ML20236T530
| ML20236T530 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 07/17/1998 |
| From: | NORTHEAST NUCLEAR ENERGY CO. |
| To: | |
| Shared Package | |
| ML20236T527 | List: |
| References | |
| NUDOCS 9807280242 | |
| Download: ML20236T530 (10) | |
Text
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REACTOR C00LANT SYSTEM BASES l
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OVERPRESSURE PROTECTION SYSTEMS (continued)
The reactor vessel material is less tough at low temperatures than at normal operating temperature. As the vessel neutron exposure accumulates, the material toughness decreases and becomes less resistant to stress at low temperatures (Ref. 2). RCS pressure, therefore, is maintained low at low temperatures and is I increased only as temperature is increased.
The potential for vessel overpressurization is most acute when the RCS is water solid, occurring while shutdown; a pressure fluctuation can occur more quickly than an operator can react to relieve the condition. Exceeding the RCS P/T limits by a significant amount could cause nonductile cracking of the reactor vessel. LCO 3.4.9.1, " Pressure / Temperature Limits - Reactor Coolant System,"
requires administrative control of RCS pressure and temperature during heatup and I cooldown to prevent exceeding the limits provided in Figures 3.4-2 and 3.4-3.
This LCO provides RCS overpressure protection by limiting mass input capability and requiring adequate 1ressure relief capacity. Limiting mass input capability requires all Safety In;ection (SIH) pumps and all but one centrifugal charging pump to be incapable of injection into the RCS. The pressure relief capacity requires either two redundant relief valves or a depressurized RCS and an RCS f vent of sufficient size. One relief valve or the open RCS vent is the
\ overpressure protection device that acts to terminate an increasing pressure event.
With minimum mass input capability, the ability to provide core coolant addition is restricted. The LCO does not require the makeup control system deactivated or the safety injection (SI) actuation circuits blocked. Due to the lower pressures in the Cold Overpressure Protection MODES and the expected core decay heat levels, the makeup system can provide adequate flow vis the makeup control valve.
PORV Requirements As designed, the PORY Cold Overpressure Protection (COPPS) is signaled to open '
if the RCS pressure approaches a limit determined by the COPPS actuation logic.
The COPPS actuation logic monitors both RCS temperature and RCS pressure and determines when the nominal setpoint of Figure 3.4-4a or Figure 3.4-4b is approached. The wide range RCS temperature indications are auctioneered to ;
select the lowest temperature signal. '
The lowest temperature signal is processed through a function generator that calculates a pressure setpoint for that temperature. The calculated pressure
..IME(f setpoint is then compared with the indicated RCS pressure from a wide range
'8 If the indicated pressure meets or exceeds the calculated (pressure channel.
value, a PORV is signaled to open. 1 I
Figure 3.4-4a and Figure 3.4-4b present the PORV setpoints for COPPS. Above 110*F, the setooints are stannered so only one valve opens during a low i 9807290242 990717 l PDR ADOCK 05000423 P PM ,
MILLSTONE - UNIT 3 8 3/4 4-16 Amendment No. 77. FF,157 0528 On- ,
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REACTOR C00LAKI' SYSTEM BASES T
( .
OVERPRESSURE PROTECTION SYSTEMS (continued)
The cold overpressure transient analyses demonstrate that either one relief valve or the depressurized RCS and RCS vent can maintain RCS pressure below limits when RCS letdown is isolated and only one centrifugal charging pump is operating.
Thus, the LCO allows only one centrifugal charging pump capable of injecting when cold overpressure protection is required.
'The cold overpressure protection enabling temperature is conservatively established at a value 1275'F based on the criteria described in Branch Technical Position RSB 5-2 provided in the Standard Review Plan (NUREG-0800).
