ML20081K774

Incore Detector Algorithm (Pidal) Analysis of Quadrant Power Tilt Uncertainties

ML20081K774
Person / Time
Site:	Palisades
Issue date:	08/14/1990
From:	Baustian G CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.)
To:
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ML18057B003	List: ML20081K774
References
NUDOCS 9107020012
Download: ML20081K774 (15)

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i PALISADES INCORE DETECTOR ALGORITHM (PIDAL) l I

ANALYSIS OF QUADRANT POWER TILT UNCERTAINTIES G.A. Baustian Consumers i>cwer Company August 14,1990 1

9107020012 910625 PnR ADOCK 05000255 P PDR

CONTENTS 1: Objective 2: Summary of Results 3: Assumptions 4: Analysis Methodology S: Analysis Results 6: Palisades Core Map i

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9 Objective The purpose of the work described by this analysis was to determine the accuracy of the full core PIDAL power distribution calculations when the true core power distribution is radially tilted. This is in response to comments made by the USNRC while reviewing the PIDAL methodology and uncertainty analysis.

In particular, the NRC requested the following:

1- A comparison of the tilt measured by PIDAL with the true or theoretical tilt.

2- Verification that the PIDAL code programming was correct by supplying theoretical detector input and comparing the resulting PIDAL solution with the original theoretical power distribution solution.

3- Determination of the Srm uncertainty component for radially perturbed or tilted powei distributions up to the full power Technical Specification limit of 5% quadrant power tilt.

4- An explanation of what assumptions are made in the Palisades Safety Analysis to cover radial peaking factor increases caused by quadrant power tilts.

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Summary and Conclusions Comparisons Laeen the quadrant power tilts determined by the PIDAL model w ere made to corresponding theoretical values. It was found that in all cases PIDAL either accurately measured the quadrant power tilt, or in some instances conservatively measured the tilts to be greater than truth.

The Srci) uncertainty component as defined in the PIDAL uncertainty analysis was recalculated for radially tilted cores, it was found that in all cases the Sr(i> value for tilted cores was bounded by the value used in the PIDAL uncertainty analysis for cores .zith quadrant power tilts up to 2.8%. It was also found that the value of the Srci) uncertainty component depended strongly on the direction and magnitude of the oscillation causing the power tilt. For cores oscillating about the diagonal core axis, the assumed PIDAL measurement uncertainty is valid for tilts up to 5%.

For the ascillation about the core major axis, the Sr(s) uncertainty component ceases to be bounded by the value assumed in the PIDAL uncertainty analysis for quadrant power tilts greater than 2.8%. Since the Palisades Technical Specifications allow for full power operation with quadrant power tilts of up to 5%, and it was clear that the current PIDAL uncertainties were only valid for tilts up to 2.8%, it was necessary to derive new uncertainties to allow use of PIDAL for tilts abcVe 2.8%. An analysis was performed, as described in Sections 3 and 4 of this report in order to determine the uncertainties in F9, F4" and F^ at the 5% quadrant power tilt threshold. These uncertainties may be found in Table #3 of Section 5 of this report.

It was shown that the coding in the PIDAL program is correct by reproducing a theoretically flat power distribution when given the appropriate theoretical incore detector values. This is in agreement with results previously obtained as part of the PIDAL Uncertainty Analysis.

Finally, it was found that quadrant power tilt is not an input to the Safety Analysis and that the increase in local or radial peaking resulting from a tilted core scenario is implied by the peaking factor or LHOR used in the analysis. There is no tilt multiplication factor applied to the peaking factors. ..

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Assump: ions The Palisades FSAR specifically talks about three types of instabilities within the reactor core: radial, azimuthal and axial. This analysis is only concerned with the first two modes. It is assumed that the use of the word " radial"in the FSAR refers to an oscillation which moves from the center of the core outward to the periphery and then back. An oscillation of this type could be depicted by the top of a single spired circus tent being raised and lowered. It is assumed that the word " azimuthal" refers to an oscillation which traverses the entire width or the core before returning back to the point of origination. In the rigorous sense of the word, this type of oscillation could hypothetically traverse circumferentially around the core as well, much like a pie tin would rotate if it were not perfectly balanced on a central point.

