ML20093D174
| ML20093D174 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 09/28/1984 |
| From: | Brandon R, Gridley R GENERAL ELECTRIC CO. |
| To: | |
| Shared Package | |
| ML112991306 | List: |
| References | |
| NEDC-30942, NUDOCS 8410110129 | |
| Download: ML20093D174 (75) | |
Text
s 1.
NEDC-30492 Class 1 September 1984 GENERAL ELECTRIC BWR LICENSING REPORT:
AVERAGE POWER RANGE MONITOR, ROD BLOCK MONITOR AND TECHNICAL SPECIFICATION IMPROVEMENT (ARTS) PROGRAM FOR MONTICELLO NUCLEAR GENERATING PLANT Approved:
Arb Approved:
9-2,7-m R. J/Krandon, Manager R. I/. Gridlef, Manager
' NucYear Services Engineering Fuel and Seriicos Licensing Operation m
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8410110129 841002 PDR ADOCK 05000263 PDR p.
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1 DISCLAIMER OF RESPONSIBluTY TNs document was prepared by or for the General Electric Company. Neither the General Electric Company nor any of the contributors to this document:
A.
Makes any warranty or representation, express or implied, with respect to the accuracy, completeness, or usefulness of me information containedin this Jocu-ment, or that the use of any information disclosed in tNs document may not infringe privately owned rights; or B. Assumes any responsibiny for liabiMy or damage of any kind which may result from the use of ar.yinformation disclosed in tNs document.
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- C NEDC-30492 CONTENTS P., a.g,e, 1.
SUMMARY
l-1 2.
INTRODUCTION 2-1
- 3.
APRM SYSTEM IMPROVEMENTS 3-1 3.1-
System Description
3-1 3.2.
System Evaluation 3-3 3.2.1 Objectives 3-3 3.2.2 Evaluations 3-4 4.
ROD BLOCK MONITOR SYSTEM IMPROVEMENTS 4-1 4.1 Current RBM System Description 4-1 4.1.1 Limitations of Current RBM System 4-3 4.2 New RBM System Description.
4-10 4.3 Rod Withdrawal Error-Analysis 4-15 4.3.1 Analysis 4-15 4.3.2 Sensitivity Analyses 4-18
. I 4.5 RBM Operability Requirement 4-35 5.
TECHNICAL SPECIFICATION CHANGES 5-1 6.
REFERENCES 6-1 l:
APPENDICES A.
RBM HARDWARE DESCRIPTION A-1.
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,1 NEDC-30492 TABLES Table Title M
- 3-1 MNGP Transiest Analysis Results 3 F 4-1 Rod Block Monitor. System Improvements 4-9 4-2 Rod Withdrawal Error Analysis Results 4-19
'4 RWE Analysis Results for Peripheral-Rod Groups 4 l 4-5 RBM System Setup
. 4-31 4-6 RBM Setup Setpoint Definitions 4-32 4
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ILLUSTRATIONS d
Figure Title Py 3 Proposed APRM Limits for MNCP 3-2 3-2 MCPR Limits (MNGP) 3-10 3-3 MNGP MAPLHCR Factor (MAPFAC) 3-11~
3 MAPFAC Limits (MNGP) 3-14 7
4-1 Conceptual Illustration of Current MNGP Flow Dependent RBM System with AC/BD LPRM Assignment 4-4 4-2 MNGP RBM Current AC/BD LPRM Assignment 4-6 3 Current RBM Limits (Typical for 106 Setpoint) 4-7 4-6 Typical RBM Channel Responses Old versus New LPRM Assignment (No Failed LPRMs) 4-14 4-7 New MNGP RBM System Core Power Limit 16 4-8 Design Basis RWE MCPR Requirement versus RBM Setpoint 4-20 4-9 Design Basis MCPR Requirement. for RWE (MNGP) 4-21 1
4-11 MNCP Neutron Monitoring System 4-25
'4-12 Rod Block Monitor Rod Group Geometries 4-26 4-13^
Results of LPRM Failure Rate Sensitivity Study 4-27 I
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SUMMARY
The'Monticello Nuclear. Generating Plant (MNGP) Average Power Range Monitor
' (APRM), Rod Block Monitor.(RBM), and Technical Specification Improvements (ARTS) program is a comprehensive project'with the objectives of:
a.
increasing plant operating efficiency,
- b.. updating thermal limits requirements and administration, c.
reducing' mechanical-duty, d.
improving plant. instrumentation responses and accuracy, and
'mproving the man / machine interface involved in plant operation.
i e.
These objectives are attained by making the following improvements.(the
- objectives met-.by each improvement are given in parentheses 'at the end of each
--item):
b.
the'APRM trip setdown requirement is replaced by more meaningful limits to reduce the need for manual setpoint adjustments and to allow more direct limits administration (improves man / machine
+
' interface, updates thermal limits administration, and increases plant operating efficiency),
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NEDC-30492 d.
the rod withdrawal error analysis is performed in a manner consistdnt with the system changes and more accurately reflects actual plant conditions (updates thermal limits administration).
f.
modern electronic components are installed in the RBM (improves instrument accuracy),
m h.
the RBM logic is improved to eliminate the need for manual trip reset (improves man / machine interface), and 1.
" Limiting Rod Pattern" is defined to simplify RBM operability requirement decisior.s (updates thermal limits administration).
The analyses which justify these changes and which determine instrument setpoints and operating limits consistent with their implementation are dis-cussed in detail in this document and the supporting references. These include transient analyses, rod withdrawal' error analyses, and loss-of-coolant accident (LOCA) analyses.
1-2
8
-3; NEDC-30492 2.-
INTRODUCTION
' Factors which restrict the flexibility of a BWR during power ascension from the low power / low core flow condition are:
9 a.
the APRM flow-referenced rod block line, b.
the RBM flow referenced rod block line, Preconditioning Interim Operating Management Recommendations c.
(PCIOMRs), and' d.
the APRM scram and flow referenced rod block setdown requirement.
If the rated load line control rod ' pattern -is maintained as core flow is
. increased, increasing xenon concentrations will result in less than rated
, power at rated core flow. In addition, fuel pellet-cladding interaction con-
-siderations may inhibit withdrawal of control rods at high power levels. The combination of-these factors may require difficult and time consuming maneuvers to achieve rated power.
