ML20137P028
| ML20137P028 | |
| Person / Time | |
|---|---|
| Site: | Paducah Gaseous Diffusion Plant |
| Issue date: | 04/02/1997 |
| From: | Polston S UNITED STATES ENRICHMENT CORP. (USEC) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9704090119 | |
| Download: ML20137P028 (8) | |
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,8 United states
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Enrrhment Corpntion Paducah Site Office j
q P.O. Box 1410 i
Paducah, KY 42001 Tel. 502 4415803 Fax: 502 441-5801 April 2,1997 United States Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC 20555-001 Paducah Gaseous Diffusion Plant (PGDP) - Docket No. 70-7001 - Preliminary Event Reports ER 97-002 and ER 97 005 Pursuant to 10CFR 76.120(c)(2), Enclosur : 1 is the required 30-day written Preliminary Event Repost covering Events97-002 and 97-005 at PGliP. Rese events relate to the Criticality Accident Alarm
' System (CAAS) cluster meter readings which were observed to be lower than Engineering requirements.
The Nuclear Regulatory Commission (NRC) was verbally notified of the events March 5,1997 and March 18,1997, respectively The investigation activities are continuing with a final report targeted for January 28,1998. Enclosure 2 is a list of commitments made in the report.
Should you require further information on this subject, please contact Bill Sykes at (502) 441-6796.
Sincerely, f
eve Polst 4
General Manager Paducah Gaseous Diffusion Plant SP:WES:mel Enclosures (2) cc:
NRC Region 111 NRC Senior Resident inspector, PGDP g
9704090119 970402 PDR ADOCK 07007001 C
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United States Nuclear Regulatory Commission Page Two l April 2,1997 4
Distribution Robert L. Woolley '
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- J. Adkins, USEC-IIQ D. Allen, PORTS
. M. Boren S. Brawmer
. J. Dietrich, LMUS-liq L. Fink, PORTS R. Gaston, PORTS
. J. Labarraque B. Lantz PORTS R. Lipfert, PORTS 4
J. Miller, USEC-11Q
- i. iv2zc S. Penrod
- 11. Pulley A. F.ebuck-Main, USEC-liq S. Routh, USEC-IlQ S. Scholl, PORTS B. Sykes R. Wells, USEC-IIQ File: 97-890-117 t
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WDocket'No. 70-7001-Page1of5 i
i' EVENT REPORTd ER-97-02 and ER-97 05 i
Preliminary Report f
l BACKGROUND l
The Criticality Accident Alarm System (CAAS) at Paducah Gaseous Diffusion Plant (PGDP)is i
det gned to detect gamma radiation levels that would result from the minimum criticality accident of concern and to warn plant personnel by activating evacuation alarms. The CAAS i
consists of clusters of three detector modules and one logic module. According to the Safety Analysis Report (SAR), Section 3.12.6, "The clusters consist of three detector modules which E
4a arm w en a gamma ose-rate of 10 millroentgen per hour (mR/hr) above background is l
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detected." Each detector module has an intemally generated s!gnal to maintain a constant operational check of the detection circuits. Currently, the background reading of the detector is initially adjusted to display a nominal 10 mR/hr on the front panel of the detector module. The alarm setpoint of each detector module is currently set at a nominal 20 mR/hr so that the detector module will go into alarm status ifit receives ra:iiation of 10 mR/hr or greater above background.
. The fault setpoint of each detector module is currently cet at a nominal 5 mR/hr so that the detector module will go into fault status if the background reading drops significantly. A fault status is indicative of equipment problems and causes the CAAS to transmit a trouble signal to the central control room. Each detector module will detect radiation independently of each other.
- The cluster is designed to minimize the number of false audible evacuation alarms that plant personnel experience by applying the following logic to interpret detector module alarm status.
To receive an audible CAAS alarm, one of two conditions must exist: (1) at least two of the three detector modules must be in alarm status simultaneously; or (2) only one detector module may be in alarm status while the other two detector modules are in fault status.
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On November 6,1996, under regulation of the Department of Energy, two CAAS detector modules in Cluster X in the C-337 building were obsen ed with low background readings of 9 l
mR/hr and 8.5 mR/hr. (Event Report PAD-1996-0054 While the actual background radiation level would be essentially zero, the CAAS indicating meter is set to indicate a background of 10 mR/hr 0.5 mR/hr. This means at an actual radiation field of 10 mR/hr, the meter will indicate 20 mR/hr i0.5 mR/hr. The alarm is set to initiate at 10 mR/hr above background. (10 mR/hr background setpoint plus 10 mR/hr radiation field equals 20 mR/hr alarm setpoint nominal
, values.) A low background reading on a module could prevent the module from going into alarm status even though it has detected a 10 mR/hr increase in radiation. As stated above, in order to
. generate an audible alarm, the cluster logic requires either (a) two detectors to be in alarm status; or (b) one detector to be in alarm status while the other two detect 6r itiedules are hi fault status.