The Palisades FSAR states that a radial oscillation in the reactor is highly unlikely and stable if it does occur. To this end, there are times when the word " radial" is used loomly, meaning either a truly radial oscillation, or sometimes meaning "about the radial plane". It is hoped that the context of the usage will clearly dictate the meaning.

There is one fundamental difference between the uncertainties derived from this analysis and the original values derived in the PIDAL Uncertainty Analysis which was brought on by the nature in which this analysis had to be performed. In the original PIDAL uncertainty analysis, it was assumed that the Sno uncertainty components contained both the measured and inferred components of the box power synthesis uncertainty. For this analysis, the Sng uncertainties calculated do not contain the same component because the detector powers supplied to PIDAL are based on theory. Since no data for significantly tilted cores exists for the Palisadet reactor, it must be assumed that recalculating the uncertainty components based purely on theoretical detector powers is valid.

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l Analpis Methodology in order to answer the questions posed by the NRC. it was necessary to supply PIDAL with incore detector signals from a variety of radially tilted configurations. It was desired to investigate the effects of quadrant power tilts on the order of 0% to 50, as well as more severely tilted cases on the order of 10G.

The 0% to 5% tilt range was chosen because this covered the range over which the Palisades reactor can operate at greater than 25G power while remaining within the quadrant power tilt guidelines set forth in Palisades Technical Specification 3.23.3. At the present time, power operation with quadrant power tilts greater than 5% is not anticipated since tilts of this magnitude are highly unlikely unless a dropped control tod or otherwise severe localized power anomaly occurs. Nevertheless,it was deemed necessary to investigate how well PIDAL performed when more severe tilts were present.

Since Palisades rarely operates with measured quadrant power tilts greater than 10, and measured incore detector signals for radially tilted cores were not available, it was necessary to find an alternate method for providing PIDAL with the required tilted incore detector data, it was decided to use detector powers derived from full core XTG sc.lutions as input to PIDAL This required that XTG cases be run which modelled rr. dial or azimuthal imbalances in the reactor core.

A total of four XTG cases were run in order to model a variety of azimuthal and radial Xenon oscillation scenarios. Three of the four XTG runs started from a restart corresponding to roughly 3/4 total cycle length. The fourth case was run at BOC. These four cases all started the transient by dropping a single control rod into the core and then leaving the rod fully inserted for a period of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> after which time the rod was rapidly pulled out. The ensuing transient was then followed for a period of 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. The only differences between the four transient cases run were which control rod was dropped and therefore which direction the oscillation took across the core.

The first two of the transient cases were run by dropping group 3 control rods into the core. The first case dropped in a group 3 outer rod (rod 3 34) while the second case dropped in the central control rod (rod 3 33). The object of the case which dropped in the 3 outer rod was to induce an azimuthal oscillation. The object of dropping the central rod wa io see if a radial oscillation could be induced.

The second two cases run both used a group 4 control rod as the transient initiator.

The object of these two cases was to initiate an azimuthal oscillation which started off of the major axis (on a diagonal). Both of the two cases which used a dropped group 4 control rod as transient initiator were identical with the exception being tbt the first case was run at 3/4 cycle length while the second case was run at BOC.

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Analysis Methodology After the XTG cases were run,it was necessary to infer theoreticalincore detector powers from the resultant three dimensional XTG power distributions. This was accomplished by writing a small utility program, XTGDET, which used the power distribution from the XTG punch file as input.

The purpose of the XTGDET program was to read in a 3 D power distribution punch file created by XTG and convert the nodal powers into equivalent incore detector powers. Subroutine EXPAND is the meat of the XTGDET program. Based on the 3 D nodal power distribution determined by XTG, it calculates the theoretical detector powers.

