Duane,
We (Exelon-Byron) have been performing a 3-legged coastdown for about 7 years.  Temperature coastdown (at full reactor power- ~2 days), followed by GV Wide-Open (temperature and power reduce together- up to ~14 days), followed by Power coastdown (Power reductions to maintain Tave above minimum allowed Tave- ~ usually 0 to 2 days, depending upon previous cycle operation.
 
As part of our uprate several years ago, we had our accident analyses evaluated for an expanded range of allowable initial HFP Tave values.  Cycle specific evaluations are required each cycle but are performed as part of the normal reload design analysis.  
 
One word of caution.  Although having longer, shallower coastdowns may appear to provide an economic benefit, we may have put our fuel into a less optimal fuel conditioning state.  If the fuel loses its full power conditioning as reactor power coasts down, it may cost you additional holds or reduced ramp rates during power ascension.  That doesn't necessarily have to be the case but it can happen, especially during missing pellet surface situations.  We have had PCMI failures when initially reaching full power during the start-up after a cycle with a significant coastdown.  Previous cycle power history at coastdown is considered in developing our power ascension profile.
 
Our two units have different HFP Tave values (U1 is 587, U2 is 581).  Consequently the GVWO portion of the coastdown is different for the two units and our Unit 2's optimal coastdown is much shorter than for Unit 1.
 
Also see below.
 
Ron Niederer
Byron Reactor Engineering
815-406-3443
 
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-----Original Message-----
From: respvr@retaqs.com [mailto:respvr@retaqs.com]On Behalf Of Gore, Duane
Sent: Tuesday, May 06, 2008 7:02 AM
To: respvr@keffective.com
Subject: [Respvr] Temperature Coastdown

STP currently performs a Power Coastdown at the end of each cycle.  I have been tasked with exploring performing a Temperature Coast (on Tavg) to the lower Safety Analysis limit, followed by a Power Coast.

 

We reach Valves Wide Open on the turbine in about 4 days (2F drop in Tavg). After that, the efficiency of the secondary side drops.

 

For those who perform a temperature coast, I would like to know:

  1. What type of temperature coast do you use (Tavg, Treference, others)? Why? [Niederer, Ronald J.] Just Tave- Operators do not change RCS conditions or turbine power until Tmin (minimum allowed Tave) is approached.  Usually coastdown operation is over around the time Tmin is reached. 
  2. What instrument recalibrations are usually necessary? [Niederer, Ronald J.]  NIS PR (daily or more) 
  3. When do you reach valves wide open on the turbine? [Niederer, Ronald J.] After about 2 days (2 degrees Tave drop)  
  4. How does the secondary side efficiency drop with steam pressure? [Niederer, Ronald J.] It does.  But by keeping reactor power higher with the GV wide open, it comes out a plus on the MWe. 
  5. What operational issues/lessons learned would you care to pass along? [Niederer, Ronald J.] I have some power point materials we've used to help our operators.  Let me know if you are interested and I can send them to you.  We also created new computer points to monitor the margin between Tave and Tmin.  Tmin is a linear function of reactor power.

 

Temperature Coastdown,

•  ~ 1 day for 1°F - no reactor power reduction
–“GV Wide Open” Coastdown
•Decrease in Tavg of 0.5 to 0.7 °F is accompanied by a decrease in power of 0.45 % each day
–Power Coastdown
•~12 MWe PER DAY (~ 1 day for -1% power )
•Ideally, Tavg rests just above Tmin, the lower Tavg limit, as power is reduced

 

Thanks in advance. 

 

Duane Gore

Supervisor, Reactor Engineering

STPNOC

 

(361) 972-8909