ML20210J333

Power Cutoff Removal Analysis

ML20210J333
Person / Time
Site:	Arkansas Nuclear
Issue date:	05/31/1986
From:	Delano B, Holman P BABCOCK & WILCOX CO.
To:
Shared Package
ML20210J326	List: 1CAN098611, Forwards Arkansas Nuclear One Unit 1 Power Level Cutoff Removal Analysis Rept. Rept Covers Analysis Pertaining to Proposed Tech Spec Changes 3.5.2.4 & 3.5.2.5 ML20210J333
References
NUDOCS 8610010113
Download: ML20210J333 (31)

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I ANO-1 Power Level Cutoff il Removal Analysis lI I

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AP&L POWER LEVEL CUTOFF REMOVAL ANALYSIS ll ll P.L. HOLMAN B.J. DELANO

!I BABCOCK & WILCOX COMPANY l NUCLEAR POWER DIVISION NUCLEAR FUEL SERVICES MAY,1986 I

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L Abstract I

Transient xenon analyses were performed for Arkansas Nuclear One Unit-1, cycles 7 and 8, to evaluate the removal of the Technical

' Specification Power Level Cutoff (PLC) requirement at 92% of full power. Results indicate that the B&W generic.1.05 total xenon factor, which is used to evaluate margin to the Loss of Coolant p Accident (LOCA) linear heat rate criteria, is conservative.

L Margins to the Initial Condition-Departure from Nucleate Boiling (IC-DNB) criteria were also evaluated, and the B&W generic 1.025 radial xenon factor was found to be conservative. This study 5 provides the basis and justification for removal of.the PLC at 92% of rated power.

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lI 1.0 Introduction 1.1 Backaround The Technical Specifications for Arkansas Nuclear One Unit-1 (ANO-1) require that the reactor operate within Limiting Condi-tions for Operation (LCOs). The LCOs related to core power distribution control consist of limits on rod index, axial power shaping rod (APSR) position, axial imbalance, and quadrant power tilt. LCO limits prevent power distributions from exceeding I accident initial condition criteria (allowable Loss of Coolant Accident - LOCA, and Initial Condition Departure from Nucleate Boiling -

IC-PNB, peaking limits). Technical Specification 3.5.2.5.4 requires a power level hold at 92% of the maximum I allowable power level unless one of two conditions is satisfied:

a. Xenon reactivity is within 10% of the equilibrium value for I operation at the maximum allowable power level and asymptot-ically approaching stability.

I b. Except for xenon free startup, when item (a) applies, the reactor must operate in the range of 87% to 92% of the maximum allowable power for a period exceeding two hours.

In addition, Technical Specification 3.5.2.4.1 requires (in part) the following:

I Except for physics tests, if quadrant tilt exceeds 3.1%,

power shall be reduced immediately to below the power level cutoff (92%). Moreover, the power level cutoff value shall be reduced 2% for each 1% tilt in excess of 3.1%.

The power level hold was originated in the licensing analysis as a means to allow less restrictive LCO limits at full power I conditions. The predicted increase in power peaking above the steady-state values at constant offset, due to transient xenon conditions, was correlated with core average xenon concentration I or worth. A basic relationship was established that once xenon reactivity was within 10% of its equilibrium value and asymptoti-cally approaching equilibrium, the increased peaking due to transient xenon would always be reduced to a level which could be I accommodated in setting non-restrictive full-power operating limits. Further analysis of unrodded operation indicated that a hold of greater than two hours is sufficient to reduce peaking I due to transient xenon to an acceptable value, independent of the xenon concentration. Thus the dual conditions of the present Technical Specifications for unrodded operation as noted above were established.

1.2 Justification for PLC Removal Because the length of a power level hold could be as long as two AP&L Power Level Cutoff Removal Analysis Page 3 of 31 I

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.E to twenty hours and many power level holds could occur in any

.E cycle, it is cost-effective to remove the PLC at 92% of m-.cimum allowed power due to the high cost of replacement power. A loss

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of 8% FP in capacity factor for two hours represents 0.16 EFPH; if replacement power costs $250,000 per day, the loss is $40,000 per event which causes a two hour hold at the PLC. This study justifies the removal of the 92% PLC by simulation of xenon transients which may be expected to occur during power opera-tion. Since the calculations are based on the ANO-1 cycle 7 and 8 fuel loadings, the applicability of these conclusions to future l reloads must be specifically verified on a cycle-by-cycle basis; E the procedure and requirements for cycle-by-cycle verification are discussed in Section 5.0.

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I AP&L Power Level Cutoff Removal Analysis I Page 4 of 31 1

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2.0 Methodoloav I 2.1 Standard Methodoloav The standard calculational methodology fcr setting the Limiting Conditions for Operation (rod index, APSR position, axial imbalance, and quadrant tilt limits) assumes that the core is at equilibrium xenon conditions prior to initiation of a transient.

To accommodate the effects of transient xenon reactivity and L distribution on power peaking, a total xenon peaking multiplier (5% for unrodded 177 fuel assembly plants) is applied during the calculation of LOCA margins at power levels above the PLC. A l

radial xenon peaking penalty of 2.5% similiarly applies to the 5 calulation of IC-DNB margins. While it is possible that transi-ent xenon peaking can be greater than 5% above equilibrium xenon peaking, the practice of utilizing a power level hold at 92% of

[ full power ensures that the factor is bounding.

