Safety Injection during Main Steam Line Break

[jim hardy]

see recent post in your homework thread

old jim

 

[FermiAged]

Jim Hardy - Did you used to work for FPL,specifically at Turkey Point? If so, I we worked together on the Turkey Point / St. Lucie Simulators.

 

[jim hardy]

Well -- Nuclear Power is a small world...

check your PM inbasket

old jim

 

[mudweez0009]

Speaking of Turkey Point, I was reading about that the other day, and how its ultimate heat sink is an elaborate canal system, miles and miles long (and I think I estimated about 10 square miles off of Google Maps)... And I thought to myself, "why in a hot, humid climate like the southern tip of Florida, would the heat sink be canal systems".. I imagine they need to be that long due to how hot it is, that heat removal is so inefficient. It seems so obvious to me, and I feel like cooling towers would have been much more appropriate and MUCH less environmentally invasive. What was the reasoning behind the canals?! It was bothering me.

On another note, getting back to natural circulation. I found the equations to use in the above post and solved the problems without issue, and posted the equations in the homework thread (for anyone else that may stumble upon this thread some day). As for another inquiry:

Does natural circulation ALWAYS happen in a power plant, or does it only happen when the plant loses all power? And I guess from an operator's standpoint, when the plant loses all power (normal and emergency), what kinds of things are monitored (alarms, gauges, etc.) to make sure that its working and the plant is becoming stable again?

 

[jim hardy]

What was the reasoning behind the canals?! It was bothering me.

Nobody had built a salt water cooling tower at the time, around 1969.

There was concern about salt spray drifting downwind and ruining the potato fields. A surprising fraction of the country's winter potatoes comes from those fields . Hurricane Donna in 1960 had put so much salt onto them it was a decade before the ones within a few miles of the bay recovered.

Also the dewpoint in S Florida runs so high there was concern over just how well they'd work in summer months, when air conditioning load strains the electric grid down there. . When temperature and humidity are both 100 your sweat just doesn't evaporate.

Actually they built a small test tower and concluded they were glad they'd opted for the canals. They cool a lot by emitting radiant heat.

images courtesy of http://lew-cabintalk.blogspot.com/2012_02_01_archive.html

Crocodiles moved right into the canals and are doing very well. An eighteen footer used to hang out on our intake. Most days you can hear them grunting adjacent the employee parking lot.
I figure they're another level of security.

FOR A SPECIES that, as recently as last year, was listed as endangered, the American crocodile is surprisingly easy to find--as long as you can get past the armed guards. More than one-fifth of the crocodile's U.S. population, about 400 juveniles and adults, lives at Florida Power and Light's (FPL) Turkey Point nuclear power facility in Homestead, Florida.

http://www.nwf.org/news-and-magazin...hives/2008/why-is-this-crocodile-smiling.aspx

 

[jim hardy]

Does natural circulation ALWAYS happen in a power plant, or does it only happen when the plant loses all power? And I guess from an operator's standpoint, when the plant loses all power (normal and emergency), what kinds of things are monitored (alarms, gauges, etc.) to make sure that its working and the plant is becoming stable again?

In a PWR it's only when power is lost to the pumps. Usually that's during a grid blackout.
The main pumps are too large to be powered by the emergency diesels - 3,000 hp diesels won't start 7000 hp pumps.

The geniuses who designed the plants (ahem - Rickover) realized that natural circulation, being natural, is pretty hard to mess up. So he placed the reactor below the steam generator to make a thermosyphon where gravity does your pumping for you. Navy plant i am told can go to some low power on natural circulation enabling the submarine to move very slow and quiet, no mechanical noise from those big pumps. Maybe somebody who knows for sure could elucidate. The premise for natural circulation is :

during a blackout
IF
you've got liquid water (term is subcooled, ie below saturation temperature) in the reactor;
and you have water in the steam generator that you can boil away,,
THEN
heat will move from the core to the steam generator by natural circulation.

The three necessary elements are primary inventory, primary subcooling, and secondary inventory.
So the operators watch reactor pressure and temperature to make sure the water there is subcooled,
they watch pressurizer level to make sure they're maintaining inventory,
they watch steam generator level to make sure there's inventory there as well,
and they watch steam generator pressure because so long as it's at saturation pressure for primary temperature, that confirms decay heat is being moved successfully.

At TMI the trouble was they let the primary system get down to saturation pressure and steam filled the top of the primary side pipes, blocking flow. They lost both primary inventory and subcooling.

After TMI, one midnight shift we pulled our reactor pressure gage and wrote adjacent each cardinal point on its scale the corresponding saturation temperature. That way we could directly verify subcooling without having to memorize the steam tables or remember where we left them.

