IR 05000336/1991006
| ML20024G901 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 04/18/1991 |
| From: | Eapen P, Moy D NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I) |
| To: | |
| Shared Package | |
| ML20024G900 | List: |
| References | |
| 50-336-91-06, 50-336-91-6, GL-88-17, NUDOCS 9105020067 | |
| Download: ML20024G901 (14) | |
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U. S. NUCLEAR REGULATORY COhihilSSION i
REGION I
ReprtNo.
50 336/91-06 Docket No.
50-336
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License No.- DPR 65 Licensee:
Northeast Nuclear Energy Company r
P. O. Box 270 Hartford. Connecticut 06141 Facility Namei Millstone Nuclear Power Station. Unit 2 Inspection At:
Walttford. Connectici!t Inspection Conducted:-
February 26 - hf arch 1. J20],
inspector:
M Of/
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D. T. Moy, Keacty Eqkineer, Systems h
dafe Section, Engineerw + ine'i, DRS
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Approved by:
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Dr. P. K. itpen, CtWef, Systems Section, -
date Engineering Branch, DRS
-Inspection Summary: Routine announced safety inspection was performed at Millstone 2 Nuclear Plant Station on February 26 - March 1,1991.
r Areas Inspected: ' Licensee actions in response to Generic Letter 8817, " Loss of Decay Heat Removal" during non-power operation. The inspection reviewed,mid loop operating
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L procedures, instrumentation, plant hardware modification and thermal hydraulic analysis as
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related to reduced reactor coolant system inventory operation.
insoectionLResults: The Generic Letter 88-17 recommendations were implemented adequately-at Millstone 2 Nuclear Station. The licensee has implemented adequate measures to prevent
loss of decay heat removal during non pot or operation.
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No violations or unresolved items were identified,
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DETAILS-1.0 Person Contracted 1.1 Northeast Utility
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- S. Brinkman, Operation Engineer, h!P2 J. Keenan, Director, hip 2 S. Scace, Director, hiillstone Station Site
- J. Smith, Operation hianager, hip 2 1.2 U. S. Nuclear Regulatory Commission
- P. Habighorst, Resident inspector, h1P2
- Denotes those attending the exit meeting on 3/1/91.
The inspector also contacted additional administrative, licensing and engineering technical personnel during the mid loop inspection.
r 2.0 Lang_Trra"regrammed Enhancement for Generic Letter (GL) 8817. "Lesi.nLDkGly ikat Removd Non-Power Ooeration Loss of Decay Heat Removal (DHR) during non power operation and the consequences of such a loss are of significant safety concern. hiany events of loss of
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DHR have occurred while the RCS has been drained down for such mid loop activities as steam generator inspection and repair of a reactor coolant pump. These activities are often in progress when the reactor coolant system and the containment are not secured.
The seriousness and continuation of these problems resulted in issuance of Generic Letter, GL 88-17 " Loss of Residual Heat Removal (RHR) while the Reactor Coolant System (RCS) is Partially Filled." This letter was issued to all licensees of operating PWRs and holders of construction permits on July 9,1987,. GL 8817 required the recipients to respond with two plans of actions:
a.
A short-term program entitled " expeditious actions" that was essentially limited to reduced inventory conditions, b.
A longer term program entitled " programmed enhancements".
The Generic letter stated that the programmed enhancements consisting of hardware
l installation and/or modification, and programmed enhancements that depend upon
hardware installation and modification, should be implemented:
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a.
by the end of the first refueling outage that is initiated 18 months or later following receipt of the GL, or b.
by the end of the second refueling outage following receipt of the GL,
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whichever occurs first. If a shutdown for refueling has been inhiated as of the
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date of receipt of this letter, that is to be counted as the first refueling outage.
Programmed Enhancements that do not depend upon hardware changes were to be implemented within 18 months of receipt of the Generic Letter.
The licensee provided the responses for " Expeditious Actions" and " Programmed Enhancements" respectively in letters dated December 23,1988 and January 31,1989.
In addition to above responses, the licensee revised the response to the Expeditious Action item 4 (Programmed Enhancement item 1) on January 30,1991. The purpose of this revision was to describe in detail the hiillstone Unit No.2 reactor water level indication system, shutdown cooling instrumentation upgrades, and performance
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monitoring that will be used to monitor the reactor coolant system level during a reduced inventory condition.
