Tokyo/Fukushima: The courageous Japanese workers on Wednesday plugged a 12-inch hole, which led to the seepage of highly toxic water into sea from a concrete pit at the quake-hit Fukushima nuclear plant. However, the workers were confronted with a new problem of gas build-up at one of the six reactors which may cause another hydrogen blast.

The plant's operator, Tokyo Electric Power Company (TEPCO), said the leak of radioactive water from a seaside pit located near the No.2 reactor, which was detected on Saturday
last, was stopped at 5.38 am local time after workers injected some 6,000 liters of chemical agents, including "water glass" or sodium silicate.

TEPCO also prepared to inject nitrogen into the No.1 reactor's containment vessel where hydrogen gas was building up.

Hidehiko Nishiyama, a spokesman for the Nuclear and Industrial Safety Agency, said the move is being considered in a bid to stop a possible hydrogen explosion "in advance" and
that it does not mean there is an "immediate danger," reports said.

The nitrogen injection process is expected to take several days, and may lead to the release of radioactive substances in the air.

Last month's hydrogen blasts at the No.1 and No.3 reactors following the March 11 mega quake and tsunami destroyed reactor buildings, causing leaks of radioactive steam from the plant.

According to national broadcaster NHK, TEPCO plans to infuse a total of 6,000 cubic metres of nitrogen gas during a 6-day period and is now checking procedures with the government.

The government's nuclear safety agency said it has ordered TEPCO to keep monitoring the concrete pit, from where the toxic water release was halted, to check whether leakage
into the Pacific Ocean has completely stopped, nearly four weeks after the magnitude-9 quake and tsunami struck Japan's northeast leaving nearly 30,000 people dead or missing.

There is a possibility that the water, which has lost an outlet, may show up from other areas in the plant's premises, it said.

The highly radioactive water is believed to have come from the No.2 reactor's core, where fuel rods have partially melted, and ended up in the pit.

The pit is connected to the No. 2 reactor's turbine building and an underground trench connected to the building, both of which were found to be filled with high levels of contaminated water.