05000265/LER-2012-004

LER-2012-004, Drywell Radiation Monitor Failed Downscale

Docket Number
Event date:
Report date:
2652012004R00 - NRC Website

PLANT AND SYSTEM IDENTIFICATION

General Electric - Boiling Water Reactor, 2957 Megawatts Thermal Rated Core Power Energy Industry Identification System (EIIS) codes are identified in the text as [XX].

EVENT IDENTIFICATION

The 2B Drywell (DW) Radiation (Rad) Monitor Failed Downscale and resulted in one of the two divisions of the isolation logic becoming inoperable, for which both divisions are required for the Primary Containment Isolation (PCI) Group 2 isolation logic to complete its safety function.

A. CONDITION PRIOR TO EVENT

�Unit: 2 Event Date: September 21, 2012� Event Time: 1449 hours0.0168 days <br />0.403 hours <br />0.0024 weeks <br />5.513445e-4 months <br /> � Reactor Mode: 1 Mode Name: Power Operation� Power Level: 100%

B. DESCRIPTION OF EVENT

At 1449 hours0.0168 days <br />0.403 hours <br />0.0024 weeks <br />5.513445e-4 months <br /> on September 21, 2012, Operations found the 2B DW Rad Monitor reading downscale. The affected point history data was reviewed for trends. The 2B DW Rad monitor was previously trending between 2.2 and 2.5 Rads per Hour (R/Hr), and during this event, the monitor was indicating between 0.1 to 0.3 R/Hr. Operations inserted a Division II PCI 1/2 Group 2 Isolation at approximately 1515 hours0.0175 days <br />0.421 hours <br />0.0025 weeks <br />5.764575e-4 months <br /> to comply with Technical Specification (TS) 3.3.6.1, Primary Containment Isolation Instrumentation [JM], Conditions A and B since one required Drywell Radiation [NH] Monitor [RIS] division was inoperable. Operations also entered Technical Specification 3.3.3.1, Post-Accident Monitoring (PAM) Instrumentation [IP], Condition A since one required Drywell Radiation Monitor division was inoperable.

Troubleshooting performed on the 2B DW Rad Monitor chassis indentified the chassis had failed. The chassis was replaced with a bench calibrated spare chassis. TS 3.0.5 was entered for functional testing on the new chassis. By 1837 hours0.0213 days <br />0.51 hours <br />0.00304 weeks <br />6.989785e-4 months <br /> the chassis was satisfactorily tested without any issues. Operations then exited TS 3.0.5, TS 3.3.6.1 Conditions A and B, and TS 3.3.3.1, Condition A, and the 2B DW Rad Monitor was returned to operable status.

The safety significance of this event was minimal. Since both divisions of drywell radiation monitoring are required to complete the Group 2 isolation logic, but only one was available during this event, this report is submitted in accordance with the requirements of 10 CFR 50.73 (a)(2)(v)(C), and (a)(2)(v)(D), which requires the reporting of any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to: control the release of radioactive material, and mitigate the consequences of an accident.

C. CAUSE OF EVENT

The failed 2B DW Rad Monitor was sent to the vendor for failure analysis on September 28, 2012. The apparent cause of the monitor failure was determined to be dust and dirt inside the module which caused some of the sub­ components to malfunction and result in a downscale indication. Inadequate periodic maintenance, in combination with the chassis [IMOD] ventilation design, had allowed the components inside the chassis to be vulnerable to dust and dirt buildup since its installation. The preventive maintenance (PM) program did not require the chassis to be disassembled to facilitate internal components cleaning. This buildup of dust and dirt caused some of the monitor components to malfunction. The components that malfunctioned were powered by -24 Volts Direct Current, which feeds the trip logic. An additional detailed investigation will be performed by the vendor to determine the exact cause and circuit sub-components that failed.

D. SAFETY ANALYSIS

System Design The Containment Atmosphere Monitoring (CAM) System [IK] instrumentation provides the signals necessary to indicate and alarm high radiation levels in the drywell following a Loss of Coolant Accident (LOCA). At high radiation conditions in the drywell, the DW Rad Monitors initiate: (1) Primary Containment Group 2 isolation, (2) Reactor Building (RB) [NG] Ventilation (Vent) and Control Room [NA] Vent Isolations, and (3) an auto-start of the Standby Gas Treatment (SBGT) System [BH].

Drywell Radiation - High Function: High drywell radiation, as monitored by the DW Rad Monitors, indicates possible gross failure of the fuel cladding. Therefore, when Drywell Radiation - High is detected, an isolation is initiated to limit the release of fission products. However, as provided in the TS Bases 3.3.6.1, Function 2.c (Drywell Radiation - High), this Function is not assumed in any accident or transient analysis in the UFSAR because other leakage paths (e.g., MSIVs) are more limiting.

The Drywell Radiation - High Function receives input from two radiation detector assemblies [RT] (located in capped drywell penetrations) each connected to a switch [RIS] (Radiation Indicating Switch (RIS) 2-2419-A and RIS 2-2419- B). Each switch actuates two contacts. Each contact inputs to one of four trip strings. Two trip strings make up a trip system and both trip systems must trip to cause an isolation of the PCI valves. The contacts associated with the same switch provide input to both trip strings in the same trip system. Any contact will trip the associated trip string.

