ML20028H705
| ML20028H705 | |
| Person / Time | |
|---|---|
| Site: | Maine Yankee |
| Issue date: | 01/19/1991 |
| From: | Nichols S Maine Yankee |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| MN-91-17, SEN-91-26, NUDOCS 9101290021 | |
| Download: ML20028H705 (8) | |
Text
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- I MaineYankee y tjaM. @ W:ys qpi.Qivn M LDiLO4 DRfVI
- AUGUSTA. MA!NE 04330 * (200 602 4MS January 19, 1991 MN-91-17 SEN-91-26 4
UNITED STATES NUCLEAR REGULATORY COMMISSION Attention:
Document Control Desk Washington, D. C.
20555
References:
(a)
License No. DPR-36 (Docket No. 50-309)
(b)
USNRC Letter to MYAPCo. dated November 26, 1986 (c)
MYAPCo. Letter to USNRC dated January 23,1987(MN-87-09)
(d)
USNRC Letter to MYAPCo. dated April 27, 1987 (e) MYAPCo. Letter to USNRC dated March 28, 1988 (MN-88-3.?)
(f) USNRC Letter to MYAPCo. dated July 7, 1988 (g) MYAPCo. Letter to USNRC dated February 15, 1989 (h) VSt!RC Letter to MYAPCo. dated April 24, 1989 (i) USNRC Letter to MYAPCo. dated June 7, 1989
Subject:
Maine Yankee inadequate Core Cooling Instrumentation System Gentlemen:
With Reference (f), the NRC Staff and its Technical Assistance Contractor approved the design of Maine Yankee's inadequate Core Cooling instrumentation (ICCl) system contingent upon completion of three items.
The NRC requirement and Maine Yankee's status of each item is as follows:
HRC Recuirement:
Emergency procedures and operator training in the use of the 1C01 system have not been reviewed by the staff, thuntlankee Status:
Emergency procedures which reflect the 1988 refueling shutdown modifications to the ICCI system were prepared, reviewed, and approved during the course of the refueling shutdown, and implemented at the completion of that shutdown. Operator training on the use of the ICCI system was completed in February 1989.
Training records and Emergency Operating Procedures have been reviewed by the staff and the inspection findings were transmitted by Reference (i).
NRC Requirement:
b 7,
Technical Specifications for the ICCI system have not been submitted and reviewed by p
the staff.
rD V
Maine Yankee Status:
i Technical Specifications for the ICCI system were submitted with Reference (g) and approved by the staff with Reference (h),
9101290021 910119
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PDR ADOCK 05000309 0
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[dRflieYarilcee UNITED STATES NUCLEAR REGULATORf COMMISSION MN-91--17 Attention: Document Control Desk Page Two NRC Recitirgstni:
An inplementation letter report as required by item II.F.2 of NVREG-0737 must be submitted by MYAPCo (see attichment to Reference (f) for details of implementation l e t t.c r).
Maine fankee Status:
1hc Maine Yankee loplementation Letter Report consistent with the NRC implementation letter report requirements of the attachment to Enclosure (1) of Reference (f) is provided as Attachment A.
Please contact us should you have any questions regarding this matter.
Very truly yours, h/!hfcb b S. E. Nichols, Manager Nuclear Engineering & Licensing SEN:SJJ Attachment c:
Mr. Thomas T. Martin Mr,. (.. H. Trottier Mr. Charles S. Marschall SEN9126.LTR
Mtachment A Maine Yankee ICCI System Implement _ation letter Repatt 1.0 Sy1Let'L(pmole t io!!
The Maine Yankee inadequate Core Cooling System (ICCl) was upgraded / installed during the refueling outage ending in December 1988. -The ICCI Systems, consisting of the Core Exit Thermocouples (CETs), Saturation Margin Monitor (SMM), and Primary Inventory Trend Systems (PITS) have been calibrated and functionally tested as required.
Test results are available on-site for NRC Inspection.
Based on the calibration and functional test results, the ICCI System performs according to design.
2.0 ICCI System Calibrationtfunctional Testino Results 1C01 System Calibration / functional Testing consisted of:
a.
Performing-individual component calibration; b.