2.2 Cycle / Transient-Snecific Methodoloav L

If the fuel cycle design yields more restrictive power peaking, as is frequently the case when economic improvers are utilized in 5 the design, the standard analysis may be replaced by a transient xenon-specific analysis. Xenon transients are simulated, and limiting power distributions with implicit transient xenon r peaking are then used to calculate the LOCA and IC-DNB margins.

The benefit of this methodology is to improve restrictive operating limits by taking credit for whatever reduced transient xenon peaking (relative to that obtained from the analysis using standard bounding factors) may be available from the power distributions. However, the transient xenon-specific analysis does not include complete calculations to demonstrate the PLC can be removed, even though the transient peaking penalty can be reduced.

2.3 PLC Removal Methodoloav To remove the 92% PLC, additional transient xenon-specific analyses are required. The analyses must ensure that LOCA and IC-DNB margins used to calculate the LCO limits (when based on standard methodology including the xenon peaking multipliers) are more limiting than those margins derived using transient xenon distributions without the benefit of a power level cutoff.

The following parameters were considered as making significant contributions to transient xenon peaking:

1) Magnitude of power level change

2) Regulating rod positions allowed during the transient

3) APSR positions allowed during the transient

4) Imbalance allowed during the transient

.5) Time in cycle for occurrence of the transient AP&L Power Level Cutoff Removal Analysis Page 5 of 31

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I 6) Effect of different core loading patterns

7) Burnable Poison Rod Assembly (BPRA) lengths These parameters were addressed by considering the transients I summarized in Table 1. ANO-1 cycles 7 and 8 were used as the bases for this analysis because:

1) Short-stack BPRAs were utilized in cycle 7, which can produce higher peaking in the fuel region above the BPRAs as control rods are withdrawn.

2) It is desirable to ensure applicability of the PLC study to the earliest possible cycle, in this case cycle 7.

3) Both cycles 7 and 8 utilize black APSRs, which are most l restrictive for the computation of transient xenon effects. If I

gray APSRs are utilized in future cycles the results of this analysis should be bounding.

I 4) Cycles 7 and 8 are unrodded, feed and bleed, partial very low g leakage (VLL) cores utilizing a modified in-out-in shuffle 3 scheme. This scheme is typical of what Arkansas Power and Light (AP&L) has indicated as desirable for future reloads.

2.4 Calculational Models I

! The calculational strategy for performing the analysis was based

,I on use of the three-dimensional FLAME nodal computer code to simulate xenon transients. Radial-local peaking factors, which account for the ratio of peak pin-to-assembly average power, are computed with the two-dimensional pin-by pin two-group PDQ07 I model and are input to the MARGINS code for computing LOCA and IC-DNB margins. The MARGINS code accesses a FLAME history tape with simulated three-dimensional limiting power distributions, and calculates the difference (expressed in percentage) between the allowable linear heat rate (LHR) limit and the limiting calculated pellet linear heat rate. If the resultant peaking I margin is positive, operation is allowable without exceeding the LOCA or IC-DNB peaking limits. If the peaking margin is nega-tive, operation at the corresponding conditions of time in cycle, I power level, rod index, APSR position, imbalance, and quadrant tilt must not be allowed. The MARGINS code computes LOCA and IC-DNB margins for every node in the core, and provides the most restrictive value of each. Centerline fuel-melt (CFM) and I steady-state DNB margins are not a concern to Limiting Condition for Operation limit generation because they are based on immedi-ate ' fuel damage criteria, and are therefore prevented by the Reactor Protection System trip functions.

The simulated xenon redistribution was assumed to be caused by a transient which gave the worst axial power shift and which also lI assumed appropriate long-term use of control components.

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" Appropriate" actions are those taken by an operator over periods r of many hours to control power distribution in a correct rather L than an adverse way. Some degree of misoperation is allowed to account for anticipated operational occurrences or long-term j operational modes known to have occurred in the past. With these l general quidelines, transients that result from long-term maloperation or deliberate sabotage are not considered by this study. Specifically, it is assumed that APSRs are moved in response to non-equilibrium xenon conditions and are not deliber-ately used to initiate a transient xenon condition.

Damping of axial renon oscillations typically requires periodic movement of rods or APSRs by +/-5% withdrawn. Transients of this type are covered by 100-50-100 type " design" transients and by the 100-15-100 power maneuver transient. For transients simula-

[ ting power maneuvering from 90-100% full power, which is in the L range of the PLC hold, the 100-90-100 transient is conservatively simulated by rod movement with no power level changes to maximize local xenon burnout. These transients, which will be referred to as 100-100-100 transients, require regulating rod movement to

] accommodate small power level changes or to maintain power level j during chemical shim adjustment operations. Typical rod and APSR movements for the 100-100-100 transient are bounded by +/- 7.5%

withdrawn from the nominal Rod Operation Recommendation posi-tions. For this study, movements as large as +/-13% withdrawn i r' are considered.

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3.0 Sn==arv of Transients Detailed analyses to evaluate transient xenon included examina-tion.of the following transients:

1) Three 100-50-100 type transients were evaluated ,one for ANO-1 beginning of cycle (BOC) 7 and two for cycle 8. Of the two cycle 8 design transients, one is at - BOC and one is at end of cycle (EOC). For the cycles 7 and 8 BOC transients, no APSR motion was assumed and imbalance limits were allowed to be violated, since ANO-1 operators are allowed up to 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> response time. The third design transient at EOC 8 was performed assuming APSRs would only be moved when imbalance alarms were reached during the first six hours of.the transient. After six

! hours upon return to full power, APSRs were used to damp the xenon oscillation induced by the design transient, and imbalance

[ was maintained as close to zero as possible. This case was run since it was desired to evaluate the effect of uncovering p shadowed fuel by the APSRs when they are moved in response to a L significant transient. EOC was chosen since the shadowing of fuel is maximized.