Therefore, low background readings on two detector modules simultaneously could prevent an
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- Docket No. 70-7001 4..
Page 2 of 5 audible alarm from being generated even though one or more modules has detected 10 mR/hr radiation. For that reason, clusters are currently declared inoperable if two of the three detector l
modules have backgrond readings below 9.5 mR/hr. As a result of the above event, long term orders (LTO) were issued to establist weekly field monitoring of CAAS clusters to determine if
_ the low background readings constitute a system problem. The LTO instructs operators that -
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" modules with readings found belcw 9.5 mR/hr are to be promptly reported to the Plant Shift 9geout." On February 4,1997, another incident Superintendent and a work order initiated for of two CAAS modules with low background a ings occurred (Problem Report: PR-CO 0556). CAAS clusters U and AK located in the C-337 building were observed with two modules each reading 9.0 mR/hr. The clusters were declared inoperable and the LCO action steps as
- defined by Operational Safety Requirements, KY/D-3971, Section 3.1.2.3, were implemented.
- The modules were replaced by Instrument Maintenance and Clusters U and AK were declared l
operable on February 4 and 7, respectively.
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DESCRIPTION OF EVENT j
On March 4,1997, dering the weekly field monitoring of the CAAS clusters, Cluster V in C-337 l
process building was found with two of the three detector modules displaying background F
readings below 9.5 mR/hr. Detector Module Serial No. 580054 and Detector Module Serial No.
580105 were both reading 9.0 mR/hr. The cluster detector units are Model GCM-650 gamma criticality monitors, manufactured by Nuclear Research Corporation. Cluster V was declared inoperable. Since the Technical Safety Requirement (TSR) Section 2.4.4.2a defines the Limiting Condition for Operation (LCO) as " Criticality accident detection shall be operable" and since Cluster V does not have complete overlapping coverage from adjacent clusters, the LCO action steps as defined by the TSR Section 2.4.4.2 were implemented. Due to this "as found" inoperable condition, an event notification was made on March 4,1997, pursuant to 10CFR76 120(c)(2)(i).
An investigation team was established to determine the root cause and corrective actions that 2
wW prevent recurrence. The team focused on two primary areas: (1) determination of the proper setpoints for the detectors; and (2) determining the cause(s) of the changes in the detector 4
l readings.
The selection of the proper alarm setpoint for detectors is necessary to ensure that the detector j
will detect the level.of radiation generated during the minimum criticality accident of concern,
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while not generating excessive false audible alarms. The setpoint also determines the radius of i
i coverage for detection. While the detectors may still function for a nearby criticality, the distance at which the minimum criticality would be detected, decreases as the background i
reading decreases. The ' detectors have a fault' alarm; however it currently does not come in until
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- Docket No. 70-7001 Page 3 of 5 i
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- the reading falls to a nominal SmR/hr. At this level, the detectors would not alarm at a 10 mR/hr increase in radiation level (15 mR/hr total is less than the radiation alarm setpoint of 20 mR/hr) j
'An Engineering Evaluation is being prepared to support the changing of these setpoints to ensure the alarm / fault setpoint combination results in acceptable detection capability and timely
- awareness of detector modules which may not be able to meet their intended function. This -
awareness of a low background reading on a detector module will allow timely replacement of that single module, thereby reducing the probability of clusters being inoperable due to low background readings. In support of this Engineering Evaluation, Instrument Maintenance has completed testing to verify that a linear relationship exists between background settings and radiation levels. Additionally, the selection of the proper alarm setpoint and fault setpoint combination shall ensure that the dermed range of acceptable background readings has
' adequately considered equipment / calibration constraints and limits of error. Based on the hypothesis that variations in background readings currently being experienced are actually within the expected limitations of the equipment / calibrations, proper setpoint changes which increase L
the range of acceptable background readings while maintaining acceptable detectability may resolve the current problem. This will not be known until an Engineering Evaluation is complete to support the setpoint changes.
To evaluate possible cause(s) of fluctuations in background readings, several initiatives are being pursued concurrently. Data bases are being developed using data from both the weekly field monitoring of the CAAS clusters and from maintenance records in the Instrument Shop to provide trending of CAAS data by such fields as building and module serial number. Based on the results of this trending, additional testing will be defined, as required. The detector mcdules i
which exhibited the low readings were checked in the Instrument Maintenance shop; no cause of the low reading could be determined. Additionally, detector modules which have exhibited problems in the field were sent to the vendor for evaluation related to the low background incident. Trending data has also been provided to the vendor to assist in the troubleshooting. An environmental chamber is on order which will allow the Instrument Maintenance shop to do more extensive testing of detector modules under simulated conditions similar to the temperatures which may be experienced in the field. Issues which result from the above testing / trending initiatives will be tracked to completion through the Corrective Action Program.