The Monticello Extended Load Line Limit Analyses (ELLLA) are described in Reference 2 and provide the analytical bases for raising the APRM rod block lines at the bottom of the flow control range by reducing the flow referencing slopes'from 0.65 to 0.58 and for extending the operating envelopes to include the region bounded by the new 108% APRM rod block line, the rated power line, and the wated load line.
This report supplements and builds on those documents to allow full uti-lization of the extended operating region, to update thermal limits adminis-tration, to reduce mechanical duty, to improve instrumentation response and accuracy, and to improve the 7an/ machine interface for plant operation.
2-1
8 3
s NEDC-30492 The bases for these changes are provided by the MNGP APRM/RBM/ Technical Specifications Improvement Program (MNGP ARTS) which is described in this report. This document provides the analytical bases for:
e.
introduction of an improved rod withdrawal error analysis.
2-2
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3.
APRM SYSTEM IMPROVEMENTS
.3.1 SYSTEM DESCRIPTION
. The functions of the APRM. System are,to:
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generate trip signals which will automatically scram the reactor
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'during bulk neutron flux. level transients before the actual bulk neutron flux level exceeds the safety ' analysis design bases to prevent fuel damage from single operator errors or equipment malfunctions; and
' b.. block control rod withdrawal when core power significantly exceeds design bases and approaches the scram level; and t
c.
provide an indication of the bulk thermal power level of the reactor in the power range.
The MNGP APRM System' calculates an average of in-core LPRM chamber signals using' analog electronics.' The LPRMs are averaged such that the APRM signal is proportional to core average neutron flux, and can be calibrated as a means of
. measuring core thermal' power. The APRM signals are compared to a recirculation drive flow referenced scram trip and. a recirculation drive flow referenced control rod. withdrawal block trip. Shown in Figure 3-1 are the MNGP APRM scram-and rod block trips as they will exist following implementation of the ELLLA.
The MNCP Technical Specifications' require that the flow referenced APRM trips be lowered (set down)* when the core maximum fraction of limiting power density (MFLPD) exceeds the fraction of rated power (FRP). The basis foe this "APRM setdown" requirement' originated from the obsolete Hench-Levy Minimum i
Critical Heat Flux Ratio (MCHFR) thermal limit criterion.
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.* Alternately accomplished by APRM gain increases..
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.o NEDC-30492 140 FLOW R EFERENCED Hi FLUX SCRAM 1038WD + 62) 120 g
100% POWER LINE 100% INTERCEPT POINT (100/87) g,g 3
(1 W 106)
APRM ROD BLOCK
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LINE (0.58WD*
TYPICAL POWER ASCENSION PATH 80 e
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0.58WD + 82 100% FLOW LOAD LINE I
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Figure 3-1.
Proposed APRM Limits for Monticello 3-2
's s
JThe change to GETAB/GEXL with its deemphasis of local thermal hydraulic
. conditions and the move :to -secondary reliance on flux scram for licensing
~
. transient evaluations -(for. transients. terminated by anticipatory or direct scram) has provided more effective and operationally acceptable alternatives
~
The MNGP ARTS program uses transient analyses to to the setdown requirement.
define thermal limits initial conditions (operating limits) which conserva-tively assure that all licensing criteria are satisfied without setdown of the
t 3.2 SYSTEM EVALUATION ~
3.2.1 Objectives
.The objective of this evaluation is to justify removal of the peaking factor setdown requirement. Those licensing areas which might be affected by-the' elimination of the setdown' requirement are:
a.
fuel thermal-mechanical integrity, and 1
b.
loss-of-coolant accident.
The following criteria assure satisfr.ction of the applicable ilicensing requirements and were applied to demonstrate the acceptability of r -
elimination of the setdown requirement.
MCPR safety limit shall not.be violated as a result of any abnormal a.
operating transient, b.
'All fuel thermal mechanical design bases shall remain within the licensing limits described in GESTAR-II,
- and 9
lc.
peak cladding temperature and maximum cladding oxidation fraction following a LOCA shall remain within the limits defined by the applicable regulations.
3-3
-s NEDC-30492 3.2.2 Evaluations
.The' safety evaluations include abnormal operational transients and LOCA analysis.
3.2.2.1 Loss-of-Coolant Accident Previous LOCA analyses applicable to MNCP are documented by References 4, 5
6, and 7.
References 5 and 6 evaluate the effects' of a LOCA initiated from less than rated core flow for all classes of GE BWRs. Standard exposure dependent maximum average planar linear heat generation rate (MAPLHGR) limits.
are generated from LOCA analyses initiated from rated power and flow condi-tions. For core flows lower than a critical value, boiling transition at the limiting fuel node occurs sooner than during the standard LOCA evaluations;
-this phenomenon is referred to as early boiling transition (EBT). The EBT increases the low heat transfer period before final water level recovery. If the initial fuel heat flux is high enough, the resultant peak cladding temper-ature (PCT) can exceed the standard LOCA results.
In this case, it may be necessary to apply an "MAPLHGR multiplier" for operation in'certain flow Previous LOCA core flow effects analyses,6 assumed that the core is 5
ranges.
operated on or below the proposed flow referenced APRM rod block line, (0.58WD + 50) + (FRP/CMFLPD). This represented a conservatively higher power In addition, than the typical (0.65Wg + 43)'. (FRP/CMFLPD) APRM rod block line.
fractional recirculation drive flows, W, in the equation was conservatively D
taken as core flow, W, since the fraction of rated drive flow is always less C
than or equal to the fraction of rated core flow. These assumptions result in FRP MAPLHGR limit an initial MAPLHGR given by (0.58WC + 50) x GFLPD
- 100 3-4
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NEDC-30492 were re-examined to determine their adequacy / necessity
program'.
The pre-ARTS MAPLHGR multipliers for MNGP were 0.94 for core flows below 90% and-0.91 for' core flows below 70%.
[Two multipliers were necessary for
-MNGP because'the_most limiting break size is 34% design basis accident
] 'A conservative LOCA analysis,5-determined that a 0.91 multiplier 4
. (DBA').
was necessary to avoid EBT at core flows below 70% of rated; the 0.94 multi-plier for core flows less than 90% of rated resulted from the DBA analysis.
I An analysis'was performed at 62% core flow which shows that EBT does not occur at 62% flow and that the 0.91 multiplier is not necessary if the ARTS program is in effect. An analysis was also performed at 80% core flow.