( 2) One 100-100-100 transient (cycle 7) was simulated by with-drawing Group 7 from the nominal rod index of 287 to 293.5 for six hours, then returning it to 287. APSRs were moved to maintain imbalance within LCO limits. This transient was included to determine the effect of operating with Group 7 above the portion of fuel shadowed by BPRAs.

3) One 100-100-100 transfent (cycle 8) was simulated by inserting Group 7 from the nominal rod index of 287 to the unerror-adjusted insertion limit (rod index = 277). This transient was performed to simulate chemical shim adjustments or smaller power level changes. This transient was run at full power to maximize xenon burnout in the core center.

4) A 100-15-100 transient was run to simulate the largest load swing contolled by the Integrated Control System (ICS). The large power reduction was chosen to maximize the buildup of xenon h at reduced power and to simulate a transient with high radial power peaking.

1 Control rod and APSR scans were performed at limiting times

[ during the transients. For the transients that modeled power level changes, these were times that corresponded to maximum and minimum xenon concentrations. These times usually accompanied

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the most negative and positive imbalance limits, respectively.

For the 100-100-100 transients, rod and APSR scans were run at the times of most positive and negative xenon offsets. By comparison of the transient margins and corresponding offsets to

{ margins and offsets for equilibrium cases including the 1.05 total xenon peaking factor (LOCA), and 1.025 radial xenon peaking AP&L Power Level Cutoff Removal Analysis Page 8 of 31

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I factor (IC-DNB), the adequacy of the generic peaking factors could be determined. '

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AP&L Power Level Cutoff Removal Analysis Page-9 of 31 I

Table 1 Transient Descrintion Transient Number EFPD Load Chance Comment H

1 4 100-53.7-100 design transient,

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q no APSR movement, cycle 7 2 4 100-58.3-100 design transient,

( no APSR movement, cycle 8 3 380 100-62.0-100 design transient, APSRs used to damp xenon oscillation, cycle 8 4 4 100-100-100 Group 7 withdrawal

[ transient, with APSR movement, cycle 7 I 5 4 100-100-100 Group 7 insertion b

transient, with APSR movement, cycle 8 6 4 100-15.0-100 power maneuver transient, with APSR movement, cycle 8

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4.0 Transient Analysis' Details and Results 4.1 ANO-1 Cycle 7 Transient Analyses Cycle 7.is of interest in this PLC study for two reasons. First, ,

(- cycle 7 is currently in operation and it is desirable to imple-ment the results of this study as soon as possible. Secondly, the implementation of short stack BPRAs (117 in.) in cycle 7 affords

[ the opportunity to evaluate peaking in the region above the top L of the BPRAs when the regulating rods are withdrawn.

Two transients were evaluated for cycle 7 and compared to the

( equilibrium xenon power distributions with appropriate generic LOCA and IC-DNB factors.

4.1.1 100-53.7-100 Desian Transient

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Transient #1 (refer to Table 1 and Figure 1) was a standard design transient which modeled the Group 7 regulating. rods

[ inserted to 50% withdrawn (rod index of 250) and a corresponding reduction in power level. A 50% rod insertion causes the most severe power shift to the bottom of the core. Although the APSRs

( are repositioned to minimize negative imbalance, they are not able to compensate completely for the regulating bank effects, the imbalance departs from nominal, and a significant axial xenon redistribution can occur. While the reactor operates with a rod

[ index of 250, core average xenon reactivity is assumed to be compensated by RCS boron concentration changes (accomplished by feed and bleed). After six hours at reduced power, xenon reaches its maximum core average concentration. At this point in the transient, Group 7 rods are withdrawn to return the plant to 100%

of rated power. Upon return to full power, xenon burns out and reaches its minimum core average concentration at approximately 14 hours1.62037e-4 days <br />0.00389 hours <br />2.314815e-5 weeks <br />5.327e-6 months <br /> into the transient.

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For this type of transient, the power level associated with Group

- 7 insertion is that value which maintains critical k-effective at its pre-transient value. For cycle 7 the design transient was calculated at 4 EFPD, since the LOCA linear heat rate limits (kw/ft limits) are most restrictive near the core bottom for the first 1000 MWD /mtU of core average burnup, and power peaking is greatest at BOC. The resulting 100-53.7-100 transient is shown in Figure 1, and is typical of design transients in general.

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Both core average xenon density and xenon offset are plotted in Figure 1. The normalized xenon density is an indi"ator of the

[ global xenon reactivity effect; the axial distribution of xenon xenon offset is an indicator of in the core, and represents the excess xenon density existing in the core top or bottom. A positive xenon offset indicates more xenon has built up in the top core half than the bottom core half, hence a tendency toward a negative power imbalance.

AP&L Power Level Cutoff Removal Analysis Page 11 of 31

To generate extreme power distributions at limiting points in the r transient (when peaking is highest), instantaneous rod scans and L APSR scans are simulated with the three-dimensional FLAME code.

The rod and APSR scans represent possible combinations of rod index and APSR position at maximum and minimum xenon conditions.

