>>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.
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.
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?
The plant may also have as part of its design basis an assumption w/r to the initial heat sink temperature in their accident analyses. The reservoir/river in conjunction with a pond or some other impoundment/basin would serve as part of the ultimate heat sink complex for the plant. The complex would serve as the repository for the station’s residual energy during an accident.
As part of the plant’s operation specifications you could not continue operating outside of the assumptions in your accident analyses. Typically you select your initial conditions as bounding values in your analyses, but sometimes those bounding values may not have enough chubby in them. If you exceed them, you have to get them back in line and failing that, commence a shutdown.
There isn’t anything in this article to suggest they are outside analyzed space.
Browns Ferry is a boiling water reactor. It also has mechanical draft cooling towers (rather than natural/forced draft tower). The cooling towers are likely not credited in any accident analyses (loss of offsite power assumption would shut them down, the expense to make the cooling towers qualified makes it unlikely— however it is the TVA). The cooling tower water basin may count.
You can always re-analyze to a higher initial water temperature— not an insignificant undertaking, mind you. Though not uncommon (I’ve done it twice in the past 12 years at two separate companies).
I got the impression from the article that they were using a “once through” cooling system, which would explain the high incoming temperature causing a shutdown. Evaporative cooling towers would NOT have a problem with makeup water being warm, as it would be quite effectively cooled by the tower’s intended purpose.
DH is a SRO at a S. Florida plant, 72 hours is normal to get a reactor through startup and obviously longer if they have to enter containment for repairs etc.
Between cooldown and startup, 7 days is not unusual.
Reducing power is quite easy compared to shutting down (boron anybody?) however that would only reduce the amount of water to the outakes, not the temp.
Surprisingly as hot as we are down here, we’ve never tripped or shutdown due to intake water temps being too hot.
The article (deliberately?) DIDN’t say whether the “too hot” exit temperatures were against condenser design limits or arbitrary ecological limits.
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.