ML20206G375

Safety Evaluation Supporting Amends 244 & 235 to Licenses DPR-77 & DPR-79,respectively

ML20206G375
Person / Time
Site:	Sequoyah
Issue date:	05/04/1999
From:	NRC (Affiliation Not Assigned)
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ML20206G373	List: ML20206G369 ML20206G375
References
NUDOCS 9905070185
Download: ML20206G375 (7)

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO. 244 TO FACILITY OPERATING LICENSE NO, DPR-77 AND AMENDMENT NO. 235 TO FACILITY OPERATING LICENSE NO. DPR-79 TENNESSEE VALLEY AUTHORITY SEQUOYAH NUCLEAR PLANT. UNITS 1 AND 2 DOCKET NOS. 50-327 AND 50426

1.0 INTRODUCTION

l The Tennessee Valley Authority (TVA, the licensee) requested amendments to Operating Licenses DPR-77 and DPR-79 for Sequoyah Nuclear Plant (SON), Units 1 and 2, respectively, in a letter to the U.S. Nuclear Regulatory Commission (NRC) dated January 15,1999. The amendments would revise the SQN Units 1 and 2 Technical Specifications (TSs) for the reactor individual control rod position indication (RPI) systems. Specifically, the proposed change adds

- a new action statement to TS 3.1.3.2, " Position Indicating Systems - Operating," that eliminates the need to enter TS 3.0.3 whenever two or more individual RPIs per bank may be inoperable, while maintaining the appropriate overalllevel of protection. It would also allow additional time to determine the position of the nonindicating rod (s).

2.0 BACKGROUND

SON has 53 full-!angth rod control cluster assemblies (RCCAs) or control rod assemblies. The RCCAs are' designated by function as the control banks and shutdown banks. The shutdown RCCAs provide a large negative shutdown reactivity insertion on a reactor trip. They ensure the reactor remains subcritical and that shutdown margin is maintained immediately following the trip. The control RCCAs are used to change reactivity in the core, thereby changing fuel temperature and subsequently moderator temperature to maintain average temperature (Tavg) on program during power operation. There are four control banks and four shutdown banks.

Each set of banks are labeled A, B, C, and D. With the exception of Shutdown Banks C and D, each bank is comprised of two groups, although the banks are normally operated and controlled as a unit. The axial position of the RCCAs may be controlled manually or automatically. The

- RCCAs drop into the core following actuation of reactor trip signals. The shutdown banks are ahuays in the fully withdrawn position during normal operation, and are moved to this position at a constant speed by manual control prior to criticality. A reactor trip signal causes them to fall by gravity into the core. The control banks are the only RCCAs that can be manipulated under automatic control. Each control be,1k is divided into two groups to obtain smaller incremental 9905070185 990504 PDR ADOCK 05000327 P

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. reactivity changes per step. All RCCAs in a group are electrically paralleled to move simultaneously. The two groups within the same bank are stepped so that the relative position of the groups will not differ by more than one step. The control banks are programmed so that I

withdrawal of the banks is sequenced and overlapped in the following order: Control Banks A, B, C, then D. The programmed insertion sequence is the opposite of the withdrawal sequence, i.e.,

the last control bank withdrawn (Bank D) is the first control bank inserted.

The indication of RCCA position is a Regulatory Guide 1.97 Category 3 variable (i.e., non-Class 1E performance grade). Two separate systems are provided to sense and display control RCCA positions as described below.

Demand Position Indicatino System (DPIS)

The bank DPIS counts the pulses from the rod control system that moves the RCCAs. There is one step counter for each group of RCCAs. Individual rods in a group receive the same signal to move; therefore, they should all be at the same position indicated by the group step counter j

for that group. The DPIS is considered highly precise (i 1 step ori 5/8 inch). If an RCCA does I

not move one step for each demand pulse, the step counter will still count the pulse and incorrectly reflect the position of the RCCA.

4 Rod Position Indication System (RPIS)

The RPlS provides an indication of actual control rod position, but at a lower precision than the step counters. This system is based on inductive analog signals from a series of primary and secondary coil stacks spaced along the rod drive pressure housing in which the rod drive shaft acts as the core. The higher the shaft (and RCCA) is out of the reactor, the stronger the coupling between the transformer coil stacks. This produces an analog secondary voltage, which is directly proportional to the position of the drive rod. The maximum uncertainty is i 12 steps.

The D,PIS and the RPIS are separate and independent systems as a result of operational requirements. Operating procedures require the reactor operator to compare the demand and indicated (actual) readings from the RPIS so as to verify the operation of the rod control system.

In addition, the RPIS system provides an input to the control rod deviation alarm circuit. A rod position deviation alarm would be generated if an individual rod position deviated by more than 12 steps from its RCCA bank position. Also, RPIS provides warning of misalignment of any two RCCAs within the same bank by i 12 steps, and " rod-at-bottom" (rod drop).

