05000305/LER-2006-010
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| 3052006000R00 - NRC Website | |
EVENT DESCRIPTION
On an 18 month frequency, radiation monitoring [IL] channel R-19 has been previously calibrated using three Cesium sources. However, the three previous sources were incapable of producing sufficient counts to test detector response above the High Level alarm (trip) setpoint. As part of this calibration, the set points were functionally tested by exposing the detector tube to a flashing LED, which simulates radiation. The functional portion of the calibration tests the entire channel from the detector through the electronics to the output relays and alarms. The limited frequency of the LEDs required the setpoints to be lowered for the functional test and then returned to their desired value after the test. KPS assumed, from vendor information, that the detectors had a linear response up to 1E+7 cps (which encompasses the Alert and High Alarm setpoints). Therefore, since digital setpoints do not drift, this method was considered acceptable for functional testing.
Due to NRC concerns with the above calibration method, Cesium sources of sufficient strength to calibrate the detector above the High Level alarm setpoint were recently obtained. The calibration using these new sources discovered that R-19 responds linearly up to 5.68E+5 cpm, but non-linearly at 1E+7. The monitor response is approximately 25% low at approximately 1E+7 cpm. Subsequent consultation with the original vendor of the radiation monitor detector confirmed that non-linearity occurs above approximately 1E+6 cpm.
Current setpoints for R-19 are as follows:
ALERT is 1.30E+4 cpm ALARM is 1.29E+6 cpm Therefore, for the purposes of determining reportability, for the past three years this detector has not been properly calibrated and has been inoperable. However, during that time, channel counts never exceeded the highest known linear level (5.68E+5 cpm), and the detector was reading accurately below this level. Thus, this event has a low safety significance.
EVENT ANALYSIS
R-19 monitors the liquid phase of the secondary side of the steam generator for radiation, which would indicate a primary-to-secondary system leak, providing backup information to the condenser air ejector gas monitor. Samples from the bottom of each of the two steam generators are mixed in a common header and the common sample is continuously monitored by a scintillation counter and sample volume chamber assembly. Upon indication of a high radiation level, each steam generator is individually sampled in order to determine the source. This sequence is achieved by manually selecting the desired steam generator to be monitored. The steam generator blowdown [WI] and air ejector [SH] radiation monitors are interconnected, such that either monitor isolates the blowdown and reroutes the air ejector exhaust. A high radiation signal closes the isolation valves in the blowdown lines and sample lines.
The requirements for R-19 were relocated from the Technical Specifications (TS) to the Off Site Dose Calculation Manual (ODCM), but the minimum frequency for its functional check and calibration were retained in Item 19 of TS Table 4.1.1. Item 19 was left in this table for those radiation channels in which automatic actions occur or immediate operator actions are required to assure plant safety.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) Kewaunee Power Station 05000305 YEAR This event is being reported under 10 CFR 50.73(a)(2)(i)(B) as an operation which was prohibited by the plant's Technical Specifications, and also as a Special Report required per TS Section 6.9.b.3 and Step 3.0.3 of the ODCM.
SAFETY SIGNIFICANCE
For the last three years, channel counts never reached the highest known linear level (5.68E+5 cpm), and the detectors were reading accurately in the range below this level. Thus, this event is has a low safety significance.
CAUSE
The apparent causes are:
O An inaccurate interpretation of the requirements of a radiation monitor channel calibration, o A reliance on conforming to common industry practices, and o A failure to validate that the channel responds linearly up to 1E+7 cpm
CORRECTIVE ACTIONS
Immediate corrective action was to declare this radiation monitor channel inoperable and initiate compensatory measures as directed by Table 3.1 of the ODCM.
Long term corrective actions are to operate and calibrate the channel (including the setpoints) within it's known linear range.
PREVIOUS SIMILAR EVENTS
None