At STP we use one of our power range nuclear instrumentation (NI) detectors for our reactivity measurements during low power physics testing (LPPT). Recently, on the Unit 2 cycle 15 startup, the gamma compensation (bucking) current for the lower half of the NI detector used for the measurement behaved in an atypical manner. Although the reactivity computer passed its checkout test, the measured worth of the heaviest control rod bank was over-predicted using Westinghouse’s Dynamic Rod Worth Measurement (DRWM) technique.

Ultimately, we shutdown the reactor, checked connections and cables, and switched NI detectors.  The reactor was restarted and LPPT was successfully performed. Since the NI detectors are used in such a low current range for LPPT, it was decided that the detector which was unsuitable for LPPT use would still provide proper response at power.

Once previously we replaced all of the NI detectors after connecting the reactivity computer and examining detector responses. The detector cables were inspected found to be degraded. In that case, we had not begun reactor startup.

Questions:

1. Have you had a similar experience?
2. As a result, did you change out the NI(s) that cycle?
3. Does this phenomenon foretell detector failure in subsequent cycles?
4. What procedural guidance in your LPPT procedure is given to decide if an NI is not usable and a new NI should be selected?
5. Does your plant I&C group have a method, other than using the reactivity computer, to determine if an NI is acceptable for use for LPPT?

Thank you in advance for your responses.

Duane Gore

Supervisor, Reactor Engineering

STPNOC

(361) 972-8909