In a major breakthrough at the crisis-stricken Fukushima Daiichi power plant, engineers who had been desperately struggling to contain radioactivity at the plant’s units today managed to stop highly radioactive water leaking into the Pacific Ocean. Tokyo Electric Power Co. (TEPCO) says, however, that it will continue to pump contaminated water into the sea for lack of storage capacity, and efforts are under way to begin injection of nitrogen into the primary containment vessel of Daiichi 1.

A radioactive water leak confirmed to be flowing from around a seaside pit at the Daiichi 2 reactor water intake on Saturday was blocked at 5:38 a.m. JST today after TEPCO workers injected nearly 6,000 liters of coagulation agents (or liquid glass) into holes drilled around the pit. TEPCO had identified the leak path from the Daiichi 2 turbine building to the sea via a series of tunnels used to provide power to the seawater intake pumps and supply of service water to the reactor and turbine buildings.

The water is believed to have come from the Daiichi 2 reactor core, where fuel rods have partially melted.

TEPCO told the International Atomic Energy Agency (IAEA) on Monday that a tracer used to determine where the radioactive water was coming from was injected into two new bore holes drilled near the pit, and on Tuesday, it was confirmed that the tracer was seen leaking from the crack into the sea. Work continues to prevent any further uncontrolled releases of radioactive water into the sea.

TEPCO, meanwhile, continued to dump massive amounts of water tainted with low levels of radioactivity from inside a nuclear waste disposal facility at the site, as well as water from around Daiichi 5 and 6. In total, 11,500 tons of water will be disposed by Friday: 10,000 tons from the radioactive waste treatment plant and 1,500 tons of subsurface water from the sub-drain pits of the two units. The water is being discharged to provide room for water with higher levels of radioactivity.

TEPCO has estimated that the discharges would increase the effective dose to a person eating seaweed and seafood contaminated by the discharge every day for a year by 0.6 millisieverts, the IAEA reported.

The international agency, which said the situation at the facility remained “very serious,” said that freshwater was being continuously injected into the Daiichi 1 pressure vessel through a feedwater line (at an indicated flow rate of 6 m3/h using a temporary electric pump with off-site power). At Daiichi 2 and 3, freshwater is being injected through the fire extinguisher lines, using a temporary electric pump with offsite power.

Though temperatures at Daiichi 1 have been decreasing, conflicting readings inside the reactor pressure vessel indicate that some instrumentation may not be working properly. TEPCO is expected to soon begin—as a precaution—the injection of nitrogen into the containment vessel to displace oxygen and reduce the risk of explosion due to the combustible combination of hydrogen and oxygen. The injection could last several days and release radioactive substances into the air.

At Daiichi 2, more water was injected into the spent fuel pool via a spent fuel cooling system line by a temporary pump on Monday. Power, meanwhile, has been restored to instrumentation in Unit 2.

The 9.0-magnitude earthquake and subsequent 14-meter-high tsunami that hit the northeast coast of Japan on March 11 have left nearly 2,8000 people dead or missing. Thousands more are homeless, and the world’s third-largest economy has been shaken. The seawater deluge knocked out all emergency diesel generators at the six-reactor Fukushima Daiichi plant, resulting in the loss of key cooling functions. The reactor cores of Daiichi 1, 2, and 3 have since partially melted, and all three reactors have experienced hydrogen explosions.

According to a confidential assessment (dated March 26) prepared by the U.S. Nuclear Regulatory Commission (NRC) and obtained by the New York Times, the crisis is far from over. An array of threats plagues the facility, including stresses placed on the containment structures as they are filled with cooling water, making the containment vessel vulnerable to further damage from aftershocks. Others include the possibility of explosions due to the release of hydrogen and oxygen from seawater pumped in to cool the reactors. Concerns have also been expressed about how the partially melted fuel rods and salt buildup from seawater could block the flow of freshwater meant to cool the reactor cores.

According to the Times, the document—prepared for the NRC’s Reactor Safety Team, which is helping the Japanese government and TEPCO quell the crisis—also recommends that TEPCO should continue adding boron to cooling water to help prevent the cores reaching criticality.

Sources: POWERnews, TEPCO, IAEA, The New York Times