The Fukushima Daiichi site is now populated by large numbers of storage tanks
Workers at the damaged Fukushima nuclear power facility in Japan have to deal with huge volumes of contaminated water. The difficulties of handling this water, some of which has leaked from storage tanks, has prompted the authorities to consider measures such as an "ice wall" to better contain the area around the troublesome plant.
What is the cause of the continuing leak problems at the plant?
The ongoing problem with water seems to be coming, in the main, from poorly constructed storage tanks. Tepco, the company that operates Fukushima, is using huge volumes of water every day to cool the reactors that once generated electricity at the plant. When the water comes in contact with fuel rods at the heart of the reactors, it becomes highly radioactive and has to be stored in large containers on the site where the water is then processed to remove some of the most dangerous elements.
Every day, the company has an extra 400 tonnes of irradiated water to store. This is roughly a 10th of an Olympic-sized swimming pool. The water is held in some of the 1,000 water tanks the company has erected on site. But there are problems with these tanks. There have been at least five leak events from the tanks, with the most recent, in August, being the largest. This saw contaminated water flood a walled concrete pad under the faulty tank, and then pass through a rainwater valve to soak the surrounding soil.
Officials say the problem may have been related to the fact that the tank was moved after its original installation at the site and was not reassembled properly, and/or that its plastic seals became damaged in the process. Tepco workers have since been trying to clean up the spilled water and remove any contaminated soil.
How does Fukushima's location exacerbate the water problems?
There are steep hills behind the Fukushima plant. Rainwater runs down from those hills, through the ground on which the plant is built and out into the ocean. Radioactivity readings offshore have not fallen in the way many experts say they should have, indicating that this groundwater is picking up radioactive elements on its journey. Tepco is working on a series of plans to stop more radioactive water getting into the ocean, including erecting steel barriers and injecting chemicals into the earth to create an impermeable layer.
What is the ice wall that is being proposed?
This is the latest solution being proposed. It involves running pipes through the affected ground, and pumping coolant through them. "This causes the local groundwater to freeze, forming a barrier to the movement of contaminated ground water," explains Prof Neil Hyatt from the University of Sheffield, UK. "A similar process is used in underground uranium mining, to prevent flooding of a working area at depth, so the basic engineering principles are quite well understood. However, this is a very energy intensive process to maintain, so there will need to be careful design and trial work to produce an effective barrier that minimises energy demand."
What is the eventual solution to the stored water issue?
Tepco has changed its inspection protocols for the storage tanks and claims this should pick up any future problems much earlier. But given the huge volumes of water being handled at the site, it is likely that Tepco will have to introduce bigger, more robust storage tanks at some point - similar in scale to the crude oil depot tanks for which there is good local technology in Japan. The water itself needs to be filtered, to take out the most radioactive elements. This "cleaned" water is likely then to go into the Pacific. Even before then, such is the volume of contaminated cooling water coming out of the reactors that Tepco may be forced simply to release some water unfiltered into the ocean.
Water from the storage tanks has seeped into the groundwater and then into the sea. Efforts to use a chemical barrier to prevent sea contamination have not worked.
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