While Nuclear Street focuses on the 90 percent of the ice wall at Fukushima No. 1 that is working (in the previous post), Simply Info analyzes the possible cause of the failure of the other 10 percent of the ice wall to freeze.
” Japan’s chief cabinet secretary called it “the devil’s scenario.” Two weeks after the 11 March 2011 earthquake and tsunami devastated the Fukushima Daiichi Nuclear Power Plant, causing three nuclear reactors to melt down and release radioactive plumes, officials were bracing for even worse. They feared that spent fuel stored in the reactor halls would catch fire and send radioactive smoke across a much wider swath of eastern Japan, including Tokyo.
Thanks to a lucky break detailed in a report released today by the U.S. National Academies, Japan dodged that bullet. The near calamity “should serve as a wake-up call for the industry,” says Joseph Shepherd, a mechanical engineer at the California Institute of Technology in Pasadena who chaired the academy committee that produced the report. Spent fuel accumulating at U.S. nuclear reactor plants is also vulnerable, the report warns. A major spent fuel fire at a U.S. nuclear plant “could dwarf the horrific consequences of the Fukushima accident,” says Edwin Lyman, a physicist at the Union of Concerned Scientists, a nonprofit in Washington, D.C., who was not on the panel.
After spent fuel is removed from a reactor core, the fission products continue to decay radioactively, generating heat. Many nuclear plants, like Fukushima, store the fuel onsite at the bottom of deep pools for at least 5 years while it slowly cools. It is seriously vulnerable there, as the Fukushima accident demonstrated, and so the academy panel recommends that the U.S. Nuclear Regulatory Commission (NRC) and nuclear plant operators beef up systems for monitoring the pools and topping up water levels in case a facility is damaged. It also calls for more robust security measures after a disaster. “Disruptions create opportunities for malevolent acts,” Shepherd says.
At Fukushima, the earthquake and tsunami cut power to pumps that circulated coolant through the reactor cores and cooled water in the spent fuel pools. The pump failure led to the core meltdowns. In the pools, found in all six of Fukushima’s reactor halls, radioactive decay gradually heated the water. Of preeminent concern were the pools in reactor Units 1 through 4: Those buildings had sustained heavy damage on 11 March and in subsequent days, when explosions occurred in Units 1, 3, and 4.
The “devil’s scenario” nearly played out in Unit 4, where the reactor was shut down for maintenance. The entire reactor core—all 548 assemblies—was in the spent fuel pool, and was hotter than fuel in the other pools. When an explosion blew off Unit 4’s roof on 15 March, plant operators assumed the cause was hydrogen—and they feared it had come from fuel in the pool that had been exposed to air. They could not confirm that, because the blast had destroyed instrumentation for monitoring the pool. (Tokyo Electric Power Company, the plant operator, later suggested that the hydrogen that had exploded had come not from exposed spent fuel but from the melted reactor core in the adjacent Unit 3.) But the possibility that the fuel had been exposed was plausible and alarming enough for then-NRC Chairman Gregory Jaczko on 16 March to urge more extensive evacuations than the Japanese government had advised—beyond a 20-kilometer radius from the plant.
Later that day, however, concerns abated after a helicopter overflight captured video of sunlight glinting off water in the spent fuel pool. In fact, the crisis was worsening: The pool’s water was boiling away because of the hot fuel. As the level fell perilously close to the top of the fuel assemblies, something “fortuitous” happened, Shepherd says. As part of routine maintenance, workers had flooded Unit 4’s reactor well, where the core normally sits. Separating the well and the spent fuel pool is a gate through which fuel assemblies are transferred. The gate allowed water from the reactor well to leak into the spent fuel pool, partially refilling it. Without that leakage, the academy panel’s own modeling predicted that the tops of the fuel assemblies would have been exposed by early April; as the water continued to evaporate, the odds of the assemblies’ zirconium cladding catching fire would have skyrocketed. Only good fortune and makeshift measures to pump or spray water into all the spent fuel pools averted that disaster, the academy panel notes.
At U.S. nuclear plants, spent fuel is equally vulnerable. It is for the most part densely packed in pools, heightening the fire risk if cooling systems were to fail. NRC has estimated that a major fire in a U.S. spent fuel pool would displace, on average, 3.4 million people from an area larger than New Jersey. “We’re talking about trillion-dollar consequences,” says panelist Frank von Hippel, a nuclear security expert at Princeton University.
