Um, the plants were closed because of loss of power? Don’t they generate thier own power? Does not compute!
Yes, it does. Nuclear power reactors need electricity for cooling purposes. From here:
Wow… wierd. Backup systems would make sense thou.
Not very informative replies. Nuclear plants do indeed generate their own electricity and are fully capable of operating “islanded” or removed from the electrical grid. However, they never do this for more than a short period of time. Why/
Because “house loads” utilize only a few percent of the plants output. Nuclear plants (and power plants in general) are not designed to operate very low power output. Since this makes operation unstable, the plant is procedurally required to shut down if a certain minimum load cannot be maintained.
There is another factor in play here. Nuclear plant safety and shutdown systems are electrically operated and must remove heat - a lot when the plant is first shut down and less as time goes by. If an event forces the plant to shut down, this electricity is preferably provided by the electical grid (called offsite sources). If offsite sources are not available, emergency diesel generators installed at the plant are used. There are at least two independent DGs each with 100% capacity to shut the plant down. If the DGs don’t start the plants are designed to shut down safely for 4 to 8 hours with no outside power at all until the systems can be repaired.
Because loss of the electrical grid is loss of one of the safety systems, the plants are required by regulation to shut down and begin cooling off because the normal safety systems are degraded.
I think your linked story was rewritten and corrected after you quoted from it. 6 plants were shut down, but only one was in Ohio. The Fermi plant is in Michigan.
This just raises more questions for me - Why would they have to operate very low power output? Surely if power is out they would want to offer as much power as possible.
They don’t want to island themselves in a sense of not offering power, just ‘island’ in a sense that they can operate without external power in but still offer power out
Could it have to do with the fact that in the improbable chance of there being an emergency, communication and transportation systems would be unreliable. Imagine a meltdown when there are no TV or radio networks up to warn people…
I think KenGr is talking about a situation where the power plant is no longer connected to a functional power grid. (Which is the case in the New York power outage - a failure of the power grid itself, not just a shortage of available power.) In that case the plant cannot send its power output through the grid, and the grid cannot act as a backup power supply for the plant’s safety systems.
Yes, the requirements for the plants include that they be connected to the grid by two separate transmission lines that leave the plant in two separated directions. In the unlikely event that a tornado “circles” the plant or the grid itself collapses, the plant can’t transmit any power out.
And also remember that nuclear plants take a minimum of 24 hours to come back on line when they are shut down. The local news kept saying that 9 nuclear plants in New York State were down.
This means that there is a possibility of brown-outs or even black-outs this afternoon if conservation is not used to keep loads down during the day before the plants can come back on line.
Note that this really has nothing whatsoever to do with the plants being “nuclear” - it’s mainly the media hearing the “N” word, and deciding that there’s some sort of story. Coal plants will most assuredly trip if their load is suddenly reduced to zero, the same as a nuclear plant will, or a baseloaded natural gas turbine. Think about it this way - think of a 500MW coal or nuclear plant. At full load operation, it’s requiring more than 670,000 horsepower to turn that turbine against the electrical load of the grid. OK?
Now, assume that due to an outage, downed main lines, exploded trasnformers, whatever, that load is removed. You’ve got 670,000 horsepower with nowhere to put it.
A good analogy would be cruising along the highway at 80mph, holding your gas pedal constant, and shifting into neutral. The engine will spool up wildly, and most likely redline or explode (unless you have a rev limiter).
And you can’t just “disconnect” the steam from the turbine either. That 670,000 hp worth of steam enthalpy (actually more than that, since the turbine is likely only 30% - 60% or so efficienct) has to go somewhere. Your condenser can’t handle 2,000,000 lbm/hr of superheated steam, so the unit “trips”, and shuts down - sometimes, alarmingly fast.
Una, who has been there and seen it happen.
Actually, correcting myself, closer to 3,500,000 lbm/hr for 500MW…