Q&A: Fukushima leak problems
Our environment correspondent Matt McGrath assesses the ongoing efforts to deal with contaminated water at the damaged Fukushima nuclear power plant in Japan. It has been revealed that some of the storage facilities used in the clean-up have been leaking highly radioactive water.
This isn't the first water leak at the plant. What is going on?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, says Prof Neil Hyatt, from the University of Sheffield, UK.
"To keep up with the rate at which radioactive cooling water is accumulated, Tepco has opted to use containment tanks incorporating plastic seals. Seepage from these joints was the cause of the latest leak of radioactive water."
It is believed that about a third of the storage is constructed in this way. Four previous, smaller leaks all came from these type of tanks.
Finding the small leaks is very difficult, according to Prof Hyatt.
"It is very challenging. They have a real problem with the high level of background radiation, so the small leaks are hard to find."
Officials have said that the level of radiation close to this latest leak is extremely high. The water is said to be 8 million times above the safe level for drinking water.
According to Prof Paddy Regan, at the University of Surrey, UK, this leak must be kept in some perspective.
"The numbers reported for dose from these concentrated sources are high - standing there for any more than a few minutes would not be encouraged - but the risks are measurable and the potential doses received should be monitored by workers in the immediate area," he said.
The overall level of radiation that has been emitted by the Fukushima disaster also needs to be kept in perspective. According to Dr Ken Buesseler, senior scientist at the Woods Hole Oceanographic Institution, US, the release of the radioactive element caesium from Fukushima is between a 10th and a third of what was released from the accident in Chernobyl, and perhaps one fortieth of what was released by nuclear bomb testing globally in the 1950s and 1960s.
The proposal to raise the incident level from one to three means it's more serious than we thought?Raising the threat level to three on the International Nuclear Event Scale (Ines) makes it the most serious nuclear incident since the reactors themselves melted down in the wake of the tsunami in 2011.
Each step on the seven-step scale represents a tenfold increase in severity. Level 3 means the danger is contained on the site and there is no imminent threat to the public.
According to the International Atomic Energy Agency (IAEA), Level 3 can be assigned when there is "severe contamination in an area not expected by design, with a low probability of significant public exposure."
Prof Andrew Sherry from the University of Manchester, UK, says the measures taken by Tepco are the right ones.
"Though serious, this leak is a long way from the Level 7 incident we were facing in 2011. The approach taken by Tepco to drain the tank, pump leaked water to temporary storage, and protect the drainage of contaminated water to ground water, is entirely sensible," he explained.
Is storing water the only problem?If only. The Fukushima site suffers from its location, where groundwater from the hills surrounding the plant flows down and into the radioactive areas. Tepco wants to channel some of this water, with low levels of radiation, into the sea, but local fishermen are strenuously opposing this.
Given the plant is so close to the sea, 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.
But given that the plant is in an active earthquake zone, there is a danger that further tremors could spill much of the stored water.
"It is a potential; it is realistic, " said Prof Neil Hyatt. "I would be saying to the government and to Tepco to make clear that in the event of an emergency, there are plans in place to deal with this."
Underlying all the water problems is the key issue of what to do with the damaged reactor cores. If these can be dealt with, then the water becomes much less of a problem Tepco had planned to remove some of the 400 tonnes of highly irradiated spent fuel in Reactor No 4 later this year. This won't be easy.
"It is on another scale entirely, a nightmare really," said Prof Hyatt.
In essence, they already have. In May last year, the Japanese government injected $12.5bn (£8bn) in return for more than half the shares in the company. It was part of a 10-year restructuring plan.
According to the trade and industry minister, Yukio Edano, the capital injection was needed to ensure the utility company could continue to supply electricity and pay for compensation and decommissioning costs.
Where will all this end?In the very short term, Tepco will have to invest in more robust storage tanks, and cope with continuing problem of groundwater. Its ultimate hope is that this water can be treated and cleaned, and then released into the sea.
Tepco will then have to tackle the fuel rods, which a slow and tricky task. The IAEA says it could take 40 years. Some experts believe the whole clean-up will take more than a century.