Following component calibration, inputs were simulated to'the ins _trument calculating subsystem and all outputs and alarms were verified.
This testing encompassed as much of the instrument system as is practical.
Inputs were simulated over their full range and inputs were provided that simulated the outputs over their full range. This testing verified items such ast 1.
The ability of the PITS, SMM and CET calculating circuitry to provide the required outputs for a given se of input conditions inclusive of recorders, indicators, and computer inputs.
2.
The ability to provide the required alarms.
3.
The ability of the switching circuit used to automatically shift.the i
PITS from CET density compensation (used for subcooled and saturated conditions) to pressurizer compensation (used for superheat d
conditions).
I 4.
No interactive effects on alternate channels within the same system.
System input instrumentation loop testing to-insure:
c.
1.
Correct sensor response to known inputs; 2.
Demonstration of loop integrity; 3.
No interaction existed between input channels; and 4.
Inputs common to PITS and SMM interfaced properly.
SEN9126.LTR 1
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4 This testing overlapped with subsystem input simulation testino to insure the total system was completely tested in a logical sequence.
d.
System level testing demonstrating that:
1.
Each channel's Cris could be selected and displayeu in less than three minutes (less than six minutes for Doth channels) as per NUREG-0737, Section ll.f 2.-
2.
The ability of the PITS sensors to measure a known reactor vessel water level.
3.
PITS test circuit interfaces and alarms.
l The results of calibration and functional tests were compared against predetermined test acceptance criteria to demonstrate that the ICCI system performed in accordance with design expectations.
3.0 Desian and Deviation Descriptions The following provides a brief summary of each system design and a description l
of any changes from previous design descriptions, j
- .1 Core Exit Thermocouples (CETs) Desian Summary Maine Yankee's design provides for 45 Type K CETs in a bottom entry configuration. The CETs are located at the top of the core fuel alignment plate measuring the temperature of core exit flow.
Sixteen of the CFTs are used in the ICCI design.
T5ese CEis have been separated into two separate channels.
Two CETs in each channel measure temper'ture in each core quadrant providing up to four qualified thermocouples
, nuadrant.
l A selector switch at the Main Control Board (MCB) for each cr.>
al allows the operator to seloct CETs one at a time for digital display on the MCD.
The operator can select and monitor all eight CETs per channel in less than three minutes, less than six minutes for both channels, as required by NUREG-0737. The selected CET from each channel is also inputted to the associated channel of SMM and PITS for determination of margin to saturation and vessel fluid density. CETs representing average core exit temperature are selected by operators for input to the SMM and PITS. The CET orirery display is also available via the plant computer and SPDS j
thrcuss electrical isolation devices where required.
3.2 CET Desian Chanaes/Additiona1'Information a.
CET Nll (Channel A, Quadrant Ill) is currently unavailable to measure core exit temperature.
This is not a significant concern since 15 other CETs are available for display on the MCB and an additional 28 l
CETs are avaliable via the pltnt computer and SPDS.
Maine Yankee l
plans to fill CET position Hll with a new qualified CET at a future refueling outage.
1 i
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i b.
The CET uncertainties were recalculated in: February 1989.using _the-1 0.oot-Sum-Sqeare=(RSS) method. This changed the total uncertainty of the CETir.n w aments from 12.2'F to il6.7'F.- This small change in uncertaint-s acceptable, = has been included in-the overall-uncertainty a the SMM and PITS channels and can be accountea for in the EOPs. - -CET calibration uncertainties _ are scheduled to: be addrersed in 1991' as part of Maine Yankee's instrumentttion awuracy and setpoint uncertainty program.-
c.
It was not possible to color code each channel of CETs on the SPOS CET map because of computer limit:tiens.
The purpose of the color coding was to improve identification of Channel-A CETs versus Channel B CETs.
The color coding is considered optional-and not a requirement for 1C01.
3.3 Saturation Marain Monitor (SMM) Desian Summary Maine Yankee's_-SMM System-consists of two separate microcomputer-based' modules which use CET and pressurizer r 7ssure _ inputs to = provide-a-continuous indication of vessel core region temperature. margin toi saturation. These two independent channels _are ranged to monitor between 200'F subcooling to 50*F uperheat._ Each channel receives up to eight1 selectable CET inputs (two frum each quadrant) and one pressurizcr (RCS) pressure input.