PORY Performance for both three astd four RCS tects UNisoIAted The 10CFR50 Appendix G analyses how that the vessel is protected against non-ductile failure when the PORVs are set to open at the values shown in Figures 3.4-4a and 3.4-4b within the tolyance allowed for the calibration accuracy. The curves are derived. by analysesithat model the performance of the PORV cold overpressure protection system (COPPS), assuming the limiting mass and bat transients of one centrifugal charging' pump injecting into the RCS, or the energy addition as a result of starting an RCP with temperature asymmetry between the RCS and the steam generators. These analyses consider pressure overshoot anc undershoot beyond the PORV opening and closing, resulting from signal processing
( and valve stroke times. i The PORV setpoints in Figures 3.4-4a and 3.4-4b will be updated when the P/T ;
limits conflict with the cold overpressure analysis limits. The P/T limits are periodically modified as the reactor vessel material toughness decreases due to neutron embrittlement. Revised limits are determined using neutron fluence projections and the results of testing of the reactor vessel material irradiation surveillance specimens. The Bases for LCO 3.4.9.1, " Pressure / Temperature Limits
- Reactor Coolant System (Except the Pressurizer)," discuss these evaluations.
The PORVs are considered active components. Thus, the failure of one PORV is assumed to represent the worst case, single active failure.
RHR Suction Relief Valve Performance !
The RHR suction relief valves do not have variable pressure and temperature lift i setpoints as do the PORVs. Analyses show that one RHR suction relief valve with a setpoint at or between 426.8 psig and 453.2 psig will pass flow greater than that required for the limiting cold overpressure transient while maintaining RCS i pressure less than the isothermal P/T 1 lait curve. Assuming maximum relief flow l requirements during the limiting cold overpressure event, an RHR suction relief valve will maintain RCS pressure to i 110% of the nominal lift setpoint.
Although each RHR suction relief valve is a passive spring loaded device, which meets single failure criteria, its location within the RHR System precludes meeting single failure criteria when spurious RHR suction isolation valve or RHR
(. suction valve closure is postulated. Thus the loss of an RHR suction relief
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, BASES
(
OVERPRESSURE PROTECTION SYSTEMS (continued)
- c. When COPPS is armed, PORV undershoot is analyzed for mass injection transients limited to one charging pump. LCO 3.4.g.3, " Reactor Coolant System - Overpressure Protection Systems," provides this protection by requiring both safety injection pumps and all but one charging pump to be incapable of injection into the RCS.
In order to provide protection for the RCP #1 seal, a PORV setpoint of 1595 psia for temperatures 1 160 degrees F must be met. This minimum setpoint is derived by adding the applicable train' uncerhinty and valve undershoot to the required minimum RCS pressure required for seal integrity. Due to the differing instrument uncertainties for the two trains of PORV COPPS, the train with the highest uncertainty is paired to the high setpoint curve.
LCQ This LCO requires that cold overpressure protection be OPERABLE and the muimum mass input be limited to one charging pump. Failure to meet this LCO could lead to the loss of low temperature overpressure mitigation and violation of the Reference 1 isothermal limits as a result of an operational transtesi..
f To limit the mass input capability, the LCO requires a maximum of one centrifugal A charging pump capable of injecting into the RCS.
The elements of the LCO that provides low temperature overpressure mitigation through pressure relief are:
]
A PORV is OPERABLE for cold overpressure protection when its bloc alve is i open, its lift setpoint is set to the nominal setpoints provided Figure 1 3.4-4a or' 3.4-4b and when the surveillance requirements are met.p
An RHR suction relief valve is OPERABLE for cold overpressure protection when its isolation valves from the RCS are open and when its setpoint is at or between 426.8 psig and 453.2 psig, as verified by required testing.
An RCS vent is OPERABLE when open with an area of 15.4 square inches.
Each of these methods of overpressure ~ prevention is capable of mitigating the limiting cold overpressure transient.
(
Insert A:
The use of a PORV for Cold Overpressure Protection is limited to those conditions when no more than one RCS loop is iso!ated from the reactor 1 vessel and, whenever an RCP is running, the temperature signal from the isolated loop is removed from the PORV opening logic. When two or more loops are isolated, Cold Overpressure Protection must be provided by either the two RHR suction relief valves or a depressurized and vented RCS.