This showed that-EBT did not occur above 80% core flow. The initial MCPR assumed for this analysis was chosen consistent with a rated operating limit of 1.28.**
The 0.94 MAPLHGR multiplier need only be applied below 80% core flow as long as the operating limit MCPR is at least 1.28.. The multiplier must I
3.2.2.2 Transients A large data base was used to study the trend of transient severity with-out the average power range monitor.(APRM) core peaking factor setdown. This data base was established-by analyzing limiting transients over a range of
. power and flow conditions and was used to develop plant operating limits (MCPR and MAPLHGR) which will assure that margins to fuel integrity limits are equal to or larger than those in existence at the present time.
. core flow.
3-5
e NEDC-30492 l
All transient analyses were performed using the standard reload licensing l
methodology,3 except the loss of feedwater heating (LWH). The L WH has been analyzed using methodology described in a generic LWH report submitted by GE in July 1983.8 Results from the above transient analyses were used to establish the 3.3 PLANT OPERATING LIMITS Even with the transient severity increase included, large margins still exist between the required thermal limits and expected operating plant performance at power levels. Accordingly, above PBypass, bounding power dependent trend functions have been developed. These trend functions are
. multipliers on the rated MCPR operating limits and MAPLIIGR limits. The MNGP analyses documented in this report have verified the large margins and the applicability of the multipliers.
3-6 i
a NEDC-30492 No thermal monitoring is required below 25% power.
Bypass, the actual operating limit MCPR Below P is illustrated in Figure 3-2.
The peak core average heat fluxes and maximum ACPRs for the pressuriza-tion transients are presented in Table 3-1.
Comparison of the table values with Figure 3-2 verifies that the K curve is conservative for Monticello.
p I
In the absence of the APRM scram setdown requirement, special limits are substituted to assure adherence to the fuel thermal-mechanical design bases.
This limit is derived to assure that the peak transient MAPLHGR for any transient is not increased above the rated power fuel design basis transient values.
3-7
o e
WEDC-30492 Table 3-1 MNGP TRANSIENT ANALYSIS RESULTS h
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NEDC-30492 Table 3-2 MNGP LIMITING TRANSIENT POWER DEPENDENT MAPFAC " REQUIREMENTS p
"MAPFAC "4"
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=
P MAPLHGR Limit at Rated Power bPressure Scram
" Position Scram 3-9
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NEDC-30492 3.3.3 Flow Dependent MCPR Limit Flow dependent MCPR limits are necessary to assure that the safety limit MCPR is not exceeded during flow runout evento. The design basis flow runout event is a slots flow / power increase event which is not terminated by scram, but which stabilizes at a new core power corresponding to the maximum possible core flow. The MNGP MCPR(F) limit is shown in Figure 3-4.
Flow runouts were analyzed along a constant xenon flow control line assuming an equilibrium
_ plant heat balance at each ' flow condition.
Like the power dependent MAPLP.CR factors, these factors were derived such that the peak tran-sient MAPLHCR during these events is not increased above the fuel design basis values. Figure 3-5 factors also incorporate the requirements derived from LOCA a7alyses (Subsection 3.2.2.1).
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3.3.5 Coverning overall Limit l
For Single-Loop Operation (SLO), the most restrictive of the SLO or ARTS MAPLHGRs will define'the limiting condition of operation. Any MCPR adjustments required for SLO shall be applied to overall MCPR limits as previously defined.
3-15/3-16
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.NEDC-30492 1
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-l 4.<
ROD BLOCK MONITOR. SYSTEM IMPROVEMENTS
' The function of the-Rod Block Monitor (RBM) System is to assist-the
~
i operator in safe plant. operation:in the power range by:
2 Ra.
initiating a rod block to prevent violation of the fuel integrity safety criteria during withdrawal of a single control rod, and b.
- providing a signal to permit operator evaluation of the change in local relative power during control rod movement.
4.1 ' CURRENT RBM SYSTEM DESCRIPTION 0
=To provide the measure of local power change, the RBM System uses the set:
of LPRMs that are displayed to the operator in.the four-rod display. There:
- are two RBM circuits.(designated Channel 1 and Channel 2);- one uses the LPRM
. detectors from the A&C level detectors and the other uses the B&D, level detec-
. tors. - The RBM has between four and eight ' LPRM inputs depending upon whether
~
~
it"is_ operating on a center or near periphery' rod.
._The RBM computes the analog' average of all assigned unbypassed LPRMs, - in much'the same manner as the APRM. The average of the input chambers.is modi-fied automatically.to read the same as a reference APRM by a gain adjustment in-the RBM whenever a control rod is selected. This gain adjustment factor-1can never be less than one. Thus, the LPRM average will never'be adjusted below the APRM. 'There is.a momentary rod block while the gain adjustment.is made..This gain is held ~until a new control rod-is selected.-
The RBM automatically limits ' the local thermal margin changes by allowing
.the. local average neutron flux indications-to increase by a controlled amount.
TIf the change is too great, the rod. withdrawal permissive is removed.
Only one of the two RBM channels is required to trip to prevent rod motion.
+
~~
4-1
r
's NEDC-30492 T
The RBM has three drive flow biased trip levels (rod withdrawal permissive removed). The trip levels may be adjusted and are nominally 8% of reactor power apart. Typical settings might be 108%, 100%, and 92% at 100% flow. For 4
Monticello', the high trips are cycle dependent. Each trip level is automatically varied'with' recirculation system flow to' protect against fuel damage at lower flows.-- The operator may encounter any number (up to three) of trip points
[:
depending on the starting power of a given control rod withdrawal. The lower two points may be successively bypassed (acknowledged) by manual operation of a pushbutton. The reset permissive is ' actuated (and indicated by a light) when the RBM reaches 2% power less than the trip point. The operator should then assess the. local power and either acknowledge or select a new rod. The f
. highest trip point cannot be bypassed.
A count of the active LPRMs is made automatically and the RBM declared inoperative if too few detectors are available for use. The rod withdrawal permissive is removed if the RBM is inoperative and not bypassed. Only one RBK channel may be manually. bypassed at any time.' If the reference APRM is indicating.less than 30% power, the RBM is bypassed automatically. The RBM also is bypassed if the control rod has one or more adjacent fuel bundles
- loca'ted in the outer boundary of the reactor core. In this case, the high
-neutron leakage prevents overlimit conditions. An RBM reading downscale and not automatically bypassed by the APRM low power feature is considered to
- have failed and the rod withdrawal permissive is not given.