Besides developing better systems for monitoring the pools, the panel recommends that NRC take another look at the benefits of moving spent fuel to other storage as quickly as possible. Spent fuel can be shifted to concrete containers called dry casks as soon as it cools sufficiently, and the academy panel recommends that NRC “assess the risks and potential benefits of expedited transfer.” A wholesale transfer to dry casks at U.S. plants would cost roughly $4 billion. ”
by Richard Stone, Science
The research team for simplyinfo.org put together an excellent summary report of the status of each unit at Fukushima Daiichi, along with the various decommissioning and cleanup projects, including the frozen ice wall and barrier, radioactive water filtration, muon scans, storage tank farms, evacuation lifts and compensation for Fukushima evacuees, Fukushima-related lawsuits, and radiation exposure and contamination to people, food and the environment.
” FUKUSHIMA – Tokyo Electric Power Co. plans to set up a forum for dialogue with residents in Fukushima Prefecture about the company’s work to decommission reactors at its wrecked nuclear plant, according to a Tepco executive.
In a recent interview, Naohiro Masuda, 57, head of Tepco’s in-house company for reactor decommissioning, said, “We want to ease concerns among local people about our reactor decommissioning work in order to help promote their return home” as the government’s evacuation advisories are gradually being lifted.
“As early as in April, we plan to launch a forum including myself and other members of the decommissioning division so that we can hold direct talks with residents,” he said. “We will hear from them about issues of their interest while providing information on the decommissioning work.”
Tepco is set to decommission all six reactors at the Fukushima No. 1 plant.
Three of the reactors suffered core meltdowns following the magnitude-9 earthquake and subsequent monster tsunami on March 11, 2011.
In fiscal 2017, the company is slated to remove fuel assemblies from the spent fuel storage pool in the reactor 3 building.
While noting that Tepco successfully removed fuel from the pool for the No. 4 reactor in 2014, Masuda said that a remote control system will be used for the work at the No. 3 unit because radiation levels are still very high.
“We hope that the installation of equipment for the fuel removal will be completed within fiscal 2016,” he said.
Meanwhile, the company’s work to treat radiation-tainted water at the plant through the Advanced Liquid Processing System, or ALPS, has been slow.
Masuda pointed to a delay in the construction of tanks for storing treated water.
He also said that the volume of contaminated water that needs to be treated has increased temporarily because the completion of water-shielding walls near the coast led to a rise in the amount of groundwater to be pumped up.
“It is difficult to say when the ALPS-based water decontamination work will be completed,” he said. ”
” Tokyo Electric Power Co. has unexpectedly been forced to deal with an increasingly large amount radioactive water accumulating at the crippled Fukushima No. 1 nuclear power plant after seaside walls to block the flow of groundwater were constructed in October.
TEPCO completed the walls on Oct. 26 to block contaminated groundwater from flowing into sea. The utility began pumping up groundwater from five wells dug between the walls and the plant’s reactor buildings. The plan called for releasing the less contaminated water into the sea after a purification process, but TEPCO discovered that the water had larger amounts of radiation than it had expected.
TEPCO officials said the situation has left the utility with no option but to transfer 200 to 300 tons of groundwater each day into highly contaminated reactor buildings since November, a move that could further contaminate the water.
Comprised of numerous cylindrical steel pipes measuring 30 meters tall, the seaside walls were installed on the coastal side of the No. 1 to No. 4 reactor buildings to block contaminated groundwater flowing out of the highly contaminated buildings from reaching the ocean.
To control groundwater levels, TEPCO planned to release the less contaminated groundwater from the five wells into sea after a purification process.
However, the water from four of the wells was discovered to have high levels of tritium–a radioactive substance that is hard to remove–at levels higher than 1,500 becquerels per liter, which means the water cannot be released into sea.
To compound the problem, the seaside walls have also significantly raised groundwater levels, forcing the utility to pump a lot more groundwater than it originally planned.
TEPCO has been forced to temporarily transfer large amounts of the groundwater into highly contaminated reactor buildings, where it could become contaminated to an even further degree by being exposed to melted nuclear fuel.
The utility said it suspects the high levels of radiation found in the groundwater from the wells is due to the water being exposed to highly contaminated soil near the plant’s coastal embankment.
To reduce the amount of contaminated water at the plant, TEPCO began operations in September to pump up the groundwater in wells constructed around the reactor buildings to release it into the sea after a purification process.
The company initially announced that the project had reduced the amount of groundwater flowing into the contaminated reactor buildings from 300 tons to 200 tons a day.
The increasing amount of contaminated water has been stored in tanks constructed in the plant’s compound after going through operations to reduce contamination.
TEPCO plans to increase the amount of water it pumps from wells located elsewhere on the plant site to help reduce the amount of contaminated groundwater accumulating in the seaside wells.
Company officials admitted they are not sure when it can turn things around and reduce the amount of contaminated water at the Fukushima plant. ”