WBN has no experience in this area. Mark Dicus, Reactor Engineering, 305-246-6185, mark_dicus@fpl.com, has some OE in replacing detectors at Turkey Point. This would be good foe you to review. See Below: ABSTRACT: The high flux trip setpoint for an Intermediate Range detector was determined to be non-conservative prior to startup following refueling. A review of operating data determined that the value installed during initial power ascension for the previous operating cycle had been non-conservative and in fact exceeded the Technical Specifications Allowable Value of 31%.
REASON FOR MESSAGE:
To alert the industry of the potential effects of design changes to excore nuclear instrumentation and of potential weaknesses in the process for determining trip setpoints for new operating cycles.
EVENT DATE: 12/4/2006
UNIT NAME: Turkey Point Unit 4
NSSS/A-E: Westinghouse / Bechtel
TURBINE MANUFACTURER: Westinghouse
MAINTENANCE RULE APPLICABILITY: Yes
COMPONENT INFORMATION (AS APPLICABLE):
MANUFACTURER: Imaging and Sensing Technology
MODEL NUMBER: NY-10721 and NY-10034
PART NUMBER: N/A
DESCRIPTION:
During the refueling outage prior to Unit 4 Cycle 22, an assembly containing an Intermediate Range detector with an aluminum housing (model NY-10034) was replaced with one using a titanium housing (model NY-10721). The change to a titanium housing was made due to corrosion concerns. Based on feedback from the vendor, Turkey Point is the only customer who uses the titanium housing for the Intermediate Range detector.
For Cycle 22, new Intermediate Range high flux trip setpoints were calculated and installed. Following the initial power ascension in Cycle 22, reactor engineers used measured detector current at 100% power to evaluate the installed trip setpoint. The setpoint was found to be 34.6% of the 100% power current. The Technical Specifications Allowable Value for the trip setpoint is 31%. The procedure provided instruction to calculate a new setpoint if the installed value exceeded 27.5% of the 100% power current. Engineers recognized that the installed value exceeded 27.5%, but failed to recognize that the installed value exceeded the Technical Specification Allowable Value of 31%.
At the start of the following fuel cycle, an updated trip setpoint was calculated using the setpoint from the beginning of Cycle 22 as an input. The procedure wording was not specific about whether the previous cycle setpoint to be used was the one calculated prior to startup or the one calculated once 100% power was reached. Prior to startup, Operations requested verification that the installed Intermediate Range trip setpoints were set to less than or equal to 25% power. A reactor engineer reviewed operating data from Cycle 22, expecting to find that the installed setpoints had been less than 25% of the measured current at 100% power. Instead, it was discovered that the installed setpoint for N35 had been 34.6% of the 100% power current. Since this non-conservative setpoint had been used as an input to the Cycle 23 setpoint, it was determined that the Cycle 23 setpoint was also non-conservative. A new, conservative setpoint was calculated and was installed prior to initial criticality.
CAUSES:
The primary cause was determined to be inadequate change management with respect to the use of a titanium housing for the Intermediate Range detector. The design change affected the operating characteristics in the as-installed condition that were not recognized by the vendor or the plant staff. Consequently, processes for calculation of setpoints for the Intermediate Range detectors were not changed.
A contributing cause was inadequate procedures. The procedure for determining trip setpoints was not specific regarding which neutron sensitivity values and which previous cycle setpoints to use when calculating Intermediate Range trip setpoints for a new operating cycle. The procedure used to periodically validate the installed Intermediate Range trip setpoints while at power did not require a check to ensure the installed trip setpoint was less than or equal to the Technical Specifications Allowable Value of 31%.
CORRECTIVE ACTIONS:
Since this event, a design change was made to change the Power Range detector housing to titanium, which raised the site's awareness to the impact of design changes to the excore nuclear instrumentation and resulted in lessons learned. No further internal corrective actions were considered necessary to address this causal factor. An action was created to have the vendor review this event under their corrective action program.
The procedure for calculating Intermediate Range trip setpoints for a new cycle will be revised to incorporate specific guidance for which inputs (e.g., which previous cycle setpoint) are to be used in the calculation. The procedure for validating Intermediate Range trip setpoints at power will be revised to include a comparison of installed values to the Technical Specifications Allowable Value.
SAFETY SIGNIFICANCE:
This event was determined to have low safety significance. The Intermediate Range High Flux trips at 31% power are not credited in the safety analysis; instead, Power Range detector trips at 25% power are credited. The trip logic for Intermediate Range requires 1 out of 2, and in Cycle 22 one channel (N36) had a valid trip setpoint that would have initiated a reactor trip, if called upon, prior to exceeding 31%. A new, conservative trip setpoint for N35 was installed following the initial power ascension in Cycle 22. Cycle 23 did not operate with non-conservative Intermediate Range trip setpoints
INFORMATION CONTACT:
Mark Dicus, Reactor Engineering, 305-246-6185, mark_dicus@fpl.com
-----Original Message----- From: respvr-bounces@retaqs.com [mailto:respvr-bounces@retaqs.com] On Behalf Of Michael_Fanguy@dom.com Sent: Wednesday, January 31, 2007 7:38 AM To: respvr@keffective.com; pwrrm@keffective.com Cc: Robert_A._Wright@dom.com Subject: [Respvr] Question on Power Range NI Replacement
This email is being sent out for North Anna Reactor Engineering.
North Anna (3 loop Westinghouse) is planning to replace a PRNI in their next outage. The question is do other utilities put any special measure in place for startup due to the new detector? Are your Hi Flux Trips extremely low? Is the startup performed with the NI channel in TRIP?
Any feedback would be appreciated. Please send your responses to Robert_A._Wright@dom.com.
Nuclear Analysis & Fuel Lead Reactor Engineer Surry Power Station Email: Michael_Fanguy@Dom.com Phone: 757-365-2155 Pager #: 3453
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