Japan’s NRA announced that 31 waste containers at Fukushima Daiichi have exceeded their life span.
The containers in question hold highly radioactive sludge leftover from various water treatment systems that remove contaminants from water on site. The containers were reported to have levels as high as 9 Sieverts/hr. Those levels are now known to be underestimates.
These containers known as high integrity containers or HIC containers have a troubled past. When they were initially proposed in 2012, TEPCO planned to use them without any additional testing. Japan’s new nuclear regulator, the NRA stepped in and demanded the containers be drop tested. The containers failed the drop tests, requiring the supplier to improve the containers before they could be used at Fukushima Daiichi. Two years later the crane used to move these containers in the storage yard failed. This caused no further accident but shows why the kind of testing demanded by the NRA was necessary.
In 2015 it was discovered that improper ventilation of the containers could cause them to suffer a hydrogen explosion. The pressure caused by the hydrogen accumulation made some of the containers leak radioactive water out of the top seal.
The latest problem with the sludge containers involved improper measurement of the radioactive substances in the containers. TEPCO measured radiation levels partway into the sludge inside the containers. NRA discovered that the radiation levels at the very bottom of the containers were significantly higher. The more radioactive substances in the sludge were also the heavier materials in the container so they sank to the bottom.
The radiation level of each container is used to estimate the useful life span of the container. A container that exceeds the estimated life span is more likely to fail or break. About 3000 of these containers are in use on-site, 31 have been determined to be past their useful life span. Those containers were originally assumed to be safe to use until July of 2025. TEPCO now plans to move the contents of these containers to new containers in August. The first containers were put into service at some point between 2013-2014. The oldest containers on site would be about eight years old. NRA estimated that another 56 containers would exceed their life span in the next two years.
TEPCO did not explain how the sludge would be transferred to new containers or the risk level of such work. This work requires handling radioactive sludge with potentially lethal levels of radiation and the potential for some level of hydrogen accumulation could be involved. NRA said that none of the containers have leaked so far but did not elaborate on their current condition.
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