United States Nuclear Regulatory Commission - Protecting People and the Environment

LICENSEE: ROSEMOUNT NUCLEAR INSTRUMENTS, INC
SITE: ROSEMOUNT NUCLEAR INSTRUMENTS, EN NUMBER: 35844
DOCKET: EVENT DATE: 06-18-99
RX TYPE: EVENT TIME: 10:00
VENDORS: NOTIFY DATE: 06-18-99
EMERGENCY CLASS: N/A REGION: 3 STATE: MN TIME: 12:28
OPS OFFICER: BOB STRANSKY
10 CFR SECTION: CCCC UNSPECIFIED PARAGRAPH
10 CFR PART 21 REPORT REGARDING POTENTIAL NEED FOR CAPACITOR REPLACEMENT IN SPECIFIC TRIP/CALIBRATION SYSTEMS

The following is text of a letter submitted to the NRC Operations Center via facsimile:

"The purpose of this letter is to inform you that a specific capacitor C25 used in Rosemount Trip/Calibration Systems may require replacement. This affects all Rosemount Model 510DU Trip Units (obsolete since 1987) and certain Rosemount Model 710DU Trip Units.

"Prior to 1987, there were 10 confirmed random failures of C25 capacitors used in slave and master trip unit circuit boards. In 1987, a number of tests were run on sample capacitors in an attempt to duplicate the failure modes. It was found that unless the capacitors were exposed to temperatures and voltages in excess of normal operating conditions, no failures were induced.

However, introduction of a contaminant such as moisture could possibly cause this type of failure. As a precautionary measure, a component change was instituted on the C25 capacitor beginning with manufacturer date code 8630. This change featured an enhanced moisture seal. No confirmed failures of the post 8630 date code C25 capacitor have been identified. However, Rosemount Nuclear Instruments. Inc is sending this letter to affected utilities to remind users that the model 710 and 510 trip units with pre-8630 date code C25 capacitors could be susceptible to this condition. Failure of the C25 capacitor can cause a low resistance condition between the power supply input and chassis ground. This low resistance can allow over-current conditions to cause failure of the affected unit.

"A letter [enclosed in facsimile] was issued by Rosemount Nuclear Instruments Inc to the Scram Frequency Reduction Committee of the BWR Owners' Group in response to a BWROG request for information [enclosed in facsimile], documenting the change which had been made in 1987. Since that time, there have been five additional confirmed C25 failures and fourteen additional failures possibly related to the C25 capacitor. Again, all of these failures occurred in units with C25 capacitors that had date codes of pre-8630. The total number of model 510 and 710 trip units shipped with the pre-8630 date code capacitors was 18,164. We believe that recent failures may warrant further examination of your 510DU/710DU systems to determine whether replacement of the pre-8630 date code capacitors is warranted."



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