Margins to the LOCA and IC-DNB allowable peaking limits are calculated from the scans to determine which power distributions

, can be allowed without exceeding the accident initial-condition criteria.

Examination of LOCA margin contour plots generated from regula-ting rod and APSR scans demonstrates that the most restrictive

( margins occur at maximum rod insertion allowed by the operating limits. The plots of LOCA margin versus offset in this report are generated at a constant rod index of 277.7 (Group 7 is 77.7%

I withdrawn). This rod insertion level corresponds to the unerror k adjusted rod index that would be required in the B&W analysis to duplicate the AP&L generic operating limits when appropriate error adjustments are made.

Figure 2 shows a comparison of LOCA margin versus offset for Transient #1 at conditions of maximum and minimum xenon. Also

( shown are LOCA margins for zero and equilibrium xenon conditions.

Figure 2 demonstrates that equilibrium xenon cases with the 5%.

total xenon factor bound all transient conditions except the zero xenon margins.

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It should be mentioned at this point that it is only necessary for the 5% total xenon factor to preclude conditions of negative margin. It is not. necessary for equilibrium xenon (with 1.05 factor) margins to be most limiting for conditions where positive margin exists for all transient conditions. Such situations only occurred for conditions where positive margin existed for all transients analyzed, and therefore pose no concern. For Transi-ent #1, the most restrictive LOCA margins occur when APSRs are withdrawn beyond approximately 20%. Under these conditions the offset becomes quite negative due to the insertion of Group 7 and the withdrawal of APSRs. Consequently, the power is shifted to the core bottom (about 20 inches from the bottom), to the region of most restrictive LOCA linear heat rate limits.

From an examination of power distribution behavior resulting from

{ the rod and APSR scans, the most limiting conditions for IC-DNB were always found to occur with the regulating bank completely withdrawn (rod index = 300) . This results from the restrictive nature of the Maximum Allowable Peaking (MAP) curves near the

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core top. As a result, all IC-DNB margin comparisons shown in subsequent plots were performed for a rod index of 300.

Evaluation of IC-DNB margin for this design transient indicates that the 2.5% radial xenon factor applied to equilibrium cases establishes the most restrictive conditions for operation. These AP&L Power Level Cutoff Removal Analysis Page 12 of 31 J

h results are shown in Figure 3. From Figure 3, minimum xenon e cases establish the most restrictive transient IC-DNB margin, L since this condition is associated with the most positive axial imbalance and hence the highest outlet peaking.

( 4.1.2 100-100-100 Grouc 7 Withdrawal Transient An interest was expressed in the effect of the shorter (117 in.)

P BPRAs on xenon transients. The Group 7 withdrawal transient was b simulated because of the potential impact on peaking that could occur in the region above the BPRAs, but below the regulating rod tips under conditions of rod withdrawal. Higher peaking could occur during small power level changes which cause the ICS to withdraw Group 7 above the nominal operating position. Another possible scenario for this type of transient is normal steady-( . state operation, in which the ICS will gradually withdraw Group 7 to maintain the reactor core critical as fuel depletes. After a period of time the operator will deborate and insert Group 7 to r the recommended rod operating position. APSRs were also moved L when it was necessary to maintain imbalance within Technical Specification limits. This transient is referred to as a 100-100-100 Group 7 withdrawal transient, and is listed in Table 1 as

( Transient #4.

Since the Group 7 withdrawal transient was simulated at 100% of rated power, there is not a significant enough change in the core

{ average xenon concentration to indicate the worst point in the transient for performing rod and APSR scans. For full power transients in which there can be both a significant radial and

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axial shift in xenon concentrations, imbalance and xenon offset are good indicators of the limiting time in a transient for rod and APSR scans. Examination of Figure 4 indicates that at six

( hours into the transient, the axial imbalance is approximately

+10% with a corresponding negative xenon offset of -0.5%. After Group 7 rods are returned to their normal position, a minimum imbalance of 0% is reached .with a corresponding large positive xenon offset of 5% (20 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br /> into the transient). A review of xenon offset from Transient #1 in' Figure 1 confirms that xenon offset is also a good indicator of the limiting times for rod scans for the design transient. Therefore, in the evaluation of subsequent transients a combination of xenon offset, axial imbalanco, and core averago xenon were used to determine the times for limiting conditions of xenon.

The Group 7 withdrawal transient margin is also shown in Figure

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2. Rod and APSR scans were performed at the conditions of most

- positive and negative xenon offset, and the resulting LOCA margins were compared to equilibrium and zero xenon cases. As with Transient #1, the limiting conditions are set by equilibrium

( with the 5% xenon factor applied. Comparison of rod scans with both positive and negative xenon offsets indicate that positive xenon offset scans are the most limiting transient cases. This AP&L Power Level Cutoff Removal Analysis Page 13 of 31 J

behavior is understood by noting that for this full power transient, both the imbalance and xenon offset act in a direction to maximize peaking near the core bottom where the most restric-tive LOCA linear heat rate limits occur. This effect is confirmed by noting that the scans indicate the minimum margin at 20 hrs to

[ occur 25 inches from the core bottom. Likewise, for conditions of negative xenon offset and positive imbalance at 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />, peaking occured near the top of the BPRAs, 16 inches from the top

[ of the active fuel height.

