ML20151V834

Non-proprietary Rev 2 to NSA-SSO-96-525, Jm Farley Nuclear Plant Safety Analysis IR Neutron Flux Reactor Trip Setpoint Change

ML20151V834
Person / Time
Site:	Farley
Issue date:	09/30/1998
From:	WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
To:
Shared Package
ML20138K955	List: ML20151V783 ML20151V822 ML20151V834
References
NSA-SSO-96-525, NSA-SSO-96-525-R02, NSA-SSO-96-525-R2, NUDOCS 9809150084
Download: ML20151V834 (8)

Text

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NSA-SSO-96-525, Rev. 2 September,1998 l

JOSEPH M. FARLEY NUCLEAR PLANT SAFETY ANALYSIS 1 INTERMEDIATE RANGE NEUTRON FLUX ,

i REACTOR TRIP SETPOINT CHANGE t l

Introduction i The Intermediate Range Nuclear Instmmentation System (NIS) channels are designed to provide information to the plant operators about the neutron flux and power in the reactor core when operating in the low to mid power range. The Intermediate Range (IR) channels also provide a reactor trip signal on increasing power if the power level 4

rises above the trip setpoint (typically 25% Rated Thermal Power). During controlled plant startup, the IR reactor trip is manually blocked above the P-10 permissive (typically 10% Rated Thermal Power), which is provided by the Power Range (PR)

NIS detectors. Although the safety analyses do not explicitly assume a trip from the IR NIS, it serves as a backup for the PR NIS trip at low power level, thus providing diversity in the reactor trip system (RTS). In addition, the IR provides a control interlock function (C-1) and a permissive (P-6). The C-1 control interlock blocks j control rod withdrawal at power levels greater than 20% Rated Thermal Power (RTP),

unless manually bypassed above P-10. The function of P-6 is to allow a n:anual block of the Source Range reactor trip at 10* amps on increasing power and to automatically enable the Source Range reactor trip on decreasing power.

During routine plant operations, problems with the IR trip can arise bemuse the outputs of the IR and PR detectors are subject to different [

]+"'*. The IR channels are typically calibrated one time at the beginning of the cycle, while the PR channels are normalized to a secondary side power calorimetric on a daily basis. Over time, the large magnitudes of the

[ ]+*** can potentially lead to overlap of the P-10 permissive (as measured by the PR detectors) and the IR reactor trip setpoint.  :

When such overlap occurs, routine plant evolution such as plent/ reactor shutdown j must be delayed until the NIS 1R channels can be recalibrated. Should the operators j fail to observe an overlap condition, then an inadvertent reactor trip would result. To alleviate these types of problems with this trip function it has been proposed that the Technical Specifications IR reactor trip setpoint be increased from 25% to 35% RTP to increase the operating margin between PR P-10 and the IR reactor trip setpoint. j The trip uncertainties (including the [ ]+*'*) were evaluated to support this Technical Specifications change. As part of this effort, Farley-specific data was 4 gathered and evaluated to better define the [ ]+*'*. In Page 1 9809150084 980911 PDR ADOCK 05000348 p PDR

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! NSA-SSO-96-525. Rev. 2 September.1998 i

addition, all other terms in the IR setpoint calculation were reviewed with respect to actual plant practices and equipment capabilities. This evaluation pavides the basis i for updating uncertainty terms in the IR setpoint uncenainty calculation. The results of those evaluations are described in this document and suppon raising the trip setpoint to 35% RTP.

Current IR Basis When computing the instrument uncertainties for the Farley 1 & 2 IR channels, the Westinghouse setpoint methodology (described in WCAP-13751, Rev. 0) initially assumed a [ ] "'* total uncertainty for the [

]+"'* term. This value was broken down further into [

]+"'*. The values assumed for IR trip setpoint allowances in the Farley Setpoint Study appear below.

