ML20135A902

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Rev 00-NP to Overview Description of Core Operating Limit Supervisory Sys
ML20135A902
Person / Time
Site: Palo Verde, Arkansas Nuclear, Waterford, San Onofre, 05000000
Issue date: 09/30/1985
From:
ABB COMBUSTION ENGINEERING NUCLEAR FUEL (FORMERLY
To:
Shared Package
ML19269B664 List:
References
CEN-312-NP, CEN-312-NP-R, CEN-312-NP-R-NP, CEN-312-NP-R00, CEN-312-NP-R00-NP, CEN-312-NP-ROO, NUDOCS 8509100308
Download: ML20135A902 (59)
v • d • e


Text

_. - - -.

OVERVIEW DESCRIPTION OF THE CORE OPERATING LIMIT SUPERVISORY SYSTEM (COLSS)

CEN-312-NP Revision 00-NP SEPTEMBER, 1985 COMBUSTION ENGINEERING, INC Nuclear Power Systems Power Systems Group Windsor, Connecticut 8509100308 850905 DR ADOCK O 1

F1 ABSTRACT A nuclear power plant must be maintained within its limiting conditions for op'eration as specified in the plant Technical Specifications to assure safe operation. The Core Operating Limit Supervisory System (COLSS) aids the operator in maintaining operating margin to limits on linear heat rate and departure from nucleate boiling. To do so, COLSS uses measurements of incore detector signals, CEA positions and plant thermal / hydraulic properties to determine the core power distribution and thermal performance.

This report provides a general description of the scope and methodology of the COLSS algorithms.

It is provided solely for information to be used as a reference during future reviews of submittals on the dockets of C-E supplied NSSS's that utilize COLSS.

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Table of Contents 1.'0 Introduction and Summary 8

2.0 COLSS Description 10 2.1 Purpose of the COLSS System 10 2.2 Overview of COLSS Operation 11 2.2.1 System Inputs 12 2.2.2 Process Measurement Processing 12 2.2.3 COLSS Calculations 14 2.2.3.1 Volumetric Flow Calculation 15 2.2.3.2 Core Power Calculation 15 2.2.3.3 Power Distribution Calculation 16 2.2.3.4 Secondary Calorimetric Power Calculation 17 2.2.3.5 Local Power Density Power Operating Limit Calculation 17 2.2.3.6 Thermal Margin Power Operating Limit Calculation 17 2.2.3.7 Core Power Margin Calculation 18 2.2.4 COLSS Outputs 18 2.3 Description of COLSS Algorithms 28 2.3.1 Reactor Coolant System Volumetric Flow 29 2.3.2 Primary Calorimetric Power 30 2.3.3 Turbine Power 30 2.3.4 Secondary Calorimetric Power 31 2.3.4.1 Power in Each Steam Generator 32 2.3.4.2 Power Adjustments from the NSSS 33 4

Table of Contents (Cont'd) 2.3.5 Plant Power 34 2.3.6 Core Power Distribution 34 2.3.6.1 Conversion of Flux to Power 35 2.3.6.2 Planar Radial Peaking Factors 36 2.3.6.3 Axial Power Distribution 37 2.3.6.4 Hot-Pin Integrated Radial and ASI 38 2.3.6.5 Azimuthal Tilt 38 2.3.6.6 Three-D Power Distribution 39 2.3.7 Linear Heat Rate Power Operating Limit 40 2.3.8 Thermal Margin Power Operating Limit 41 2.3.9 Thermal Margin Power Operating Limit Update 42 2.3.10 Core Power Margin 43 2.4 Uncertainties 44 2.4.1 Power Measurement Bias 44 2.4.2 Power Operating Limit Uncertainties 45 4

3.0 Constants and Supporting Data 46 3.1 Basis for Mechanical and Thermal-Hydraulic Constants 46 3.2 Basis for Core Design Constants 48 3.2.1 Conversion of Flux to Power Constants 48 3.2.2 Planar Radial Peaking Factor Look-up Tables 48 3.2.3 Axial Power Distribution Constants 49 3.2.4 Azimuthal Tilt Calculation Constants 50 3.2.5 LHR Limit Constants 50 5

Table of Contents (Cont'd) l 3.3 Basis for DNB Margin Mon..aring Constants 50 3.3.1 Derivation of the from the Loss of Flow Analysis 51 3.3.2 Other Events Analyzed to Confirm Adequate Monitoring 52 3.3.3 COLSS Penalty Factors Applied for CEA Calculators Inoperable 54 3.4 Basis for Measurement and Calculational Uncertainty Constants 54 3.5 Basis for Constants Supporting On-Line Drib Calculation 56 4.0 Conclusion 58 5.0 References 59 l

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GLOSSARY OF TERMS Anticipated Operational Occurrence A00 AS'I Axial Shape Index Control Element Assembly CEA I

CHF Critical Heat Flux Core Operating Limit Supervisory System COLSS Core Protection Calculator CPC Cathode Ray Tube (display)

CRT Departure From Nucleate Boiling DNB Departure From Nucleate Boiling Ratio DNBR Departure From Nucleate Boiling Overpower Margin DNB-OPM Differential Pressure DP F,

Planar Radial Power Peaking Factor x

Kilowatts per Foot KW/FT Limiting Condition for Operation LCO Linear Heat Rate LHR Loss of Coolant Accident LOCA Loss of Flow (event)

LOF Nuclear Steam Supply System NSSS Power Dependent Insertion Limit PDIL Power Operating Limit POL Reactor Coolant Pump RCP Reactor Coolant System RCS Resistance Temperature Detector RTD h

g 4

7

l 1.0 Introduction and Sunmary i

Maintaining a nuclear power plant within its Limiting Conditions for Operation (L'CO) is a necessary condition for safe operation and acceptable transient consequences. These LCOs are delineated in the Technical Specifications.

i There are many systems in a nuclear power plant that are used to help the operators maintain the limiting conditions for operation. One such system used in C-E supplied NSSSs is the Core Operating Limit Supervisory System i

l (COLSS). COLSS is a digital computer based on-line monitoring system that is used to provide information to aid the operator in complying with the Technical Specifications operating limits on total core power, peak Linear Heat Rate (LHR), Departure from Nucleate Boiling Ratio (DNBR), Axial Shape Index (ASI),

and azimuthal power tilt. The C-E Standard Technical Specifications discuss j

the importance and purpose of these operating limits in the bases for Section 3.2.

The system is used at the following plants:

i 1)

Arkansas Nuclear One Unit 2, 2)

San Onofre Nuclear Generating Station Units 2 and 3, 3)

Waterford Unit 3, and l

4)

Palo Verde Nuclear Generating Station Units 1, 2, and 3.

COLSS uses input from selected sensors to detennine the plant condition and displays this condition to the operator in a form which allows easy interpretation of reactor core status.

Audible alarms and visual CRT messages j

are provided to alert the operator when an operating limit is exceeded. COLSS i

is a monitoring system and does not activate any safety equipment, initiate

any automatic actions, or provide any direct input to safety systems. The major calculations performed by COLSS are:

1)

Core Power, 2)

Core Power Distribution, 3)

Margin to Minimum Departure from Nucleate Boiling Ratio, i

4)

Margin to Linear Heat Rate Limit, and 5)

Core Azimuthal Power Tilt Magnitude.