EXPAND uses the same methodology as subroutine EXPAND of PlDAL and Section 2.2.1 of the PIDAL Methodology Report should be consulted if further reference is required.

The XTGDET program was compiled and link edited four times. The program was identical for each compilation except for the incore detector location array, DETLOC. For the first compile DETLOC defined the actuallocations of the detector strings in the reactor core (i.e. DETLOC was defined just like it was in the PIDAL block data section). For the second compilation the incore detectors spatial orientation to each other was not changed, but the entire core was rotated 90 clockwise underneath them. The third and fourth compiles rotated the core 180* and 270* clockwise respectively from its true orientation to the incore c'etector strings. The reason for wanting to rotate the core about the incore detector locations will be discussed shortly.

Once the theoretical detector powers were obtained for the radially tilted conditions, they were input to PIDAL The core power distributions calculated by PIDAL were thet compated back to the original XTG solution. For each of the PIDAL cases run, the statistical analysis option was chosen in order to determine the uncertainties associated with the PIDAL calculations for the tilted conditions.

Prior to discussing the actual PIDAL cases which were run, it is appropriate to describe the temporary modifications which were made to the cycle 7 PIDAL model in order to overlay the measured incore detector signals with the full core theoretical values supplied by XTG via XTGDET, In the main program, immediately after the call to Subroutine BXPWR (which calculates the detector powers based on measured millivolt signals and the Wprimes), temporary coding was added which reads in the theoretical detector powers and detector level normalization factors produced by XTGDET. This read was activated by the IXPOW f'ag which is normally used to tell PIDAL to use theoretical detector powers from the 1/4 core XTG model that runs concurrently with each PIDAL case. Following the input of the ful! core theoretical detector powers, the IXPOW flag was turned off so that the normal 1/4 core theoretical detector power logic in PIDAL would not take effect. Note that the measured detector powers are actually overlaid by the new coding and that PIDAL assumes the full core theoretical values to be measured from this point on.

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Analysis Methodology A total of 19 PIDAL cases were run for this analysis. The first case was a non tilted base case which corresponds to the core conditions at 3/4 EOC. The XTG case used to supply the full core theoretical detector powers was the second step of the 3/4 EOC group 4 rod drop scenario. The base case is important because it serves to verify that the entire system is working as designed for this analysis. The following checks were made:

- Verifiertion that the full core XTG model for cycle 7 is working properly by comparing the full core XTG run with the 1/4 core XTG power distribution of PIDAL

- Verification that the XTGDET program is working properly by comparing the full core XTG power distribution with the XTGDET collapsed 2 D radial power distribution.

- Verification that the XTGDET program is working properly by comparing the XTGDET theoretical detector powers with those previously calculated by the 1/4 XTG which is part of PIDAL.

Verification that the full core detector signals are getting input to PIDAL correctly from XTGDET and that the PIDAL solutica is correct by comparing the PIDAL solution with the original XTG solution.

With description of the base case out of the way, discussion on the remaining 18 PIDAL cases is appropriate. The PIDAL cases run used theoretical detector powers from two of the XTG dropped rod induced transient scenarios. The first 6 PIDAL cases used powers from the 3/4 EOC group 4 rod induced transient while the second 6 used powers from the group 3-outer rod induced XTG case. ,

The first six PIDAL cases run corresponded to peak quadrant power tilts of 10%,

7.6%, 5.6%, 2.9%,1.6% and 0.3% respectively. These cases were selected because they l covered the spectrum of tilted cores for a tilt range of no tilt up to 10% tilt. Concentration on tilts between 0% and ~5% was greater because it is over this range that the reactor may l be operated without reducing power or correcting the tilt. The second six PIDAL cases all

lie within the no tilt and ~5% quadrant power tilt range.