_ +

_ a.c The above effects were treated as statistically [ ]+***

parameters, and thus were combined using [

]+"'*. Therefore, the total combination is [ ]+***. As mentioned before, the assumption in WCAP-13751, Rev. O is [

)+a,c Test and Data Analysis The IR detectors are approximately centered about the mid-plane of the core. Because of the relatively short length of the IR detectors (compared to the longer length PR detectors) the IR detectors are more sensitive to [

j + a,c As a result, the IR channels can see a significant shift in indicated power over the cycle. In addition, since each of the IR detectors is located just a few assemblies away [

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NSA-SSO-96-525, Rev. 2 September,1998

]+"'*. That is, [

j + a,c In order to better quantify the [ ]+"'*, Farlegperformed a test on Unit I to measure the effect of changes [ ]

• on the NIS IR detector currents. Reactor power was [

]+*** as test data was taken. The [

]+"'*. This test provided two sets of data; one for N35 and one for N36 [ ]+"'". Since the

'[

]+"'* in the analysis. A test performed at the beginning of the previous cycle in Parley Unit I also provided data for the N35 and N36 detectors over [

]+*' . Although the rods ] ,

were not [ +"'* the results are ve

]+"'* to the data analyzed and confirm the repeatability q of the effects described below.

Several different [

i ]*** . The resulting plots approximate [ ]+*** and demonstrate i dramatically the impact of [

]+** The data  !

shows that the N35 detector output can change from [ l

]+***. Based on Farley data, the sensitivity of the N35 detector output to power at a [

]+"'*. A similar analysis was performed for the N36 data; however, this detector exhibits different characteristics with a variation with respect to [

]+"'* It should be noted that differences between detector characteristics are not unusual and have been observed at other plants. In order to minimize the errors, and I provide for a single calibration point for both detectors, it is recommended that the calibration be performed [

]+"'*. 'Ihis results in a maximum nonconservative error of [

]+"' for the N36 detector and [ ]+** for the N35 detector.

Therefore, the maximum [

]+"'*. The maximum [

]+"'*. Since the N36 effect is more limiting, it will be used in the calculation of the setpoint uncertainty. In addition, based on a review of Farley data at 100% RTP [

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NSA-SSO-96-525, Rev. 2 September,1998

) + a,e The Farley results are similar to those seen at other plants, and the [

]+ accounted for in the setpoint uncertainty is expected to be bounding for both Unit I and Unit 2, power uprating conditions, differences in individual detector characteristics, and future core desgn changes which do not [ {

]

In addition to [ ]+ , the [ ]+"'* terms were also revised to

better reflect plant operating conditions at the Farley Units. The [ I 1

l 4

] "#. The previous [ j i

l

]+*' . In addition, the l I

data that was used to determine the [

J + a,c Use of the updated allowances (rather than an estimate based on engineering judgment) results in a more accurate PMA uncertainty calculation as noted below.

__ __ +a.c s

1 The equation for CSA for the Intermediate Range channels used in the Farley Setpoint Study (WCAP-13751, Rev. 0) has the following form.

CSA = ((PMA)* + (PEA)* + (SCA + SMTE + SD)' + (SPE)' + (STE)' + )

I (RCA + RMTE + RCSA + RD)' +(RTE)'}"' + EA l

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NSA-SSO-96-525, Rev. 2 September,1998 I

where:  !

CSA =

Channel Statistical Allowance i PMA =

Process Measurement Accuracy PEA =

Primary Element Accuracy i SCA =

Sensor Calibration Accuracy SMTE =

Sensor Measurement and Test Equipment Accuracy SD = Sensor Drift SPE =

Sensor Pressure Effects

STE =

Sensor Temperature Effects RCA =

Rack Calibration Accuracy RMTE =

Rack Measurement and Test Equipment Accuracy RCSA =

Rack Comparator Setting Accuracy RD = Rack Drift l RTE = Rack Temperature Effects EA =

Environmental Allowance l The value of CSA for Farley Units 1 & 2 in the current WCAP-13751, Rev. O is

[ )+"'* for a span defined to be 0 to 120 %RTP.

For the determination of a new uncenainty for the IR reactor trip setpoint, the latest Westinghouse CSA algorithm, which more accurately reflects plant operating practices has been used. This enhancM algorithm has the form shown below and is in concert with ISA 67.04 guidelines.

