A fuel rod in a Fukushima Prefecture plant appeared to have partially melted, the government's Nuclear and Industrial Safety Agency said Saturday.
A fuel rod in a Fukushima Prefecture plant appeared to have partially melted, the government's Nuclear and Industrial Safety Agency said Saturday.
However, the agency said pressure in the core containment vessel at Tokyo Electric Power Co.'s Fukushima No. 1 nuclear power plant has started to decrease, lowering the possibility of its rupture.
The plant was shut down after the magnitude-8.8 earthquake struck Friday. But the backup system to cool the reactor core failed to operate.
TEPCO said earlier that it had released steam from the reactor to lower the pressure within the reactor containment vessel.
The central government on Saturday declared a state of emergency at the No. 2 Fukushima nuclear power plant, located about 8 kilometers south of the No. 1 Fukushima nuclear power plant.
A state of emergency had been declared for the No. 1 plant the previous day.
Residents within 10-km of the No. 1 plant were told to evacuate.
Traces of radioactive cesium were confirmed around the plant in an inspection.
Water levels within the reactors at the No. 2 plant have been maintained, and unusual levels of radiation have not been detected.
The state of the two Fukushima plants was causing particular concern because, unlike other plants hit by Friday's earthquake, emergency generators had failed to operate after the failure of external power supplies. That crippled electric powered emergency core cooling systems (ECCS).
Those systems, which should begin operating when water levels within the reactor core fall, are critical to the safety of the reactors.
The No. 1 Fukushima plant has 13 emergency diesel generators for its ECCS, but all appear to have failed following Friday's earthquake.
The ECCS is designed to inject coolants to prevent overheating and damage to the reactor core. Several different systems are available to deliver the coolants, including a high-pressure spray from the top of the pressure container at the core and a spray mechanism that cools the containment vessel from the outside.
The first successful use of an ECCS system during an accident was in February 1991 at the No. 2 reactor at the Mihama nuclear power plant of Kansai Electric Power Co. At Mihama, the ECCS was triggered when a pipe in the steam generator was damaged and the reactor core automatically stopped.
Experts say there are three main phases of any attempt to deal with problems in a reactor core: stopping the reactor, cooling it, and containing radioactivity. The second phase of that approach failed at the No. 2 reactor at the No. 1 Fukushima plant.
Friday's earthquake stopped the emergency generators and that in turn stopped the cooling mechanism at 8:30 p.m. Friday, about six hours after the earthquake hit.
Without effective cooling, heat remaining in the reactor core could melt the nuclear fuel container, causing nuclear fuel to leak and pressure within the reactor containment vessel to rise due to the steam generated by evaporating water. That chain of events could lead to damage to the core and a major radioactive leak.
The authorities are trying to prevent damage to the pressure container by releasing gas and therefore reducing the pressure inside it.
Sources said Saturday's release of gas from the No. 1 Fukushima plant was the first time radioactive materials had been knowingly released into the atmosphere by Japan's nuclear power industry.