In m &
The wide range RCS temperature indicators rnonitor loop temperature in the portion of the RCS loop that can be isolated. If a loop is isolated, the temperature signal from the isolated loop may be significantly lower than the temperature signals from the unisolated loops. This would result in a calculated pressure setpoint for the PORV be:ow that anticipated by the operator based on the temperature in the unisolated portion of the RCS.
Since this could result in a significantly lower calculated PORV setpoint, RCP operation is not permitted under these conditions unless the temperature input from the isolated loop is removed from the auct;oneered circuit. This restriction will ensure that the #1 RCP seals are not challenged as a result of PORV undershoot. Since the PORV mass and heat injection transients have only been analyzed for a maximum of one loop isolated, the use of the PORVs is restricted to three and four RCS loops unisolated.
If one loop is isolated without removing its temperature input from the PORV calculated setpoint auctioneered circuit and at .least one RCP is in operation or if more than one loop is isolated, then the PORVs must have their block valves closed or COPPS rnust be blocked. For these cases, Cold Overpressure Protection must be provided by either the two RHR suction relief valves or a depressurized RCS and an RCS vent. l The use of the PORVs is restricted to three and four RCS loops unisolated; for a loop to be considered isolated both RCS loop stop valves must be closed. If only one loop stop valve in an RCE 69 si c!csed for an extended period, the PORVs must have their block valves closed or COPPS must 6-blocked. A single RCS loop stop valve can be stroked for short time periods for surveillance or other purposes and not affect the use of the PORVs for l Cold Overpressure Protection.
The RHR suction relief valves have been qualified for all mass injection l transients for any combination of isolated loops. In addition, the heat '
injection transients not prohibited by the Technical Specifications have also f been considered in the qualification of the RHR suction relief valves.
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t' Insert C:
,For three loops unisolated, the temperature input from the isolated loop must l be removed from the COPPS auctioneered circuitry whenever any RCP is in operation.
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__________.__i____________._________. _ . _ _ _ _ _________J
,- .' l Docket Nos. 50-243 B17057 Attachment 3 Millstone Nuclear Power Station, Unit No. 3 Change to Technical Specification Bases Cold Overpressure Protection System -
Isolated Loop Operation (TSCR 3-6-98)
Retyped Pages i
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July 1998 I i u_----______ ._ _ _ - - - - _ - -- >
E
- BASES i j
4
- 0VERPRE5SURE PROTECTION SYSTEMS (continued)
The use of a PORV for' Cold Overpressure Protection is limited to those conditions when no more than one RCS loop is isolated from the reactor vessel and, whenever an RCP is running, .the temperature signal'from the isolated. loop is removed from the PORV opening logic. When two or more loops are isolated Cold Overpressure Protection must; be provided by either. the two RHR suction relief valves or a depressurized and vented RCS.
The reactor' vessel material .is less tough at _ low temperatures than at normal operating temperature. As the vessel neutron exposure accumulates, the material toughness decreases and becomes less resistant to stress at low temperatures (Ref. 2).- RCS pressure, therefore, is maintained low at low temperatures and is increased only as temperature is increased.
The potential.for vessel overpressurization is most acute when the RCS is water
. solid, occurring while shutdown; a pressure fluctuation can occur more 'quickly '
than an operator can react to relieve the condition. Exceeding the RCS P/T l
' limits by a significant amount could cause nonductile cracking of the reactor
' vessel. LCO 3.4.9.1, " Pressure / Temperature Limits. - Reactor Coolant System,"
requires administrative control of RCS pressure and temperature during heatup and y cooldown to prevent exceeding the limits provided in Figures 3.4-2 and 3.4-3. j This. LCO provides RCS overpressure protection by. limiting mass input capability I and requiring adequate pressure relief capacity. Limiting mass input capability requires all Safety Injection (SIH) pumps and all but one centrifugal charging l pump to be incapable of injection into the RCS. The pressure relief capacity requires'either two redundant relief valves or a depressurized RCS and an RCS i vent of sufficient ' size. One relief valve or the open RCS vent is the l overpressure protection d'evice that act:, to terminate an increasing pressure '
event.