The purpose of this report is to provide a general description of COLSS for reference during future review of submittals on the dockets of C-E supplied NSSSs that utilize COLSS. To meet this purpose, the report describes:

1)

COLSS monitoring and alarms which aid the doerator in maintaining the appropriate Technical Specification operating limits, 2) sensor data and its processing for input to COLSS, 3)

COLSS algorithm functions, and 4) determination of constants for use in COLSS.

The accuracy of the information supplied by COLSS to the operator was orig-inally evaluated in Reference 1 and has been updated in Reference 5.

This subject will not be addressed further in this report.

2.0 COLSS Description 2.1 Purpose of the COLSS System The plant Technical Specifications specify Limiting Conditions for Operation of plant systems, components, and parameters. Monitoring systems are provided to assist the operator in meeting these Technical Specification requirements.

COLSS is a monitoring system that assists the plant operator in maintaining the Limiting Conditions for Operation (LCO) specified in the following Technical Specifications:

1) 3.2.1 Linear Heat Rate, 2) 3.2.3 Azimuthal Power Tilt, 3) 3.2.4 DNBR Margin, 4) 3.2.7 Axial Shape Index, and, for some plants, 5) 3.3.3.2 Incore Detector Operability.

An audible alarm and a visual CRT alarm message is initiated whenever any of the parameters indicated above do not satisfy the LCO conditions required by the Technical Specifications.

COLSS monitoring is accomplished by performing calculations using incore detector signals, CEA positions, primary and secondary coolant pressure measurements, and various temperature measurements and flow measurements to monitor the following parameters:

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1) margin to the peak Linear Heat Rate (LHR) limit, 2) margin to the Departure from Nucleate Boiling Ratio (DN8R) limit.

3) margin to the licensed total core power,

'.4) azimuthal tilt, and 5)

Axial Shape Index (ASI).

The function of the COLSS in the overall plant monitoring and protection system is illustrated in Figure 2-1.

The protection function is provided by the Core Protection Calculators (CPC) which cause a plant trip if necessary to i

avoid violation of fuel design limits on LHR or DNBR. The COLSS monitoring system reviews system behavior and alerts the plant operator to situations where LHR or DNBR have reached their monitoring limits.

In addition, COLSS alerts the operator when other plant parameters (e.g., azimuthal tilt or axial shape) are at prespecified limits. The Technical Specifications require periodic review of specific aspects of the operation of both the monitoring and protection systems relative to detailed calculations or specific measure-ments to verify acceptable operation and recalibrate as required.

2.2 Overview of COLSS Operation The COLSS algorithms provide an integrated approach to monitoring those system parameters important to the evaluation of LHR and DNBR. Rather than restricting each parameter individually, COLSS uses its inputs to simulate the l

. core power distribution which is then used to directly evaluate the current LHR and DNBR.

From this evaluation, the power margin to the DNBR limit, to the LHR limit, and to the licensed plant power are determined and compared to

.11 i

alarm setpoints which monitor the requirements of the Technical Specifications.

Additional alarm limits are provided on Axial Shape Index (ASI) and azimuthal tilt.

If an alarm setpoint is violated, an alarm sequence is initiated to arert the operator to the violation. The functional block diagram of Figure 2-2 illustrates the overall COLSS algorithm.

2.2.1 System Inputs Table 2-1 provides a typical list of COLSS monitored variables. The specific number of sensors and the sensor ranges can vary from plant to plant depending on installed instrumentation. Figure 2-3 shows typical COLSS sensor locations.

All COLSS sensors are sampled at one second intervals except for the CEA position indications, which are sampled at ten second intervals, and the incore detectors, which are sampled at two second intervals for some plants.

2.2.2 Process Measurement Processing The plant computer process control executive program processes system inputs for use by COLSS. This processing includes taking the measurements, checking the values against transducer limits, and conversion of measurements to engineering units.

If a measurement exceeds the associated transducer limits, it is identified as invalid.for use in later algorithms.

l Additional measurement validity checking is performed internal to COLSS j

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m This checking will alert the operator to the gradual deterioration of a sensor.

When data being obtained from a sensor is detennined to be invalid, the operator is informed of the sensor failure by alarm and the data is marked within COLSS as being invalid. To compensate for invalid data from a particular sensor,

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PROPRIETARY INFORMATION

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2.2.3 COLSS Calculations Portions of the COLSS calcolations are performed at one, ten, and thirty second intervals and are synchronized with data acquisition rates.

(e. g.,

incore instruments are polled at 2 second intervals but used in COLSS power distribution synthesis at 10 second intervals.) Calculations performed at one second intervals include:

1) measurement processing, 2) reactor vessel volumetric flow calculation, 3) plant power calculation based on:

a) turbine first stage pressure b) reactor coolant temperature rise across the core 4) update of the DNB power operating limit since the latest detailed calculation, and 5) comparison of the plant power to calculated limits.

Calculations performed at ten second intervals include:

1) axial power distribution synthesis, 2) azimuthal tilt calculations, m

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3) local pcwer density power operating limit calculations, and 4) comparison of ASI and azimuthal tilt to allowed limits.

Ca'lculations perforced at thirty second intervals include:

1) secondary calorimetric calculations of reactor power, and 2) thermal margin power operating limit calculations.

2.2.3.1 Volumetric Flow Calculation The volumetric flow for a single pump is based on differential pressure across the pump, pump rotational speed, and water properties from the measured values of cold leg temperature and primary system pressure. The total reactor coolant system flow is derived by summing the individual pump flows.

2.2.3.2 Core Power Calculation Core power is determined by an auctioneering process between power values calculated by a primary side calorimetric and a correlation to turbine first stage pressure, both of which are calibrated periodically to the secondary side calorimetric. The primary calorimetric power is derived from the calculated volumetric flow and water properties based on measured values of cold leg temperature, hot leg temperature, and reactor coolant system pressure. The estimate of reactor power from turbine pressure is based on a third order polynomial fit to turbine first stage pressure. The secondary 15

l calorimetric power is derived from the measured values of feedwater flow,

feedwater temperature, steam flow, and secondary steam pressure. Appropriate allowances for energy gains and losses are included.

2.2.3.3 Power Distribution Calculation Signals from the fixed in-core neutron detectors and signals from the CEA pulse counter position indicators supply the input to the power distribution calculations. The calculations performed include:

1)

Determination of planar radial peaking factors based on CEAs present in each axial plane, 2)

Calculation of a normalized 40 node axial power distribution and a 3-D power peaking factor for use in the calculation of the LHR power operating limit.

3)

Calculation of a core average axial shape index (ASI),

4)

Determination of a 20 node hot channel axial power distribution and associated integrated radial peak for use in the calculation of the thermal margin power operating limit to DNB, and 5)

Calculation of azimuthal power tilt.

Both Feedwater flow and steam flow are determined from differential pressure measurement across known flow restrictions.

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2.2.3.4 Secondary Calorimetric Power Calculation Secondary calorimetric power is derived from measurements of steam header pr' essure, feedwater flow (as differential pressure), feedwater temperature, and steam flow (as differential pressure). These inputs are used to perform an energy balance on each steam generator and then the separate results are added. Corrections are made to the secondary calorimetric power for energy additions to and losses from the system, including letdown and charging pump flows, reactor coolant pump heat input, pressurizer heat input, and heat losses from NSSS components.

2.2.3.5 Linear Heat Rate Power Operating Limit Calculation The power operating limit is based on the core average full power linear heat rate, the linear heat rate limits (historically set by the LOCA), the calculated 3-D power peaking factor, and the calculated azimuthal tilt.

The LHR limit can be provided as a function of both inlet temperature and axial position.