The RBM has outputs to recorders located on the reactor operator's console, local meters, trip units and the on-line computer.
The signal conditioning electronics for the RBM forms the average of the LPRM' chambers as described above. The detectors are assigned upon selection
- of a control rod by a selection matrix. The matrix receives a voltage signal
' corresponding to the selected rod group. The selection of the rod routes the 4
proper LPRM. signals to the meter displays and to the assigned RBM.
j 4-2
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NEDC-30492 The power for the RBM is supplied from low voltage power supplies located in the same cabinet as the RBM. Although the RBM has no reactor protection outputs, each RBM channel is assigned to a separate trip system and the ac
. power for the RBM low voltage power supply is supplied from independent sources.
The trip unit utilizes the output voltage from a flow converter to drive the linear variation of the trip set points with flow. The slope of the rod block trip is variable between 0.52 and 0.78 with a current setting of.0.65 for MNGP.
. One RBM channel may be manually bypassed by operator action. As discussed in Subsection.4.1, automatic bypass occurs if the APRM level is below a pre-scribed.value or reactor core outer boundary control rods are selected. - All trips are bypassed if the reactor mode switch is in any position other than "RUN."
A schematic of the current Monticello RBM System is presented in Figure 4-1.
4.1.1 Limitations of Current RBM System The MNGP RBM System was designed in the middle 1960's. Since that time there.have been significant technological advances in the fields of two-phase-heat transfer and. electronics.
The GETAB/GEXL Critical Power Ratio has replaced the Hench-Levy Critical Heat Flux-Ratio as the preferred means of determining departure from nucleate boiling. This means that optimum evaluation of fuel thermal margins can no longer be performed solely on a local basis, but requires both local informa-tion and information about the entire fuel bundle. For the RBM to fulfill its' intended function, changes in the RBM signal (s) must correlate closely with the thermal margin changes during control rod withdrawal. The current
'RBM signals do not always correlate well with thermal margin changes during
~ control rod withdrawal, and the system performs its function only at the expense of significant operational penalties.
4-3
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1 NEDC-30492-l I
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o TO ROM CHANNEL "B"
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i-Figure'4-1. Conceptual Illustration of Current MNGP Flow Dependent RBM System with AC/BD'LPRM Assignment i
l-4-4
' ;9:
- .n NEDC-30492 4
Y
,r For determination of trip setpoints, the
.most' responsive channel is assumed to be bypassed and the setpoints are deter-mined by-the operating (least responsive) channel. It is also assumed that.
-some of. the LPRMs assigned to the operating' channel have failed. This further diminishes the response of this chana.e1. The RBM setpoint chosen is the one twhich' blocks rod withdrawal before-violation of the safety limit minimum
. critical power ratio (SLMCPR), based on the response of the least responsive
~
channel with maximum allowable LPRM failures. However, when this setpoint is actually implemented at the plant, both RBM channels typically will be in oper-I ation and the number of failed LPRMs will be less than assumed in the analysis.
The more responsive channel actually blocks rod withdrawals at much shorter withdrawal increments and unnecessarily restricts control rod movements. This results in complex unnecessary. plant maneuvers to reach the full-power rod
. pattern.
The problem of failed LPRMs is addressed in the analysis of the rod with-
- drawal' error (RWE).
4 a
Figure'4-2 illustrates *.he current LPRM assignments.
~
When a control rod is selected, rod withdrawal is blocked by the current-RBM System until the proper LPRM signals have been routed to the RBM averaging
. electronics, and a variable gain has.been applied to the channel responses
-which calibrates (normalizes) them to read the same as the core average power. level-in percent of rated as obtained from the reference APRM channels
. Figure 4-1). -Normalization of the signal and trips to the reference APRM
(
provides am arbitrary method of mapping RBM setpoints over a broad range of 4
. power and flow coaditions (Figure 4-3). Three flow biased trip lines are provided;Lthe one selected is determined by the power and recirculation drive
' t a' given flow, the RBM trip line immediately flow at the time of selection.
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Figure 4-2.
MNCP RBM Current AC/BD LPRM Assignment 4-6
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-e NEDC-30492 i
RBM ROD 8 LOCK (1 of 3 shown)
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R8M RESET (1 of 2 shown)
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20 40 80 80 100 Figure 4-3.
Current RBM Limits (Typical for 106 Setpoint) 4-7
..n NEDC-30492
~ above theSAPRM measured power is selected for enforcement. If the APRM-
- measured power is within the 2% reset band immediately below the two lower
~
. trip-lines, the next higher RBM trip line is automatically selected for
' enforcement.. Similarly, manual reset of the lower trip to the next higher trip Lis allowed when the local power reaches tha 2% band as a result of rod with-drawal. In this case, the operator verifies that adequate thermal margins exist before resetting the trips. These reset. features are a necessary-I-
result of.the normalization of the signals.to the APRM. If the APRM power is
-just below the trip, random noise in the signals may cause the trip to be exceeded and no withdrawal will be possible. Since the flow biased trip lines.
roughly parallel the flow control lines, it would be very difficult to increase
- core power above an RBM trip-line without the reset features. Resets are
. possible only for the two lower trip lines; the high trip cannot be reset.
Since the highest trip line cannot be reset. another direct consequence of the normalization of the RBM signals to the reference APRM is that control rod withdrawal is not permitted when the reference APRM exceeds the highest RBM trip line.
--Shown in Figure 4-3 is an ideal startup path based on attaining rated t
power without control rod movement after recirculation flow has been increased
-above the minimum pump speed. ~Also shown.in Figure 4-3 are the RBM trip lines and the ideal startup path relative to.the highest RBM trip line. Because these two lines cross at low flow, the RBM prevents withdrawal of control
. rods necessary to attain the ideal startup path. These control rods must currently be withdrawn at higher powers resulting in unnecessary fuel duty.
I
' Summarized in Table 4-1 are the limitations of the current Monticello RBM
. System, the impact and the proposed improvements.
1 4
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+
e 4-8 l
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.. _ ~ -.., _....
NEDC-30492 Table 4-1 ROD BLOCK MONITOR SYSTEM IMPROVEMENTS Current Design Impact Improvement I
e Low Trip Setpoints e Unnecessary Rod Blocks J
4-9
NEDC-30492 4.2 NEW RBM SYSTEM DESCRIPTION The changes which ARTS will make to the MNGP RBM System will:
eliminate the restrictions imposed on gross core power by the a.
current flow referenced RBM trips (this function will be fulfilled by the APRM flow biased rod block),
b.
enhance operator confidence in the system by reducing the frequency of non-essential rod blocks and by making occurrence of rod blocks more predictable and therefore avoidable, and upgrade the performance of the system such that the RWE will never c.
be the limiting transient.