The NRC reviewed the above responses and documented the conclusion in NRC letters dated hiay 8,1989 and hiay 29,1990. The NRC reviewed the licensee's short terin or expeditious actions program as detailed in Inspection Reports 50-336/88-28 and 50-336/89 08.
The purpose of this inspection was to assess the adequacy of the licensee's long term l
program for Generic Letter 8817, as detailed below:
i 2.1 Instrumentation for hiid Looo Ooeration
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l Generic Letter 8817 provided the following recommendations for instrumentation for hiid Loop operation:
Provide reliable indications of parameters that describe the state of the RCS and the performance of systems normally used to cool the RCS for both normal and accident conditions. At a minimum, provide the following in the Control Room:
a.
two independent RCS level indications l
b.
at least two independent temperature measurements representative of the core exit whenever the Reacte Vessel (RV) head is in place.
the capability of cont auously monitoring DHR system performance
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whenever a DHR systern is being used for cooling the RCS
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visible and audible indications of abnormal conditions in temperature, level, and DHR system performance.
The licensee's response to the above recommendation as stated in their letter,
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dated January 31,1991, was:
" Northeast Nuclear Energy Company (NNECO) will maintain a minimum of two independent, continuous RCS water level indications operable whenever the RCS is in a reduced inventory condition.
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Instrumentation utilized to provide RCS water level indication will be as follows-1.
A single channel of RVLMS shall be operable, providing the operator with a continuous display of reactor vessel level at eight discrete points above the core. Three discrete points corresponding to the top, centerline, and bottom of the hot leg provide accurate indication of level regardless of RCS pressure perturbations. The RVLMS data will be continuously displayed in the control room.
2.
NNECO will install continuous level detection lastrumentation which will monitor hot leg water level and provide control room indication and alarm functions. These level monitoring devices will be independent, electronic-based instruments urzing an external standpipe mounting configuration."
Insoection Finding Three independent level transmitters are utilized for level indication, level trending and level alarming. These transmitters are installed to continuously monitor RCS level from the reactor vessel flange to the bottom of the hot leg.
One continuous level detection system is located in the drain line piping of the RCS No.1 Hot Leg coolant loop. The reduced RCS level alarm alerts the operator that level is approaching reduced inventory condition; i.e.,
approximately three (3) feet below the reactor vessel flange. The loop I hot leg level low alarm alerts the operator that the RCS hot leg has fallen to a point where air entrainment and possibly the onset of vortexing is likely at shutdown cooling (SDC) flows larger than 1200 GPM. At the same source location as above, local indication is provided via site glass level indicator.
This level indicator is continuously monitored via camera on a control room remote TV monitor and can be used for cross channel checks of the other RCS level sensors. This level indicator consists of a float in a standpipe. Indication is provided by high visibility magnetic ' flags' attached to the standpipe which are positioned depending on the elevation of the internal float.
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DHR System Performance Monitoring The licensee has developed an Integrated Computer System (ICS) Performance Monitoring software package. This package was installed in the MP2 process computer to monitor and trend SDC performance. It will continuously monitor nine parameters associated with Shutdown Cooling. Each of the nine parameters are scanned every second and stored in a one hour historical F'..
These functional parameters are:
SDC level indication
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Trending of SDC level indication
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System constant definitions
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Flange to mid loop reference
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SDC/P&lD display
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SDC pump status
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SDC data archive function
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SDC calculated value
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The ICS will provide appropriate displays for continuous monitoring of Shutdown Cooling System performance parameters, RCS level, and RCS core exit temperature during reduced inventory or mid loop operation. Trending is also available on demand, as required by the operator.
The inspector reviewed the implementation of the ICS monitoring software package and found it to be technically adequate for the mid loop operation.
Core Fait Thermocouple Trmoerature Indications As stated in the licensee's response, dated January 31, 1989, NNECO plans to maintain two independent channels of the reactor vessel level monitoring system (RVLMS) operable when the RCS is in a mid loop condition with the reactor vessel head located on top of the reactor vessel.
The Millstone Unit No. 2 RVLMS consists of a heated thermocouple pair at eight discrete levels above the core. When drained to mid loop conditions, four of the eight pairs of thermocouples are in contact with RCS inventory.
This provides temperature indications that are representative of the core exit conditions. NNECO considers the intent of this recommendation satisfied, provided one unheated thermocouple is operable in each channel of RVLMS.