2.2.3.6 Thermal Margin Power Operating Limit Calculation The thermal margin power operating limit calculation is based on the same methods used in the C-E developed thermal margin design computer code (CETOP) and incorporates the CE-1 Critical Heat Flux (CHF) correlation (see References 2 and 3). This calculation uses measured data from cold leg temperature sensors and reactor coolant system pressure sensors along with the hot channel axial power distribution and the primary system volumetric flow calculated 17

1 previously. The detailed calculation is performed at 30 secor:d intervals and i

is updated at one second intervals based on changes in reactor coolant system pressure, cold leg temperature, reactor coolant volumetric flow rate, az'imuthal tilt, and integrated radial peaking factors to provide the operator with a smoother response to changes in plant conditions.

2.2.3.7 Core Power Margin Calculation The core power margin calculation compares the actual power to the thermal margin and LHR power operating limits (POL) and to the licensed power limit.

Two sets of checks are done. The first set consists of two margin calcu-lations using the present value of the core power and two POLS. The second set consists of three margin calculations using running averages of both the power and the two POLS and includes calculation of the margin to the licensed power limit. These latter three margins are called " smoothed" margins.

In all, five margins are calculated and compared to appropriate limits. The smallest of the smoothed margins is selected for display on the digital panel meter and the CRT display.

2.2.4 COLSS Outputs A typical set of dedicated COLSS outputs to the plant operator are listed in Table 2-2.

These outputs include displays of core power, power operating limits, the minimum margin to any power operating limit, the COLSS master alarm, and the azimuthal tilt alarm.

The COLSS master alarm is activated when licensed power is exceeded, when either power operating limit is exceeded, or when a valid value of plant power or a power operating limit is unavailable.

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I This alarm is also activated when COLSS has been bypassed for testing.

Sample messages that can be displayed on the COLSS alarm CRT are given in Figure 2-4.

Additional displays and reports are incorporated in COLSS to assist the op'erator in monitoring the operation of the NSSS and in evaluating COLSS alarms. These additional outputs are:

1)

CRT displays of several hundred internal parameters (Figure 2-5 gives a sample of the types of parameters included),

2) a detailed printed report of all inputs and outputs, 3) an axial power distribution plot as illustrated in Figure 2-6, 4) a COLSS Failed Sensor Report listing all sensor inputs that have failed validity checking, and 5) a Test Mode Report to verify correct operation of the COLSS program.

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i TABLE 2-1 C)

TYPICAL COLSS MONITORED PLANT VARIABLES Typical Measurement Sensors Typical Number Range & Units Core volumetric flow Reactor coolant pump 2 per pump 100 - 1200 RPM rotational speed Reactor coolant pump 2 per pump 0 - 150 PSID differential pressure Core power Primary calorimetric Cold leg temperature 1 per cold leg l

Narrow range 525'- 625F Wide range 0 - 600F Hot leg temperature 1 per hot leg 525 - 675F Secondary calorimetric Feedwater flow I per generator 0 - 780 in water Steam flow AP 1 per generator 0 - 660 in water Feedwater temperature 1 per generator 100 - 500F Steam pressure 1 per generator 850 - 1050 PSIG Core power distribution In-core monitoring system 44 to 61 incore NA (power distribu-assemblies with 5 tion is provided axially stacked graphically) detectors each CEA position 1 per CEA 0 - 150 inches Reactor coolan't pressure Pressurizer pressure 2 (on pressurizer) 1,500-2,500 PSIA Turbine power Turbine first stage steam 2 (on turbine) 0-1,000 PSIA pressure i

4

l TABLE 2-2 TYPICAL DEDICATED COLSS OUTPUTS TYPICAL UPDATE OUTPUT OUTPUT QUANTITY DISPLAY RANGE UNITS FREQUENCY TYPE I

Plant Power 0 to 125

% Power 1 Sec.

Analog Power Operating Limit 0 to 125

% Power 1 Sec.

Analog based on Linear Heat Rate Power Operating Limit 0 to 125

% Power 1 Sec.

Analog based on Thermal Margin j

Minimum Margin to an

-50.0 to 125.9

% Power 1 Sec.

Digital Operating Limit a Digit Axial Shape Index

.7 to +.7 10 Sec.

Analog Margin alarm close - open 1 Sec.

Contact CPC Azimuthal Tilt alarm close - open 10 Sec.

Contact Tech. Specification close - open 10 Sec.

Contact Azimuthal Tilt alarm Axial Shape Index out close - open 10 Sec.

Contact of limits alarm 21 1

N FIXED IN-CORE HOT & COLD LEG T RATURE gfE j

INPUT,.

DETECTOR SIGNALS PRIMARY SYSTEM PRESSURE CONDARY Si PRIMARY COOLANT FLOW PROPERTIES (3 SEGMENTS)

I I

h1 1 r 1 r r 1 r 1r 1 r IN-CO RE REACTOR COLSS CPC ANALYSIS ENGINEER PROGRAM 1r 1 r i r

  • DETAILED 3-D POWER MARGIN TO LCD DISTRIBUTION VERIFICATION OF LIMITS ON:

TRIP SIGN ALS OUTPUT:

  • THERMAL MARGIN COLSS/CPC OUTPUTS
  • CORE POWER BASED ON:
  • TILT MAGNITUDE AND STORED CONSTANTS
  • PEAK LHR
  • PEAK LHR
  • BURNUP DISTRIBUTION
  • THERMAL MARGIN
  • THERMAL MARGIN
  • TILT MAGNITUDE WHERE:

OFF LINE (ON-SITE OR UTILITY OR C-E ON-LINE (PLANT OR ON-LINE (CORE REMOTE COMPUTER)

ENGINEERING STAFF CORE MONITORING PROTECTION COMPUTER )

CALCULATORS)

WHEN:

ON DEMAND AS NECESSARY SEVERAL TIMES PER SEVER AL TIMES MINUTE PER SECONO (AUTOMATICALLY)

(AUTOMATICALLY)

FUNCTION:

PROVIDE DETAILED VERIFY COLSS/CPC PROVIDE INFORMATION PROVIDE AUTOMATIC CORE INFORMATION RESULTS AND TO ASSIST OPERATOR IN PROTECTION AGAINST FOR ANALYSIS AND ACCEPTABILITY OF MAINTAINING CORE EXCEEDING LHR AND INTERPRETATION SY STORED CONSTANTS CONDITIONS WITHIN DNOR FUEL DESIGN ENGINEERING STAFF TECH. SPEC.