The-SMM_ provides displays at the HCB nd-on the SPDS. -
SPDS displays in the core cooling andLinventory Critical Safety Function (CSF) displays provide both digital-and bargraph information on the SMMs.
3.4 SMM Desian Chances / Additional Informatl2D a.
A temperature input from the react r hee neton-is available to each SMM channel for determining saturation margin in the; head region.
Head region SMM is not required by NUREG-0737;-therefore, this design feature is considered optional-and not part of the_iCCI' System.
b.
Based on the combined uncertainty _ of' the temperature and pressure inputs, and the SMM calculator modules ~, the revised uncertainties-associated with the SMM channels:are:
i Noret
,onditions: _ i21.0* F LOO.onditions:
i31.6'F (at 532*F).toli 73.0*F (at 340'F)-
The SMM channel; uncertainties have increasec' from 13.7*F to 21.0*F (normal containment) and from i37'F to 73.0*F ;(post-LOCA cor litionst at CET temperature of 340*F). The normal uncertainty _ increase can be read 1_ly accommodatedL by modifying -the xE0P setpoints.
However,-
applying - the higherJ post-LOCA uncertainties to the E0P setpoints
- would result inipotentially unachievable subc.ooling requirements. Of -
particular consequence would be an ' inability. to restart 1 Reactor" Coolant Pumps (RCPs),. an inability:to -throttle Safety injection (SI), _
and conflicts between the Low-Temperature Overpressure; Protection j
(LTOP)'dcoldown limits t.nd subcool-ing setpoints.
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g a
Currently Maine Yankee uses the post-LOCA uncertainty of 31.6'F at 532'F based on CET temperatures for hot standby rather than applying the 73.0'F to the entire LOCA temperature range.
Use of this uncertainty provides an adequately conservative setpoint for the majority of times the E0Ps are required where operators are directed to bring-the plant to hot standby.
However, for the improbable situation where the SMM is required during a post-LOCA condition, the RCS temperature is below 532'F and the instrument is actually inaccurate by greater than the 31.6'F uncertainty, the potential exists for the operators to think that the RCS is subcooled when, in fact, it is saturated or even superheated at the CETs.
The implications of this situation are not-expected to be significant since the operator will have alternate indication of vessel level and CET superheat (alternate _ rain of SMM) to assure.that the core is covered. The revised SMM uncertainties will be incorporated into the E0P setpoints document and the E0Ps by March 31, 1991.
c.
Tt.a revised uncertainties of the individual temperature and pressure inputs to the SMM are:
CETs:
il6.7*F (normal and LOCA)
Pressurizer Pressure:
148.7 psi (r.m mal)
+163.7 psi, -160.9 psi (LOCA) 3.5 Primary Inventory Tran_d System (PITS) Desian Summa,ty The PITS is an environmentally and seismically qualified, Class lE system consisting of redundant, independently powered channels.
The system is capable of providing an indication of reactor vessel level within the calculated uncertainties during a small break plant transient with the reactor coolant pumps shut'off.
Each PITS channel is ranged to measure hydrostatic head (dp) between the top and bottom of the reactor vessel. Both channels use a common sensing line. A seal pot at the top of the reference leg provides a reservoir for thermal expansion and= contraction of the reference leg water -and for displacement of dissolved gases.
The dp measarements_ are automatically compensated for average densit,y in the reactor vessel and sensing line to l
provide a display of equivalent collapsed liquid' level.
Density compensation is performed by Foxboroi Spec -200 microprocessors which calculate the specific gravity of the_ reactor vessel and censir.g 11.0 fluids. Reactor vessel liquid and steam density compensation are based on-l saturation density from CET temperature -when subcooled and saturated conditions exist.
If the core becomes uncovered and superheated conditions occur at the core exit, steam and-liquid densities are l-approximated as saturation densities using a pressurizer pressure input.
The sensing line density is approximated as saturated density using temperature measurements from RTDs attached to the sensing line.
Up to three RTDs per channel are used to monitor the sensing line temperature.