Shown in Figure 3 is the IC-DNB margin for the Group 7 withdrawal transient. These results, like those for Transient #1, demon-L strate that equilibrium xenon with the 2.5% radial xenon factor is most restrictive for IC-DNB margin. The most restrictive transient IC-DNB margins occur for conditions of negative xenon

{ offset and positive axial imbalance, which promote peaking near the top of the core.

4.2 ANO-1 Cycle 8 Transient Analyses ANO-1 cycle 8 is given extensive analysis, since it will be the

_J next operating cycle and because it constitutes a class of cycles

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that is indicative of future reloads. Additionally, black APSRs are used in cycle 8. If the gray APSR option is exercised in a future cycle, this analysis is expected to provide a conservative

{ basis for confirmatory analysis of PLC removal in fuel cycles with gray APSRs.

4.2.1 100-58.3-100 Desian Transient As for cycle 7 analysis, a BOC (4 EFPD) design transient involv-ing a 50% Group 7 rod insertion (rod index = 250) with an l associated reduction in power level was simulated. After six hours the plant is returned to full power at maximum xenon concentration. For this design transient, referred to in Table 1 j as Transient #2, no APSR movement was allowed, in order to I

promote large swings in axial imbalance and maximize peaking.

This transient is shown in Figure 5 with maximum and minimum core average xenon concentrations occuring at 6 and 14 hours1.62037e-4 days <br />0.00389 hours <br />2.314815e-5 weeks <br />5.327e-6 months <br />, respec-l tively. This design transient was calculated at 4 EFPD, since LOCA linear heat rate limits and peaking are most restrictive at this point in the cycle.

Figure 6 shows a comparison of LOCA margin versus offset for this I design transient for conditions of maximum and minimum xenon, and for equilibrium with the 5% total xenon factor applied. This figure clearly demonstrates that the equilibrium case is most conservative, as was the case with cycle 7 results. Even though a large positive imbalance and large negative xenon offset were achieved at 14 hours1.62037e-4 days <br />0.00389 hours <br />2.314815e-5 weeks <br />5.327e-6 months <br /> into the transient, conditions of minimum xenon are less restrictive than conditions at maximum xenon. This demonstrates the importance of the shape of the LOCA kw/ft AP&L Power Level Cutoff Removal Analysis Page 14 of 31 l

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l limits, which are most restrictive near the core bottom, in determining which xenon conditions will minimize LOCA margin.

L These conclusions are further evidenced by zero xenon conditions, which are more restrictive than either maximum or minimum xenon r and have peaking occuring between 29 and 35 inches from the core L bottom.

Evaluation of IC-DNB margin versus offset at a rod index of 300, shown in Figure 7, demonstrates that equilibrium conditions with a 1.025 radial xenon factor are bounding for all cases of xenon in Transient #2 except minimum xenon conditions. These results are expected, since the conditions of minimum xenon are associ-ated with outlet peaking (large positive imbalance - see Figure

5) . These results do not invalidate the conservative nature of the generic radial factor because positive margin exists for all cases.

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The benefit of using intermediate length BPRAs (121.5 in.) in

[ cycle 8 is apparent by comparing the cycle 7 and 8 IC-DNB margin L versus offset plots for design Transients #1 and #2 (Figures 3 and 7). The shape of the IC-DNB margin curve has been consider-ably flattened, since the longer length BPRAs in cycle 8 reduce peaking near the core top, where allowable peaking is most restrictive.

4.2.2 100-15-100 Power Transient A 100-15-100 power transient was run at 4 EFPD in cycle 8 and is shown in Figure 8. This transient, referred to as Transient #6, was simulated to investigate the effects of the largest load swing that can be accommodated by the ICS in automatic power operation. A unique feature of this transient is that during the xenon buildup at low power, the regulating banks are positioned so that the core remains well balanced axially. However, core average xenon buildup is large due to the large reduction in power level. This type of transient determines the significance of radial shifts in power peaking that will predominately affect IC-DNB margin.

The 100-15-100 transient LOCA margin is shown in Figure 6. The results shown by this figure demonstrate that even the large swings in core average xenon are not sufficient to violate the 5%

total xenon factor applied to equilibrium conditions.

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Results of the IC-DNB margin evaluation for this transient (shown in Figure 7) indicate that positive margin exists for all transient and equilibrium xenon conditions. Of the transient xenon conditions, maximum xenon was found to be most limiting since this transient condition is associated with the largest positive axial imbalance.

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I 4.2.3 100-100-100 Group 7 Insertion Transient The 100-100-100 Group 7 insertion transient, shown in Figure 9, was considered because of two possible scenarios. The first would I involve slight over-insertion of Group 7 in response to debora-tion from normal depletion of reactivity. This situation is feasible, since time delays are involved in core-wide reactivity I changes, especially where deboration is involved. Secondly, the rod insertion transient is also appropriate for modeling small power level changes in the range of 90-100% full power. For I conservatism, the transient was simulated at 100% of rated power to maximize xenon burnout in the core center.

I The Group 7 insertion transient was run at 4 EFPD in cycle 8 and is referred to as Transient #5. The results of the transient analysis are shown in Figure 10 and confirm that equilibrium xenon is bounding for determining LOCA margin. Of the transient I xenon cases, the condition of positive xenon offset and negative imbalance produce the most limiting conditions of LOCA margin.

Transient #5 was the most limiting transient evaluated that could impact LCO imbalance limits for cycle 8.

