Tokyo Electric Power Co. on Monday began releasing 10,000 tons of low-level radioactive water from the Fukushima No. 1 power plant into the Pacific Ocean on Monday evening to help accelerate the process of bringing the crippled complex under control.

The radical step was taken to make room for the more radioactive water that is being pumped out of the No. 2 reactor’s turbine building.

The utility also said it plans to release 1,500 tons of radioactive water being stored under the No. 5 and No. 6 reactors, which have been safely shut down.

The government said dumping the water will pose “no major health risk” and is inevitable in order to rescue the plant.

Tepco will try to minimize the environmental impact of the dump by setting up an underwater silt fence similar to an oil fence outside the seawater intake near the damaged No. 2 reactor, where toxic water is already leaking into the sea from a cracked storage pit.

The water to be dumped comes from several parts of the flooded plant — including areas close to the reactor core — and has a radiation level about 100 times the legal limit, which Tepco deems relatively low.

According to the Nuclear and Industrial Safety Agency, even if one eats 200 grams of fish caught within a 1-km radius of the Fukushima plant each day, the cumulative radiation exposure over a year would amount to 0.6 millisieverts, which is below the 1.0 millisieverts recommended in one year.

Tepco released the water because it is desperate to find storage tanks for the highly radioactive water flooding the turbine buildings of its damaged reactors.

That water is preventing workers from entering the facility to repair its critical cooling systems.

The plan for the silt fence emerged after Tepco failed Sunday to plug up a cable trench leading to the cracked storage pit where contaminated water is seeping into the ocean. An earlier attempt to plug the closet-sized pit with concrete failed because the influx of water prevented it from setting.

The source of the deadly water, its volume and exactly how it is getting from the reactor to the sea continue to baffle Tepco’s nuclear plumbers, who have confused the public by wavering on their own alarming radiation readings.

The underwater fence is the third plan that has been trotted out so far to contain the radiation leak. The Nuclear and Industrial Safety Agency said it will require a large, curtainlike silt fence to be erected underwater. Silt fences are normally used in river and bridge construction work to prevent polluted or muddy water from spreading.

The fence is suspended by a float, allowing it to hang down theoretically to the bottom of the sea and contain the contaminated water within the perimeter.

“It may not completely shut in the polluted water or keep it from spreading, but it can contain a large portion of it,” NISA spokesman Hidehiko Nishiyama said.

Nishiyama said it will take a few days to procure the silt fence and install it. If the temporary step works, it may buy Tepco time to find the source of the internal and environmental leaks and patch them.

In his news conference, Chief Cabinet Secretary Yukio Edano underscored the urgency of preventing the ocean from being contaminated further.

“If this situation continues for a long time and the amount of radioactive leakage adds up, it will have a big impact on the ocean even if it spreads and dilutes,” Edano said. “We need to stop this as soon as possible, so (the government) instructed Tepco to take steps immediately.”

Meanwhile, Tepco continued to look for the source of the radioactive water by injecting a milk-white bath additive so it can to trace the flow.

The utility believes the water is exiting the flooded No. 2 turbine building through a cable trench connected to the damaged storage pit.

Tepco poured the bath additive into the cable trench at around 7 a.m., but as of 11 a.m. hadn’t seen any milky water exiting the storage pit.

This indicates the contaminated water is coming from elsewhere and the utility is searching for other routes.

On Sunday, Tepco tried unsuccessfully to clog up the cable trench with a mix of polymer, sawdust and shredded newspaper after an attempt to fill the pit itself with concrete failed the day before.

In the meantime, Tepco is continuing the pain-staking process of pumping radioactive water out of the flooded turbine buildings of reactors 1 through 3 so workers will eventually be able to install power cables and restart the cooling systems.

Finding storage places for the hazardous water, however, is proving to be another headache.

Information from Kyodo added.