LIMITS OPERATING LIMITS l

FIGURE 2-1 OVERVIEW OF C-E CORE MONITORING AND PROTECTION SYSTEMS

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FEEDWATER TEMP. m STEAM PRES $URE SECONDARY Y

CALORIMETRIC FEEDWATER FLOW POWER ALARM / DISPLAY m

STEAM FLOW

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TURBINE POWER TURBINE 1st STAGE TURBlNE POWER AND AT POWER 9

COMPARISON 4

PRESSURE E

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CALORIMLTPIC C

POWER a

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REACTOR COOLANT m

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CORE POWER LIMIT POWER DENSITY I

AZlMUTHAL TILT Y

MAGNITUDE v AUDI8LE ALARM m

IN-CORE FLUX LICENSED PI)WER LIMIT NORMALIZED AXIAL

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POWER DISTRIBUTION FIGURE 2-2 FUNCTIONAL DIAGRAM OF THE CORE OPERATING LIMIT SUPERVISORY SYSTEM I

FIGURE 2-3 COLSS SENSOR LOCATIONS FIRST STAGE PRESSURE J L n

+ PRESSURIZER PRESSURE g

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STEAM PRESSURE STEAM PRESSURE STEAM FLOW AP j g j g STEAM FLOW AP g MAIN STEAM LINE 1 MAIN STEAM LINE 2 l

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i TEMPERATURE ap PUMP AP J L dL PUMP AP PRIMARY COOLANT LINES PRIMARY COOLANT LINES O TEMPERATURE TEMFERATURE FEE 0 WATER 4

FLOW AP FEED-J L REACTO f

WATER VESSEL FLOW AP LOOP 1A LOOP 2B RCP RCP o

1A RPM 28 RPlii

- FEE 0 WATER LINE 1 FEEDWATER LINE 2 -

PUMPAP PUMP AP O

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CONDENSATE FEE 0 WATER SEPARATORS TURBINES

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O FEE 0 WATER RC = REACTOR COOLANT PUMP TEWERATURES I

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FIGURE 2-4 TYPIC /L ALARM CRT MESSAGES Alarm 1 Messages (TIME)

XX:XX:XX ALARM COLSS DNBR POWER LIMIT EXCEEDED XX:XX:XX ALARM COLSS KW/FT POWER LIMIT EXCEEDED XX:XX:XX ALARM COLSS LICENSED POWER LIMIT EXCEEDED XX:XX:XX ALARM COLSS INSTANTANE0US DNBR POWER LIMIT EXCEEDED XX:XX:XX ALARM COLSS INSTANTANE0US KW/FT POWER LIMIT EXCEEDED XX:XX:XX ALARM COLSS LPL ALARM DURATION EXCEEDED XX:XX:XX ALARM COLSS DNBR ALARM DURATION EXCEEDED XX:XX:XX ALARM COLSS KW/FT ALARM DURATION EXCEEDED ALARM 2 and ALARM 3 Messages XX:XX:XX ALARM COLSS CPC TILT LIMIT EXCEEDED XX:XX:XX ALARM COLSS TECH SPEC TILT LIMIT EXCEEDED XX:XX:XX ALARM COLSS CPC TILT ALARM DURATION EXCEEDED XX:XX:XX ALARM COLSS TECH SPEC TILT ALARM DURATION EXCEEDED Alarm 4 Messages XX:XX:XX ALARM COLSS ASI OUT OF LIMITS XX:XX:XX ALARM COLSS ASI ALARM DUPATION EXCEEDED Other Alarm Messages XX:XX:XX ALARM COLSS REMOVED FROM SERVICE XX:XX:XX ALARM COLSS HOT LEG DEVIATION EXCEEDED l

25

FIGURE 2-5 SAMPLE PARAMETERS FOR CRT DISPLAY

' Parameter Description Usage Units COLSS HOT LEG TEMP-LOOP 1 INPUT DEG F COLSS HOT LEG TEMP LOOP 2 INPUT DEG F COLSS TURB IST STAEE PRES,PR INPUT PSIA COLSS TURB IST STAGE PRES,AL INPUT PSIA COLSS FW OUTLET TEMP, SG1 INPUT DEG F COLSS FW OUTLET TEMP, SG2 INPUT DEG F COLSS FEEDWATER FLOW OP SGI INPUT IN H2O COLSS FEEDWATER FLOW OP,SG2 INPUT IN H20 COLSS SECONDARY STEAM PR,SGI INPUT PSIG COLSS SECONDARY STEAM PR,SG2 INPUT PSIG COLSS STEAM FLOW OP, SGI INPUT IN H2O COLSS STEAM FLOW DP, SG2 INPUT IN H20 CEA REG GRP 1 MINIMUM POS INPUT IN CEA REG GRP 2 MINIMUM POS INPUT IN CEA REG GRP 3 MINIMUM POS INPUT IN CEA REG GRP 4 MINIMUM POS INPUT IN CEA REG GRP 5 MINIMUM POS INPUT IN CEA REG GRP 6 MINIMUM POS INPUT IN CEA REG GRP 1 MINIMUM POS INPUT IN CEA REG GRP 2 MINIMUM POS INPUT IN CEA S D GRP 1 MINIMUM POS INPUT IN CEA S D GRP 2 MINIMUM POS INPUT IN CEA REG GRP 1 DEVIATION INPUT IN CEA REG GRP 2 DEVIATION INPUT IN CEA REG GRP 3 DEVIATION INPUT IN CEA REG GRP 4 DEVIATION INPUT IN CEA REG GRP 5 DEVIATION INPUT IN CEA REG GRP 6 DEVIATION INPUT IN CEA P L GRP 1 DEVIATI0h INPUT IN CEA P L GRP 2 DEVIATION INPUT IN CEA S 0 GRP 1 DEVIATION INPUT IN CEA S D GRP 2 DEVIATION INPUT IN DET SENSTVTY CORR FLUX INPUT NV*E14 RCP IA SPEED OUTPUT RPM RCP IB SPEED OUTPUT RPM RCP 2A SPEED OUTPUT RPM RCP 2B SPEED OUTPUT RPM RCP 1A DIFF PRESS OUTPUT PSIO RCP 18 DIFF PRESS OUTPUT PSID RCP 2A DIFF PRESS OUTPUT PSID RCP 28 DIFF PRESS OUTPUT PSID RCS PRESSRZR PRESS OUTPUT PSIA RCS LOOP 1A COLD LEG TEMP OUTPUT DEG F RCS LOOP IB COLD LEG TEMP OUTPUT DEG F RCS LOOP 2A COLD LEG TEMP OUTPUT DEG F RCS LOOP 28 COLO LEG TEMP OUTPUT DEG F 26

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5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 BOTTOM CORE HEIGHT. PERCENT TOP FIGURE 2-6 COLSS POWER DISTRIBUTION PLOT I

2.3 Description of COLSS Algorithms As discussed in the overview (section 2.2), COLSS performs the following major ca'lculations:

1) calculation of reactor coolant system volumetric flow rate, 2) calculation of core power based on:

a) reactor coolant temperature rise across the core, b) turbine first stage pressure, and c) secondary system calorimetric, 3) calculation of core power distribution parameters including:

a) normalized core average axial power distribution.

b) azimuthat tilt magnitude, c) hot channel integrated planar radial peaking factors and 3-D peaking factors, and 4) calculation of power limits based on linear heat rate and on the departure from nucleate boiling ratio (DNBR).

This section provides additional descriptions of these calculations.

This material is intended to provide a general description of the scope and methodology implemented in the COLSS algorithms.

References are provided, as appropriate, to more detailed reports.

28

2.3.1 Reactor Coolant System Volumetric Flow The volumetric flow calculation is performed every second and provides the fl'ow input needed for the calculation of primary calorimetric power and of the power operating limit based on DNBR.

The flow through each pump is calculated based on sensor inputs of 1) pump rotational speed, 2) pump differential pressure, 3) cold leg temperature, and 4)

Reactor Coolant System (RCS) pressure.

Following validity checking of sensor inputs, the specific volume of the water entering the reactor coolant pumps is determined from cold leg temperature and RCS pressure. The differential pressure is then converted to pump head and is adjusted for the fraction of rated pump speed at which the pump is opera-ting. This result is then used to calculate volumetric flow in gallons per minute based on a polynomial fit to pump speed and the ratio of pump head divided by the square of the fractional pump speed. The coefficients of this fit are derived from pump testing. Total flow is then calculated as the sum of the flows frem each of the four pumps. A normalized vessel volumetric flow is also calculated.