Advances in electronics have made it possible to efficiently specify system performance requirements which were not possible in the mid-60s. The ARTS Program takes advantage of these advances to make changes in the Monticello RBM hardware which controls the trip logie and LPRM averaging to enhance the instrumentation accuracy and to improve the signal to thermal margin correls-tion. Further improvements in the capability of the RBM to perform its intended function of assisting the operator in safe operation of the plant are obtained by improving the methodology used to determine the required trip setpoints.
reselecting a rod, Reselection will result in a recalibration to the refer-ence signal.
4-10
e
.-4
.a.
a t-NEDC-30492 i
f i
4-11
- ,=
e NEDC-30492 6
In Figure 4-6 the individual channel responses are compared to a typical high worth control rod withdrawal.
The new MNGP REM System is easily understood, possesses readily T
predictable behavior, and will limit the thermal margin reduction during rod withdrawn 1s, but will not restrict rod withdrawals on the basis of gross core 4-12
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Figure 4-6.
Typical RBM Channel Responses. Old versus New I
4-14
. power level-(Figure 4-7).
Limitations on gross core power levels are now imposed by the APRM flow biased rod block; this system will remain unchanged.
1 The RWE eval'uations necessary to establish the CPR limit and the trip
. setpoints for each power interval are discussed in the following subsections.
4.3 ROD WITHDRAWAL ERROR ANALYSIS 4.3.1 Analysis The deterministic, bounding, cycle specific analy::is is replaced with a statistical analysis valid for applica-tion to all MNCP. cores utilizing General Electric fuel designs through P8x8R.-
The data base was, drawn from actual plant operating states and covers the spectrum of plant designs and power densities (BWR/2, 3, 4, and 5) and
- currently utilized fuel designs. Cases were selected with low MCPRs and high MAPLHCRs in bundles near deep control rods to yield meaningful results. All State A cases were selected near rated power and rated flow. The actual rod
- patterns were modified to reduce the MCPR(s) of bundle (s) near the deep rods to approximately 1.20.- To cover the power flow map, two other power / flow points were included in the database.
1
+
1 4
4-15
_._. _,, _ _-.~._.. _ _._.__ _ _._. _._..,_.. _._ _.. _ _ _ _ _.
.i
)
9; NEDC-30492 120 t
e NO ROM POWER LIMIT ON ROD WITHDRAWAL e POWER LIMIT LEFT TO APRM ROD BLOCK
,/
100
=
IDEAL STARTUP PATH
/
/
[ /*,
.0 lw e
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NATURAL I
CIRCULATION TWO-PUMP MIN SPEED
~
/
/
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s, ->
0 20 40 80 80 100 CORE FLOW (%)
Figure 4-7.
New MNGP RBM System Core Power Limit 4-16
NEDC-30492 A sensitivity study was also performed on LPRM failures and is discussed in Subsection 4.3.2.2.
This,
study shows that the new system is fairly insensitive to LPRM failure rates.
The RBM responses were generated for both channels for each RWE analyzed.
From these responses, error rod position at the rod block trip level was gener-ated as a function of RBM setpoint. The results were tabulated as functions of RBM setpoint.
The overall results were determined for each power / flow point for each RBM channel.
4-17
7 Q:
e NEDC-30492 5
The results for both RBM channels for each power flow state for a range of RBM setpoints are summarized in Table 4-2.
Also shown is the bounding MCPR requirement fdr each setpoint. This bounding MCPR requirement was used to generate the design basis MCPR requirement as a function of REM setpoint (Figure 4-8).
The results in Table 4-2 show that, for setpoints of interest, the MCPR lLaits do not vary significantly over the power flow map.
3 This value was chosen to assure that RWE will not signifi-cantly limit plant operation. Figures 4-8 and 4-9 were used to determine the RBM setpoints such that the RWE required MCPR is less than or equal to the core wide transient power dependent MCPR requirement. The RBM downscale trip setpoint was selected to detect abnormally low RBM signal conditions.
Control rod withdrawal is blocked when the RBM is downscale. The resultant MNCP power dependent RBM setpoint requirements are shown in Figure 4-10.
4.3.2 Sensitivity Analyses 4.3.2.1 Peripheral Rod Groups The RBM setpoints given in the previous section(s) were based on analysis of RWEs occurring in four rod cells surrounded by four LPRM strings. The RBM 4-18
e NEDC-30492 i
Table 4-2 ROD WITHDRAWAL ERROR ANALYSIS RESULTS I
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Figure 4-8.
Design Basis RWE MCPR Requirement versus RBM Setpoint 1
1
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Figure 4-9.
Design Basis MCPR Requirement for RWE (MNGP ARTS) 4-21 l
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NEDC-30492 i.
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I Figure 4-10.
HNGP RBM Setpoints versus Power l
l 4-22 s
... ~... _. -. _. _ _,, _ _. -, -., _,,, _, _
o.
NEDC-30492 cells near the core periphery may possess fewer than four control rods and have one, two, or three LPRM strings.
A study was performed to verify that the results obtained in the previous sections are valid for peripheral cells with less than four LPRM strings. The locations of LPRM strings and control rods in the MNGP cores arc shown in Figure 4-11.
The rod group geometries and error rods studied are shown in Figure 4-12.
A single case was selected from the database used to establish the RBM setpoints. This case was reanalyzed with the various geometries of Figure 4-12 substituted for the standard 4-string geometry. For this study, the RBM setpoint was fixed at 108. The results are given in Table 4-3 and show no significant differences between the base (4-string) case and the limit-ing peripheral geometries.
4.3.2.2 LPRM Failures A study was performed to determine the sensitivity of the MCPR requirement to the failure probability.
and "20" MCPR requirement for a 108% RBM setpoint are shown as functions of LPRM failure probability in Figure 4-13.
A low sensitivity to LPRM failure probability is demonstrated in Figure 4-13.
It i~ concluded that the RBM setpoints are adequate for any realistically expected incidence of LPRM failures.
4-23 l.