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Analysis Methodology There were two reasons for using the two different transient scenarios as suppliers of the theoretical detector powers. First, the dropped group 3 outer rod scenario did not result in quadrant power tilts greater than 5% Juring the oscillatory period. Therefore, it was necessary to use cases from the dropped group 4 rod secuario in order to get results on tilts up to 10E Secondly, the oscillations between the two scenarios were quite different.

The dropped group 3 outer rod oscillated about the major symmetric axis while the dropped group 4 rod scenario oscillated about the diagonal axis. Consideration of both is important because the majority of the symmetric incore detector locations are rotationally symmetric (and not generally symmetric about either major axis or diagonal) and therefore oscillations about differing axis' could have differing effects on the accuracy of the PIDAL quadrant power tilt algorithm.

Expanding on this last statement, it was decided to further investigate the effects of tilt location on the PIDAL solution. In the case of the dropped group 4 rod induced transient, the power peak used for the PIDAL cases 1 through 6 occurred in quadrant 2.

What if the power peak was in one of the other three quadrants? In other words, what if the power distribution was the same, just rotated 90*,180* or 270 ? Since the incore

- detectors are not equally distributed over ine quadrants, it is not expected that the power distributions as measured by PIDAL would be the same for the rotated cas:s. The same questions can be asked for the group 3-outer rod induced transient as well.

The XTGDET program allowed for use of the same XTG case for each of the four possible symmetric oscillations induced by individually dropped group 4 rods. In a similar l

fashion, the existing group 3 outer dropped rod XTG case could be used for three additional symmetric transient scenarios.

Six additional PIDAL cases were then run. Three of the cases were for the 5% tilted group 4 rod induced oscillation at rotations of 90,180 and 270* clockwise from the original power distribution. The other three cases were for the 5% tilted group 3 outer rod induced i transient at rotations of 90,180* at d 270*.

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The results of the three transient cases which caused azimuthal xenon transients are summarized in Table #1. From this table it is apparent that the core is less stable at beginning of cycle than at EOC azimuthally. This is in agreement of Section 3.3.2.8 of the Palisades FSAR which states that it appears that the azimuthal mode is the most easily excited at beginning of life even though the axial mode becomes the most unstable later.

From Table #1 it is also clear that the oscillation resulting from tne group 4 rod drop is more severe from a quadrant power tilt standpoint than for the group 3 outer rod drop.

The reason for this is that in the group 3 outer induced transient, the power peaking is symmetric along the quadrant lines, and therefore the peak tilt is actually distributed over two adjacent quadrants. in the case of the dropped group 4 rod transient, the power peaking is symmetric about the diagonal which lies within a single quadrant.

Table #2 presents the results of the PIDAL cases which were run and it is this data that will be used to answer the questions asked by the NRC. The first NRC request was for comparison of the tilt measured by PIDAL with the true or theoretical tilt. For the dropped group 4 rod case, the agreement be: ween the PIDAL solution and the original XTG quadrant power tilt was very good. For the true tilts between 0% and 10%, the error was on the order of 0.72% or less.

For the dropped group 3 outer rod induced transient, the quadrant power tilt was not as accurately measured, however it was measured conservatively in each case. For true quadrant power tilts of ~4% or less, the PIDAL tilt was still within 1% of the original XTG.

When the true tilt rose to greater than 5% the error in the PIDAL tilt calculation reached 1.23%. Again it should be noted that the PIDAL tilt fer these cases was always higher than the true tilt and therefore conservative.

j The second NRC comment asked that the PIDAL code programming be verified correct by supplying theoretical detector input and comparing the resulting PIDAL solution with the original theoretical power distribution solution. In actuality, this comment had already been addressed by the PIDAL Uncertainty Analysis. The S g,y uncertainty component represents the error in the PIDAL solution when PIDAL is given detector l powers from a known power distribution solution. For the entire data base, the Sr uncertainty component was 0.0022. This value is in excellent agreement with the individual case S n ,) uncertainty components found on the statistical summary edit following each of the PIDAL runs performed for this analysis.