The volumetric flows are also used to determine the mass flow rate for each cold leg as the ratio of the volumetric flow rate to the specific volume of the cold leg water. The total vessel mass flow which is the sum of the flows through the four cold legs, is provided for operator information.

  • allowance is made for core bypass flow in the DNBR calculation 29

-... - =.

i-(

i 2.3.2 Primary Calorimetric Power 1

i The primary calorimetric pcwer calculation is performed every second. This calculation of power uses the volumetric flow already calculated for each pump plus sensor inputs of:

1)

RCS pressure, i

2) cold leg temperature, and 3) hot: leg temperature.

i

?

f O

i The primary' calorimetric power calculation begins with the compensation of i

- tach of the four cold leg temperature indications for sensor time response and plenum mixing time. For each cold leg the compensation uses a digital filter 4

i which is implemented using the present and previous values of cold leg k

temperature and the previous value.of the compensated cold leg. temperature.

1

]

The coefficients of this filter are explicitly determined from the time i

l responses and the period of the calculation.

f i-I The enthalpy of the water in each hot leg and cold leg is determined from polynomial fits to the measured. hot leg temperatures, the compensated cold leg i

- temperatures, and the reactor coolant systen pressure.. Power is then

~

calculated from the enthalpy change between the cold and hot legs.

4 i

i 1

2.3.3 Turbine Power t

i The turbine power. calculation is performed every second. The only measured

~

input to this calculation is turbine first stage pressure.

The calculated 4

T l

30

,.L.-

. - + - - - -

-e

,c,-

y,

,re-.-w4 n.--,,,-..,-

-,ywy 3,wn-,y-m. m.

q-

.-.r+4,p-.7m-w_- m,,,

.w

-w-->-ew--

r.y'rTF-t="-'

i l

l power is given by a third order polynomial fit to the turbine pressure. All l

coefficients in the fit are determined empirically.

2.3.4 Secondary Calorimetric Power The secondary calorimetric calculation is performed once per 30 seconds using input values that are averaged over the previous 10 seconds to reduce the t

impact of sensor noise *. The measured inputs to this calculation for each steam generator are:

l 1) feedwater flow pressure drop, 2) steam flow pressure drop, 3) feedwater temperature, and 4) secondary steam pressure.

The calculated secondary calorimetric power is the sum of the power transferred to each steam generator and the energy lost from the system, less energy additions to the system.

  • For the Palo Verde COLSS, secondary calorimetric power itself is averaged over several calculations rather than using averaged input parameters.

~

31

2.3.4.1

-Power in Each Steam Generator The power transferred to each steam generator is calculated from feedwater en'thalpy, feedwater pressure, feedwater mass flow rate, feedwater specific volume, steam mass flow rate, and steam generator pressure. The calculated feedwater pressure (performed separately for each feed train) is the secondary j

steam pressure corrected for pressure losses from the feed injection point i

back to the pressure transducer.

The feedwater specific volume (derived from the feedwater temperature and pressure using standard water properties) is used to convert the measured feedwater flow pressure drop to mass flow for each feed train. A small temperature correction is provided in this conversion to account for changes 4

in flow resistances.

The measured secondary pressure for each steam generator is corrected for pressure losses between the steam generator and the sensor to obtain steam generator pressure.

The steam mass flow rate is calculated as the feedwater flow minus the blowdown mass flow rate (an input constant).

The power transferred to each steam generator is then calculated as the difference between enthalpy removal via the steam and blowdown mass flows, and enthalpy entry via the feedwater mass flow.

The quality of both steam flow and blowdown flow are properly accounted for.

32

2.3.4.2 Power Adjustments from the NSSS The calculated secondary calorimetric power is adjusted for power losses and po'er credits to the NSSS. The power losses are determined from input w

constants. On-line measured data is not used directly. The power losses that are included are:

1) letdown mass flow rate and enthalpy, 2) reactor coolant pump seal cooling mass flow rate and enthalpy, 3) cooling water mass flow rate and enthalpy, 4) mass flow rate of other primary coolant water leaving the system and 4

fts enthalpy, j

5) power loss from the pressurizer 6) power loss from primary coolant piping, 7) combined power loss from steam generators, and 8) other energy losses from the NSSS.

Similarly, the power credits to the system are also based on input constants.

The power credits that are included are:

I 1) charging pump mass flow rate and enthalpy, 2) total power input from active reactor coolant pumps, 3) power input from pressurizer heaters, 4) other sources of power input from electrical equipment, and 5)

'all other power input to the NSSS.

33

The final calculation of secondary calorimetric power is then simply the sum of the two steam generator powers plus the' net NSSS power losses (i.e., total losses minus total gains).

2.3.5 Plant Power Both primary calorimetric power and turbine power are calibrated using a correction factor based on the most recently performed secondary calorimetric calculation of power.

,,e

=

The larger of the two calibrated powers is 4

selected as plant power for display to the operator, for use in margin calc.ulations, and for use by the Power Dependent Insertion Limit (PDIL) CEA F*

Application Program. g F-

_J w

2.3.6 Core Power Distribution The major steps in deriving the core power distribution include:

1) conversion of incore flux measurements to assembly relative power by axial region, 2) determination of planar radial peaking factors from CEA position, l

3) synthesis of a core average axial power distribution, 34 i

4) calculation of azimuthal tilts, and 5) synthesis 'of a pseudo hot pin power distribution.

2.3.6.1 Conversion of Flux to Power Using methodology that is essentially identical to that used in CECOR

-(Reference 4 ), this algorithm converts incore detector compensated neutron flux to assembly relative power at each incore detector location at 10 second intervals.

l The flux to power conversion uses the incore detector compensated fluxes at cach of the five axial levels of each of the incore detector strings along with the CEA group positions.

The CEA group positions are used to provide an additive correction to the conversion factor to account for shadowing of a specific detector by a CEA in the same assembly.

For each string a power dependent correction factor is determined as a linear function of plant power.

The final conversion factor for a string is then the sum of the CEA shadowing correction plus the product of the burnup dependent correction factor and the power dependent correction.

4 The burnup dependent component is calculated daily using t!.e integrated power 4

at a detector location.

This " integration" is done stepwise assuming that the power has been constant over each 10 second interval. The depletion of fuel in the vicinity of a given detector location is taken to be proportional to i

i 35

the integrated power. The burnup dependent flux to power correction factor is then given by a polynomial in burnup.

2.'3. 6. 2 Planar Radial Peaking Factors The appropriate planar radial peaking factors are detennined for each axial node by a table lookup process based on indicated CEA group positions. This calculation is performed once per ten seconds and is done in two parts.

I 1)

Planar radial peaking factor tables are stored for each of the possible CEA configurations.

For each configuration, the table M

D contains the planar radial peaking facto i

I m

ama 2)

Penalty factors are applied to the radial peaking factors based on the determination of out-of-sequence CEA group insertion and excessive CEA deviations within any group. A pre-calculated CEA out-of-sequence penalty multiplier is applied if any out-of-sequence condition exists.

i l

A second penalty factor accounts for CEA deviations within a group.

The penalty factor for each CEA group is determined as a piece-wise Ifnear function of the size of the deviation.

The final deviation 36

penalty factor is the product, over all groups, of the individual penalty factors.

The magnitude of the penalty factor applied depends on the CEA group in which the deviation is occurring.