TVA proposes to modify the SON Units 1 and 2 TSs by adding a new action statement to TS 3.1.3.2, " Position Indicating Systems - Operating," that would eliminate the need to enter TS 3.0.3 (which would require a plant shutdown) whenever two or more individual RPIs may be inoperable per bank, while maintaining the appropriate overall level of protection and would add flexibility to the initial determination of the position of the non-indicating rod (s).

RCCAs (or control rods) are considered operable when they are mechanically free to drop into the reactor core, their control system will perform its required function to free them to drop, and when they are in their requl red position as assumed in the accident analyses. Operatig

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experience has demonstrated that when the control rod RPI system indicates the control rods to be out of position, alternate measurement techniques typically demonstrate that the rods are indeed in the correct position, but the RPIS itself has problems. Operating experience has shown that failure of an individual RPI to properly indicate control rod position typically does not result in control rods being out of position. RPIS Inoperability is normally the result of a difference of more than 12 steps (the current limit specified in the TSs) occurring between the analog RPIS and the associated DPIS or" step counter." Such discrepancies have occurred several times in the past during reactor startups (see, for example,1VA Licensee Event Report l

No. 50-327/96011, dated December 18,1996.) The proposed change is similar to one being proposed by the nuclear industry as a change to the Westinghouse Standard TSs (NUREG-1431).

Specifically, the proposed changes to TS 3.1.3.2 would revise Action Statements (a) and (b) that currently require the initial determination of position of the non-indicating rod (s) indirectly by the j

movable incore detectors from "at least once per 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />" to "at least once per 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />." The proposed changes to TS 3.1.3.2 would also add a new Action Statement (b) that would read:

With more than one rod position indicator per bank inoperable either:

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Determine the position of the non indicating rod (s) indirectly by the movable incore 3

detectors at least once per 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />, and immediately after any motion of the non-l Indicating rod which exceeds 24 steps in one direction since the last determination of the rod's position, and Place the control rods under manual control, and monitor and record reactor coolant system (RCS) average temperature (Tavg) at least once per hour, and l

Restore the rod position indicators to OPERABLE status within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> such that a maximum of one rod position indicator per bank is inoperable, or Be in HOT STANDBY within the next 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.

As a result of the above proposed change, the current existing Action Statement (b) would become Action Statement (c).

3.0 EVALUATION TS Bases Section 3/4.1.3," Movable Control Assemblies," states that the specifications of this section ensure that;

a. Acceptable power distribution limits are maintained,

b. the minimum shutdown margin is maintained, and

c. the potential effects of rod alignment on associated accident analyses are limited.

The proposed TS changes provide compensatory measures to assure that the Bases are maintained. The compensatory actions require that rod position be determined indirectly via the movable incore flux detectors and that the RCS temperature be monitored and recorded, since

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. temperature is.a function of control rod positions. These actions address items a and b above.

Also, rod control is placed in manual, which limits automatic rod motion. This addresses item c above.

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The TVA request addresses the condition wherein the RPIS is inoperable with several l

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L compensatory actions'being taken during a 24-hour allowed outage time (AOT). However, if, l

during this period of time, an actual misalignment or rod control operability problem were to occur, then additional limiting conditions of operation (LCO's) would apply (e.g., LCO 3.1.3.1.b

- or c). The additional time to determine the position of the nonindicating rod (s) is requested to

. remove the interference between reducing power and determination of rod position while trying i

to troubleshoot the problem, since reducing power changes the neutron flux profile.

l The proposed action statement has an AOT of 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, as well as compensatory measures to use the movable incore detectors to ascertain rod position, to monitor and record RCS temperature, to place rod control in the manual mode (which limits automatic rod motion), and to allow the use of other reactivity control mechanisms such as boration and dilution. The 24-hour AOT provides sufficient time to troubleshoot and restore the RPIS to operation while avoiding plant challenges associated with an unnecessary plant shutdown. Monitoring and recording the RCS temperature would allow early detection of actual mispositioned or dropped rods. Overall plant safety would be enhanced by maintaining steady-state operation, as compared with the large rod movements and potential challenges required during an unnecessary plant shutdown

- in conjunction with the loss (inaccuracy) of a srr all number of RPI signals. The new action would avoid unnecessary plant shutdowns per TS 3.0.3, when operators would be challenged to use the rod control system with degraded rod indication capabilities.

Because no design changes are involved with this amendment request, the impact on the plant

- safety analysis design basis wou!d be one involving a reactivity transient induced by operator error associated with the loss of position indication. The analysis results for these events in Final Safety Analysis Report (FSAR) Sections 15.2.1 through 15.2.3 are not dependent upon operator action. The assumed reactivity insertion rates are based on conservative, worst-case i

scenarios independent of whether they are due to equipment malfunction or human error. Loss of RCCA. position indication would not affect the assumed reactivity insertion rates. Further, the protection systems assumed in the analysis of these events (power range neutron flux high and low settings and Overtemperature-Delta T) are unaffected since no design changes are involved.

The worst-case reactivity transient of this nature, the withdrawal of a single RCCA, has been analyzed in FSAR Section 15.3.6 assuming that operators ignore RCCA position indication.

i Whether indication is lost, as is the case covered by this new action statement, or disregarded, l

does not change the method of analysis or the outcome of this event. Warning of rod bank insertion limits would be available to the operator from the rod bank demand position system.

