ML18153D199

Proposed Tech Specs Re RCS Loop Stop Valve Interlocks

ML18153D199
Person / Time
Site:	Surry
Issue date:	12/11/1992
From:	VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:
Shared Package
ML18153D198	List: ML18153D197 ML18153D199
References
NUDOCS 9212170251
Download: ML18153D199 (13)

Text

ATTACHMENT 1 SURRY POWER STATION PROPOSED TECHNICAL SPECIFICATION CHANGES REACTOR COOLANT SYSTEM LOOP STOP VALVE INTERLOCKS

e TS 3.17-1 3.17 LOOP STOP VALVE OPERATION Applicability Applies to the operation of the loop stop valves.

Objective To specify those limiting conditions for operation of the loop stop valves which must be met to ensure safe reactor operation.

Specifications

1. The loop stop valves shall be maintained open unless the reactor is in COLD SHUTDOWN or REFUELING SHUTDOWN.

2. A hot or cold leg stop valve in a reactor coolant loop may be closed in COLD SHUTDOWN or REFUELING SHUTDOWN for up to 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> for valve maintenance or testing. If the stop valve is not opened within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />, the loop shall be isolated.

3. Whenever a reactor coolant loop is isolated, the stop valves of the isolated loop shall have their AC power removed and their breakers locked open.*

4. Whenever an isolated and filled reactor coolant loop is returned to service, the following conditions shall be met:

a. A source range nuclear instrumentation channel shall be operable and continuously monitored with audible indication in the control room during opening of the hot leg loop stop valve, during relief line flow, and when opening the cold leg stop valve in the isolated loop. Should the count rate increase by more than a factor of two over the initial count rate, the hot and cold leg stop valves shall be re-closed and no attempt made to open the stop valves until the reason for the count rate increase has been determined.

• Power may be restored to a hot or cold leg loop stop valve in an isolated and filled loop provided the requirements of Specifications 4.b or 4.c are met, respectively. Power may be restored to a loop stop valve in an isolated and drained loop provided the requirements of Specifications 5.a and b are met.

Amendment Nos.

TS 3.17-2

b. Before opening the hot leg loop stop valve.

1) The boron concentration of the isolated loop shall be greater than or equal to the boron concentration in the active loops.

Verification of this condition shall be completed within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> prior to opening the hot leg stop valve in the isolated loop.

c. Before opening the cold leg loop stop valve.

1) The hot leg loop stop valve shall be open with relief line flow established for at least 90 minutes at greater than or equal to 125 gpm.

2) The cold leg temperature of the isolated loop shall be at least 70°F and within 20°F of the highest cold leg temperature of the active loops. Verification of this condition shall be completed within 30 minutes prior to opening the cold leg stop valve in the isolated loop.

3) The boron concentration of the isolated loop shall be greater than or equal to the boron concentration in the active loops.

Verification of this condition shall be completed after relief line flow for at least 90 minutes at greater than or equal to 125 gpm and within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> prior to opening the cold leg stop valve in the isolated loop.

5. Whenever an isolated and drained reactor coolant loop is filled from the active volume of the RCS, the following conditions shall be met:

a. The isolated loop shall be drained. Verification of this condition shall be completed within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> prior to partially opening the hot or cold leg stop valve in the isolated loop.

b. The Reactor Coolant System level shall be at least 18 ft. during opening of the loop stop valves and during the filling of the isolated loop. If Reactor Coolant System level is not maintained at 18 ft. or above, the loop stop valves shall be closed.

Amendment Nos.

TS 3.17-3

c. The hot and cold leg stop valves shall be fully opened within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> after the isolated loop is filled.

d. A source range nuclear instrumentation channel shall be operable and continuously monitored with audible indication in the control room during the backfill evolution. Should the count rate increase by more than a factor of two over the initial count rate, the hot and cold leg stop valves shall be re-closed and no attempt made to open the stop valves until the reason for the count rate increase has been determined~

The Reactor Coolant System may be operated with isolated loops in COLD SHUTDOWN or REFUELING SHUTDOWN in order to perform maintenance. A loop stop valve in any loop can be closed for up to two hours without restriction for testing or maintenance in these operating conditions. While operating with a loop isolated, AC power is removed from the loop stop valves and their breakers locked opened to prevent inadvertent opening. When the isolated loop is returned to service, the coolant in the isolated loop mixes with the coolant in the active loops. This situation has the potential of causing a positive reactivity addition with a corresponding reduction of shutdown margin if:

a. The temperature in the isolated loop is lower than the temperature in the active loops (cold water accident), or

b. The boron concentration in the isolated loop is insufficient to maintain the required shutdown margin (boron dilution accident).