1 NEDC-30492 Table 4-3 RWE ANALYSIS RESULTS FOR PERIPHERAL ROD GROUPS b
4-24
NEDC-30492 MONTICELLO
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33 36 37 39 41 43 46 47 49 S1 h'LPRM LOCATION (Letter indicates TIP Machine.
$ LPMM LOCATION (Common Location for All TIP Machines, h IRM LOCAT,0Ns 6 snM LOCAtloNs
$ sounCa LoCATroNs Figure 4-11.
MNCP Neutron Monitoring Systern 4-25
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TYPICAL FOUn-STRING:
3 2
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IRMOR ROD
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TYPICAL THREE47 RING:
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g STRING 1 4
STRING 1 (4 MIS $1NG)
--] (COME EDGE)
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(MIS $1NG 2)
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(MIS $ LNG 4)
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Figure 4-12.
Rod Block Monitor Rod Group Geometries 4-26 t
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Figure 4-13.
Results of LPRM Failure Rate Sensitivity S s
i
NFDC-30492
)
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- The setpoints here are " Analytical Limits." Other adjustments are recommended for inaccuracy calibration, and drift effects to obtain the " Nominal Trip Setpoint." Some adjustment r9nges have been fixed by design such that sur-j veillance can be performed by simply establishing that the adjustments are in the limiting position.
4-28
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NEDC-30492 4.5 RBM OPERABILITY REQUIREMENT The RBM System design objective is to block erroneous control rod withdrawal initiated by the operator before the safety limit MCPR is violated.
When any control rod in the core would violate this limit upon complete with-drawal, operability of the RBM System was required. Such a condition is a
" limiting control rod pattern" because RBM operation is required. The RBM System basis is limited to consideration of single control rod withdrawal errors and cannot accommodate multiple errors. Therefore, in defining
" limiting control rod patterns," only single control rod withdrawals are considered.
a.
First the MCPR changes were determined for complete rod withdrawal:
s b.
Then.the pre-RWE MCPR requirement was determined:
Safety Limit MCPR = 1.07 was used.
The following limiting MCPR values were determined to provide the required marF n for full withdrawal of any control rod:
i I
4-35
Y
- s.
- e.
d 4
. Whenever operating MCPR is below the preceding values, the plant is on a
" limiting control: rod pattern" requiring that the RBM System be operable; whenever.the operating MCPR is above these values, complete RBM bypass is
, justified.
e s
9
+
+
e 4-36
NEDC-30492 5.
TECHNICAL SPECIFICATION CHANGES The following changes to the MNGP Technical Specifications are required for implementation of the ARTS Program:
a.
Delete the requirement for setdown of the APRM scram and rod blocks.
b.
Change slope and intercept of APRM flow biased rod block to 0.58 and 50, respectively; change slope and intercept of APRM flow biased scram to 0.58 and 62, respectively.
I d.
Add new RBM bypass requirements including definition of limiting rod pattern.
I f.
Replace K with new MCPR
- F F
1.
Delete or modify affected bases.
I 5-1/5-2
i as ;
I s
6.
REFERENCES 1..
General Electric Standard Application for Reactor Fuel, April 1983 (NEDE-240ll-P-A-6).
2.
General Electric Boiling Water Reactor Extended Load Line Limit Analysis for Mc ticello Nuclear Generating Plant, Cycle ll, March 1984 (NEDC-30515).
3.-
General 'lectric Standard Application for Reactor Fuel (Supplement for United States), April 1983 (NEDE-240ll-P-A-US-6). -
4.
-Loss of Coolant Accident-Analysis Report fer Monticello Nuclear Generating mPlant, December 1980 (NEDO-24050-1) ~as amended.
5.
' R. L. Gridley (GE), Letter to D. G. Eisenhut (NRC), " Review of Low-Core Flow, Effects on LOCA Analysis for Operating BWRs," May 8,1978.
6.
D. G. Eisenhut (NRC), Letter to-R. L. Gridley, enclosing " Safety Evalua-tion Report Revision of Previously Imposed MAPLHGR (ECCS-LOCA) Restric-tions for BWRs at Less Than Rated Flow," May 19, 1978.
7.
Monticello Nuclear Generating Plant, Final Safety Analysis Report Docket 50-263.
' 8.
Letter, J. S. Charnley (GE) to F. J. Miraglia (NRC), " Loss of Feedwater -
Heating Analysis," July 5, 1983 (MFN-125-83).
4 6-1/6-2
NEDC-30492 APPENDIX A o
a RBM HARDWARE DESCRIPTION A.1 DESCRIPTION'
" A.1.1.
General The Rod Block Monitor-(RBM) System is designed to automatically detect
'and block control rod withdrawal that could violate Technical Specification
^
safety limits during a single' control rod withdrawal error (RWE) transient.
Upon operator selection of a control rod for withdrawal, the system begins comparing RBM signals to predefined trip levels. The RBM signals are the
' averages.of local power range monitor (IERM) in-core signals in the:immediate L
core region of the selected control rod. An increase.in the RBM signals during control rod withdrawal indicates a local power increase, and will, therefore.
inversely. correlate to local thermal margins changes. Rod block trip levels are. determined by analysis to limit the thermal margin reductions to assure fuel limits are not violated. It is assumed that the core is operated in compliancewithplant_TechnYealSpecificationsbeforetheRWEevent. The plant
~
operator is relied on to verify that he is in compliance with Technical Speci-
'fication fuel thermal limits before resetting the rod block trip. Once reset, the RBM System reinitializes'and allows furthe,r control rod withdrawal con-sistent with the design basis fuel thermal margin reduction increments. Design--
basis-fuel thermal margin reduction increments represent the differences-
.between the Technical Specification safety. limits, and the Technical Specifica-
" :Mion operating limits.
p-
, In addition to the above function, the REN provides continuous display of
--RBM signals to the operator as an indication of local power change during rod 1 movement.
i A.I.2 Application The following addresses thos major features of the modified RBM System.
Areas not addressed are, unchanged from the standard RBM design.
c s
A-1
0 o
NEDC-30492 A.1.2.1 LPRM Assigbhent and Functional Description circuit as in the standard system for a typical central region control blade.
Note, however, that some control rods near the core edge do not have the complete complement of 16 surrounding LPRMs 'and that some control rod selec-tions result in 8 LPRMs in each'RBM, others have 6 LPRMs in each RBM and finally some result in only 4.
r This gain is held until a new control rod is selected. The RBM automatically limits the local power change by allowing the local average peutron flux indications to increase by a controlled amount. If the change is too great, the rod withdrawal permissive is removed. This is accomplished by implementing the rod block upscale trips relative to the same reference source signal used for RBM signal noimalization.
