Are Nuclear Plants Required to Shut Down During Tornado Warnings?

[ppnl]

Do nuclear plants shut down during a tornado warning? I have heard that they do but really can't think why.

 

[Simon Bridge]

People get upset when you don't.

 

[SteamKing]

Do nuclear plants shut down during a tornado warning? I have heard that they do but really can't think why.

What do you mean by 'shut down'? I don't think conventional power plants shut down during just a tornado warning. Where I live we have tornado warnings occasionally and the lights stay on unless a transformer blows or a power line is knocked down by the wind or by debris.

 

[Simon Bridge]

It's unusual, but it happens.
There are dangers to plants in extreme weather conditions ... i.e.
Tornado Forces Shut Down Of Two Reactors At 1.6 Gigawatt Surry Nuclear Power Plant
From Reuters: "Dominion Virginia Power said the two nuclear reactors at its Surry Power Station shut down automatically when a tornado touched down and cut off an electrical feed to the station. The U.S. south was hit by violent storms over the weekend. No radiation was released during the storm and shutdown, the NRC and the company said. The situation was described as an "unusual event", the lowest of the four NRC emergency classification levels."
04/18/2011

 

[ppnl]

I mean purposely taken off line during a tornado warning. if so how often does this happen?

 

[SteamKing]

I would say almost never, unless some mechanical failure intervenes. After all a tornado warning is just one step in the alert system.

See: http://en.wikipedia.org/wiki/Tornado_warning

Even after a tornado has formed and passed through an area, residents are always warned that downed power lines can still be energized and that touching them can result in a fatal shock.

 

[Thermalne]

I would say almost never, unless some mechanical failure intervenes. After all a tornado warning is just one step in the alert system.

See: http://en.wikipedia.org/wiki/Tornado_warning

Even after a tornado has formed and passed through an area, residents are always warned that downed power lines can still be energized and that touching them can result in a fatal shock.

Another thing to add is nuclear plants have very robust structures (containment building, turbine building, control rooms) that can survive even the most severe tornadoes.

 

[Astronuc]

Nuclear plants may reduce power or go to hot zero power (HZP) in the event of a severe storm that may damage the switch yard or transmission lines (grid issues, e.g., loss of load, loss of offsite power, . . . .).

http://bigstory.ap.org/article/extra-inspectors-nuclear-plants-storm-nears

Otherwise, nuclear units can be brought down quickly if a storm damages the grid.
http://www.reuters.com/article/2011/04/27/us-utilities-tva-storms-idUSTRE73Q98920110427

 

[ppnl]

How long does it take to come back from a hot zero?

 

[Astronuc]

How long does it take to come back from a hot zero?

Several hours. Assuming the fuel in the core is conditioned, the balance of plant is the limiting factor. The turbine has to be synched with the grid, which happens around 14% power or so. Plants can come back to 50% power pretty quickly, and almost as quick to 100% power with conditioned fuel.

 

[QuantumPion]

Another thing to add is nuclear plants have very robust structures (containment building, turbine building, control rooms) that can survive even the most severe tornadoes.

The Surry tornado caused considerable damage to the switchyard, they operated on EDG's for a while.

Pictures here: https://plus.google.com/photos/1007...45063115537?banner=pwa&authkey=CNjO7szfopKOGg

 

[gmax137]

The offsite electric power system is not designed to withstand 'natural phenomena' such as tornadoes (as those photos make very clear). That's one reason why General Design Criterion 17 requires an independent onsite electrical system in addition to the offsite system, and why GDC 2 requires that the onsite system must be designed for the maximum natural phenomena (including earthquakes, floods, hurricanes, tornadoes, etc.). Further, the plant safety systems must accommodate failure of either the offsite system or the onsite system, in addition to a single failure elsewhere.

Some of the coastal plants will shut down in advance of a hurricane; this avoids the forced shutdown that occurs when the offsite power system is damaged. For severe hurricanes, the distribution system will typically be damaged to the extent that there is nowhere for the power to go anyway (so they might as well shut the power plant down). Hurricane tracking allows this in an orderly fashion (since normally the hurricane path is well known within 24 hours). I don't know if any plants require shutdown in advance of a tornado (and that might not even be feasible given the short warning time).