The return to service of an isolated and filled loop is done in a controlled manner that virtually eliminates any sudden positive reactivity addition from cold water or boron dilution. A flow path to mix the isolated loop with the active loops is established through the relief line by opening the hot leg stop valve in the isolated loop and starting the reactor coolant pump. The relief line flow is low enough to limit the rate of any reactivity addition due to differences in temperature and boron concentration between the isolated loop and the active loops. In addition, a source range instrument channel is required to be operable and continuously monitored to detect any change in core reactivity.

Amendment Nos.

TS 3.17-4 The limiting conditions for returning an isolated and filled loop to service are as follows:

a. A hot leg loop stop valve may not be opened unless the boron concentration in the isolated loop is greater than or equal to the boron concentration in the active portion of the Reactor Coolant System.

b. A cold leg loop stop valve can not be opened unless the hot leg loop stop valve is open with relief line flow established for at least 90 minutes at greater than or equal to 125 gpm. In addition, the cold leg temperature of the isolated loop must be at least 70°F and within 20°F of the highest cold leg temperature of the active loops. The boron concentration in the isolated loop must be verified to be greater than or equal to the boron concentration in the active portion of the Reactor Coolant System.

c. A source range nuclear instrument channel is required to be monitored to detect any unexpected positive reactivity addition during hot or cold leg stop valve opening and during relief line flow.

If an isolated loop is initially drained, the above requirements are not applicable. An initially isolated and drained loop may be returned to service by partially opening the loop stop valves and filling the loop in a controlled manner from the Reactor Coolant System. Prior to partially opening the loop stop valves, the following measures are required to ensure that no sudden positive reactivity addition or loss of Reactor Coolant System inventory occurs:

a. The isolated loop is verified to be drained, thus preventing dilution of Reactor Coolant System boron concentration by liquid present in the loop.

b. Reactor Coolant System level is verified to be :2:18' elevation to ensure Reactor Coolant System inventory is maintained for decay heat removal. Reactor Coolant System inventory is required to be maintained at > 18' elevation during the backfill evolution. In addition, the filling evolution is limited to one isolated loop at a time.

c. A source range nuclear instrument channel is required to be monitored to detect any unexpected positive reactivity addition.

Amendment Nos.

TS 3.17-5 After an initially drained loop is filled from the Reactor Coolant System by partially opening the loop stop valves, the loop is no longer considered to be isolated. Thus, the requirements for returning an isolated and filled loop to service are not applicable and the loop stop valves may be fully opened without restriction within two hours of completing the loop fill evolution.

The initial Reactor Coolant System level requirement has been established such that, even if the three cold leg stop valves are suddenly opened and no makeup is available, the Reactor Coolant System water level will not drop below mid-nozzle level. This ensures continued adequate suction conditions for the residual heat removal pumps.

The safety analyses assume a minimum shutdown margin as an initial condition.

Violation of these limiting conditions could result in the shutdown margin being reduced to less than that assumed in the safety analyses. In addition, violation of these limiting conditions could also cause a loss of shutdown decay heat removal.

Reference (1) UFSAR Section 4.2 (2) UFSAR Section 14.2.5 Amendment Nos.

e ATTACHMENT 2 SURRY POWER STATION DISCUSSION AND SIGNIFICANT HAZARDS CONSIDERATION PROPOSED TECHNICAL SPECIFICATION CHANGES REACTOR COOLANT SYSTEM LOOP STOP VALVE INTERLOCKS

e e DISCUSSION OF CHANGES INTRODUCTION Surry Power Station has administrative limits and hardware interlocks which control the operation of the Reactor Coolant System loop stop valves. The existing restrictions were created based on the original design basis that included the ability to operate the unit at power with a Reactor Coolant System loop isolated. Although an isolated loop could only be returned to service with the unit shut down, these restrictions and interlocks were established to protect against inadvertent cold water and boron dilution accidents during the startup of an isolated loop. The proposed Technical Specification changes restrict the operational modes wherein a Reactor Coolant System loop may be isolated or returned to service to cold shutdown and refueling shutdown. The proposed changes remove the interlock operability requirements, require removing the AC power and locking open the breakers of the stop valves in an isolated loop, and provide temperature and boron concentration requirements that will prevent any significant positive reactivity addition when returning an isolated loop to service. In addition, a requirement to establish and maintain a minimum inventory is being included to preclude the loss of shutdown cooling during loop backfill evolutions. Eliminating the interlock operability requirements will remove a requirement which is not applicable to the current operation of the loop stop valves.

The proposed changes will continue to adequately protect against inadvertent cold water, boron dilution, and loss of shutdown cooling accidents.

BACKGROUND The original plant design allowed power operation with less than three reactor coolant pumps running or with an isolated loop. However, this configuration is no longer permitted by Technical Specifications. Therefore, the application of Technical Specification 3.17 should now only be directed to controlling the operation of the loop stop valves in cold shutdown and refueling shutdown.