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3,6 plTS Desian Chanoes/ Additional Informati_qn d
~
Each PITS channel is displayed on _the MCB by separate and independent a.
level indicators scaled for 0%- to 100% c level in ~ 2% increments, included on the scale = and also on the MCB-reactor vessel mimic are plant elevations corresponding-to top of vessel, top of hot legs, and-i top / bottom of the reactor core, For trending purposes, level:
{
readings are recorded on separate single pen recordtrs located in each auxiliary logic cabinet.in-the Main Control-Room, b.
A test switch has been installed across each dp transmitter to permit a channel check while the -plant-is running (RCPs on). - When-in-the test position, the transmitter circuit is-open, and a : diode and i
resistor network limits the current -into the foxboro microprocessor to simulate a _ known dp_ (level) in the reactor vessel, ALreference curve is used to verify the correctness of the level reading based on the measured core exit temperature.
The test switches are mounted directly below their associated channel indicator on the MCB, An annunciator' alarm is energized to inform operators when the associated level channel is-inl test, A heatup evaluation was-performed for the portion _of sensing _line' c.
located in.the reactor cavity.
This-portion of the seraing line is-subject to heatup - from the thermal Einertia of the reactor vessel l
shell and loop piping.
The purpose of the evaluation' was to j
calculate the worst case temperature in the cavity compartment (and sensing line) to deter.nine the amount-of overpressure needed to keep 1
the water inside.the line from boiling-(subcooled)' during a small break l.0CA in this evaluation, the temperature in - the cavity 'is maximized by -
assuming no credit for the Containment Ventilation System which normally supplies sair flow to the cavity or for containment spray flow. -Heat sources in the cavit the hot legs and the cold legs,y compartment are the vessel shell, and the bulk-containment accident-temperature.
Conservative fluid tempe, 'ures,__ which represent conditions of inadequate core cooling, were' sumed in-the vessel and loop piping,-
4
' Natural convection flow develops through the reactor cavity from the -
incore instrument access shaft and up alongside the reactor vessel,-
Convective flow exits: the cavity throuah a 2 circumferential. gap ~
between the supplementary neutron; shielding aM the-floor--of the refueling ccvity, If the incore instrument = sump = is flooded,-
convective-flow i_s blocked and cooling occurs by evaporation of the water in the sump, The most. limiting condition occurs when-the sump is blocked.due!to-containment flooding.: T. a flood water is conservatively : assumed 'tol be 150'F subcooled liquio at an elevation high enoegh to block the-
-convecti_on path, but not high enough to submerse and quench the lower t
portion of the vessel, SEN9126,LTR 5
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Analyzing this conservative scenario indicates that the maximum expected temperature of the cavity and associated section of PITS sensing line is approximately 332*F occurring at about three hocrs into the scenario. Consequently, a pressure of greater than 91 psig (106 psia) would be necessary to keep the sensing line subcooled.
Based on the conclusions of the heatup evaluation, boi~ ling of the reference leg will not occur during accident coiditions if any one of the following conditions exist:
1.
Containment Ventilation System is providing ventilation to the cavity.
2.
Containment Spray System is spraying containment (CSAS).
3.
Actual RCS pressure is equal to or greater than 110 psig.
These operational requirements for PITS have been labeled on the MCB directly under the level indicators to inform operators of the limitations on PITS use.
The purpose of the PITS is to indicate reactor verel ievel during a small break and/or TMI event.- Limiting the uw of PITS to these conditions does not detract from its intended function, d.
E0P/FRP reactor level verification points have been established and calculations performed to determine the design uncertainty for these verification points.
Use of the E0P/FRP level verification points and the associated uncertainty has been reviewed to ensure adequacy of the PITS-EOP interface.
4.0 Technical Specifications Modification of Maine Yankee Technical Specifications to include all ICC instrumentation for accident monitoring has been requested and approved.
5.0 NRC Aporoval of installation Maine Yankee's ICCI system has been found _to be acceptable contingent upon NRC approval of this response to Reference. (f).
6.0 E0Ps Maine Yankee E0Ps used for operator training conform to the technical content of the NRC approved E0P guidelines.
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