Posted on 08/17/2007 3:03:37 AM PDT by Inge_CAV
ATHENS, Ala. -- The Tennessee Valley Authority shut down one of three units at the Browns Ferry nuclear plant on Thursday because water drawn from a river to cool the reactor was too hot, a spokesman said.
The nation's largest public utility shut down Unit 2 about 5:42 p.m. CDT because water drawn from the Tennessee River was exceeding a 90-degree average over 24 hours, amid a blistering heat wave across the Southeast.
"We don't believe we've ever shut down a nuclear unit because of river temperature," said John Moulton, spokesman for the Knoxville, Tenn.-based utility.
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TVA: http://www.tva.gov
(Excerpt) Read more at waff.com ...
Both have to be taken into consideration. Even thermal pollution is considered with the license, and in extreme cases high temps in the condenser tube sheet can cause fouling or scaling, reducing effectiveness.
Tell me about it—on my first boat we did a flank run from Japan (31 degree injection temp) to Guam (85 degree injection temp).
The reduction in speed was amazing.
Brown’s Ferry does not have cooling towers, they use direct cooling from the river. If the water is too hot, you get turbine trips from low condenser vacuum. If that was the reason, then the other two units would have to shut down also. The real reason for completely shutting down one unit is to not violate an EPA rule about too high cooling water discharge temperature back into the river.
I have worked at several nukes where we would have to cut back power in the summer for just that reason. At Dresden, we would de-rate 200-300 MWe (~30%) to keep the river discharge temperature below 93 degrees. You can safely run a reactor at less than full power. A light water reactor can be stable at 10% power, especially if the power block has been designed for load following. If the power block is optimized for baseload generation, then below about 30% power, you start to get transients than can trip the turbine. Chernobyl happened when they tried running a coastdown test at an unstable point at very low power and then had a sudden inrush of relatively cold water into the reactor, causing a HUGE reactor power spike.
Boron shim is NOT used in BWR’s. Power is controled with rods only. The nly boron in a boiler is the no-$h_t emergency shutdown bron injection system using a squib valve.
Those look more like helper CT’s designed more to draw in circ water discharge and cool it a bit before letting it go back into the river.
Thanks Grammy. You should see Norris Lake right now, there’s no water, it’s been sucked dry. I guess because of using more water for generating, along with no rain.
BTW, TVA is the leader in managing tailwaters for the benefit of aquatic resources. The agency has been applauded for efforts to maintain minimum flows, and building structures to create oxygen for tailwater fish. That’s why this state’s tailwater trout fishing is some of the best in the country. See Diana, yet another reason you need to move to Tennessee (grin).
I suspected but wasn’t sure—boron is a real PITA. Just ask that (former) engineer over at Davis-Besse.
I can see where boron would not play well with the secondary components.
I live close to the plant(Browns Ferry).The water temps at this point are in the 90-92 degree range on the boat thermometer.They can’t cool the reactors with water that hot and return it to the river,at a temp below 90 degrees,as law mandates.
They could release cool water from Tims Ford dam to help,but that water would be below the streamflow of the plant.
They may have to release cooler water upstream,to lower the lake water temps.
I’m paddling as fast as I can, GA! :)
Under normal operation, BFN uses a once-through circulating water system to dissipate heat from the main turbine condensers. Water is drawn from the Tennessee River by the plant intake system and is discharged back to the river. In addition, BFN currently has four mechanical draft cooling towers which can be operated to assist in heat dissipation (helper mode) primarily during summer hot weather periods.
Excerpted from here.
There was a power plant in Thailand, designed by a local engineering firm, that had an unusual problem with cooling.
They used river water for cooling, standard upstream pipe for intake, downstream pipe for the heated water.
However, the problem was that during part of the year, the river ran backwards, causing the heated water to be sucked into the intake and ejected “upstream”.
This little detail wasn’t discovered until after construction was complete, and required cooling towers to be installed.
One of the things I like most about FR is the pool of folks who work in many complex industries who can and do supply commentary as to the whys and wherefores of how this kind of advanced stuff works, and why some decisions are made the way they are. I understand some, not all of it, but I find it really interesting. Thanks, all!
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.
A powerplant is a “heat engine”, and works due to the temperature difference between the reactor-generated steam and the condenser, which last is what the river water is cooling. Greater delta-T is “good”, so a higher condenser temperature means lower output/efficiency.
The water that is turned to steam is in a closed loop. The river intake water, used for cooling the condenser downstream of the steam turbine, is completely separate.
As Freedom Poster says, the river water is NEVER part of the heat generation cycle. The river/cooling water is part of a separate loop that carries the “no longer useful” (latent heat at discharge pressure) heat away so the working fluid can condense.
Apparently, the problem is mainly that the plant is not allowed to discharge water into the river at more than 90 degrees, even though the water the plant is drawing from the river is 92+ degrees.
Don’t you just LOVE bureaucrats
In their submittal to the NRC it appears TVA’s design expected the likelihood they would have to derate because of thermal limits.
Their cooling towers are along the waterfront to the right of containment structures.
From ADAMS:
Cooling towers are operated as necessary to meet thermal
discharge temperature limits. For EPU operation of three BFN
units, use of cooling towers is expected to increase, and on
those rare occasions when NPDES permitted thermal limits
cannot be met with the operation of the cooling towers, the
plant would be derated to remain in compliance.
Your answer makes perfect sense. Thanks.
DERATE is something a plant does when it is no longer economical for it to attempt to operate at optimal capacity. Derating is the DEATH KNELL for a plant.
The plant in question is forced to REDUCE OUTPUT temporarily
due to environmental considerations (the plant CANNOT discharge water into the river COOLER [under 90 degrees]than it is DRAWING it {92 degrees} FROM the river).
“They’re clearly visible from Google maps. See here:”
From this picture you can see the shallow banks of the river if you back out to about the seventh level. The main dredged channel is clearly visible.
It was TVA that used the word “derate” in their submittal to the NRC. Semantics in the nuke biz is important, but if Tech Specs says you can operate at X%FP where X < 100 %FP then that is a defacto derate until you clear the limit.
I wouldn’t say a DERATE is a DEATH KNELL either. If, for example, you have to derate by 20 %FP until some piece of equipment is fixed, and the NaRCs agree to it— I’d take a cycle of operation at 80% to being shutdown.
Dispatching at $6/MWh, a nuke is always in the money, even if it’s output is < 100%FP.
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