The existing Technical Specifications require the interlocks to be operable whenever startup of an isolated reactor coolant loop is initiated. The proposed change will limit reactor coolant loop isolation to cold shutdown and refueling shutdown only. During cold shutdown and refueling shutdown, administrative restrictions are adequate to fulfill the safety function of the interlocks. The loop stop valves are normally closed to isolate and drain a loop for maintenance purposes. Therefore, opening a loop stop page 1 of 6

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e e valve in an isolated loop to provide a source of water is not included as a contingency in station procedures. Returning an isolated and drained loop to service by opening the loop stop valves and filling the loop in a controlled manner from the RCS reduces reactor coolant pump operation and minimizes the amount of time the loop is required to be isolated. Therefore, the proposed change specifies limits for operation of the stop valves in this manner. For the return to service of an initially filled loop, administrative restrictions equivalent to the controls provided by the loop stop valve interlock system are specified.

DESCRIPTION OF CHANGES The existing Technical Specifications for the loop stop valves apply to isolated loop startups and not to maintenance operations during cold shutdown and refueling shutdown. The proposed Technical Specification changes remove the interlock operability requirements, which will remove a requirement which is not applicable to the current operation of the loop stop valves, while maintaining controls which virtually eliminate the potential for a positive reactivity addition accident. The proposed Technical Specification changes will allow a small difference in temperature between the loop being restored to service and the active portion of the Reactor Coolant System.

Specifically, the proposed Technical Specification changes will require:

• The loop stop valves to be open while the Unit is above cold shutdown. This will be accomplished by limiting loop isolation to cold shutdown or refueling shutdown. An allowance is included for closing a single loop stop valve for valve maintenance or testing in cold shutdown or refueling shutdown.

• The loop stop valves to be closed with the AC power removed and the breakers locked open while a loop is isolated. Thus, the need for measuring the boron concentration of the isolated loop on a routine basis can be eliminated.

• An isolated and filled loop to meet the following condition prior to opening the hot leg stop valve:

The boron concentration of the isolated loop to be greater than or equal to the boron concentration of the active loops.

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e e

• An isolated and filled loop to meet the following conditions prior to opening

• the cold leg stop valve:

The hot leg loop stop valve to be open with relief line flow established for at least 90 minutes at greater than or equal to 125 gpm.

The cold leg temperature of the isolated loop to be at least 70°F and within 20°F of the highest cold leg temperature of the active loops.

The boron concentration of the isolated loop to be greater than or equal to the boron concentration of the active loops.

• The loop stop valves to be fully opened within two hours of completely backfilling an isolated and drained loop from the active portion of the Reactor Coolant System. If the valves are not opened within two hours, the loop must be treated as an isolated and filled loop and the applicable requirements met before opening the loop stop valves.

• A minimum Reactor Coolant System water level to be established and maintained during the backfill of an initially isolated and drained loop.

• A source range nuclear instrumentation channel to be operable and continuously monitored for increasing count rate with audible alarm capability: 1) when opening a hot leg loop stop valve to establish relief line flow, 2) during relief line flow, 3) when opening a cold leg loop stop valve, and 4) when backfilling an isolated and drained loop from the active portion of the Reactor Coolant System.

SAFETY SIGNIFICANCE This proposed Technical Specification changes will eliminate the operability requirements for the Reactor Coolant System loop stop valve interlocks. Therefore, the major safety concerns considered were the potential for positive reactivity additions when returning an isolated loop to service and the potential for loss of shutdown cooling when opening a loop stop valve to fill a drained loop.

Technical Specification requirements are being incorporated to require that the loop stop valves be maintained open unless the unit is in cold shutdown or refueling shutdown. When a loop is isolated the loop stop valves will have their AC power page 3 of 6

e e removed and the breakers locked open, thus ensuring that a valve is not inadvertently opened. When returning an isolated and filled loop to service, the revised Technical Specifications require that the boron concentration of the isolated loop be verified greater than or equal to that of the active loops before opening a hot leg loop stop valve, thus precluding boron dilution. To provide a controlled mixing between the isolated loop and the active loops, the isolated loop will be operated with relief line flow of at least 125 gpm for 90 minutes, while monitoring neutron flux to identify any unexpected reactivity addition. Before opening the cold leg loop stop valve, the cold leg temperature is required to be at least 70°F and within 20°F of the highest cold leg temperature of the operating loops. The isolated loop boron concentration is again verified greater than or equal to that of the active loops. Specifying a minimum isolated loop temperature of 70°F prevents any pressurized thermal shock concern for Reactor Coolant System operation in cold shutdown with the system pressure above 200 psig. Limiting the temperature differential between the isolated and active loop cold legs to 20°F limits the positive reactivity addition, minimizes thermal stress to the reactor vessel, and minimizes reactor vessel nozzle fatigue. Calculations have shown that an inadvertent loop startup at cold shutdown will not result in an inadvertent criticality regardless of the temperature differential between loops. At cold shutdown conditions the positive reactivity effects of changing moderator temperature are substantially less than the available shutdown margin.