The 2.5% IC-DNB radial xenon factor is conservative from results shown in Figure 11. Analysis of this figure indicates that all IC-DNB margins are positive.

4.2.4 100-62-100 APSR Transient One final transient was simulated to examine the impact of uncovering shadowed fuel when APSRs are moved in response to a xenon transient. APSRs are moved under normal coditions of I operation in response to a change in axial imbalance caused from some initiating event. In order to properly model APSR use, a I design transient was simulated to create transient xenon condi-tions. After maximum xenon conditions were achieved at 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />, APSRs were then moved as necessary to maintain imbalance as close to zero as possible, thereby damping the xenon oscillation. The initiation of a xenon oscillation, followed by appropriate APSR I movement, was calculated at 380 EFPD in cycle 8 just prior to APSR withdrawal. End of cycle (EOC) conditions were chosen to E maximize the effect of shadowed fuel in APSR locations. The 5 design transient at EOC is referred to in Table 1 as Transient #3 and is shown in Figure 12.

From Figure 12 it is observed that Group 7 is withdrawn to a rod index of 300 at 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> into the transient in order to maintain k-ef fective upon return to full power conditions. When Group 7 is withdrawn, the APSRs must also be withdrawn slightly to maintain a near zero imbalance. It is assumed that within the next half hour xenon burnout causes the ICS to insert Group 7 to its nominal pre-transient position, after which APSR insertion is I required to maintain an appropriate imbalance. As xenon burns AP&L Power Level Cutoff Removal Analysis Page 16 of 31 I

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out in the top of the core, the imbalance becomes more positive and APSRs must be slightly withdrawn to damp the xenon oscilla-

< tion. After 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> into the transient the xenon oscillation has been damped, as evidenced by almost constant imbalance with time.

' LOCA margin versus offset for Transient #3 is shown in Figure

13. These results demonstrate that equilibrium xenon with the 5%

total xenon factor is conservative with respect to transient F

xenon conditions. Margin for all transients is positive. It will be noted that for conditions of maximum xenon and negative xenon offset, a significant axial pinch effect is apparent which p results in a LOCA margin of approximately 4% at an offset of only L -9%. This pinch effect is verified by noting that the limiting LOCA margin conditions for maximum xenon occur for peaking located near the core midplane. The pinch effect is caused by Group 7 insertion to a rod index of 277.7 (77.7% withdrawn),

( APSRs located on the core bottom, and a negative xenon offset (more xenon in the core bottom forcing power to the core mid-F- plane). However, APSRs have insertion limits that prevent L reaching conditions there a pinch effect could impact operating limits. APSR insertion limits also prevent APSRs from being inserted past their position of maximum worth, thereby limiting their ability to control axial imbalance. If gray APSRs are implemented in future cycles, then pinch effects will be signifi-cantly reduced or eliminated. This occurs due to the decreased reactivity worth of gray APSRs and their increased length.

{

Evaluation of IC-DNB margin for Transient #3 indicates that a r minimum of 5.5% margin at 380 EFPD exists. Because all transient L conditions have positive margin (figure not shown), equilibrium cases with the 2.5% radial xenon factor are appropriate for analysis purposes, since operating limits will not be defined by

( IC-DNB margin.

s w

AP&L Power Level Cutoff Removal Analysis Page 17 of 31

L e

5.0 Conclusions I

The results of this comprehensive PLC removal study indicate that the 5% total xenon factor applied in the computation of LOCA margin provides conservative operating limits. Similarly, the 2.5% radial xenon factor applied in the evaluation of IC-DNB margin was also demonstrated to be conservative. This analysis is based on evaluation of transients that could occur during normal operation.

, Based on the conservative nature of both xenon factors, it is c concluded that ANO-1 Technical Specification 3.5.2.5.4 may be L deleted in its entirety, or it could be reworded to raise the power level cutoff from 92% FP to 100% FP. The latter change effectively deletes the PLC requirement, but retains the wording

{ structure in the Technical Specifications should future fuel cycle designs require it. In addition, Technical Specification 3.5.2.4.1 may be reworded to retain the present structure (but r replacing 92% FP by 100% FP), or the reference to the power level L cutoff in the first sentence may be deleted as follows:

Except for physics tests, if quadrant tilt exceeds 3.1%,

power shall be reduced immediately 2% for each 1% tilt in excess of 3.1%.

The applicability of these results to future fuel cycles will require cycle-specific verification. The applicability to cycle 8 was assured because cycle 8 comprised a large portion of the database for the PLC study, and the results were factored directly into the Maneuvering Analysis which generated LCO I

l limits. Therefore, the cycle 8 LCO limits benefit from both removal of the PLC hold and reduced xenon peaking factors defined during the PLC analysis. For cycle 9 and beyond, the cycle-specific verification of PLC hold removal using the I

j standard generic xenon peaking factors can be accomplished within the base-scope Maneuvering Analysis.