~
j With minimum mass input capability, the ability to provide core coolant addition is restricted. The LC0 does not require the makeup control system deactivated i or the safety injection (SI) actuation circuits blocked. Due to the lower pressures in the Cold'0 overpressure Protection MODES and the expected core decay ;
heat. levels, the makeup system can provide tdequate flow via the makeup control '
valve. l PORV Requirements i
. As- designed,'the PORY Cold Overpressure Protection (COPPS) is signaled to open 10 the RCS pressure approaches a limit Mtermined by the COPPS actuation logic.
"'he COPPS actuation logic monitors hoth RCS temperature 'and RCS pressure and
' determines when. the nominal,'setpoint of Figure 3.4-4a or Figure 3.4-4b is approached. J The wide : range RCS temperature indications are auctioneered to select the ' lowest temperature signal.
JMILLSTONE - UNIT 3 8 3/4.4-16. Amendment No. fp, pp, Jp7, 0593 V. - AY _
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,7 REkCTOR COOLANT SYSTEN BASES l .
L l OVERPRESSURE PROTECTION SYSTEMS (continued)
I l PORV Requirements (continued)
The lowest temperature signal is processed through a function generator that l- calculates a pressure setpoint for' that temperature. The calculated pressure L setpoint is then compared with the indicated' RCS pressure from a wide range-( pressure channel. If- the indicated pressure meets or exceeds the calculated value, a PORV is signaled to open.
L The wide range RCS temperature indicators monitor loop temperature in the portion l _of the RCS loop that can be isolated. If a loop is isolated, the temperature j signal from the isolated loop may be significantly lower than the temperature signals from the unisolated loops. This would result in a calculated pressure setpoint for the PORV below that anticipated by the operator based on the temperature in the unisolated portion of the RCS. Since this could result in a significantly lower calculated PORV setpoint, RCP operation is not permitted under these conditions unless the temperature input from the isolated loop is removed from the auctioneered circuit. This restriction will ensure that the #1 l RCP seals are not challenged as a result of PORV undershoot. Since the PORV mass E
and heat injection. transients have only been analyzed for a maximum of one loop isolated, the use of the PORVs is restricted to three and four RCS loops l unisolated.
n L If one. loop is isolated without removing its temperature input from the PORV i calculated setpoint auctioneered circuit and at least one RCP is in operation, L or if more than one loop ~is isolated, then the PORVs must have their block valves l closed or COPPS must be blocked. For these cases, Cold Overpressure Protection b
must be provided by either the two RHR suction relief valves or a depressurized RCS and an RCS vent.
The use of the PORVs is restricted to three and four RCS loops unisolated; for a loop to be considered _ isolated, both RCS loop stop valves must be closed. If
!: - only one loop stop valve in an RCS loop is closed for an extended period, the L^ PORVs must have their block valves closed or COPPS must be blocked. A single RCS loop stop valve can be stroked for short time periods for surveillance or other
- purposes and not affect the use of the PORVs for Cold Overpressure Protection.
The RHR suction relief valves .have been qualified for all mass injection
- transients for any cenbination of isolated loops. In addition, the heat injection transients not prohibited by the Technical Specifications have also been considered in the qualification of the RHk suction relief valves.
= Figure 3.4-4a and Figure 3.4-4b present ~ the PORV setpoints for COPPS. Above J
110*F, the setpoints are staggered so only one valve opens.during a low MILLSTONE - UNIT S 8 3/4'4-15a headment No. pp, pp, Jpf, 0693
s REACTOR COOLANT SYSTEN BASES .
OVERPRESSURE PROTECTION SYSTEMS (continued) 4 The cold overpressure transient analyses demonstrate that either one relief valve l or the depressurized RCS and RCS vent can maintain RCS pressure below limits when )
RCS letdown is isolated and only one centrifugal charging pump is operating. 1 Thus, the LC0 allows only one centrifugal charging pump capable of injecting when cold overpressure protection is required.