4 2.3.6.3 Axial Power Distribution 4

A forty node core average axial power distribution is calculated based on in-core detector power signals using a five mode Fourier series expansion.

This calculation is performed once per ten seconds to provide the power distribution used in the LHR calculation.

i For each of the 5 detector levels, the assembly relative powers calculated previously (see section 2.3.6.1.) are averaged over all incore locations with valid signals.

These average powers at each level are then normalized to have a sum of 100%. The normalized detector signals are transformed into fiva Fourier series weighting coefficients by evaluating the matrix product of a prestored " coefficient matrix" and the vector of detector signals. This prestored matrix depends only on the integral of the five Fourier modes over the axial length of the incore detectors. The 40 node power distribution is i

then constructed by forming the sum, at each axial node, of the Fourier functions (prestored in an array) times their respective coefficients. The axial power distribution is normalized so that the average value of the axial distribution is unity.

l 4

37

i Once the axial power distribution is available, the core average ASI is determined as the difference between the lower and the upper half core power fractions.

m

==

i

=

i 2.3.6.4 Hot-Pin Integrated Radial and ASI 7

A hot pin power distribution is deterinined as the product of the axial

.l power distribution and the planar radial peaking factor for each of th j

nodes. The integrated radial peaking factor is then calculated as the average of the hot pin power distribution over the axial nodes. The hot pin ASI is calculated in the same manner as the core average ASI except for the use of the hot pin power distribution.

2.3.6.5 Azimuthal Tilt The core average azimuthal tilt is calculated from the assembly average powers

)

once per 10 seconds using methodology that is essentially identical to that in CECOR (Reference 4).

The incore detectors are divided into " tilt groups" of 1

l 38

four detectors with appropriate symmetry properties. Depending on the plant, I 'l there are between nine and twelve tilt groups at each axial detector level, p

a.

For each tilt group, the sum and difference of the signals in opposite quadrants are calculated.

These sums and differences and a set of detector location dependent constants are used to calculate an azimuthal tilt for each group.

The average azimuthal tilt at each level is then calculated as an " arithmetic average" of the magnitude of the individual group tilts at that level.

In some plants,

=

b The core average azimuthal tilt is calculated by averaging the 5 level tilts using a weighting factor for each level that is based on the number of valid sets of detectors at that level.

If the calculated azimuthal tilt is higher than either the Technical Specification limit or the allowance used by the Core Protection Calculators (CPCs), then an alarm is initiated.

2.3.6.6 Three-D Power Distribution The 3-D power peaking factors are calculated for use in the linear heat rate power operating limit calculation.

The 40 n' ode 3-D power distribution is then 39

determined as the product of the radial peaking factor (Section 2.3.6.2) and the value of the 40 node core average axial power distribution (Section 2.3.6.3) at each node. The maximum value of these products is the 3-D power pe'aking factor which is made available for operator information.

2.3.7 Linear Heat Rate Power Operating Limit The core power operating limit based on the Linear Heat Rate (LHR) limit is calculated once per 10 seconds.

This calculation is used to monitor the LHR limit normally established by Loss of Coolant Accident (LOCA) considerations.

The linear heat rate is calculated for each of the 40 nodes of the 3-D power distribution.

This linear heat rate is the product of the normalized power fraction in the node, the core average linear heat rate at rated power, and the fraction of core power at which the plant is operating. Correction factors are applied to account for the azimuthal tilt and modeling uncertainties.

The power operating ifmit at each node is calculateo as the product of plant power, a correction factor to account for failed incore detectors, and the LHR limit divided by the calculated linear heat rate for that node. The minimum value calculated in this manner is the LHR power operating limit.

It is this value which is compared to the current value of plant power.

I l

\\

\\

~

40

h 2.3.8 Thermal Margin Power Operating Limit Th'e thermal margin power operating limit is based on maintaining the calculated DNBR above a specified minimum value (based on the CE-1 CHF correlation) and maintaining the fluid quality below a specified maximum value at the point of minimum DNBR. The thermal hydraulic model used to evaluate this limit is i

based on the C-E proprietary code CETOP and the CE-1 CHF correlation (see references 2 and 3). This calculation is performed once per 30 seconds with a I

dynamic update provided once per second.

The thermal-hydraulic modeling uses I

~

J m

41

The calculation proceeds in an iterative manner in that an estimate of the poweroperatinglimit(POL)isusedtodeterminetheminimumDNBRandthe quality at that point.

If both the DN3R and the quality are within their re'spective limits, the algorithm raises the POL estimate and recalculates the DNBR and quality. Similarly, if either of DN8R or quality are not within their limits, the POL is lowered. This iteration continues until it finds the maximum POL that meets both DNBR and quality limits.

I The details of this calculation have been amply described in references 2 and 3 and will not be repeated here. The calculation does incorporate an adjust-ment to account for the margin required for the Loss of Flow event in the resultant POL. This adjustment is discussed further in Section 3.3.

2.3.9 Thermal Margin Power Operating Limit Update The detailed thermal margin calculation is only performed once per 30 seconds.

An approximate update to the most recent detailed calculation is performed once a second to provide the operator with a smoother indication of the core performance. The updated DNBR power operating limit (POL) is based on changes in several measured and derived parameters including:

1) primary pressure,
2) maximum compensated cold. leg temperature,
3) core flow rate,
4) integrated radial peaking factor, 42
5) azimuthal tilt, 6)
7) quality at the node of minimum DNBR,
8) most recently calculated power operating limit,
9) POL derivative with respect to quality, and i

i

10) POL derivative with respect to DNBR.

I 2.3.10 Core Power Margin The core power margin calculation compares the actual power to the thermal margin and LHR power operating limits (POL) and to the licensed power limit.

Two sets of checks are done. The first set consists of two margin calculations using the present value of the core power and the two POLS. The second set consists of three margin calculations using running averages of both the power and the two POLS and includes calculation of the margin to the licensed power limit. These latter three margins are called " smoothed" margins.

In all, five margins are calculated and compared to appropriate limits. The smallest of the smoothed values is displayed on the digital panel meter and CRT display and is referred to as MARGIN.

If any of the 5 calculated _ margins is less than its respective limit, an alarm is initiated.

Before being used in these comparisons, the calculated power operating limits are adjusted for power measurement biases. These biases are dependent on the measured power level and on which of the three calculated powers have been used to determine plant power (see section 2.4.1).

43

2.4 Uncertainties The calculation of DNB and LHR power operating limits requires numerous me'asured inputs and calculated constants.

Each of the measured inputs (i.e.,

temperature, pressure, etc.) and the calculated constants (i.e., fuel and poison rod bow, system parameters, etc.) can have some uncertainty associated with it. These uncertainties are applied in a conservative fashion to reduce the predicted power operating limits to ensure that adverse combinations of uncertainties do not prevent alarms when limiting conditions for operations are violated. References 1 and 5 describe the methods used to determine these uncertainties.

2.4.1 Power Measurement Bias The accuracy of the power measurement is a function of the frequency of calibration and the method for determining the present power output. The secondary calorimetric is the most accurate measure of reactor power and generally has a net uncertainty of less than or equal to 2% of rated power

=

1-near full power increasing to no more than of rated power at low power.

% J The primary calorimetric and the turbine first stage pressure determinations of power are less accurate having a typical uncertainty of 3.5%.

Biases are applied to the POLS to account for these uncertainties. All of the bias terms are calculated as a function of plant power.

Before a bias is applied to the power operating limits, the validity of each of the three methods for deriving plant power is determined.