The trip strings are arranged in a one-out-of-two taken twice logic. As used in TS Table 3.3.6.1-1, Function 2.c, Drywell Radiation High, a channel is considered to include a radiation detector assembly, a switch, and one of two contacts. Under this design, two divisions of Drywell Radiation - High Function are available, and are required to be operable to complete the PCI logic for a Group 2 isolation from DW radiation monitoring.

The PCI function is initiated based on the specific plant accident conditions that have been sensed. The actions in Group 2 are initiated on sensing any one of the following conditions, per TS Table 3.3.6.1-1:

• reactor low water level (>1= 3.8") (Level Transmitters [LT] 2-263-57A/B and 2-263-58A/B);

• high drywell pressure (4= 2.43 psig) (Pressure Switches [PS] 2-1621-A to D); and

• high drywell radiation ( The signals for Group 2 primarily provide for isolation of the containment drywell/torus [NH] atmosphere and include isolation valves for:

• Residual Heat Removal (RHR) [BO] shutdown cooling supply, reactor [AD] head spray, and discharge to radwaste [WD];

• drywell vent, purge, and sump isolation [VB];

• drywell pneumatic supply;

• drywell air sampling (oxygen analyzer valves);

• traversing in-core probe (TIP) [IG] withdrawal and isolation; and

• reactor building main vent isolation [VA].

In addition to providing isolation, the Group 2 signal initiates other actions designed to limit radioactive releases, e.g., the Group 2 isolation signal automatically starts the SBGT system, and trips the Reactor Building floor drain and equipment drain pumps.

Safety Impact Since the 2B DW Rad Monitor was inoperable, this one of the two divisions of the isolation logic was unavailable, whereas, both divisions are required for the PCI Group 2 isolation logic to complete its safety function. Each DW Rad Monitor (2A and 2B) is designed such that a downscale failure mode does not result in an automatic conservative/tripped condition to its associated two channels; rather the chassis provides no automatic channel actuation functions when downscale. Therefore the safety function for Group 2 isolation from DW radiation monitoring was not met during the 2B DW Rad Monitor downscale event.

The 2B DW Rad Monitor had failed downscale for approximately 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />. During this period, a high drywell radiation condition would not have initiated a Group 2 isolation, however, a Group 2 isolation remained available from reactor water low level, or high drywell pressure signals.

Although a loss of PCI Group 2 isolation function occurred for this brief period .when the 2B DW Rad Monitor failed, this condition did not create any actual plant or safety consequences since the Unit was not in an accident or transient condition requiring use of DW Rad Monitors during this period of time.

Risk Insights The plant Probabilistic Risk Assessment (PRA) model credits the drywell rad monitors, hence the as-found conditions contributed to an increase in risk. Since the 2B DW Rad Monitor had failed downscale for approximately 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />, during this period, a high drywell radiation condition would not have initiated a Group 2 isolation for valves that are modeled in the PRA model, however, all of these valves are normally closed, except for one valve, 2-1699-7 (Air Operated Unit 2 Vent to Reactor Building), which is normally open.

As such, the risk contribution was calculated as follows: the effect on change in Core Damage Frequency (CDF) is:

1.27E-09/yr, which is insignificant; the effect on change in Large Early Release Frequency (LERF) is: 4.03E-10/yr, which is also insignificant.

In conclusion, the overall safety significance and impact on risk of this event were minimal.

E. CORRECTIVE ACTIONS

Immediate:

1. Replaced the 2B DW Rad Monitor with a new drywell radiation monitor to restore function.

Follow-up:

1. Vendor to perform detailed failure analysis on the failed 2B DW Rad Monitor to determine which circuit subcomponent had failed.

2. Add a PM procedure to clean the DW Rad Monitors on an increased frequency.

3. Develop a PM to Replace/Refurbish the DW Rad Monitors.

F. PREVIOUS OCCURRENCES

The Station Events Database, LERs, and ICES were reviewed for similar events at Quad Cities Nuclear Power Station. This event was a downscale failure of a DW Rad Monitor caused by inadequate periodic maintenance, in combination with the chassis ventilation design which allowed the components inside the chassis to be vulnerable to dust and dirt buildup since installation. Similar events observed are below, however, there were no previous events identified that would have prevented this event.

1. Station Events Database, Issue Report (IR) 868559: On 01/19/09, a lowering trend on the 2B DW Rad Monitor was observed. The 2B DW Rad Monitor was previously trending between 2.2 and 2.4 R/Hr, and during this event the monitor indicated between 1.5-1.9 R/Hr. Bench testing was performed on the failed chassis/power supply and no performance issues were found. No clear cause was found for the failure of the 2B DW Rad Monitor. The 2B DW Rad Monitor chassis was replaced. A dust related cause for failure was possible, but the exact failure mode could not be identified. The issue did not reoccur during troubleshooting.

2. Station Events Database, IR 900795: On 3/31/09, the 1B DW Rad Monitor was observed to be trending downscale. The cause of this event was determined to be an internal cable degradation, or cable connection issue that caused the intermittent spurious downscale indications. This failure was specific to the cable involved and did not appear to be related to a dust buildup in the chassis.

G. COMPONENT FAILURE DATA

Drywell Radiation Monitor Radiation Indicating Switch (RIS) 2-2419-B:

Component Manufacturer: General Atomics/Sorrento Component Model Number: RP-2CM Component Part Number: RP-2CM, Range - 1 to 108 R/hr.

This event has been reported to ICES as Failure Report No. 300720.