Antinuclear

Beyond the earthquake itself, a well understood series of events and external factors contributed to the meltdown of three of Fukushima’s six reactors, an incident that has been characterized by nuclear authorities as the world’s second worst nuclear power accident only after Chernobyl. It’s a label that warrants context, given the scale, complexity, and expense of the decontamination and decommissioning of the plant.

How does a plant and its engineers move on from such devastation? The recovery initiatives have faced major challenges, constantly being confronted by issues involving radioactive contamination of everything from dust to groundwater. And those smaller issues ultimately complicate the remediation effort’s long-term goal: to locate and remove the nuclear fuel that was in the reactors……….

the severity of the accident is probably most keenly felt in the scale of the cleanup. The incident has necessitated the ongoing cleanup and decommissioning of the plant—something that Tokyo Electric Power Company (TEPCO), the plant’s owner and operator, is responsible for. Even though the plant is seven years into the cleanup and has accomplished a great deal, we won’t see a conclusion for decades yet. ……

Reactor investigations

While they’re now stable in terms of nuclear activity, Units 1-3 remain highly contaminated. As such, while the structural integrity of these buildings has been restored, relatively little work has been undertaken within them. (One notable exception is removal of contaminated water from condensers, completed last year.)

Over recent years, a variety of remotely operated devices and imaging technologies have performed investigations of these units. The intention has been to gather information on internal physical and radiological conditions of the PCVs—the heavily reinforced bell-shaped structures that host reactors. TEPCO wants, and needs, to understand what has happened inside. Some things are known: once melted, fuel mixed with structural materials including steel and concrete to form something known as corium. But precisely where the corium ended up, how much there is, and whether it’s submerged are just some of the questions in play.

The International Research Institute for Nuclear Decommissioning (IRID), which was established in April 2013 to guide R&D of technologies required for reactor defueling and decommissioning, is supporting TEPCO in seeking answers. IRID is composed of multiple stakeholders, including Japanese utilities and the major nuclear vendors Hitachi, Mitsubishi, and Toshiba.

Naoaki Okuzumi, senior manager at IRID, described for Ars the investigative approaches and technologies. Early work utilized Muon tomography, which Okuzumi described as “a kind of standard practice applied to each unit… to locate high density material (fuel) within PCVs.” It yielded low-resolution data on the approximate location of corium. But with pixels representing 25cm-square cross-sections, the information has been useful only in so far as validating computational models and guiding subsequent robotic investigations.

The latter task hasn’t been easy. In addition to the challenge of navigating the dark, cramped labyrinths of tangled wreckage left behind, TEPCO has had to contend with radioactivity—the high levels act something like noise in electronic circuits. The wreckage has made access a challenge, too, although varying points of ingress have been established for each PCV.

The circumstances mean that TEPCO hasn’t been able to simply purchase an off-the-shelf kit for these investigations. ”An adaptive approach is required because the situation of each PCV is different… there is no standard with investigating the PCVs by using robots,” said Okuzumi, describing an approach that has translated into devices being specially developed and built in response to conditions of each PCV.

But they’re making progress. As recently as January 2018, corium was identified for the first time inside Unit 2 using an enhanced 13m-long telescopic probe and a revised approach designed to overcome problems encountered during investigations in 2017. The situation was hardly easier at Unit 3, where the PCV is flooded to a depth of around 6.5m. Here, it took a remotely operated, radiation-shielded submersible called ‘Little Sunfish’ to locate corium in July 2017.

Altogether the investigations—featuring a litany of robotic devices—have revealed that little fuel remains in any of the cores of Units 1-3. In Unit 2, a large amount of corium is present at the bottom of the RPV; in Units 1 and 3, almost all fuel appears to have melted through the RPVs entirely and into the concrete floor of PCVs beneath. The information is crucial, as we’ll come to see, for future deconstruction work at the reactors, but it continues to be extended as investigations continue…………

the amount of contaminated water now being generated—a mix of groundwater, rainwater and water pumped into reactors for cooling—has decreased from about 520m3/day to about 140m3/day between last December and February. Even so, treating that amount of contaminated water is proving taxing.

Water treatment is happening at large-scale facilities that have been built onsite, including a multi-nuclide removal facility. Here, a so-called Advanced Liquid Processing System (ALPS) reduces concentrations of cesium isotopes, strontium, and other radionuclides to below legal limits for release. But one radionuclide remains: tritium……….

