Answers to STP questions:
1. What type of temperature coast do you use (Tavg, Treference, others)? Why?
TMI-1 is a Babcock & Wilcox-designed plant, which runs at constant Tavg of 579 F when above ~20% power (when the once-through steam generator level rises above the low level limit setpoint). Tavg is one of the controlled parameters during normal operations. Therefore the TMI-1 temperature coastdown is a Tavg coastdown. The analysis for cycle stretch by Tavg reduction allows operators to reduce Tavg by up to 7 F. The procedural limit is a bit less than that.
2. What instrument recalibrations are usually necessary?
Power range nuclear instruments require more frequent recalibration. As Tavg is reduced, Tcold goes down, downcomer density increases, and thus fewer neutrons escape to be read by the out-of-core neutron detectors. This drives the NIs in a non-conservative direction.
3. When do you reach valves wide open on the turbine?
They never go wide open. TMI-1 turbine is designed to operate w/ the control valves nearly full open. We have found that they don't control well when open more than 85%. As Tavg decreases, the turbine control valves will tend open up. To preserve some "bite" on the control valves, we will reduce power during the Tavg reduction to keep the turbine control valves less than 85% open.
4. How does the secondary side efficiency drop with steam pressure?
We maintain steam pressure. During normal operation, the once-through steam generators (OTSGs) provide about 50 F of superheat. As Tavg decreases, the amount of superheat in the steam to the turbine drops slightly.
Another phenomenon is that OTSG operating range "level" (more like OTSG delta-P) increases as Tavg decreases. This is one result of slightly colder/higher flow of feedwater entering the OTSG.
Carnot implies that as Tavg drops (with attendant drops in Thot and Tsteam), plant efficiency declines. However, there are some compensations. For example, the heat losses are slightly lower due to reduced temperatures and flow losses through the turbine control valves are lower because they are open a bit more. Bottom line: we did not observe any change in plant efficiency during the Tavg reduction at the end of the last cycle.
5. What operational issues/lessons learned would you care to pass along?
As implied above, stay on top of NI calibrations and turbine control valve position.
Bill Stanley TMI-1 Reactor Engineering (717) 948 8695
-----Original Message----- From: Crawford, Howard C. Sent: Tuesday, 06 May 2008 12:12 To: Stanley, William S (TMI) Subject: FW: [Respvr] Temperature Coastdown
Please respond to this request for TMI. Thanks
-----Original Message----- From: Buckley, Fred Sent: Tuesday, May 06, 2008 8:13 AM To: Crawford, Howard C.; Tsai, Robert W. Subject: FW: [Respvr] Temperature Coastdown
Thought you might be interested in this question. -----Original Message----- From: respvr@retaqs.com [mailto:respvr@retaqs.com]On Behalf Of Gore, Duane Sent: Tuesday, May 06, 2008 8:02 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?
2. What instrument recalibrations are usually necessary?
3. When do you reach valves wide open on the turbine?
4. How does the secondary side efficiency drop with steam pressure?
5. What operational issues/lessons learned would you care to pass along?
Thanks in advance.
Duane Gore
Supervisor, Reactor Engineering
STPNOC
(361) 972-8909
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