W Trip time delay is short enough to limit rod movement well below that which could cause a thermal limits violation.
8 I
A-2
/
O O
NEDC-30492 As in the original system, the downscale trip automatically detects abnormally low RBM signals and also removes the rod withdrawal permissive.
A count of the active LPRMs is automatically made and the RBM declared inoperative if too few detectors are available for use. Up to half of the input LPRMs are allowed to be bypassed in an RBM channel (circuit) before a
~
channel is declared inoperative.
The rod withdrawal permissive is not issued if the RBM is not operative and not bypassed. During operation with a limiting rod pattern, only one RBM channel may be manually bypassed at any time. Analyses performed for the rod block trips assume that only the least responsive RBM channel is in operation.
At some low reactor power, fuel damage cannot occur for any single rod with-drawal; hence, if the reference APRM is indicating below this value, the RBM System is automatically bypassed. The RBM is also automatically bypassed if the control rod has one or more adjacent fuel bundles comprising the outer boundary of the reactor core. In this case, the high neutron leakage prohibits i
overlimit conditions. In addition, an REM reading downscale and not automati-cally bypassed by the APRM low power feature is considered to have failed and the rod withdrawal permissive is not given.
A.I.2.2 Signal Conditioning Equipment The signal conditioning electronics for the RBM forms the average of the ISRM chambers as described above. The detectors are assigned upon selection i
of a control rod by a selection matrix. The matrix receives a signal corresponding to the selected control rod group. The selection of the rod routes the proper LPRM signals to the meter displays and to the RBM.
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NEDC-30492 A signal generated at g
the change.of rod selection causes the RBM to reinitiate the null sequence.
]
The rod withdrawal permissive is not present during the nulling sequence.
Once the gain adjustment is accomplished,, this gain setting is maintained until a'new gain adjust required signal (new rod selection) is received. The RBM has outputs to recorders located on the reactor operator's console, local meters, trip units and the on-line computer. The output to the upscale trip-unit can be delayed for a short time to allow small rod adjustments despite abnormally high noise.
The accuracy of RBM outputs in percent of full scale including drif t, environmental changes, and supply voltage variation within the normal oper-ating conditions is at least as good as the standard RBM designs. The'averag-ing circuit response time is also equal to or shorter than standard RBM designs.
Overall system quality equals or ' exceeds that of the REM being replaced. The overall reliability of the REM' System in performing its rod bleck function when required is equalled or increased.
- A.l.2.,3 Power Supply and Trip Characteristics r
The power for the RBM is supplied from low voltage power supplies located in the same cabinet as the RBM. Although the RBM has no reactor protection outputs, each RBM is nominally assigned to a separate trip system and the ac power for the' low voltage power supply'is supplied from that source. There is no required difference in RBM circuitry between large and small plants.
However, variations in RBM circuits may exist from plant to plant to accommo-
-date plant specific configuration requirements, solid state versus relay components, or other unique plant features.
The trip unit allows for adjustment of the three power biased trip levels -
and the power ranges over which each is implemented as previously discussed.
The accuracy of the trips (the point at which the trip circuits operate) equals or exceeds that of the RBM System being replaced. The trips (functions i
A-4 9
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NEDC-30493 described in the previous section) include: too few inputs, downscale rod withdrawal block, upscale rod withdrawal block,' instrument inoperative, mode switch in other: than " Operate," a module removed, number of unbypassed inputs
~too few, and-failure to null to the reference source signal. The response time of:the trip logic and drift of the setpoints equals or fa less than that of the logic-being replaced.
A.1.3', Interface f
The RBM compares the signal of each channel with a preset alarm level which is chosen in respect to-the magnitude of the reference signal. If the
'RBM signal exceeds the alarm level, a rod block signal will be provided to the Reactor Manual Control System. The RBM also provides the averaged values to the Process Computer.
A.2 DESIGN / PERFORMANCE OF ELECTRONICS HARDWARE A.2.1; The REN has been designed to provide information about the local neutron flux level in the vicinity of a control rod that has been selected for with-drawal or insertion and to provide alarm signals used to inhibit rod withdrawal
'if the signal change reaches a predetermined level.- This level shall be one
-of three RBM upscale trip levels which are to be enforced over the range of core power level fron 30% to 100%. The RBM shall provide appropriate readout..
and annunciation for operator action and attention. The number of REN channels
^
is two (RBM Channels A and B).
'A.2.2 Input Sianals The RBM equipment.has been designed so that upon selection of a rod for
' withdrawal or insertion, a group of 16 (maximum) conditioned LPRM signals are
- automatically fed into the two RBM channels.
A-5
_. _. _._. _ _ _ _ _ _... _.. ~.... _. ~... _ _.. -.. _, _ _ _ _ _ _.,. _. _. _., _., _ _..,.., _
D D
A.2.3 circuit Isolation The equipment has been designed so that any single short or open circuit of any single LPRM input to the RBM shall not affect any other LPRM inputs to the same RBM.
A.2.4 LPRM Auto-Bypass The RBM has been designed so that each LPRM input level is sensed and compared with a predetermined reference level. If the LPRM input signal to an
- RBM averaging circuit is below this level, the LPRM input in question is automatically removed from the RBM signal conditioner and the gain of the signal conditioner automatically adjusted to compensate for the bypassed LPRM input signal. The bypass function in no way affects the LPRM from which the signal bypass was derived. White indicator lights are associated with the LPRM Meter Group Display and are ailluminated when the RBM/LPRM input auto-bypass occurs.
If the number of auto-bypassed LPRM inputs to the RHM averaging circuit exceeds the number specified, the RBM instrument inoperative alarm will be actuated.
A.2.5 Reference Signal Each RBM is furnished with a reference APRM signal. This reference signal will be used to automatically select the corresponding RBM upscale trip. One APRM signal from each RPS bus supplies this reference signal for the RBM on the same bus. In the event of APRM bypass, another APRM on the A-6
?
-e o
NEDC-30492 same reactor. protection bus is substituted automatically.
'l i
i A.2.6 Bypass 9
A'.2.6.1 The RBM equipment is designed such that when peripheral rods are selected for withdrawal or. insertion, the RBMs are automatically bypassed.