A continuous display of each pair of RVLMS thermocouples will be available in the control room. Additionally, core exit thermocouples (CET) could be utilized to satisfy this recommendatio i
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jnspection Findings The inspector verified that for mid loop operation, the licensee has taken adequate idministrative and procedural steps (Station Procedure.OP 230lE, Rev.15, Step 5.1.lc) to provide at least two independent continuous coolant temperaturt indicators that are representative of the core exit conditions. The licensee mmitors the core exit temperature using thermocouples. Both
" inadequate, core cooling cabinets" will be operable during mid loop operation.
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The plant operating procedure requires the operator t., record both RCS level and temperature hourly when the vesselis drained belo e the reactor vessel flange for reduced inventory operation (OP 230lE, Rev.15, a note prior to procedure step 5.1.6.1).
2.2 Review of Mid loon Ooeratine Procedutts
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Generic Letter 88-17 recommended the development and implementation of
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procedures to cover reduced inventory operation and to provide an adequate basis for entry into a reduced inventory condition. These include:
procedures that cover normal operation of the NSSS, the containment.
a.
and supporting systems under conditions for which cooling would normally be provided by DHR systems.
b.
procedures that cover emergency, abnormal, off normal or the equivalent operation of the NSSS, the containment, and supporting systems if an off normal condition occurs while operating under conditions for which cooling would normally be provided by DHR
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l systems, administrative controls that support and supplement the procedures in c.
items above, and all other actions identified in this communication.
The licensee provided the following respor.se in this regard:
1.
At Millstone Unit No. 2, operation with the RCS in a reduced
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inventory condition is governed primarily by two procedures. These procedures are: OP 2301E, " Draining the Reactor Coolant System,"
and OP 2310, " Shutdown Cooling System." OP 2310E has been revised, and OP 2310 will be revised to incorporate GL 88-17 recoinmendations prior to the start of the next refueling outage which is scheduled to begin on February 4,1989.
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A procedure covering abnormal / emergency operation of NSSS, containment, and support systems also currently exists at Millstone Unit No. 2. The development of AOP 2572, ' Loss of Shutdown Cooling,"
began near the issuance of, but independelt of, GL 8712, This procedure has been in effect since October 20,1988. An'y additional recommendations contained in GL 8817 not previously addressed in the procedure will be incorporated prior to the apcoming refueling outage.
3.
The necessary administrative controls to support reduced invertory RCS operations and supniement procedure revisic ns discussed above have been drafted and will be issued prior to the start of the next refueling outage, inspection Finding The inspector selected the following mid loop plant operating procedure for review; Steion Procedure, OP 3270, Rev.16, Plant Cool down
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Sta. in Procedure, OP 2301E, Rev.15, Draining the RCS
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Static Procedure, OP 2301, Rev.14, Shutdown Cooling System
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These procedures provide instructions for operations and surveillance whenever there is fuel in the reactor vessel and the reactor coolant systc n is in a reduced inventory condition (36 inches below the top of reactor Gange or 43 inches above the mid plane of the hot leg).
For each of the above station operating procedures, the inspector verified that the precautions, limitations, and entry conditions to mid loop operation were adequately stated.
As recommended by Plant Engineering during mid loop operation to prevent air entrainment (vortex formation) into the SDC suction piping, the Mil' stone 2 operating limit requires that SDC flow be restricted to no more than 1200 GPM when RCS is drained down to mid loop. As required by the station operating procedures, all mid loop procedures should be implemented prior to
- draining RCS water level down to mid loop level.
As stated by the licensee, the station procedures for mid loop operation were developed using th: guidelines from nuclear industry and plant experience.
Prior to draining to three (3) feet below reactor vessel flange, the following must be assured:
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At least two channels of RVLMS unheated junction thermocouples are
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in contact with RCS inventory and is being monitored in control room ICC RCS level display for RCS temperature.
Hot leg no. 2 RCS narrow range mid loop level instrumentation is
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At least two continuous reactor coolant system water level indications.
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One high pressure safety injection pump and at least two charging
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pumps must be operable.
Communication between the control room and auxiliary operator in
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containment will be established.
The inspector found the above to be technically adequate and responsive to the recommendations stated in Generic T etter 88-17.
2,3 Beview of RCS Inventory Addition Generic 1.xtter 8817 recommended the following in this regard; a.