I l

However, determination of the allowable peaking factor reduction for future cycles will require simulation of a cycle-specific 100-100-100 Group 7 insertion transient (similar to Transient I

l

1. 5). The analysis required to define the allowable factor reduction and to establish PLC removal verification with the reduced xenon peaking factors can be accomplished within the resources of the VLL option adder.

t l

AP&L Power Level Cutoff Removal Analysis Page 18 of 31

I Figure 1. 100-53.7-100 Design Transient ANO-1 Cycle 7 (4 EFPD)

I 100 -

Power 75 -

Level 50 -

I (% FP) 25 -

I 0 i i i ' ' ' ' ' ' '

0 2 4 6 8 10 12 14 16 18 20 Time (hours) 1.2 -

ty (Nonnalized) A 0.9 -

0.8 ' ' ' ' ' ' ' ' ' ' '

I Group 7 Position 100 -

I 80

(% WD) 60 -

40 -

I Group 8 Position 20 b , , ,

l

+10 -

Scan ance

(%)

-5 -

I -10

-15 p Scan f t

(%) 0

-2 -

Scan I 4 I ' I ' f I t '

0 2 4 6 8 10 12 14 16 18 20 Time (hours)

I Page 19 of 31 I

I

t r r _, r __1 w r r r _

r c r r i 1 r <v Figure No. 2 ANO-1 Cycle 7 Transient Comparisons e

M

• *". . ) . ,

.;;[,,... .. LEGEND E p g j,-- ,,,,i. ^ Equilibrium Xe gz e

/.

/ .e 7

i

/ f/

7 g

//

Zero X_e Trans (1 Max Trans

Xe Min Xe

[ [d' l ~~5a~n's f)i~ 'e~d'ie 6 1 .6 , $,/', ,

"" W i.ii..G. f 'iiss .i. s'6.'E y ,* ,. ,

M m

/

/ .- ),/

f l

/ -

8<

&, .I

/ /- / p' /

?'

\/ /

O m --2 -

no W

8 o 7

[ -25 -20 -15 -10 -5 0 5 10

~

0# set - %

u r r r r r - ~ ~ r cw Figure No. 3 ANO-1 Cycle 7 Transient Comparisons S

\

\\

b LEGEND

\ '..

Equilibrium Xe N... Trans fl Min Xe M  %, .. Trans (1 Max Xe ,_'

I Ia- "x Trans 4 Pos Xe 0.S.

'y \ \ [,'Ea'h~s'f_d',dXe

~

61~~

u  %.  %,,.3 \,

. . . . .Z..e. ..r. .o. .. .X. . .e. .n. . .o. . .n. .. . . . .. .

x so .

\ \ . s s* Qq,.. :g

% 3 is

~ T

[ -15 -10 -5 0 5 10 15 20 25 30 Offset - 7.

._ ~

L r'

Figure 4. 100-100-100 GP7 Withdrawal Transient ANO-1 Cycle 7 (4 EFPD)

L 100 r Power 75 ~

L Level Constant 100% Power

(% FP) 50 -

p 25 -

L 0 ' ' ' ' ' ' ' ' ' ' '

0 2 4 6 8 10 12 14 16 18 20 Time (hours) r L 1.2 -

Xenon Density 11 ~

(Normalized) 1.0 d.9 - N Xenon Change = 0.0 Xenon = 1.0 0.8 Group 7 100 .

Position -

[ 80 -

(%IID) 60 -

~

Group 8 --

Position 20 -

0 ' ' ' ' ' ' ' ' ' ' '

Scan I

+10 -

__ Scan Axial

[ Imbalance 0 l

-10 -

(%)

-20 -

[ -30 i i > > > > i i i > '

Scan 4 -

l

+2 Xenon Offset f

{ (%)

0 2 .

4 i , , S9an , , , , , , ,

( 0 2 4 6 8 10 12 14 16 18 20 J Time (hours)

[

l Page 22 of 31

{

[

Figure 5. 100-58.3-100 Design Transient ANO-1 Cycle 8 (4 EFPD)

I I Power Level 100 75 I (% FP) 50 25 0 ' ' ' ' ' ' '

I 1.2 -

O 2 4 6 8 10 Time (hours) 12 14 16 18 20 Dens ty  ;*

(Normalized) 0.9 -

0.8 ' ' ' ' ' ' ' ' ' ' '

Group 7 100 -

Positon 80 1 I (%l10)

Group 8 60 40 20 -

Position ,

+ "

Axial - Scan

+20 -

[

I Imbalance

(%)

+10 0

I

-10 - ' ' ' ' ' ' '

I +10 -

"- Scan I Xenon Offset

(%)

+5 -

_ Scan

\

-5 -

M Scan

-10 ' ' ' ' ' ' ' ' ' ' '

0 2 4 6 8 10 12 14 16 18 20 I Time (hours)

Page 23 of 31 I

Figure No. 6 ANO-1 Cycle 8 Transient Comparisons

/ .

LEGEND

[ \..

Equilibrium Xe Zero Xe

-: A h

.'4 ' Trans f6 Min Xe

\

g .,.-P . .\, '

. . .T.r.a.n. .s. f.6. .M. .a.x. .X. .e. . .

/f '

i. , ',

) .. Tran.. s #2. Min X..e

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1 u /ii- , .

.... .Tr...a. .n.. .s. . . f.2. . . .M. . .a.. .x.

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r

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C i ,. 9 e i . . . . -l' l

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% Offset - %

M

i 1 O m W m O m. O M Tm rm m rm ra_ r- A im r -- , r-Figure No. 7 ANO-1 Cycle 8 Transient Comparisons O_e LEGEND

. Equilibrium Xe

- > s.h

' Trans #6 Max Xe

- - 1 g ' '.

\  :.

. s Tr.ans f6 Mi.n Xe.-_.

l l

i %f.= ==

_ _ _T_r_a_n_ _s_ f_2_ _M_ _a_x_ _X_ _e_ _

. . . . . . .. . T

.......r...a..n...s... X

j. 2.. .M....i..n..... ..e.......