A.2.6.2 One RBM channel may be manually bypassed by operating the remote RBM bypass switch.
A.2.6.3 Bypass is indicated on a local indicator by a white light and remotely indicated by a white display pilot light.
A.2.7 Signal Conditioning Equipment '
The signal conditioning equipment for the RBM has been designed to process, condition and control with signals provided from the selected LPRMs.
the reference APRM, rod selection switch, and bypass and other controlling functions, as illustrated in Figure A-1.
A.2.7.1. The number of conditioned LPRM signals selected as input to the RBM channel may vary from a minimum of two to a maximum of eight. Over this range of number of inputs, the equipment has been designed to meet ~the performance requirements specified.
A.2.7.2 The LPRM signals are allowed to vary their full range allowance.
A.2.7.3 The signal conditioning equipment of the RBM is designed to have a sensitivity compatible with the minimum LPRM signal, the accuracy requirements and the minimum number of LPRM inputs.
'A.2.7.4 The RBM equipment has been designed so that the signal conditioner gain is automatically adjusted with the output level of the RBM signal A-7
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Figure A-1.
RBM Functional Block Diagram (Illustration Only) 4
e NEDC-30492 conditioner always corresponding to a constant 1evel whenever a control rod is selected. This gain is held until a new control rod is selected. The change of the RBM signal is constrained within the limit specified by an upscale alarm setpoint which varies with the APRM reference value.
A.2.7.5 During the period that the gain of the RBM signal conditioner is being adjusted, withdrawal of the selected control rod will be inhibited. The period of time required for this gain adjustment shall not exceed If gain adjustment is not accomplished during this interval, an instrument inoperative alarm is initiated.
A.2.7.6 Over the normal control room environmental range, the actual RBM output does not deviate from the specified output (defined as tha design center output) by more than A.2.7.7 The RBM signal conditioning equipment has been designed so that at the design center environmental conditions, the short-term (10-min) drif t does not exceed A.2.7.10 Input signals required by the RBM and furnished by the Reactor Manual Control System are serialized signals which will allow the RBM to determine which rod is selected, and from that, determine whether:
a.
No rod is selected.
b.
One rod is selected.
A-9
-4 g> -
Rod is part of a group. surrounded by'two or-three LPRM detector
~
2 c.
' assemblies, i
i.
d.1 Peripheral rod selected.
- A.2.7.'11'. The ' signal conditioning equipment has been designed to provide the following signal outputs at the levels indicated.
A.2.7.11.1 An appropriate signal to provide for readout, on, or near, the.
signal. conditioning equipment. :The signal is switchable and switching shall not affect the operation of the RBM.
A.2.7.11.2 ~A 0-to-1.0-volt signal for 0 to 125% (full scale) has been provided
.for use by a remotely located recorder. The signal is switchable and switching shall not affect the operation of the RBM.
F.
'A.2.7.11.3 A 0-to-160-mV signal for 0 to 125% (full scale) has been provided to the Performance Monitoring and Control System.
e A.2.7.11.4. An inhibit withdraw (RBM Cain Adjust in Process) signal has been provided for, use by the Reactor. Manual Control System.
A.2.7.11.5.A 0-to-1-volt signal for 0 to 125%'of the constant reference signal level and proportional to the' average of LPRM inputs of each REN channel has been provided. This signal is presented to a recorder and is switchable. The
, switching does not affect the alarm level setting.
A.2.8L Trip Function' b
The REN provides the alarm functions listed in this paragraph.
q.
A.2.8.1' All alarms are of the nonseal-in type (nonlatching) except the upscale level alarm (Rod Block) which can be reset by activating a reset switch 7
.or selecting another rod. Signal gain adjustment occurs only on rod selection and is not a function of the reset.-
)
A-10 l-u
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~
A.2.8.2 _ Locally Mounted Alarm bisplay Lights. The equipment is designed so that locally mounted alarm status display lights are located on or near the RBM signal conditioner. These display lights are color ceded as follows:
Upscale Level Alarm (Rod Block) - Amber a.
b.
Downscale Level Alarm
- White c.
Instrument Inoperative Alarm
- White l
l A.2.8.2.1.3 Accuracy. Over the normal control room environmental range, the actual alarm level does not deviate from the ideal alarm level more than e
of full scale, i
A-11
c.
i v
5 A.2.8.2.1.4 ; Calibration. The quality of freedom from error to which the alarm level is calibrated.with respect to the true desired setting does not exceed of full scale.
.A.2.8.2.1.5 Drift. The alarm level drift does not exceed of full scale over the maximum surveillance test period.
A.2.8.2.2 Downscale Level Alarm
- Design is unchanged from the current RBM design.
E
- A.2.8.2.3-' Instrument Inoperative Alarm In the event that'a particular RBM channel is out-of-service, an instru--
ment inoperative alarm will be activated. Conditions causing an instrument inoperative alarm are as follows:
a.
Calibrate-operate switch in other than operate position.
g b.
Any interlock in the equipment open.
3 Auto-bypassed LPRM exceeds the number spe'cified.
c.
A.4.8.2.4 Remotely mounted display pilot lights and annunciators are unchanged 7 -
from the current RBM design.
F A.2.8.2.5' Bypass RBM upscale, downscale and inoperative alarms are automatically bypassed in the event that the channel is bypassed.
A-12
.. _. _ _ _... _ ~ _... -, _,..,,. _. _ -. _ _ _. -. _ _ _. _ _,.. _ _ _ _. - _ _ _ _. _, _ _,. _,,...
-o.
A. 2. 9' Environment Requirements The equipment.has been designed to function within the normal control room environmental conditions.
A.2.10 Power Distribution Powe'r connection to the REN is unchanged. Power bus separation is desired and the equipment is designed to prevent inadvertent power bus interconnection.
'A.2.11 : Susceptibility The equipment is designed such that interaction between the systems and subsystems of the Neutron Monitoring System (NKS) is minimized. In addition, the l equipment is designed to operate within these specifications in the ap' pro-
-priate nuclear power plant environment. Sufficient equipment testing has been performed during the design of the equipment to assure that these requirements are met.
A.2.12 Statement of Accuracy The statements of accuracy contained herein pertain to equipment upon
' which statistical determination of accuracy has been made. The accuracy of the equipment is within the figure stated herein with a probability,
95%.
r A.2.13. Maintainability The RBM channels require normal customer maintenance.
A-13/A-14 II