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Other considerations beside return temperature is the fact that if intake temperature is too high, it actually degrades performance (low condenser vacuum) to the point where the plant can’t run nearly as efficiently as well.

But the primary reason in this case is discharge temperature—the other two units heat burden is taxing the system to the max, and that’s WITH the cooling towers cut in.

The previous poster was also right—although I’m wondering why a FULL shutdown was necessary. Could they have simply down-powered to a self-sustaining condition (about 15 percent) and just stayed there until weather conditions improved and returned to full power ops? Probably didn’t want to if they didn’t know how long they’d be.

At least we don’t have that problem up here... :)

You are correct, OCCASparky - it's a discharge temperature issue. There are environmental rules governing this.

The problem with reducing power that significantly is they they still use up the fuel at a rate similar to if they were running at 100%. Nuclear is different than fossil fuel in that regard - lowering output will still effect the longevity of the fuel rods just as if it was running at full output. So, it's more efficient to take one of 3 units off line and run the other two at or near full output than to run all 3 at a significantly reduced output.

Hopefully, they'll get some rain in eastern Tennessee - it's effecting their hydro generation as well as nuclear. I'm certain that it's effecting other plants along the river as well.

From what I remember seeing of the plant while flying it is located in a slow flow area of the Tennessee River. That is between two dams, Wilson Dam in Muscle Shoals and Guntersville Dam. At this time of year, middle of a hot August, with lower water levels you can see just how shallow the river is near Decatur, AL. During normal weather years this would not be a problem but this year is dryer and hotter than we usually have to endure.

Another factor could be that they've decided to use this outage to do work on the unit, rather than use the fuel for little return. I worked at one of TVA's nuclear sites for some years (IT). If an outage was forced or probable, there would be a lot of planning going on while they decided what maintenance and upgrade work they could fit in the outage window. They would always take advantage of any outage time to the fullest.

Could be xenon poisoning. Remember that when you're running at full power you're producing a heckuva lot of xenon in the fuel. Run long enough and you reach equilibrium so it's not a problem, but if you runback power the equilibrium is disturbed in the direction of poisoning the core. Eventually you re-establish equilibrium but it takes time. You could compensate for the xenon poisoning at reduced power if you have enough excess reactivity, but it might require significant changes in the control rod positions, which in some cases is not good if you have to withdraw those rods close to their full extent (recall that this was one of the exacerbating conditions at Chernobil). The operators may have judged that simply shutting down for a bit was the best option rather than run at low power with a xenon-poisoned core.

Power demand in the TVA service area peaks during summer months so it behooves them to upgrade the cooling capacity of these plants. Since it is a BWR using low-profile cooling towers, perhaps simply cascading some additional cooling units would do it?

You would think that a higher intake temperature would make the system run a little more efficiently. The intake water is turned into steam to run the turbines to generate electricity. It is going to become super-heated water anyway. I find it hard to believe that anything in the intake piping is going to get damaged when the water is several degrees warmer.

I can see how the river water might be too warm already to accept the outflow water.