after the disaster and over the past few years. The bottom line is this — until now, Tepco has never been able to approach this area of the containment vessel because all earlier attempts to manually drive robots ended in failure long before the vehicles reached this point (as seen in the image below):


Last summer, Tepco released the results of its muon tomography, showing that Reactor #2 still had a significant mass of material at the bottom of its reactor pressure vessel, or RPV. We already knew the reactor had breached, because pressure inside the reactor fell after the initial disaster, without any action on the part of plant personnel. Nuclear reactors are an example of defense-in-depth,  a strategy in which multiple types of defense mechanisms are used to limit the risk of various disasters. In this case, specifically, there is no evidence that core slag melted through the containment vessel. Each of Fukushima’s destroyed reactors exhibits different failure characteristics because the impact of the disaster played out differently in each. Reactor #1’s Reactor Core Isolation Cooling System (an emergency security cooling system meant for emergencies) ran for 70 hours in Reactor #2, while it failed quickly in Reactor #1. This means that when the reactor vessel failed, there was a tremendous amount of pressurized steam within the RPV.


Unless the muon tomography was wrong, most of the mass of fuel is still within the reactor. Hiddencamper notes that “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.”

“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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