05000458/LER-2014-003
| River Bend Station - Unit 1 | |
| Event date: | 06-10-2014 |
|---|---|
| Report date: | 08-11-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(b) 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(i) |
| 4582014003R00 - NRC Website | |
REPORTED CONDITION
On June 10, 2014, with the plant operating at 100 percent power, technicians performing a scheduled surveillance test found that one instrument channel in the reactor protection system (JC) failed its time response acceptance criterion. This was the second of two such tests that failed in similar fashion. Since it is conceivable that the second tested channel was out of specifications at the time the first channel was tested, this condition caused independent redundant channels in the same trip system to be inoperable at the same time.
The actions required by the applicable Limiting Condition for Operation were not taken since the operators were not aware of the latent condition at the time of the first surveillance test failure. This condition is reportable in accordance with 10CFR50.73(a)(2)(i)(b) as operations prohibited by Technical Specifications, as well as 10CFR50.73(a)(2)(vii), a potential common-cause inoperability of independent trip channels.
BACKGROUND
One of the functions in the reactor protection system is the initiation of a reactor scram in the event of a closure of the main steam isolation valves (MSIVs). Limit switches on each of the eight MSIV actuators provide input to the individual, redundant RPS trip channels if the valve moves to a nominal 12 percent of stroke length in the "close" direction. The design of the RPS system requires that a channel respond to an input from its MSIV limit switch and generate a trip signal. The maximum response time specified by the Technical Requirements Manual is 90 milliseconds. The calibration frequency is four years (24 months on a staggered test basis).
The arrangement of the instrumentation includes 16 individual channels. Inboard and outboard MSIVs in each of the four main steam line are instrumented with redundant limit switches monitored by independent trip channels of the RPS system.
IMMEDIATE ACTIONS
In the calibrations performed in 2010, a degrading trend in the response times was noted in the four channels containing the Agastat relays (**94**). In the 2010 tests, the response time of each of the four channels was 89 milliseconds. The as-found response times found in the recent tests ranged from 90 to 102 milliseconds. In each case, the Agastat relay was replaced and the response time was then verified to be within specifications. The response times for the channels with no Agastat relays ranged from 41 to 51 milliseconds.
An engineering evaluation of this condition was performed, and the RPS system was declared operable with compensatory measures.
Until this issue is resolved, the frequency of the calibration tests in the channels with Agastat relays has been increased to once per year. This action is being tracked in the station's corrective action program.
CAUSAL 'ANALYSIS The cause of this event remains under investigation. Corrective actions to prevent recurrence will be developed following the evaluation of future test results. These will be reported in a future supplement to this LER.
SAFETY SIGNIFICANCE
Due to the design redundancy of the independent channels of the RPS system, it is unlikely that this condition would not have prevented the system from performing its safety function. Had an actual full MSIV isolation occurred with the channel response times in their as-found condition, the reactor scram signal would have likely have still occurred within the specified instrument response time. This will be confirmed in the completion of the event investigation.
(NOTE: Energy Industry Component Identification codes are annotated as (**XX**).)