Feedwater-bypass during stretch-out ?

[Marco]

This a special question dedicated to the Operators around here at Nukeworker.com.
A fellow RO from the Dutch Borsselle NPP (PWR) told me, that they're going to test a new procedure during stretch-out this year. For those who are not familiar with Operations-stuff, we call the time after the natural end of the cycles (EOL), stretch-out. So my buddy Hans told me, that they want to bypass the high pressure feedwater heater, to have a lower reactor inlet temperature, and as a result, they can use the negative temperature coefficient and operate the plant a couple of weeks longer after EOL. Beside this they decrease coolant temperature set point step by step, with the same consequences as explained.
Decreasing coolant temperature set point is also part of our stretch-out procedure, but bypassing feedwater heater ? I really have my doubts. 
I've never heard before about this procedure, so I thought some of you guys are experienced in this topic and would share your knowledge with me.

Every contribution is appreciated. Thank you.

[Roll Tide]

The reason we use feedwater heaters is because of the increase in efficiency. Any lowering of the temperature of the feedwater would result in lower efficiency, and seems counterproductive.

I would be interested in seeing the calculations that have proven this choice.

[bigstew]

Quite true that the positive reactivity added by the lowered feedwater temp will allow the station to extend the run at 100% power (or close to it). Removing feedwater heaters is a necessary evil to accomplish this.

A typical plant runs at about 30% efficiency ( from reactor themal power to electrical output)and feedwater heating is a part of this equation but you're losing 100% of some very expensive electrical megawatts and using the stored energy of some even more expensive nuclear fuel that you otherwise wouldn't have been able to burn. The fuel is already paid for and the bundles that have been burnt more than twice (for a boiler anyway) are going into permanent storage, never to have their energy extracted.

Go visit the desk of one of your reactor engineers (or equivalent) and ask them to show you a long form heat balance. It will graphically display the factors involved and show that you're not losing as much thermal efficency as you're gaining in electrical generation.

[HydroDave63]

2 other items to consider:

Differential temperature on your feedwater nozzles and S/G internals can have detrimental effects. Increased inspections might be warranted of secondary-side S/G internals.

Lowering primary temperature will give better moderator reactivity effect, but will also affect your xenon burnout and equilbrium levels (changing some of the terms of the 6-factor formula). Coupled with the increased Pu buildup at EOL in the fuel, it can give you different reactor performance. All of this is bounded in the fuels calcs, of course, but it could a bit of a surprise for any junior RO.

By the way, Marco, do you all use SAP for planning at Neckar?

[Marco]

Hello colleagues and fellows,

thank you all for your comments and contributions. It seems I'm not alone with my opinion, that this procedure is quite unusually. From the metallurgy point of view are a lot of things to consider, and I have to admit, I didn't consider the most in my deliberation.   Recently I've heard that Goesgen NPP in Switzerland tried the same procedure. I'll contact Hans from Borsselle NPP, and try to get more information on this topic. Maybe a web-search might also help ?   

@ HydroDave63:
Yes we use SAP/R3.


Thanks again and keep on discussing, Nukeworker.com is the best place for nuclear professionals.   

Greetings from Germany,   

[Kernwerker]

Hi Marco

As i am only a RP tech  please explain how they want to bypass the feedwater lines? yes i know it is a common practice in our plants to lower feedwater temps by lowering the high pressure preheater temperature...but bypass? How can they feed enough water in the reactor by bypass?

In German: Also kapiere ich ehrlich gesagt nicht...wie soll das gehen? Gut ich kann der Speisewasser Temperatur senken über die HD Vorwärmer...aber bypass? versuch mal ein paar infos zu bekommen

[Marco]

Hi Kernwerker,
I try to reply in English, because of the other non-German members. When we talk about bypassing the feedwater heater, it doesn't mean that the lines are also bypassed, only the heater. In a PWR you can bypass the heater in case of a heater leakage for example or when heater fill level is out of operational range. This will happen automatically. My origin question was, that I heard from a Dutch and a Swiss NPP that they bypass there feedwater heater during stretch-out or so-called down-coasting. I'm not familiar with BWR stuff but I think it's similar. Correct me if not.

I prefer to discuss in English, because I want to improve my English skills, if you mind.

Greetings,

[Roll Tide]

Hi Kernwerker,
I try to reply in English, because of the other non-German members.

I find it fascinating that you both know a second language well enough to discuss technical matters in it. I find it quite impressive!

I would be interested in seeing an overall comparison in cost vs. "coastdown" where you let power sag at end of life. If you factor in some of the concerns of HydroDave, it could be a little costly...

[Kernwerker]

Hi Kernwerker,
I try to reply in English, because of the other non-German members. When we talk about bypassing the feedwater heater, it doesn't mean that the lines are also bypassed, only the heater. In a PWR you can bypass the heater in case of a heater leakage for example or when heater fill level is out of operational range. This will happen automatically. My origin question was, that I heard from a Dutch and a Swiss NPP that they bypass there feedwater heater during stretch-out or so-called down-coasting. I'm not familiar with BWR stuff but I think it's similar. Correct me if not.

I prefer to discuss in English, because I want to improve my English skills, if you mind.

Greetings,

Of course  I dont mind Marco! Sorry for the German lngo Yeah this an American site and lets keep the proper language

[Marco]

Hi folks,
Sorry for my late reply, I've just returned from Switzerland.      The big advantage in down coasting is you can produce energy with a minimum amount of fuel. You can stretch the cycles, therefore we call it stretch-out. This is a usual procedure in KWU-designed reactors. 
I assume the US-plants don't do this in the same manner. A very interesting item is the HP-FW heater-bypass from which I've never heard before. I'm lacking in core design knowledge, so I can't point out the costs or the detailed advantages of this procedure.     
Sorry  Roll Tide, probably is this not a satisfying answer.     


Regards,

[Roll Tide]

The big advantage in down coasting is you can produce energy with a minimum amount of fuel. You can stretch the cycles, therefore we call it stretch-out. This is a usual procedure in KWU-designed reactors. 

Sorry  Roll Tide, probably is this not a satisfying answer.   

Thanks for the info Marco. And I am satisfied with this answer!

[Kernwerker]

Hi folks!

Well to this subject! Well what I understand is that downcoasting is the practice to let the reactor gradually lose power until EOC. Stretchout on the other hand means to let the reactor run at 100% or a bit lower by lowering the feedwater temperature. Of course lowering the feedwater temperature means to lower the HP/FW preheater temperature. So that means according what i have heard of is that downcoasting = gradually losing power towards EOC ( much lesser then 100%) and stretchout= keeping the reactor at 100 or nearly 100% by lowering the FW temperature considering the moderator coefficient, xenon burnout, boron concentration etc....Well that is for  a  PWR...... well here in Germany the main coolant pumps of a BWR( 9 of them in a standard Siemens/KWU BWR, construction series 69) are installed directly in the reactor instead of a "LOOP"...I am lacking some info here what a stretchout would cause in a BWR...well my guess would be a lower FW temp in a BWR would of course give the reactor some more "reactivity" due to the  less void coefficient...if the void coefficient gets too low (sorry folks i refuse to use the terms positive and negative reactivity due to the political situation here in Germany ) you could  have lower RPM's with the main coolant pumps compensating too low void coifficient

Best regards Bob


Bob