Provide adequate operating, operable, and/or available equipment of high reliability for cooling the RCS and for avoiding a loss of RCS cooling.
b.
Maintain suffielent existing equipment in an operable or available status so as to mitigate the loss of DHR or loss of RCS inventory, should they occur. This should include at least one high pressure injection pump and one other systcm. The water addition rate provided by system shouM be at least sufficient to keep the core covered.
c.
Provide adequate equipment for personnel communications that involve activities related to the RCS necessary to maintain the RCS in a stable and controlled condition.
The licensee's response to above recommendations was:
Prior to the start of the next refueling outage, all procedure revisions and administrative controls will be in place to ensure continued reliability of the SDC systems and the availability of event mitigating equipment. As previously discussed in our response to the expeditious actions, Millstone Unit No. 2 will maintain one HPSI pump and sufficient charging pumps available for event mitigation.
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Inspectio i Findings The inspector verified that the licensee has procedures and administrative control (OP 2301E, Rey,15, Step 3.11.2 & 3.11.3) to provide at least two j
adequate means of adding inventory to the RCS in addition to other components that are a part of the normal DHR systems. One source of inventory makeup is the high pressure safety injection pumps with suction from the refueling water storage tank. The RWST is used to provide a sufficient upply of borated water to the safety injection, charging and shutdown cooling pumps during the injection mode of ECCS operation. The injection now into the RCS via the safety injection system is approximately 315 GPM per pump, (a total of two pumps for safety injection). The second source of makeup is the low pressure safety injection system.
The inspector verified that each injection flow is higher than the core boil off rate and will provide sufficient flow to keep the core covered during mid loop operation.
The inspector further verified the operability of the additional inventory now path components by reviewing the surveillance testing reports. All of these surveillance tests are required per Section 4 of the licensee's Technical Specification. These reports document the details of the tests conducted to meet the technical specifications.
The selected sample of technical reports were:
High pressure safety injection pump operability test data, dated
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12/20/89.
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Charging pump operability test, dated 12/27/89.
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High pressure safety injection system valve alignment, dated 4/20/89.
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Low pressure safety injection system electrical alignment check, dated 1/3/90.
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Low pressure safety injection valve alignment verification, dated
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1/3/90.
Low pressure safety injection and safety injection tank operability test,
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dated 5/16/90.
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The inspector found the licensee's actions to be consistent with the guidelines of GL 88-17 and had no further concerns in this regard.
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2.4 Review of Thermal Hydraulie Analysis The Generic letter recommended that licensees conduct analyses to supplement existing information and develop a basis for procedures, instrumentation installation, and NSSS interactions. The analyses should encom* pass thermodynamic and,hysical states to which the hardware can be subjected and
should provide sufficient basis. Emphasis should be placed upon obtaining a
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complete understanding of NSSS behavior under non-power operation.
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The licensee provided the followir.g response for this recommendation:
Analyses have entrently been performed (RCS hot side vent area and makeup flow rate requirements) to support procedure improvements relating to loss of SDC scenarios. Additional analyses will be performed as necessary to support procedure improvements relating to operation at reduced inventory conditions and loss of SDC scenarios. We believe that the procedure improvements will further minimize the potential for a loss of SDC event and will minimize the consequences of postulated loss of SDC events.
Insoeetion Findings The inspector reviewed the thermal hydraulic analyses, performed by the licensee to predict RCS behavior following the loss of SDC system cooling during mid loop operation. The analyses predict the time to core heat up and
. core boil off rates, the time to core uncovery for various RCS configurations, and the maximum RHR draindown rate as a function of reactor cavity water level.
' The inspector selected the following thermal hydraulic calculations for mid loop operation review:
1.
Calculation No. W2 517-821-RE. Rev. O, " Time to Bulk Boiling and Core Uncovery for h1P2-Loss of RHR with'a Partially Filled RCS,"
dated 9/18/1987, 2.
Calculation No. W2-517-889 RE, Rev, 2, "h1P2 Loss of Shutdown Cooling -Vent Area / Time after Shutdown Requirement," dated
3/6/1989.
3.
Calculation No. 89 033-718Ghi, Rev.1, "hlillstone Unit 2, Determine the available NPSH for the LPSI pumps during mid loop operation and
. minimum height above the LPSI inlet pipe which would prevent air entrainment," dated 8/17/1990.