Our operators used to quip: " It's only water. And we'll keep it that way."

hope above helps.

old jim

 

[FermiAged]

Turkey Point was REQUIRED to construct the canals to reduce the amount of warm water releassed to Biscayne Bay. Cooling towers are expensive! A benefit of the canals is that they serve as habitat for the only remaining American Crocodiles.

The driving head from temperature (density) difference is always there but it is small relative to that provided by the Reactor Cooling Pumps. In the NuScale design, it IS the only driving force and offers a simpler design at the cost of reduced power density.

Following loss of forced circulation, core Delta T will initially rise until the density differences leads to sufficient flow to maintain a heat balance. Peak Delta T will approach full power Delta T. When decay heat (in PWRs, loss of RCPs will trip the plant), Delta T will remain stable and steadily decrease.

Natural circulation flows will be a small fraction of that provided by pumps, about 2-3%. Usually, this flow is too small to be seen on flow or Delta P instruments. So nat circ is monitored by differences between cold and hot leg temperatures and Core Exit Thermocouples. It is important to maintain secondary side heat sink, so steam generator levels and pressures will also be monitored. A Steam Bypass System does this automatically but needs to be monitored.

If the RCS is overcooled by excessive steam flow out the steam generators (malfunctioning Steam Bypass or zealous operator), pressurizer level will also be watched to look for formation of steam voids in the the RCS (these can form at the top of the SG tubes or the Reactor Vessel Head.

I provided a derivation of the nat circ relations on the other thread.

 

[mudweez0009]

Thank you all for clearing that up. Makes sense. I figured since its dealing with saturation temp/pressure that monitoring temperature and pressure would be dead giveaways.

 

[jim hardy]

Turkey Point was REQUIRED to construct the canals to reduce the amount of warm water releassed to Biscayne Bay.

That is so.

The two fossil units there were built in early sixties. They drew cooling water from the bay and discharged it back into the bay a couple miles south. A small "bald spot" appeared on the bay bottom at the discharge point, it is unclear whether that was from the heat or just the velocity. I can attest that in winter the fishing was fabulous right at the plant's warm water side, the fish and manatees loved the warmth. But in summer they headed for cooler water. One winter day a sailfish had got lost and showed up in our intake - i never heard of one in inland waters but there he was, a small guy four or five feet long.

Environmentalists didn't like the idea of a nuclear plant at all and seized on the warm water and shallow Biscayne Bay issue. We add about fifteen degrees to the water and in summertime the shallow parts of the bay get up 90 degrees F, maybe a few degrees more.

Anyhow the utility was told they couldn't dump that much warm water into the bay.
So they had to provide for cooling some other way.
Their first plan was to dilute the cooling water to reduce its exit temperature. They bought five huge pumps, like twelve feet in diameter, and had them delivered to the site. That would have lowered the temperature but increased the flow rate, the figure i remember is 2200 cubic feet per second which is a decent sized river.

The environmentalists did further studies of the bay and objected to circulating that much water over the fragile "Turtle Grass" that grows there. They may have been right on that one. It's a staple in manatee diet.

So the utility had to do something quick and they were skittish about building cooling towers that might not work.

Well - some corporate entity owned a few tens of thousands of acres of bayfront just South of the plant and had plans to put an oil refinery there. Seadade i think it was.
Wouldn't THAT have made the environmentalists happy - dredging a channel clear across the bay !
So the utility was able to get that property for their cooling canals.
And that's how the canals came about.

The crocodiles were not forseen. They are great PR , but are becoming so numerous they're spreading. We had a huge one cruising our neighborhood in the Keys. Think for a minute about crocodiles in your kids' swimming hole... If you read early history of Miami , Miami Beach was an awful place where people went mostly to hunt crocodiles. I recommend "Commodore's Story" by Ralph Munro, an early Miami pioneer .

The last time i visited South Biscayne Bay it was looking great. Plenty of grass on the bottom and lots of wildlife. It's now a marine sanctuary of some sort and fishing is not allowed,
but when a huge city springs up adjacent a natural paradise you are faced with choices -you either let the people wreck the place or you put in rules that some folks are going to regard as Draconian.
Google "Columbus Day Regatta" . It happens just across the bay from the plant.
http://www.floridayacht.com/events/columbus-day-regatta/img/aerial.jpg
old jim

 

[mudweez0009]

That is very interesting Jim, and Fermi! I heard about the habitat it had formed, which generates pros and cons, but we won't get into those! I can't even imagine a 12 foot diameter pump. That would have been very impressive to see.

