The state of the two Fukushima plants was causing particular concern because, unlike other plants hit by Friday's earthquake, emergency generators wouldn't operate after external power supplies failed. That crippled the electric powered emergency core cooling systems (ECCS).
The state of the two Fukushima plants was causing particular concern because, unlike other plants hit by Friday's earthquake, emergency generators wouldn't operate after external power supplies failed. That crippled the 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 of the Mihama nuclear power plant operated by 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.