Pits pose problems at Fukushima

Monday, 15 April 2013

Tepco is emptying the three water storage pits that were found to be leaking at Fukushima Daiichi. In around two months the contaminated water should all have been removed to tanks.

Levels of radioactivity in the leaked water were 6 MBq/l and 300 MBq/l - enough to be classified as intermediate-level radioactive waste in most countries. It is not yet known exactly how much water was released, or how far it may have travelled underground.

The defective pits are part of a huge array of water storage at the Fukushima Daiichi site. Water is continuously circulated to maintain low temperatures, passing through the remains of the reactor cores and picking up tiny but highly radioactive particles en route. The water makes its way through the lower levels of the reactor buildings and accumulates in the adjacent turbine buildings, from where it is pumped out for decontamination at a large on-site facility built rapidly after the accident. Tokyo Electric Power Company (Tepco) is allowed to recirculate the water for cooling, but may not discharge it even when decontaminated to a quality fit for drinking.

Schematic of the sub-surface water storage pits and their linings (Image: Tepco)

Most stored water is in cylindrical storage tanks, but in areas of the site unsuitable for these - such beneath power lines - Tepco opted to use covered basins instead, setting up a total of seven of them lined with two layers of polyethylene, a layer of bentonite clay and a foundation of concrete. They include sensors to monitor for leakage, which alerted the company that water had indeed escaped through the layers at pit 2 and pit 3. Tepco thinks that the leaks were actually caused by damage to the lining in the areas where detectors were installed.

Initially, Tepco pumped water from leaking pit 2 into empty pit 1, but this was found to have a leak as well. The current situation sees some 15,500 cubic metres of water across defective pits 1, 2 and 3. Some of this is being shifted to existing tanks straight away, while more is being moved to undamaged pits in advance of new storage tanks expected to be in place by early June. In the meantime, Tepco has begun to use the leak detection holes to extract leaked water and return it to the pits.

Researched and written
by World Nuclear News