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Analysis Results The third comment made by the NRC requested that a determination of the Sg ,,

uncertainty component for tilted cores be made. To this end, the PIDAL statistical analysis routines, which calculate the individual case uncertainty components, were activated for each of the eighteen tilted core PIDAL runs made.The individual results are presented in Table

1. 2. When looking at these values, the reader should keep in mind the overall S ng uncertainty component of 0.0277 for the entire data base arrived at in PIDAL Uncertainty Analysis. Based on the results presented in Table #2 it can be concluded that the uncertainty component S n ,) bounds core measurements up to quadrant power tilts of 2.8%

(linear interpolation between cases 9 and 10). Furthermore, depending on the direction of the oscillation, the PIDAL measurements are bounded to above the current 5% quadrant power tilt Technical Specification limit.

For the oscillation symmetric about the core -diagonal, the PIDAL measurement uncertainty previously determined is valid for tilts up to 5%. For the oscillation about the core major axis, the S n ,) uncertainty cornponent ceases to bound the value assumed in the PIDAL uncertainty analysis for quadrant power tilts greater than 2.8% This means that the uncertainties derived in the PIDAL Uncertainty Analysis are not valid for all cases when quarter core tilts are greater than 2.8%.

• Because it was shown that the current uncertainties do not bound all tilted cases, it was namary to find new uncertainties which take power distributions with tilts greater than 2.8% into ac:ount. This was done by utilizing the PIDAL statistical processor program, to combine the data from PIDAL cases 13 through 18. The PIDAL statistical program, which was developed and documented as recorded in the PIDAL Uncertainty Analysis, can take statistical data output by individual PIDAL cases and combine it to represent an entire population. Cases 13 through 18 were used as the basis for the new tilted core uncertainty because they all were based on theoretical tilts of roughly 5% (actually 5.58% and 5.11%).

The 5% quadrant power tilt cut off was specified because Technical Specification 3.23.3 allows for full power operation of the reactor for quadrant power tilts up to 5%, without any compensatory action.

The results of the statistical combination for the tilted cases may be found in Table

1. 3. The non tilted data presented is taken from the previous PIDAL Uncertainty Analysis.

The F9, Fa h and F^ data presented in Table #3 is the basis for the revised Technical Specification Table 3.23.3.

L In response to the fourth NRC comment, a diccussion on how quadrant power tilt effected the Palisades Safety Analysis took place with members of the Palisades Transient Analysis Group. It was learned that quadrant power tilt is not an input to the Safety Analysis and that the increas t in local or radial peaking resulting from a tilted core scenario is implied by the peaking factor or LHGR used in the analysis. There is no tilt l multiplication factor applied to the peaking factors.

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b- a Analysis Results l

Intile.# 1 Step Hours Group 3 Outer Group 4 Group 4 hom dron 3/4 EOC TILT 3/4 EOC TILT BOC TILT 1 0 1.0000 1.0000 1.0000 2 0 1.0627 1.0708 1.0708 3 72 1.0488 1.0542 1.0505 4 73 1.0191 1.0410 1.0458 5 74 1.0329 1.0697 1.0777 6 75 1.0424 1.0892 1.1011 7 76 1.0483 1.1007 1.1162 8 77 1.0510 1.1057 1.1238 9 78 1.0511 1.1054 1.1251 10 79 1.0495 1.1013 1.1212 11 80 1.0459 1.0941 1.1133 12 81 1.0416 1.0854 1.1025 13 82 1.0369 1.0757 1.0898 14 83 1.0318 1.0657 1.0761 15 84 1.0266 1.0558 1.0621 16 85 1.0217 1.0463 1.0484 17 86 1.0171 1.0374 1.0354 18 87 1.0129 1.0294 1.0236 19 88 1.0092 1.0222 1.0132 20 89 1.0060 1.0160 1.0043 21 90 1.0033 1.0108 1.0104 22 91 1.0011 1.0065 1.0145 23 92 1.0006 1.0030 1.0173 24 93 1.0018 1.0036 1.0189 25 94 1.0027 1.0045 1.0194 26 95 1.0033 1.0051 1.0190 27 96 1.0036 1.0054 1.0177 28 97 1.0038 1.0054 1,0159 29 98 1.0037 1.0053 1.0136 Table #1 Peak quadrant power tilts for three scenarios each initiated by dropping a control rod, leaving it inserted for 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> and then rapidly withdrawing it. Values l predicted by Palisades cycle 7 full core XTG model.