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At the time of the inspection, Calculation W2-517 821RE, Rev. O and W2-517 889RE, Rev. 2 were being updated to Rev. I and Rev. 3, respectively, for the outage in 1992.
Based on the review of the above thermal analysis and vortex / air entrainment analysis done by the licensee, the inspector noted that the potential for air entrainment due to vortexing increases significantly as the reactor coolant system level is reduced below the center line of the hot leg (mid loop level).
J Based on the above analyses, the licensee established the following operating
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The RCS level must not be reduced below the center line of the hot leg l
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until shutdown cooling inventory flow has been reduced to equal or less than approximately 1200 GPM (Engineering Calculation No. 89 033-718GM).
Due to the elevation of the top of the shutdown cooling suetion piping
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line with respect to the center line of the hot leg (minus sixteen inches),
the shutdown cooling system should not be operated with RCS level greater than three (3) inches below centerline of the hot leg.
Alarms are set three inches below the mid loop for hot leg level low indications for loops 1 and 2. If the RCS must be drained greater than minus three (-3) inches below the hot leg centerline, full core off load is required per Station Operating Procedu ' OP 2301E,
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The inspector reviewed the above calculations and concluded that all three thermal hydraulic analyses were technically adequate.
No errors or derivations were noted.
2.5 Reactor coolant Synem Perturbation Generic Letter 88-17 recommended that the licensec should consider training, procedures, and controls that reasonably avoid perturbing activities when RCS inventory is low and decay heat is high.
The licensee stated that NNECO has committed to various procedural and administrative changes concerning RCS perturbations. These procedures will specify allowable evolutions. Any perturbations not specifically included will require additional management review and approval. These procedures and administrative controls will be revised or appropriate to incorporate lessons learned from the use.
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Inspection Findings i
The inspector verined that the licensee has implemented procedures and administrative controls to avoid operations that cause RCS perturbations.
Abnormal Plant Operating procedure AOP 2572, Rev. 3, lists the following activities which could cause perturbation of the RCS:
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Any cycling of vent and/or drain valve connected to RCS
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Changing major input to RCS,i.e., charging / letdown, SDC Dow. Minor
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changing, i.e., RCP seal injection, may be made if done slowly and compensated to control vessel level Low or oscillating SDC Dow
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Significant RCS temperature variation, and
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any instrumentation and control maintenance or testing affectmg
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charging, letdown, and SDC operation or interlocks to prevent affecting mid loop RCS level instruments Based on the system walkdowns with the reactor operators, the inspector concluded that the operators are adequately trained to preclude unnecessary RCS perturbations. The inspector interviewed the training supervisor, verified the content of mid loop operation training and concluded that the Decay Heat Removal training for non power operation is adequate.
2.6 Technical Snecifications Generic Letter 88-17 recommended that any Technical Specification that restrict or limit the safety benefit of the actions identined in Generic Letter should be identified and appropriate changes should be submitted to NRC for approval.
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The licensee's response to this was:
The Millstone Unit No. 2 Technical Speci0 cations are being reviewed to determine if any changes are required that will enhance the reliability and availability of the RHR system and provide additional sources of inventory makeup in the event of loss of RHR system while in a partially drained condition. Appropriate Technical Specification changes will be submitted to the NRC for approval.
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i Inspection Findings At the time of this inspection, the licensee completed the review, identified the technical specincations requiring changes, and was in the process of preparing the technical specincation change request to the NRC. The inspector found this action to be consistent with the licensee's commitment in this regard.
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3.0 Encineering Support for Mid Imn Opentlign I
Licensee management support was evident for the activities related to mid loop operation. The activities were detailed and technically sound. The site engineering
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personnel and first line supervisors were knowledgeable in the safety concerns and regulatory position discussed in Generic Letter 88-17. The inspector concluded that engineering support for mid loop operation was adequate.
4.0 Plant Tours The inspector toured the Millstone 2 plant, including the control room, safety, and injection pump rooms to observe any work in process, housekeeping and cleanliness.
No unacceptable conditions were found during the plant walkdown.
5.0 Management Meeting On March 1,1991, an exit interview was conducted with the licensee's senior site representatives (denoted in Section 1) to summarire the observations and conclusions of this inspection. The licensee did not indicate that this inspection involved any i
proprietary information.
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