CSA = [

j+a,c In addition, the IR setpoint calculation in WCAP-13751, Rev. O made certain

. assumptions about the IR channel racks which have been revised to better reflect the actual calibration and performance capabilities of the equipment. Due to the fact that the IR channel is designed to provide information over a span of 8 decades and that readings are taken from a log meter, the channel tolerances and allowances must be relatively large in order to be achievable. This revised calculation has, therefore, included increased values for the rack calibration accuracy, the rack temperature I

effect, and the rack drift. A reference accuracy (RRA) has also been included to account for the [ ]** '* .

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a NSA-SSO-96-525, Rev. 2 September,1998 Using the new values determined above for [

]+"'* in conjunction with the enhanced uncertainty algorithm, the new CSA for the IR trip setpoint is [ ]**' for the 120% RTP span. Table I lists the various components of the CSA calculation.

As discussed earlier, this trip is not explicitly credited in any safety analysis; therefore, there is no defined Safety Analysis Limit (SAL). With the nominal trip setpoint at 35% RTP and a CSA of [ ]+"'*, the trip is assured of actuating before reaching 60% RTP. This bounding value is sufficient to provide a diverse backup (consistent with WCAP-7306, Reactor Protection System Diversity in Westinghouse Pressurized Water Reactors) for the Power Range Iow Setpoint which has a nominal trip setpoint of 25% RTP and a SAL of 35% RTP.

The Allowable Value (as specified in the Technical Specifications) is associated with

- uncertainties in rack electronics. The intent of the Allowable Value is to provide the plant with a way to assess the operability of the process racks. With an upper calibration limit of 35% RTP and a lower calibration limit of 29.5% RTP, an Allowable Value of 40% RTP will accommodate the [

]+"' The present Allowable Value in the Farley Unit 1 & 2 Technical Specifications is 30% RTP with a trip setpoint of 25% RTP. The new recommended Allowable Value is 40% RTP with a Nominal Trip Setpoint of 35% RTP.

This analysis has no impact on the C-1 control interlock or the P-6 permissive, which are to remain at 20% RTP and 10" amps, respectively. Explicit uncertainty calculations are not traditionally performed for these functions since there are no SALs associated with interlocks and permissives which do not provide trips or ESF.

Conclusion The purpose of this analysis is to support the revision of the NIS IR nominal trip setpoint from a value of 25% RTP to 35 % RTP. In addition, this analysis supports an Allowable Value of 40% RTP.

The revision to the IR trip setpoint required alterations to the [

]+"'* in the CSA calculation. The value of the [ ]+^'* term is based on the assumption that the IR detectors are calibrated [+a,c Page 6 l

NSA SSO-96-525. Rev. 2 September,1998 l

I The revised setpoint unceitainty calculation results, using Westinghouse methodology, I justifies the proposed Technical Specification changes to the NIS IR nominal trip setpoint and Allowable Value. Increasing the trip setpoint results in increased operating margins between NIS excore PR and IR trip functions, which reduces the likelihood of inadvertent plant trips during plant shutdown. l 4

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NSA-SSO-96-525 Rev. 2 September,1998

. , TABLE 1 INTERMEDIATE RANGE, NEUTRON FLUX Parameter Allowance

• Process Measurement Accuracy - - **

Primary Element Accuracy sensor calibration I

[ ]+=,c j l

Se sor Measurement & Test Equipment Accurac ,

Sensor Pressure Effects Sensor Temperature Effects

[ ]+a.c sensor Drift i

[ ]+..c Environmental Allowance l Rack Calibration Rack Accuracy [ ] ** *

• Measurement & Test Equipment Accuracy i

Rack Reference Accuracy [ ] ** ' ' l 4

comparator [ ] ** ' '

Rack Temperature Effects [ ] ** '

• Rack Drift

[ ]+a.c In % span (defined to be 120% Rated Thermal Power) channel statistical Allowance =

_..e

+

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