Workers are scrambling to contain highly radioactive water and prevent another explosion at the quake-devastated Fukushima Daiichi nuclear power plant. Meanwhile, the situation—which still remains “very serious,” according to the International Atomic Energy Agency (IAEA)—has improved only incrementally, the chairman of the U.S. Nuclear Regulatory Commission (NRC) told lawmakers.

"The NRC continues to characterize the status of the Fukushima site as static—meaning that while we have not seen or predicted any new significant challenges to safety at the site, we have only seen incremental improvements towards stabilizing the reactors and spent fuel pools," Gregory Jaczko, NRC chairman, said in testimony prepared for a congressional hearing on Wednesday.

A major problem workers have been dealing with is the massive amount—about 70,000 metric tons—of stagnant water with high levels of radioactivity in the basement of the turbine buildings of Units 1, 2, and 3. Stagnant water from the turbine building of Unit 6 was transferred to a temporary tank on Sunday, and work is also under way to block the Unit 2 trench pit.

At Unit 1—where injections of nitrogen into the containment vessel continue as pressure in the reactor pressure vessel increases—a PackBot robot confirmed that there was no significant leakage of water from the primary containment vessel.

A plan to ramp up efforts to cool the reactor cores of Units 1, 2, and 3 by filling their pressure vessels with thousands of tons of water was postponed last week amid concerns that the water could result in dangerous leaks.

Freshwater injection into the reactor pressure vessels of Units 1, 2, and 3 continues. Feedwater nozzle temperatures of the reactor pressure vessels today were 142C at Unit 1, 118C at Unit 2, and 99C at Unit 3.

On Tuesday, about 55 metric tons of freshwater was injected into Unit 2’s spent fuel pool using the fuel pool clean-up system.

Meanwhile, spraying of anti-scattering agent  at the site continues: On Sunday, workers covered an area of 1,000 square meters on the south side of the Unit 4 turbine building and an area of about 4,400 square meters on the surface of the slope around the former main office building.

Sources: POWERnews, TEPCO, IAEA, NRC