If the secondary calorimetric calculation is valid then its bias is chosen since the other powers are periodically calibrated to 44

match it.

If the secondary calorimetric is invalid, but either of the other two power determinations is valid, then an appropriate, larger bias is applied. The bias tenn is subtracted from the calculated power operating ifmits to obtain the biased power operating limits.

2.4.2 Power Operating Limit Uncertainties Other uncertainties associated with the calculation of the power operating limits are accounted for in a conservative fashion in the power operating limit algorithms by applying additive and/or multiplicative adjustment factors.

The uncertainty factors considerea in the generation of these terms for DNBR and LHR power operating limit calculations are:

1) uncertainty in in-core detector signal measurement,
2) uncertainty in Control Element Assembly (CEA) position measurement,
3) uncertainties in temperature, pressure, and flow measurements,
4) uncertainty in verification of tabulated planar radial peaking factors (Fxy) using CECOR, 5) impact of the COLSS power distribution synthesis on the LHR algorithms and DN8 overpower margin,
6) uncertainty in COLSS ONB-0PM algorithm with respect to design calculations,
7) computer processing uncertainties,
8) fuel and poison rod bow uncertainties,
9) global axial fuel densification uncertainty, and
10) engineering factors due to manufacturing tolerances.

45

l The generation of these uncertainty terms is discussed briefly in Section 3.4.

A more detailed description can be found in Reference 5.

3.0 Constants and Supporting Data To support the COLSS algorithms, numerous constants based on plant design characteristics must be generated for incorporation into COLSS. These constants can be divided into 5 major categories:

1) constants related to plant mechanical and thermal hydraulic design, 2) constants related to core design, 3) constants related to monitoring margin to limiting conditions for operation, 4) constants related to measurement and calculational uncertainties, and 5) constants required to support on-line DNBR calculations.

Each of these areas will be discussed to provide some background into the basis for the constants in that area. General descriptions of the types of analysis used to determine the constants will be provided where appropriate.

3.1 Basis for Mechanical and Thermal-Hydraulic Constants Calculations of the RCS volumetric flow rate and calibrated power depend on constants which are based on the NSSS thermal hydraulic and mechanical design.

The volumetric flow calculation is determined by a polynomial fit to measured values of RCP differential pressure and pump speeds. The constants for this 46

calculation are based on a curve fit of experimental pump characteristic data obtained from operation of the RCP's in a test loop.

RCS rated flow and RCP rated speed are also used in the flow calculation.

The primary calorimetric power is based on calculated fluid enthalpfes and measured flows, temperatures, and pressures. No significant constants are required to support a strictly static primary calorimetric power calculation beyond standard water property tables. However, this calculation includes a dynamic compensation of variations in cold leg temperatures. The cold leg temperature compensation depends on the cold leg temperature sensor time constant and the calculated plenum time constant based on RCS design (sensor location, flow path, and flow rate). The core rated power is provided as a data base constant to permit normalization of the calculated power to percent of rated power.

The secondary calorimetric power is based on measurements of feedwater flow, steam flow, feedwater temperature, and steam pressure. Most of the constants used in the power calculation are derived from or confirmed by field data obtained during power ascension testing. These constants relate feedwater pressure to secondary steam pressure and steam flow, relate steam generator pressure to steam header pressure and steam flow, and quantify energy losses from and credits to the system (including the gain associated with operation of the RCP's).

The relationship of feedwater mass flow rate to feedwater temperature, feedwater flow, and feedwater specific volume is based on venturi 47

characteristic test data.

The calculated turbine power is based on a polynomial which is fit to the data obtained during power ascension testing.

3.2 Basis for Core Design Constants 3.2.1 Conversion of Flux to Power Constants The conversion of the flux signal for each incore detector to relative power uses correlation coefficients that reflect detector location, local ge: metry, and local burnup. These coefficients are the same as these used in the CECOR off-line power distribution calculation (Reference 4). The System 80 plants require additional adjustments in those bundles which have both a CEA and an in-core detector string. Other C-E plants using COLSS do not have CEA's entering instrumented assemblies and do not require these adjustments.

3.2.2 Planar Radial Peaking Factor Look-up Tables Prior to startup, neutronics calculations are performed to determine the maximum expected planar radial peak for each CEA configuration allowed by the CEA Power Dependtnt Insertion Limit (PDIL).

Detailed calculations are generally performed for the unrodded core and for CEA configurations containing only the part length CEAs and the first two lead regulating banks.

Conservative, bounding values are determined for other configurations including those which involve insertion of shutdown CEA banks.

48

l The maximum radial peak expected is installed in COLSS for each configuration.

During start-up testing, measurements are performed with CECOR to verify the peaking factors for the CEA configurations that are pe7nitted at higher po'wers. Adjustments to the stored constants are made if appropriate. CECOR calculations are performed periodically during the cycle to verify the continued adequacy of the installed constants as required by the Technical Specifications.

Penalty factor constants for CEA banks out of sequence and CEA misalignment are determined to assure an alarm if the CEA misoperation degrades the margin below the allowed LCO. These constants are based on analyses using standard neutronics methods to determine the change in power distribution due to the CEA misoperation.

An additional, addressable, multiplicative penalty factor on the radial peak is available to compensate for special circumstances requiring change after the COLSS constants have been installed.

3.2.3 Axial Power Distribution Constants The incore detector signals are converted into a 40 node core average power distribution using two arrays of constants. The first array converts the planar averages of the incore detector signals to amplitude coefficients of a Fourier series approximation of the axial power distribution. These constants depend only on the axial location and the length of the incore detectors, and on the Fourier modes used. The second array is a tabulation of Fourier mode 49

i values at each of the axial locations which are precalculated to reduce the COLSS calculation time.

3.'2.4 Azimuth-Tilt Calculation Constants 1

The azimuthal tilt calculation requires detector location dependent constants for each " tilt group" of four detectors and appropriate averaging factors.

These factors are used primarily to account for geometric effects (detector location) but also include an average radial tilt sensitivity from 3-D neutronics calculations. CECOR is run at regular intervals to verify the accuracy of the COLSS azimuthal tilt calculations.

3.2.5 LHR Limit Constants The maximum allowed steady state LHR limit specified in the Technical Specifications and monitored by COLSS is typically based on the Loss Of CoolantAccident(LOCA). This limit is specified as a function of core inlet temperature in the COLSS of some plants.

3.3 Basis for DNB Margin Monitoring Coretants The Limiting' Conditions for Operation (LCO) in the Technical Specifications assure that sufficient margin is available to cover the degradation in DNB margin that can occur during any Anticipated Operational Occurrence (A00).

Such a margin loss can be caused by an increase in local power or temperature, by a decrease in core flow or pressure, or by an adverse change in the core 50

-_--u-------_---

power distribution. The margin assured by the LCO is sufficient to cover co'ntinued adverse changes from the time the event begins until either corrective action is taken or a power reduction caused by a reactor trip be' gins to recover margin.

COLSS monitors the margin required by the LCOs through the use of an is sufficient to accomodate A00s without violating a fuel J

q.

design limit. The is defined as a function of ASI to reflect the

.s sensitivity of the margin loss during some A00's to the initial axial power distribution.

Historically, the Loss of Flow (LOF) analysis has determined the acceptable U

This section provides a brief overview of the types of A00 that could limit the thermal margin requirement.

