On 20 March 1978 a failure of power supply for the plant's non-nuclear instrumentation system led to steam generator dryout. (ref NRC LER 312/78-001). In an on-going study of "precursors" that could lead to a nuclear disaster if additional failures were to have occurred,<1> in 2005 the United States Nuclear Regulatory Commission concluded that this event at Rancho Seco was the third most serious safety-related occurrence in the United States<2> (Behind the Three Mile Island accident and the cable tray fire at Browns Ferry).
Link:
http://en.wikipedia.org/wiki/Rancho_Seco_Nuclear_Generating_StationAnd... From 1986...
We Almost Lost Shasta WatershedRancho Seco Near-Disaster Covered Up by Utility
<snip>
On December 26th, 1985 at 4:17 A.M., the Rancho Seco reactor located southeast of Sacramento came extremely close to a meltdown. The reactor with a hundred accidents under its belt (twice the average for reactors its age) and three of the most serious cooldowns in the industry has created a political and economic crisis for the Sacramento Municipal Utility Commission (SMUD) that may lead to the permanent shutdown of the reactor or even bankruptcy of the utility. It has been shutdown since the accident, and only has a 40% chance of coming back on line by June 1987. The accident did not come to the public's attention until after the Chernobyl disaster. What's most chilling however, is the fact that the public never learned just how serious the accident was.
In piecing together the events from the NRC's own investigation, it becomes clear that the event was far more serious than a mere mechanical foul-up, as claimed by SMUD. The accident, caused by a loose wire, cut off power to the Integrated Control System (ICS) for 26 minutes. The ICS is the computer system that regulates the temperature of the reactor and the steam system to the turbine generator. In other words, the operators lost control of the reactor from the control room.
Workers were dispatched to various parts of the facility in attempts to head off a loss of coolant accident (LOCA). LOCA's are the dreaded scenario almost sure to lead to a meltdown. Two of the workers were contaminated in their attempt to reach critical valves.
Operators were successfull at stopping the LOCA but did not have accurate preocedures for maintaining control of the reactor from outside the control room. As a result, the reactor vessel cooled down to quickly. When this occurs, the 8 inch thick walls of the reactor may shatter, crack or become embrittled reducing the usable lifetime for the unit. During the accident the following serious events ocurred:
<snip>
More:
http://www.energy-net.org/01NUKE/RSECO85.HTM:evilfrown:
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