There is a 5 percent uncertainty margin between the power peaking factor measured by the incore detector system and the design power peaking factor assumed in the analysis of American Nuclear Society Condition I and 11 transients. The movable incore detectors are capable of revealing any situation that causes power shapes to be peaked in excess of the

- design value. Asymmetric power distributions can also be detected by the excore neutron flux detectors and core exit thermocouples. FSAR Sections 7.7.1.9 and 7.7.2 provide further i

discussions on the capabilities of these systems.

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% TS Bases Section 3/4.1.3 states that control rod positions and operability of the rod position indicators are required to be verified on a nominal basis of once per 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> with more frequent verification if an automatic monitoring channel is inoperable. Standard TS (NUREG1431) Bases i

Section B 3.1.8 (equivalent to Section 3/4.1.3) states the period of 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> for verification of

. RCCA position is reasonable based upon experience, normal power operation not requiring J

excessive movement of banks, and the small probability of simultaneously having a rod significantly out of position and an event sensitive to rod position. The 12-hour period for verification (not applicable to the rod position deviation monitor being inoperable) is reasonable in that:

lt is consistent with shift frequency (chift durations are now 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />), a period typically used as the basis of the 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />, and It is consistent with the rod position surveillance frequency currently in the TS.

It removes the interference between TS 3.1.3.2 Actions a.1 and a.2 or b.1 and b.2. If power is going to be reduced, control rod position determination cannot be made during the power reduction. This change would provide more time for troubleshooting the problem and/or, allow the determination to be made after the power is reduced, and It is consistent with the 12-hour timeframe allowed to verify shutdown margin when a rod is misaligned from its group step counter height by more than i 12 steps in TS 3.1.3.1.

SQN's operating experience has been that RCCAs have not actually been found misaligned when movable flux detectors have been utilized to indirectly determine their position.

Rather, the RPIS itself has had problems due to signal variations as a result of nonlinear circuit resistance in one of the RPIS loops, also referred to as the rod bow phenomenon.

The NRC staff has reviewed TVA's proposed changes and agrees that the changes are justified given the compensatory actions that will be taken and conclude that the proposed changes have minor safety significance. The proposed changes to TS 3.1.3.2 for SON Units 1 and 2 are, therefore, acceptable.

4.0 STATE CONSULTATION

in accordance with the Commission's regulations, the Tennessee State official was notified of the proposed issuance of the amendments. The State official had no comments.

5.0 ENVIRONMENTAL CONSIDERATION

The amendment changes a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20 and changes surveillance requirements. The NRC staff has determined that the amendment involves no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. _ The Commission has previously issued a proposed finding that the amendment involves no significant hazards consideration, and there has been no public comment on such finding (64 FR 9201, dated February 24,1999). Accordingly, the

- amendments meet the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9).

Pursuant to 10 CFR 51.22(b), no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendments.

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6.0 CONCLUSION

The Commission has concluded, based on the considerations discussed above, that: (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.

Principal Contributor: Ronald W. Hernan Dated: May 4, 1999 i

4 Mr. J. A. Scalice SEQUOYAH NUCLEAR PLANT Tennessee Valley Authority cc:

Senior Vice President Mr. Pedro Salas, Manager Nuclear Operations Licensin' and Industry Affairs g

Tennessee Valley Authority Sequoyah Nuclear Plant 6A Lookout Place Tennessee Valley Authority 1101 Market Street P.O. Box 2000 Chattanooga, TN 37402-2801 Soddy Daisy, TN 37379 Mr. Jack A. Bailey Mr. D. L. Koehl, Plant Manager Vice President Sequoyah Nuclear Plant Engineering & Technical Services Tennessee Valley Authority Tennessee Valley Authority P.O. Box 2000 6A Lookout Place Soddy Daisy, TN 37379 1101 Market Street Chattanooga, TN 37402-2801 Mr. Melvin C. Shannon Senior Resident inspector Mr. Masoud Bajestani Sequoyah Nuclear Plant Site Vice President U.S. Nuclear Regulatory Commission Sequoyah Nuclear Plant 2600 Igou Feny Road Tennessee Valley Authority Soddy Daisy, TN 37379 P.0, Box 2000 Soddy Daisy, TN 37379 Mr. Michael H. Mobley, Director TN Dept. of Environment & Conservation General Counsel Division of Radiological Health Tennessee Valley Authority 3rd Floor, L and C Annex ET 10H 401 Church Street 400 West Summit Hill Drive Nashville, TN 37243-1532 Knoxville, TN 37902 County Executive Mr. N. C. Kazanas, General Manager Hamilton County Courthouse Nuclear Assurance Chattanooga, TN 37402-2801 Tennessee Valley Authority SM Lookout Place 1101 Market Street Chattanooga, TN 37402-2801 Mr, Mark J. Burzynski, Manager Nuclear Licensing Tennessee Valley Authority 4X Blue Ridge 1101 Market Street Chattanooga, TN 37402-2801