Without a feasible alternative for cleaning up the tritium, the (only) solution for ALPS-treated water has been storage. Well over a thousand tanks, each holding 1,200 cubic meters, now store tritium-laced water at the south end of the plant.  ………

The ultimate plan for stored water is unknown; tritium has a half life of a dozen years, so physics won’t clean up the water for us. Some kind of controlled, monitored discharge—the likes of which is typical within the nuclear industry—is possible, according to Barrett. Indeed, the International Atomic Energy Agency has endorsed such a plan, which was proposed by the Atomic Energy Society of Japan in 2013. The plan involved diluting tritiated water with seawater before releasing it at the legal discharge concentration of 0.06MBq/L and monitoring to ensure that normal background tritium levels of 10Bq/L aren’t exceeded.

Discussions at both national and international levels would need to come first. Part of the difficulty here harkens back to societal dynamics surrounding risk and contamination: “In nuclear there is no such thing as absolute zero—sensitivity goes down to the atom. This makes discussion about decontamination or levels of acceptable contamination difficult………

Toward permanent solutions

In some sense, much of the restoration of order at Fukushima has been superficial—necessary but concerned with handling consequences more than root causes (see, TEPCO interactive timeline). Ultimately, Fukushima’s reactors must be decommissioned. Broadly, this work involves three phases: removing used fuel assemblies that are stored within ten-meter-deep spent fuel pools of each reactor building, management of melted-down reactors and removal of corium debris, and deconstruction of reactor buildings and the greater plant.

At Unit 4, spent fuel removal operations took around 13 months and concluded in December 2014. ……In all, 1,533 fuel assemblies were removed and transferred to a common spent fuel pool onsite.

Defueling of pools at Units 1 through 3, which suffered meltdowns, isn’t going to be as straightforward. For one, there’s some expectation of debris and circumstances requiring extraordinary removal procedures. “I wouldn’t be surprised if we find some structurally bent fuel assemblies caused by large pieces of concrete or steel,” said Barrett. Additionally, although radiation in Unit 3 has been reduced sufficiently to allow rotating shifts of workers to install defueling equipment, the already painstaking operations will have to be conducted remotely. The same is likely true for Units 1 and 2.

At Unit 3, the next in line for defueling, preparation is already well underway. In addition to decontamination and installation of shielding plates, TEPCO has removed the original fuel handling crane, which had fallen into the pool seven years ago, and installed a new fuel handling crane and machine. An indication of extraordinary containment methods being used, workers have built a domed containment roof at Unit 3. TEPCO’s Kohta told Ars, “Removal of spent fuel [at Unit 3] is scheduled to begin from around the middle of 2018;” meanwhile, Unit 1 is also in a preparatory stage and Unit 2 will be handled last.

Further down the line still, corium will have to be removed from melted-down reactors. It’s a daunting task, the likes of which has never been undertaken before. The reactors held varying, but known, amounts of uranium oxide fuel, about 150 tonnes each. But how much extra mass the fuel collected as it melted through reactor vessels is uncertain.

“At TMI there was exactly 93 tonnes in the reactor. Once we were done digging out fuel debris, we’d removed 130 tonnes. At Fukushima, I expect maybe a factor of five to ten more mass in core debris. It’s an ugly, ugly mess underneath the PCVs,” suggested Barrett.

High-powered lasers, drills and core boring technologies for cutting, and strong robotic arms for grappling and removing corium are already under development, according to IRID, but precise methodologies remain undecided……….

Without doubt, the road ahead of TEPCO is a long one, beset with challenges greater than those faced to date. The Mid- and Long-Term Roadmap—the Japanese state-curated document outlining the decommissioning of Fukushima—envisions operations stretching a full 30-40 years into the future. Some have suggested it’s an optimistic target, others say that the plan lacks details on key, long-term issues such as permanent solid-waste storage beyond the onsite repository currently being employed. Certainly it is the case that key decisions remain. ……..

By all accounts, it is hard to gauge the costs for the Fukushima clean-up. Kohta told Ars that works completed to date have cost about 500.2 billion yen, or $4.7 billion—a tremendous sum, to be sure, but fractional compared to the estimate of 8 trillion yen ($74.6 billion) approved by the Japanese state last May for the complete decommissioning of Fukushima Daiichi.  https://arstechnica.com/science/2018/05/remediating-fukushima-when-everything-goes-to-hell-you-go-back-to-basics/