05000387/LER-2003-001
| Susquehanna Steam Electric Station - Unit 1 | |
| Event date: | 01-29-2003 |
|---|---|
| Report date: | 02-11-2003 |
| 3872003001R00 - NRC Website | |
EVENT DESCRIPTION
At 19:36 on January 29, 2003 a Stack Monitoring Hi Hi Alarm was received from the Ventilation Stack Monitoring System (EIIS Code: IL) in the Unit 1 Control Room. The alarm was investigated and it was determined that high indicated iodine levels from the Unit 1 Turbine Building Ventilation Exhaust Stack were causing the alarm. Control Room personnel responded to the alarm per plant procedure. The Unit 1 Turbine Building Iodine channel indicated a maximum release rate of 483 microcuries per minute.
The Shift Manager declared an Unusual Event per Emergency Action Level (EAL) 15.1.a.A.2 based on the following: indicated release rates were not decreasing, alarms had been validated, noble gas readings had increased by a factor of 100, actions initiated via off normal procedure had not resulted in a change in release rate, and the time the elevated readings were present was approaching 60 minutes. This EAL requires the declaration of an Unusual Event when radiological gaseous effluents exceed 2 times the limits listed in the Technical Requirements Manual for 60 minutes or longer. This condition is evidenced by a valid Ventilation Stack Monitoring System indication of Iodine-131 greater than 208 microcuries per minute.
CAUSE OF EVENT
The most probable cause is a simultaneous failure of the two in-series drain valves on the delay line drain pot in the Unit 1 Offgas System (EIIS Code: WF). The offgas system is designed to reduce the radioactivity in the offgas stream. The system utilizes catalytic recombination for volume reduction and control of hydrogen concentration. Selective adsorption of fission product gases on activated carbon is used to provide time for delay of short-lived radioisotopes before release. The offgas stream passes through a long stretch of piping called the "delay line". At a nominal flowrate of 21.8 scfm, the delay line provides approximately 12.9 minutes of decay time for radioactive products in the offgas stream.
A drain pot is located at the beginning of the delay line to collect entrained moisture and drains to the Turbine Building Central Area Sump (EIIS Code: WK). A float switch maintains drain pot water level. Two level control valves act in conjunction to control the water level. If both level control valves remain open due to a failure or combination of failures, the drain pot could completely empty of water. Without water, the offgas stream can flow through the drain pot. Investigations thus far indicate that these valves may have remained open, providing a flow path from the Unit 1 Offgas system to the Turbine Building Central Area Sump. The Turbine Building Filtered Exhaust System (EIIS Code: VK) services this sump. Air handled by this system passes through two 50 percent capacity filter housings. Each filter housing contains a prefilter bank, a HEPA filter bank, and a charcoal adsorber filter bank. After filtration, the air is discharged through the Unit 1 Turbine Building Ventilation Exhaust Stack. Gases flowing through the stack are monitored by the Ventilation Stack Monitoring System. The system monitors and records particulate, iodine and noble gas levels. Alarms initiate when high levels of any parameter are detected.
CAUSE OF EVENT (continued) The system also performs continuous sampling on a particulate filter and silver zeolite cartridge that provides a capability to determine the isotopic composition of particulates and iodine within the gas stream. As a result of this sampling and analysis, it was determined that the release stream did not contain an iodine component.
The iodine channel uses an integrating radiation monitor for alarm and indication. PPL Susquehanna has concluded that there are several short-lived noble gas nuclides that will produce alarms on the iodine channel, either directly, or as a result of their particulate decay products accumulating in the silver zeolite cartridge. Short- lived Nitrogen-13, a fraction in the ammonia form, is produced during plant operations and if released, it can react with and accumulate on the silver zeolite cartridge to produce alarms. During this event, Nitrogen-13 accumulated on the silver zeolite cartridge leading to the alarms.