 

[Thermalne]

The Surry tornado caused considerable damage to the switchyard, they operated on EDG's for a while.

Pictures here: https://plus.google.com/photos/1007...45063115537?banner=pwa&authkey=CNjO7szfopKOGg

I didn't mention the switch-yard for a reason.

 

[Hiddencamper]

US BWR worker/nuclear engineer here. Our tornado/high winds offnormal procedure does NOT require us to shut the facility down. I do know for plants that are on costal regions, sustained hurricane force winds require the plant to shut down. My plant is not anywhere near an ocean though. When we do have tornado winds, or if there are warnings/watches, we protect critical equipment in the plant and halt/back out of maintenance which could affect safety systems required to ensure safe shutdown.

Browns ferry in the last year or two was on diesels for a few days when they lost like 9 offsite power connections. It was considered impossible for them to lose all offsite power, so it was kind of crazy.

By shut down, I mean a manual scram (or insertion of all control rods).

To the best of my knowledge, there is no requirement that a plant shutdown for tornados, but if extensive damage is expected then a shutdown would be prudent (and would occur regardless due to a load reject, loss of auxiliary/standby power, or failure of circulating water pumps).

During a large tornado like event, depending on what/where the tornado hits, there may be reduced electrical demand on the grid, this could cause the plant to have to reduce output and potentially take the turbine/generator set offline. During Hurricane Sandy, I know that several plants had reduced power, some very deeply, due to Volts/hertz or reactive generator voltage limits (there wasn't enough demand on the grid to satisfy all the power going out), and I know one plant who was very close to de-syncing their main generator as they were close to the high voltage grid/generator trip setpoint.

As astronuc stated, if your fuel is preconditioned, you can go down/up rather quickly. One thing to remember is that fuel preconditioning starts to wear off after 4 hours, so the timeline for a rapid load reduction and restoration is pretty small, and in most cases would still likely require some preconditioning or a separate rod sequence (I'm a BWR nuke) to return to full power operation in a timely fashion.

There really aren't benefits from a risk or operating perspective to shutting down for a tornado ahead of time. Typically tornados are events that occur quickly, so you wouldn't be able to anticipate a tornado heading towards the plant, shut down, and cool down a reactor from hot standby to cold shutdown prior to the tornado hitting(which is a much 'safer' condition to be in). So that means regardless of whether you scram early or not, you have a hot core with a large decay heat load.

If you scram early, that tornado may never even come close to the plant, and now you just put the plant into a transient (and any transient, even a reactor scram, has risk associated with it), and also put thermal cycles on all your equipment, you also now have a reportable event you didnt need, it counts towards your INPO rating, and you challenge your plant systems needlessly. If I scram early, the only benefit is that I can start my cooldown on non-safety systems (condenser with steam bypass). But it wouldn't matter, because if/when the tornado causes a loss of offsite power I still would need my diesel generators to start and I would have to use SRVs, RCIC/HPCI/IC for pressure control (or atmospheric dumps for PWRs).

The main risk with tornados is station blackout (loss of offsite power and a failure of all diesel generators responsible for decay heat removal), which the US has 4 hour coping requirements and blackstart plans/generators to mitigate (as well as the new FLEX/fukushima requirements and the old b.5.b requirements). The next large risk is damage to plant auxiliary systems, usually emergency service water pumps or other things which are more 'external' to the plant, but it could also be station vents, external water tanks (like those used for RCIC/HPCI), main transformers.

Anyways that's my blurb.

 

[APSDesEng]

I agree with Hiddencamper. I was at Calvert Cliffs Nuclear plant in '02 when the LaPlatta, MD tornado hit. The storm spawned a waterspout that came within 200 yrds of the plant, but the running unit never shutdown. The containment buildings of a PWR are built to withstand (so I am told) a 747 full of fuel running into them. The main concern is the loss of offsite power for the same reasons as stated by Hiddencamper. I was working at Peachbottom nuclear when Hurricane Isabelle struck, and though the plant was far from the ocean, a gust measured 5 miles from the plant was over a set threshold, so for safety reasons they began a controlled shutdown to be safe.