9 .....**- '

% N ~. s ........... ...... ' .... 2..e...r..o....X...e..n...o...n......... ..... ....

g _

j ,,

g s .

7

!e a

w / . . . . .. .. . . . . . . . . . .

I O

m 2

~ o

-10 -5 0 5 10 15 20 25 30 0

O nset - 7.

m.

Figure 8. 100-15-100 Transient ANO-1 Cycle 8 (4 EFPD) 100 -

Power Level 75 -

p (% FP) 50 -

L 25 -1 0 i i i i e i i i i i i

[ 0 2 4 6 8 10 12 Time (hours) 14 16 18 20 1.4 .

[ Xenon Density 1.3 1,2 .

(Normalized) 1.1 -

1.0 0.9 -

s 0.8 '

Group 7 100 -

Position 80 - \ GP6 60 -

( (% WD) 40 .

/-GP8 ,

Group 8 , 20 -

/- GP7 Position

( 0

( +5 -

Axial 0 Imbalance -5 -

N S:an

[ (%) -10 -

Scan

-15 i i i i i i e i i i i

+4 S

/ can

[ Xenon +2 - g Offset 0 '

p

(%)

-2 - -

f L -4 i i e i !i e e i i i i 0 2 4 6 10 12 14 16 18 20

[, Time (hours) i Page 26 of 31

I Figure 9. 100-100-100 GP7 Insertion Transient ANO-1 Cycle 8 (4 EFPD)

I Powe 100 75 -

100% Power Through dntire Transient

(% FP) 50 -

25 -

0 ' ' ' ' ' ' ' ' ' ' '

0 2 4 6 8 10 12 14 16 18 20 Time (hours) 1.2 -

Xenon

~

Density 1.0 I (Normalized) 0.9 0.8 s 1.0 Through Entire Transient 100 -

Group 7 Position 80 7

- I 60 -

(% WD) 40 -

Group 8

]

I Position 20 0

+5 r Axial F 0

I Imbalance

(%)

-5

-10 Scan k can S

-15 ' ' ' ' ' ' ' ' ' ' '

+4 -

[ Scan [ Scan I Xenon Offset

+2 0

(%)

I -2

-4 0

2 4 6

8 10

. . . . i 12 14 16 18 20 Time (hours)

I Page 27 of 31 (I

'I

1 , - , , , ,

, , , , < r, r, < mm m Figure No.10 ANO-1 Cycle 8 Transient Comparisons

~

/' ^.. s. l

/ ,M '

i

/ LEGEND

/ '

/ ' .

! Equilibrium Xe l

"f2 / ,

,/,- (! Trans f5 Pos Xe 0.S.

Trans (5 Neg X_e 0.S.,_

. / / -

', g'

.E,l ,.

) ,',' / ' '-

. . .Z.e.r.o. .X. .e.n. o. .n. . . . . . . . . . . . .

h io

) \.

m / ,-

M j ,/ / )

A o ,

• /

• /

Oo '/

,o* l j 1 s

/

ers /

l l l o 3

n2 I m -25 -20 -15 -10 -5 0 5

- OMset - %

M

r r v, r r r r r r rtr r __r w r r r r 1-Figure No.11 '

ANO-1 Cycle 8 Transient Comparisons S

~ ' '

N. -

. LEGEND u N - Equilibrium Xe I D.N,,s, N -

,,\ Trans_#5 Nea Xe 0.S.

j

' y~N. ~

-f- Trans #5 Pos Xe 0.S.

as u N .D -h- -

- M' . . .Z.e.r.o. .X. .e.n.o. .n. . . . . . . . . . . . .

We- ,

N _

ca

% /

i O

m l

l l

l l

.u l $

~ O

\

• -5 0 5 20 l le 15 Offset - 7.

S

Figure 12. 100-62-100 Design Transient With APSR fiovement ANO-1 Cycle 8 (3800)

I

~

Power Level 75 -

I (% FP) 50 -

25 -

0 ' i e , ,

0 2 4 6 8 10 12 14 16 18 20 Time (hours) 1.2 .

De ty

[

(Normalized) -

0.9 -

0.8 i , , , , , , ,

Group 7 100 -

Position 80 -

(% WD) 60 -

~

Group 8 _

20 -

l Position 0 ,

+5 -

Axial 0 #

N

(

-5 1 Scan

-10 -

% Scan

-15 i e i , , , , , , ,

I +4 .

• SC30 N

1. 2 et 0

(.)

. Scan

-2 -

d ' ' ' ' , , , , , ,

0 2 4 6 8 10 12 14 16 18 20 Time (hours)

I I e 20 c< 21 l

I

~

{= mm en amm um aus em en amm em aus em um um een am amm en aus i

i i

I Figure No.13 l ANO-1 Cycle 8 Transient Comparisons a

l c_

u -

- ~

s 3

[=

[

r

[)\, 5' ' N g

\,

g

', i g

LEGEND Equilibrium Xe Transj3 Max Xe -

Trans (3 Min Xe i

) $* / , p , \j) _ _ _Z_ erg _Xe_Ig gn_ _ _ _ _ _ _ _,

e -

,9

, i ul i :E ,/

' t 1 l < .- \

l 1 o ,

oo n ,

\ a- s. <

i c i

/

y I

\

I  % o 30 -25 -20 -15 -10 -5 0 g Offset - %

1 3

I i