For loops that have been isolated and drained for maintenance, requirements are being incorporated to verify that a drained loop is indeed empty before opening the stop valves. This ensures that the Reactor Coolant System will not be diluted by liquid initially present in the loop when the loop is filled. A minimum Reactor Coolant System water level is required to be maintained while opening the loop stop valves to fill the loop. Establishing a minimum water level in the Reactor Coolant System and maintaining this level will ensure that a loss of shutdown cooling does not occur during the loop backfill evolution. The initial Reactor Coolant System level requirement has been established such that, even if all three drained loops cold leg loop stop valves are suddenly opened and a makeup source is not available to maintain level, the Reactor Coolant System water level will not drop below mid-nozzle level. This ensures continued adequate suction conditions for the residual heat removal pumps.

Also, within two hours of filling the loop, the loop stop valves must be fully open to ensure that there are no significant temperature or boron differences. These requirements will virtually eliminate the potential for a positive reactivity addition accident.

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C, SIGNIFICANT HAZARDS CONSIDERATION Virginia Electric and Power Company has reviewed the proposed changes against the criteria of 10 CFR 50.92 and has concluded that the changes as proposed do not pose a significant hazards consideration. Specifically, operation of the Surry Power Station in accordance with the proposed Technical Specification changes will not:

1. Involve a significant increase in the probability of occurrence or consequences of an accident previously evaluated.

The proposed changes do not increase the probability or consequences of a loop startup accident or a loss of shutdown decay heat removal. The risk of inadvertent criticality in cold shutdown or refueling shutdown is sufficiently low that administrative controls can be used in lieu of the loop stop valve interlock system in those conditions. The proposed restrictions on loop stop valve operation preclude a significant positive reactivity addition or loss of Reactor Coolant System inventory when an isolated loop is returned to service. Locking closed the stop valves of an isolated loop with power removed prevents inadvertent opening and potential boron dilution or loss of inventory.

Requirements for boron concentration, temperature, and relief line flow for an isolated and filled loop ensure no significant positive reactivity addition occurs when returning the loop to service. Verifying that a drained loop is initially empty ensures that liquid present in the loop does not dilute the Reactor Coolant System. Requiring a minimum Reactor Coolant System water level precludes a loss of RHR pump suction when filling a drained loop by opening the loop stop valves.

2. Create the possibility of a new or different kind of accident from any accident previously evaluated.

The proposed changes do not introduce any new failure modes or accident precursors. Eliminating the operability requirements for the loop stop valve interlocks does not create any new or different kind of accident scenarios. The loop startup accident has been analyzed with no credit taken for loop stop valve interlock operation. The loop stop valves will not be operated in a different manner from which they were designed to operate. Furthermore, the plant page 5 of 6

.... . . .... e e conditions for which loops may be isolated have been limited to cold shutdown

_ and refueling shutdown.

3. Involve a significant reduction in a margin of safety.

The proposed Technical Specification requirements for return of an isolated loop to service fulfill the function of the loop stop valve interlocks. Adequate controls have been established to virtually eliminate the potential for an inadvertent valve opening in an isolated loop, causing a boron dilution or cold water addition accident. These controls also virtually eliminate the potential for a loss of shutdown cooling caused by the opening of a stop valve in an isolated and drained loop. Furthermore, startup of an isolated loop is now limited to only cold shutdown or refueling shutdown. Calculations have shown that an inadvertent loop startup at cold shutdown will not result in an inadvertent criticality regardless of the temperature differential between loops. At cold shutdown conditions the positive reactivity effects of changing moderator temperature are substantially less than the available shutdown margin.

Therefore, the margin of safety as defined in any Technical Specification is not reduced.

Using the examples identified in the Federal Register, Volume 51, No. 44, of March 6, 1986, that are considered not likely to involve significant hazards considerations, the proposed changes are similar to example (ii).

Example (ii) is a change that constitutes an additional limitation, restriction, or control not presently included in the Technical Specifications, e.g., a more stringent surveillance requirement. Although the proposed changes eliminate the loop stop valve interlock operability requirements, the changes will restrict isolation of a reactor coolant loop to cold shutdown and refueling shutdown conditions. The proposed changes will require the stop valves to be locked closed and the AC power removed when a loop is isolated. The proposed changes will also provide additional restrictions on temperature and boron concentration for restoring an isolated loop to service. These requirements ensure that a positive reactivity addition accident or a loss of shutdown cooling due to stop valve operation will not occur.

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