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Analysis Results TaNe *2 Case initiating XTG PIDAL  % Tilt Srm S %,

Rod Tilt Tilt Error BASE - - - - 1.0000 1.0000 0.0000 0.0010 0.0008 1 4 1.1013 1.0959 0.54 0.0376 0.0321 2 4 1.0757 1.0721 -0.36 0.0280 0.0242 3 4 1.0558 1.0533 0.25 0.0198 0.0180 4 4 1.0294 1.0284 -0.10 0.0101 0.0102 5 4 1.0160 1.0158 0.02 0.0077 0.0066 6 4 1.0030 1.0037 0.07 0.0089 0.0044 7 3 Outer 1.0511 1.0634 1.23 0.0495 0.0445 8 3 Outer 1.0416 1.0520 1.04 0.0409 0.0367 9 3-Outer 1.0318 1.0403 0.85 0.0313 0.02S9 10 3 Outer 1.0217 1.0282 0.65 0.0219 0.0211 11 3 Outer 1.0092 1.0132 0.40 0.0112 0.0112 12 3 Outer 1.0006 1.0014 0.08 0.0083 0.0035 13 4 1.0558 1.0486 0.72 0.0239 0.0217 14 3-Outer 1.0511 1.0606 0.95 0.0529 0.0476 15 4 1.0558 1.0533 -0.25 0.0207 0.0188 16 3-Outer 1.0511 1.0634 1.23 0.0490 0.0439 17 4 1.0558 1.0486 0.72 0.0228 0.0205 18 3 Outer 1.0511 1.0606 0.95 0.0533 0.0480 Table #2 - Quadrant power tilts and detector power uncertainty components for for PIDAL for radially tilted cores.

Note: For all scenarios, PIDAL correctly identified the quadrant in which the maximum quadrant tilt occurred.

Cases 13 and 14 were for a core rotated 90* CW under the incores.

Cases 15 and 16 were for a core rotated 180* CW under the incores.

Cases 17 and 18 were for a core rotated 270* CW under the incores.

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f 0>r" Analysis Results Table #3 Statistical Standard Degrees of Tolerance

_ Variable Tolerance Deviation Freedom Factor Limit F(s) # 0.0393 1800 --

F(sa)# 0.0351 360 --

F(r) # 0.0026 408 --- -

F(s)

• 0.0306 3415 --- --

F(sa)' O.0241 683 -- --

F(r)

• 0.0021 969 - --

F(s) 0.0277 8768 - ---

F(sa) 0.0194 1754 --

F(r) 0.0022 2754 -

F(z) 0.0151 1122 -

F(L) 0.0135 188 ---

P # 0.0443 2487 1.703 Fo" #

0.0795 0.0383 489 1.766 0.0722 Ff # 0.0352 364 1.785 0.0695 P

• 0.0368 3822 1.692 0.0664 F*"

• 0.0277 877 1.733 0.0526 Ff

• 0.0242 694 1.74 6 0.04 )

P 0.0344 4826 4 1.692 0.0623 Fh 0.0237 1225 1.727 0.0455 Ff 0.0195 1790 1.712 0.0401 Table #3 Summary of PIDAL Statistical Component Uncertainties.

1. - values to be used when quadrant power tilt exceeds 2.8?c' but is less than or equal to 59c.

- values for cores with once-burnt reused incore detectors.

Note: For the final tolerance limits, penalty factors of .0041, .0046 and .0067 for P, P)" and F^ respectively were included to account for up to 25% incore detector failures.

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