Iodine detector sensitivity to Nitrogen-13 was previously recognized. In February of 2002, an event occurred which is very similar to the subject of this LER. Although the indicated release associated with the 2002 event did exceed the Unusual Event EAL for approximately 10 minutes, this did not meet the 60-minute criterion required for Unusual Event declaration. The root cause analysis developed in response to the event concluded that the most probable cause was a malfunction of the drain pot level control system. The Root Cause Analysis (RCA) team identified six (6) corrective actions to prevent recurrence. Four (4) of these corrective actions to prevent recurrence had not been completed on January 29, 2003. Corrective actions not completed were: (1) testing & inspecting the drain pot control systems, (2) the development of an improved process to respond to Ventilation Monitoring System alarms, (3) improving the computer system presentation of release data, and (4) modifying the PPL Susquehanna Emergency Plan to remove iodine channel alarm related EAL criteria.
A Root Cause Analysis (RCA) Team has been established to conduct a thorough review of the 1/29/2003 event.
The following potential root causes are under evaluation by this team:
The most likely cause is a simultaneous failure of the two in-series drain valves on the offgas delay line drain pot.
There were no procedure steps to validate iodine release using a noble gas/iodine ratio in time to avoid entering the Unusual Event.
The offgas delay line drain pot flowpath was not identified as a potential release path in procedures for operator action in response to a high iodine alarm.
REPORTABILITY DETERMINATION/ASSESSMENT OF SAFETY CONSEQUENCES
This report is submitted as a voluntary LER per the Susquehanna Steam Electric Station Emergency Plan Table 5.2 in that an Unusual Event was declared. In accordance with the guidance provided in Susquehanna Steam Electric Station Emergency Plan Table 5.2, the required submission date for this report is February 12, 2003.
There were no safety consequences from this event. Subsequent sampling and analysis determined that the indicated high iodine levels were caused by Nitrogen-13 and release rates were well below Technical Requirement Manual limits.
The Unusual Event was terminated within six hours at 01:25 on January 30, 2003. Although not required by the PPL Emergency Plan, during the Unusual Event the Technical Support Center (TSC) and the Emergency Operations Facility (EOF) were activated to assist Control Room personnel and to improve information flow to the offsite agencies. Feedback from the Control Room staff and the offsite agencies indicates that the TSC and EOF personnel staffing was beneficial. Prior to terminating the Unusual Event, the emergency response organization verified that no iodine had been released, ensured that the most likely pathway leading to the increased indications was identified and understood, and that actions had been taken to isolate this pathway.
CORRECTIVE ACTIONS
Interim actions have been taken to isolate the drain valves and provide for manual control of the level valves on appropriate drain pots until final corrective actions can be implemented.
A Root Cause Analysis (RCA) Team has been established to conduct a thorough review of this event. Additional corrective actions are anticipated from this review.
A systematic troubleshooting plan to determine the malfunction on the drain pots has been developed and has been implemented. As a result of this effort, several components associated with level control on the drain pots have been replaced or rebuilt. At this time interim actions remain in place and drain valves are closed under operator control.
Procedure changes have been developed and issued to improve the process for responding to a Ventilation Monitoring System alarm. These changes aid the operator in discerning detector response to Nitrogen-13.
Prior to this event, PPL had submitted proposed changes to the PPL Susquehanna Emergency Plan (PLA-5511 dated 9/6/2002). These changes require NRC prior approval for implementation. Among the changes proposed is removal of the EAL criteria based on iodine and particulate channel alarms. Pending NRC approval, PPL expects to implement this change before June 30, 2003.
A supplement to this LER will be provided if the conclusions of the Root Cause Analysis Team significantly change the information provided herein.
ADDITIONAL INFORMATION
Failed Component Information: The troubleshooting effort mentioned above resulted in the replacement/rebuild of the following components:
Solenoid Valves — SV17106, SV17107 � Level Valves — LV17106, LV17107(rebuilt actuators, Manufacturer — ASCO � Manufacturer — Valtek � valves tested leak Model Number —HT8345C11 � Model Number — Mark I � tight) Level Switch — LSL17107, LSH/LSHH17106 Manufacturer — Mercoid Model Number — 195-6WT-7806-06 Past Similar Events: PPL Susquehanna has had similar events. None of these previous events required an Emergency Plan entry. On February 5, 2002, an event occurred which is very similar to the subject of this LER.
The Emergency Plan was not entered for this event because an Emergency Action Level criterion was not reached. The release during this event exceeded the Unusual Event EAL for 10 minutes, but not for the required 60 minutes. The RCA Team is reviewing operating experience as part of their evaluation of the 1/29/2003 event.