, During the course of the investigation, another cluster was observed with two modules exhibiting i
background readings below 9.5 n'R/hr. On March 18,1997, at 1305, Cluster N in C-337A building was observed by the NRC Inspector with two detector modules reading 9.0 mR/hr (Detector Module Serial No. 580133 and Detector Module Serial No. 580142). Cluster N was
? declared inoperable and the the LCO action steps as defined by TSR Section 2.4.4.2 were implemented. Due to this "as found" inoperable condition, an event notification was made on March I8,1997, pursuant to 10CFR76120(c)(2)(i). The modules weie replaced by Instrument
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Docket No. 70 7001 Page 4 of 5 Maintenance and Cluster N was declared operable at 1710 on March I8,1997. The modules were returned to the Instrument Maintenance Shop for testing.
CAUSES OF EVENT
.The direct cause(s) and associated root cause(s) have not been determined at this time.
Therefore, this report will be supplemented when the root cause has been determined. Target date for a supplemental report is January 26,1998.
i CORRECTIVE ACTIONS A. Conective Actions Taken
- 1. On January 2,1997, Operations issued Long Term Orders to perform weekly checks of the CAAS clusters to check for low background readings. Modules with readings found below 9.5 mR/hr were to be promptly reported to the Plant Shift Superintendent and a work order issued for changeout.
- 2. On March 4,1997, Instrument Maintenance replaced two modules in Cluster V and the cluster was returned to an operable status at 2330.
- 3. On March 7,1997, Instrument Maintenance returned three detector modules to the vendor for further diagnostic tests and evaluation.
- 4. On March I8,1997, Instrument Maintenance replaced two modules in Cluster N and the cluster was returned to an operable status at 1710.
B. Corrective Actions Planned
- 1. By May 23,1997, Engineering will complete an Engineering Evaluation to support a change in CAAS detector module setpoints. Engineering will a'so generate required docu ientation to support associated procedure changes.
- 2. By September 23,1997, Instrument Maintenance will complete changes on se: points of all CAAS detector modules as defined in the Engineering Evaluation in Planned Corrective Action No.1. Affected Instiument Maintenance procedures will be' modified,-
. as required, prior to changing setpoints.
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- Docket No. 70-7001 Page 5 of 5
- 3. By December 19,1997, Engineering will complete an end-point assessment to determine
- the effectiveness of the corrective actions.
EXTENT OF EXPOSURE OF INDIVIDUALS TO RADIATION OR TO RADIOACTIVE MATERIALS No criticality accident occurred during the time period that the two detector modules were
. reading low. Therefore, there was no exposure ofindividuals to radiation as a result of this event.
t NUCI FAR SAFETY SIGNIFICANCE A low background reading on a detector module could prevent the module from alarming at a 10 mR/hr radiation increase. Two modules in a cluster with low background readings could prevent the cluster from alarming at a 10 mR/hr radiation increase. To reach the alarm setpoint of the claster, the detector would have to receive radiation which exceeded 10 mR/hr by an amount that would offset the low background reading. While the cluster may still detect and alarm upon a criticality, the distance at which it will detect the minimum criticality is reduced if the background reading drifts down.
LESSONS LEARNED The selection of the proper values for the fault and the alarm setpoints is necessary to ensure that the CAAS can (a) adequately detect the required radiation level; (b) provide indications when it cannot perform its intended function; and (c) minimize the number of false alann activations.
Proper values for fault and alarm setpoints must adequately consider equipment / calibration i
constraints and limits of error.
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Docket No. 70-7001 Page1of1-i
. Event Reports ER-97-02 and'ER-97-05 l
List of Commitments A. Corrective Actions Taken i _
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- 1. On January 2,1997, Operations issued Long Term Orders to perform weekly checks of the CAAS clusters to check for low background readings. Modules'with readings found i
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.below 9.5 mR/hr_were to be promptly reported to the Plant Shift Superintendent and e
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- work order issued for changeout.
i-2, On March 4,' 1997, Instrument Maintenance replaced two modules in Cluster V and the cluster was returned to an operable status at 2330.
- 3. On March 7,1997, Instrument Maintenance returned three detector modules to the vendor
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4 for further diagnostic tests and evaluation.
2 4. On March I8,1997, Instrument Maintenance replaced two modules in Cluster N and the cluster was returned to an operable status at 1710.
l B. Corrective Actions Planned
- l. By May 23,1997, Engineering will complete an Engineering Evaluation to support a i
. change in CAAS detector module setpoints. Engineering will also generate required documentation to support associated procedure changes.
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- 2. By September 23,1997, Instrument Maintenance will complete changes on setpoints of all CAAS detector modules as defined in the Engineering Evaluation in Planned i
4 Corrective Action No.1. Affected Instrument Maintenance procedures will be modified, as required, prior to changing setpoints.
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- 3. By December 19,1997, Engineering will complete an end-point assessment to determine the effectiveness of the corrective actions.
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