TVA reactor shut down; cooling water drawn from river too hot

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[gondramB]

I have not read up this case but I have a small bit of experience that may be relevent...

90 degree water can absolutely be used for cooling but there are environmental limitations on how hot the water can be when it is returned to the river - they don’t want to kill the fish etc.

So its possible this is not a safety hazard at all but a fish hazard.

Of course it will likely be framed as George Bush causing a near meltdown for letting Karl Rove cause global warming.

But then we are used to that.

[DB]

Why can’t they just reduce output instead of completely shutting down?

[driftdiver]

“Why can’t they just reduce output instead of completely shutting down?”

Something to do with the dylithium crystals Captn.

:)

[EBH]

Two other units at the plant were operating, as well as towers to cool the water. But searing temperatures and a lack of cooler water in the upper part of the Tennessee River system made it too difficult to provide cool water for all three reactors. There was no safety threat posed by the shutdown.

It looks like you are correct. It is a cooling water issue and not a safety issue. The water is too hot coming into the system. They don't know when it can come back online due to the incoming water being too warm.

[gondramB]

>>Why can’t they just reduce output instead of completely shutting down?<<

With the disclaimer that I am not familiar with this particular site...

Short answer: They can reduce capacity but they doing by shutting down a reactor. With only three reactors that’s really gonna effect production

Long answer: You can’t shut down part of a reactor nor can you quickly restart a reactor. BTW, that’s how Chernobyl happened - they were ordered to restart a reactor too quickly. American reactors don’t share the fatal design flaw in Chernobyl but its still irritating to shut down a reactor because of the long restart time.

A nuclear plant will have cooling towers for water that has been used to cool the reactor. Those towers have a maximum cooling capacity. The cooling capacity plus the storage capacity determines how much hot water the system can absorb before they would be forced to return water to the river that is over the environmental limit.

Its protocol to shut down a reactor at that point. Problems with cooling towers have forced shutdowns before. What’s unusual here is that the incoming water temperature caused this.

Disclaimer #2 - Its possible there is a bigger problem, I’m not discounting that.. but I wanted to point out there is a simple, not so bad cause that is likely.

[EBH]

If I am reading the article correctly. It looks like the river water is too warm due to drought conditions. This would be possible as the high heat and lowered runoff decrease the water level allowing the water to warm more quickly. This situation also lowers the available cool bottom waters.

[gondramB]

That sounds reasonable - hopefully the media will have a rational reading of the situation. Droughts happen.

They don’t say its just an environmental issue though. It would be nice if they clarified.

[Inge_CAV]

“90 degree water can absolutely be used for cooling but there are environmental limitations on how hot the water can be when it is returned to the river - they don’t want to kill the fish etc.”

I will agree with you. TVA does not want to become known for mass killing fish.

[DCPatriot]

Thanks for putting that in perspective. (fish dying)

[TN4Liberty]

That sounds reasonable - hopefully the media will have a rational reading of the situation. Droughts happen.

Between the terms "heat wave" and "nuclear plant," the chances of the media handling this in a rational manner is about zero.

[gondramB]

Happy to help. Hopefully so we will get a more clear report.

I tried to get us a view of the plant but darned if Google maps doesn't seem to show an empty field

[gondramB]

I stand corrected - it looks like they did try to tune down the reactors a bit first.

>>“When the water temperature remains at an average of 90 degrees or higher over a 24-hour period, we have to take steps to try and reduce that temperature,” Johnson said. “We had reduced power output from all three units by 15 percent earlier, but that did not bring the temperature down enough.”<<

This new article does make it sound like its a release water temperature problem and thus environmental rather than safety.

>>The massive cooling towers at the plant usually are sufficient to control the temperature of water released from the plant, Johnson. Due to the historic heat wave currently enveloping much of the Tennessee Valley, however, it now may be necessary to bring cooler water down from tributaries in East Tennessee, Johnson said.<<

http://www.enewscourier.com/local/local_story_228222435.html

[OCCASparky]

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... :)

[aruanan]

No, how it will later be reported is that the reactor was shut down because it was making the cooling water effluent too hot, not that the cooling water in the river was too hot to be used for cooling (for whatever reason).

[meyer]

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.

[OCCASparky]

Yeah, I hadn’t really thought of the fuel depletion rate but now that you mention it, that makes sense. Does BFN (a BWR plant) typically run the same boron concentrations as do commercial PWR plants?

[Inge_CAV]

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.

[OCCASparky]

Good point as well. Like I said, good thing we don’t have that problem. In fact, in winter we have to PREHEAT the water.

It might be an untested condition related to service water loads (safety train loads).

[chrisser]

I wish I knew a bit more about how nuclear plants work, but I’m assuming that the river water, before being returned to the river, is cooled in the cooling towers using ambient air.

Besides the high input temperature of the water, are the higher air temperatures this time of the year also a contributing factor?