Anyway, back to hammering you guys with questions..

what are the concerns if HPSI and charging is lost? I was reading about these:
http://www.nrc.gov/reading-rm/doc-collections/gen-comm/info-notices/1988/in88023s5.html

I imagine you would lose the abilility to inject boron, lose seal water injection flow to the RCPs, wouldn't be able to maintain pressurizer level and prevent make-up to the RCS, resulting in fuel damage, etc. Are these all reasonable consequences?

 

[FermiAged]

For a Large Break LOCA the RCS would depressurize to the point where Accumulators discharge (about 600 psia) and ultimately where Low Pressure Safety Injection can take place (about 300 psia). Both of these systems would replenish inventory and provide boron.

For a Small Break LOCA, HPSI and charging are more important as the RCS pressure will level off above Accumulators and LPSI. This is Functional Recovery territory. I would first try heat removal with the SGs while sending someone to vent/start the HPSI pumps. If that is insufficient, the RCS would have to be depressurized using pressurizer relief valves. The idea would be to get the RCS pressure low enough for Accumulators/LPSI.

You are talking about losing 2 or 3 HPSI pumps and 2 or 3 Charging Pumps. Its hard to believe that the pump that had the most recent surveillance would be gas bound.

Can I ask what your background is? You sound like an engineering student.

 

[mudweez0009]

For a Large Break LOCA the RCS would depressurize to the point where Accumulators discharge (about 600 psia) and ultimately where Low Pressure Safety Injection can take place (about 300 psia). Both of these systems would replenish inventory and provide boron.

For a Small Break LOCA, HPSI and charging are more important as the RCS pressure will level off above Accumulators and LPSI. This is Functional Recovery territory. I would first try heat removal with the SGs while sending someone to vent/start the HPSI pumps. If that is insufficient, the RCS would have to be depressurized using pressurizer relief valves. The idea would be to get the RCS pressure low enough for Accumulators/LPSI.

You are talking about losing 2 or 3 HPSI pumps and 2 or 3 Charging Pumps. Its hard to believe that the pump that had the most recent surveillance would be gas bound.

Can I ask what your background is? You sound like an engineering student.

So basically, for small break LOCA, loss of HPSI/charging is more severe because the RCS pressure doesn't drop enough to allow other remedial operations to initiate?

What about my guesses of losing boron injection and RCP seal injection? It seems from your description that you only lose boron injection on a small-break? Are either of these major concerns that could result in further damage, or are they minor compared to pressure concerns?

And yes, I am an engineering student. I am currently halfway through grad school, on my way to a masters degree in mechanical engineering. Just took a nuclear engineering course out of curiosity and now realized that one course can't even cover the BIG picture items that I would like to know. I really am trying to learn the components of the plant and how they interact. (If situation A happens, what happens to B, C, and D) type of stuff..

 

[mudweez0009]

For a Large Break LOCA the RCS would depressurize to the point where Accumulators discharge (about 600 psia) and ultimately where Low Pressure Safety Injection can take place (about 300 psia). Both of these systems would replenish inventory and provide boron.

For a Small Break LOCA, HPSI and charging are more important as the RCS pressure will level off above Accumulators and LPSI. This is Functional Recovery territory. I would first try heat removal with the SGs while sending someone to vent/start the HPSI pumps. If that is insufficient, the RCS would have to be depressurized using pressurizer relief valves. The idea would be to get the RCS pressure low enough for Accumulators/LPSI.

Also, do accumulators deliver boron when there is a loss of all power?

 

[FermiAged]

Yes. They are essentially tanks of borated water driven by a charge of nitrogen (at a typical pressure of about 650 psia). They discharge to the RCS through check valves (like a door that only swings one way) that are closed or opened by the differential pressure across the valve. Under normal operating conditions the RCS pressure of 2250 psia keeps them closed. When RCS pressure falls below 650 psia, the valves open and the tanks discharge based upon the RCS pressure and level in the tank (which determines the elevation head and N2 pressure).

The accumulators (a Westinghouse term - CE designed plants call them Safety Injection Tanks - I don't know what B&W calls them) require no electrical power to function. Because there is no pump that requires time for power source alignment, start signal and start up, the accumulators can be the first source of safety injection in some accidents.

 

[mudweez0009]

That is what I figured. Thanks for confirming!

 

[jim hardy]

RCP seal cooling used to be PWR's Achilles heel.
Westinghouse came up with an improved one that's passive, but it was after I'd retired so I never saw one.. That would seem to be be a comfort.

http://www.westinghousenuclear.com/Products_&_Services/docs/flysheets/NS-FS-0106.pdf

We added some fittings (I think back in the 90's) whereby one could connect a portable diesel driven pump to cool the seals should the unthinkable happen.