3.3.1 Derivation of the from the loss of Flow Analysis

=

.i For C-E plants, the Loss of Flow event has historically been the most limiting with respect to thermal margin. During the few seconds after the pumps begin 51

t slowing down and prior to a significant power reduction due to the CEA insertion, the reduced flow causes a rapid decrease in DNB margin. Several seconds into the event the heat flux / flow combination results in the minimum CNBR that will be experienced during the transient. The specific time and vclue of this minimum is a function of the axial power distribution and the initial thermal and hydraulic conditions in the core.

P 9

.i Numerous power distributions and initial conditions are used to determine the e-9 over the ASI range of interest using the HERMITE (Reference 6) or CESEC (Reference 7) transient l

codes.

The

, calculated in this manner is represented in COLSS by a piece-wise linear function of ASI which bounds the values determined in the transient cases.

3.3.2 Other Events Analyzed to Confirm Adequate Monitoring l

1 Other A00's can also result in degradation of thermal margin.

If the safety analysis were to indicate that one or more A00s require more margin than the e

p LOF, then the would be adjusted so that COLSS monitors the larger thermal margin requirement. Two events that could require more thermal margin than LOF are the Asymmetric Steam Generator Transient (ASGT) and the CEA Drop.

An Asymmetric Steam Generator Transient may result from the inadvertent closure of a Main Steam Isolation Valve. The resulting asymmetric core inlet 52

temperature distribution results in increased core power peaking on the cold side. This event is protected by an asymmetric steam generator transient trip on cold leg temperature difference (AT) in the CPC, but also requires that adequate thermal margin be available to cover temperature asymmetries that occur prior to trip actuation. This event is simulated by design transient codes for different values of the AT setpoint.

The increase in the radial peak used in the safety analysis is calculated as a function of the temperature tilt using standard physics methodology. The selected AT setpoint and the available margin monitored by COLSS are shown to be adequate for the ASGT event or the is adjusted to allow for the extra margin.

2 Plants which include the Core Protection Calculator System as part of the Reactor Protection System have the capability to accommodate deviated CEAs via penalty factors generated by the CEA calculators.

If a SAFDL violation is conservatively predicted by the CPCs following application of the penalty, then the reactor will trip.

If not, operation can continue in accordance with the Technical Specifications. As a result, COLSS has not been required to verify that adequate margin has been set aside to cover margin degradation during a dropped CEA event when the CEA calculators in the CPCs are operable.

Recent modifications in analysis methods on several plants have demonstrated that the thermal margin assured by the LCOs is sufficient to accommodate a dropped CEA event. This has eliminated the need for a reactor trip during CEA insertion events or CEA drops. Thus, comparisons have been made using standard neutronic methods to assure that the COLSS DNBR-POL calculation using the is conservative for these events.

For these plants, COLSS has been modified to incorporate i.

4 53

I I

3.3.3

_COLSS Penalty Factors Applied for CEA Calculators Inoperable If the CEA calculators of the CPCs are not in operation, automatic trip protection for CEA deviation events is not provided. Therefore, adequate margin must be set aside per the Technical Specifications.

In the COLSS calculation, this margin degradation during CEA related transients is accounted for by an addressable input constant to the DNB calculation. The value of this constant is determined by simulating the CEA misoperation distortion factor using neutronics codes.

3.4 Basis for Measurement and Calculational Uncertainty Constants Two uncertainty penalties are calculated for COLSS; one which is used in calculating the linear heat rate power operating limit and the other which is used in calculating the DNBR power operating limit.

The LHR adjustment accounts for the composite modeling uncertainty in the COLSS determination of the 3-D peak and for the various engineering factors.

This modeling error is determined from a set of several thousand comparison cases between COLSS and design codes covering suitable ranges of power level, core burnup, CEA position, and primary system fluid properties. The overall adjustment factor accounts for the effects of fuel rod bow, poison rod bow, design code modeling uncertainty, COLSS power algorithm uncertainty, CECOR F 54

measurement uncertainty, and computer processing uncertainties.

In the COLSS algorithm, this adjustment is applied as a multiplier to the core average linear heat rate. This has the effect of reducing the linear heat rate power op'erating limit.

Similarly, the DNBR adjustment accounts for the composite modeling uncertainty in the COLSS determination of the hot pin power distribution and power as well r

e as the f

m" i

jThiscompositemodeling.rrorisbatedonthesamesetofcomparison cases between COLSS and design codes used for the LHR uncertainty calculation.

The overall adjustment factor includes the effects of fuel rod bow, poison rod bow, design code modeling uncertainty, CECOR F measurement uncertainty, xy COLSS DNB algorithm uncertainty, and computer processing uncertainties.

For most COLSS plants, the system uncertainties are combined statistically and included in the minimum DNBR limit that is established for use with the CE-1 CHF correlation. The uncertainties accounted for include inlet flow distribution uncertainties, fuel pellet density uncertainties, fuel pellet enrichment uncertainties, fuel pellet diameter uncertainties, random and systematic uncertainties in fuel clad diameter, random and systematic uncertainties in fuel rod pitch, and CHF correlation uncertainties.

In the cases where the statistical combination method is not used, the various listed uncertainties are accounted for by a multiplicative adjustment to the power operating limit.

55

retails of the methodology used to determine the measurement and calculational uncertainties for COLSS can be found in Volet.a 3 of Reference 5 for the statistical combination of uncertainties method or in reference 1 for the arternate methods.

3.5 Basis for Constants Supporting On-Line DNB Calculations The DNB calculations performed in COLSS use a simplified, faster running version of the design CETOP code called CETOP-1. Most of the constants used in CETOP-1 are identical to those in CETOP or are the product of CETOP constants which are provided to reduce computer calculation time. Three significant differences exist between CETOP-1 and CETOP to reduce the computer run time, mum i

M I

56

ammaspk 4

e 57

4.0 Conclusion The preceding discussions have provided an overview of the COLSS program as us'ed in recent C-E NSSS designs. This system uses measurements of incore detector signals, CEA positions, and plant thermal-hydraulic properties to provide an on-line determination of the core power distribution and thermal margin performance. The results of these calculations are provided to the plant operator through various displays to aid him in maintaining the plant within the Limiting Conditions for Operation as specified in the Technical Specifications.

58

l 1

5.0 References 1.

" Assessment of the Accuracy of PWR Operating Limits as Determined by the Core Operating Limits as Determined by the Core Operating Limit Supervisory System (COLSS)", CENPD-169, July 1975.

2.

"CETOP-D Code Structure and Modeling Methods for San Onofre Nuclear Generating Station Units 2 and 3", CEN-160(S)-NP, September 1981.

m-3.

"C-E Critical Heat Flux - Critical Heat Flux for C-E Fuel Assemblies with Standard Spacer Grids", Part 1 CENPD-162-A September 1976, Part 2 CENPD-207-A December 1984.

4.

" INCA /CECOR Power Peaking Uncertainty", CENPD-153, Rev. 1-A, May 1980.

5.

" Statistical Combination of Uncertainties - Uncertainty Analysis of Limiting Conditions for Operation of the San Onofre Generating Station Units 2 and 3",

Part 3, CEN-283(S)-NP, October 1984.

6.

"HERMITE - A Multi-dimensional Space Time Kinetics Code for PWR Transients: CENPD-188, March 1976.

7.

"CESEC - Digital Simulation of a Combustion Engineering Nuclear Steam Supply System" Enclosure 1-NP to LD-82-001, January 6, 1982.

  • Plant specific references which are intended to be typical of similar references appropriate to other plants 59

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