The chairman of the U.S. Nuclear Regulatory Commission told a Senate committee hearing on Tuesday that Japan's damaged nuclear reactors at the Fukushima Daiichi power plant is "static" but remains "unstable" with just an improvised cooling system.

Gregory B. Jaczko told the Senate, "We don't see significant changes from day to day,. However, he clarified that the risk of additional radiation releases diminishes everyday.

At the hearing, Jaczko told members of the Senate Environment and Public Works Committee that Japanese engineers have not yet re-established a long-term regular cooling of the reactors or a regular system to deliver water to the spent-fuel pool.

He made his assessment that the situation at the nuclear power plant is "not stable" and this would prevail until "that kind of situation would be handled in a predictable manner."