At VC Summer we have been having cycle length prediction errors for many cycles, with at least 4 different IFBA concentrations.  Since we do our own core design with APA, we have done a lot of investigations of the problem.  W also did a root cause on why so many units are having problems with this.  That’s part of the reason behind the upgrade to ANC 9.0.  Unfortunately, not all of the reactivity errors are going in the same direction, so when they benchmark ANC, they try to hit the average.  A major factor in this issue is each utility’s method for determining calorimetric reactor power.  In other words, each unit may be operating at a different place within the ~2% calorimetric uncertainty, and a 1% change in core power will cause a reactivity change of around 35 ppm at the end of the second fuel cycle (at least in our unit – 3-loop core at 2900 MWth).

We tend to have a reactivity shortfall of about 300-600 pcm at the end of cycle.  In the last few cycles we have implemented a bias factor on the eigenvalue, based on the average of the last 3 cycles.  This has given us fairly good results, as long as there are no core modeling or fuel design changes.

I have given some presentations at the RxE seminar and the Tech User’s Group on this issue.  The BEACON User’s Group has also discussed it, since the reactivity bias is programmed into the software.  I’ll be happy to discuss it further if anyone is interested.

Damon Bryson

(803) 345-4814

dbryson@scana.com

From: pwrrm@retaqs.com [mailto:pwrrm@retaqs.com] On Behalf Of Gore, Duane
Sent: Thursday, July 10, 2008 5:44 PM
To: pwrrm@retaqs.com
Subject: [Pwrrm] Cycle Length Prediction Error when using Westinghouse 1.5xIBFAs

STP has been experiencing a difference between predicted cycle lengths and actual EOC burnup (specifically, over-predictions of cycle length).  This effect seems to have begun when we started using 1.5x (B-10 loading) IFBA. Westinghouse uses their APA package for our core design. Has any other utility had a similar experience?

Duane Gore

Supervisor, Reactor Engineering

STPNOC

(361) 972-8909