The cold overpressure protection enabling temperature is conservatively ,
established at a value 1275'F based on the criteria described in Branch ;
Technical Position RSB 5-2 provided in the Standard Review Plan (NUREG-0800).
l PORV Performance The 10CFR50 Appendix G analyses show that the vessel is protected against non- l ductile failure when the PORVs are set to open at the values shown in Figures 3.4-4a and 3.4-4b within the tolerance allowed for the calibration accuracy. The curves are derived by analyses for both three and four RCS loops unisolated that l model the performance of the PORV cold overpressure protection system (COPPS), 4 assuming the limiting mass and heat transients of one centrifugal charging pump injecting into the RCS, or the energy addition as a result of starting an RCP with temperature asymmetry between the RCS and the steam generators. These analyses consider pressure overshoot and undershoot beyond the PORV opening and closing, resulting from signal processing and valve stroke times.
l The PORV setpoints in Figures 3.4-4a and 3.4-4b will be updated when the P/T j limits conflict with the cold overpressure analysis limits. The P/T limits are l periodically modified as the reactor vessel material toughness decreases due to '
neutron embrittlement. Revised limits are determined using neutron fluence i projections and the results of testing of the reactor vessel material irradiation !
surveillance specimens. The Bases for LCO 3.4.9.1, "Passure/ Temperature Limits
- Reactor Coolant System (Except the Pressurizer)," discuss these evaluations. l The PORVs are considered active components. Thus, the failure of one PORY is ,
assumed to represent the worst case, single active failure. !
RHR Suction Relief Valve Performance The RHR suction relief valves do not have variable pressure and temperature lift setpoints as @ the PORVs. Analyses show that one RHR suction relief valve with '
a setpoint at or between 426.8 psig and 453.2 psig will pass flow greater than that required for the limiting cold overpressure tralisient while maintaining RCS pressure less than the isothermal P/T limit curve. Assuming maximum relief flow requirements durir.g the liniting cold overpressure event, an RHR suction relief valve will maintain RCS pressure to 5110% of the nominal lift satpoint.
t
( Although each RHR suction relief valve is a passive spring loaded device, which t
meets single failure criteria, its location within the PHR System precludes meeting single failure criteria when spurious RHR suction isolation valve or RHR suction valve closure is postulated. Thus the loss of an RHR suction relief 1
MILLSTONE - UN!1 3 B 3/4 4-20 Amendment No. U /,
r 0594 l
L__._=__-________________-_-________________. _ _ - :- _ _____a
- & '~ -
E.
REACTOR COOLANT SYSTEM BASES .
OVERPRESSURE PROTECTION SYSTEMS (continued)
- c. When COPPS is armed, PORV undershoot is analyzed for mass injection transients limited to one charging pump. LCO 3.4.9.3, " Reactor Coolant System - Overpressure Protection Systems," provides this protection by requiring both safety injection pumps and all but one charging pump to be incapable of injection into the RCS.
In order to provide protection for the RCP #1 seal, a PORV setpoint of 1595 psia for temperatures 2 160 degrees F must be met. This minimum setpoint is' derived by adding the applicable train uncertainty and valve undershoot to the required minimum RCS pressure required for seal integrity. Due to the differing instrument uncertainties for the two trains of PORV COPPS, the train with the highest uncertainty is paired to the high setpoint curve.
~ LG.Q This LC0 requires that cold overpressure protection be OPERABLE and the maximum )
mass input be limited to one charging pump. Failure tc meet this LCO could lead to the loss of low temperature overpressure mitigation and violation of the Reference 1 isothermal limits as a result of an operational transient.
l To limit the mass input capability, the LC0 requires a maximum of one centrifugal charging pump capable of injecting into the RCS.
l
- The elements of the LCO that provides low temperature overpressure mitigation i through pressure relief are: '
j
A PORV is OPERABLE for cold overpressure protection when its block valve is-open, its lift setpoint is set to the nominal setpoints provided for both three and four -loops unisolated by Figure 3.4-4a or 3.4-4b and when the surveillance requirements are met. For three loops unisclated, the temperature input from the isolated loop must be removed from the COPPS auctioneered circuitry whenever any RCP is in operation. ,
- 2. Two OPERABLE RHR suction relief valves; or I An RHR suction relief valve is OPERABLE for cold overpressure protection when its isolation valves from the RCS are open and when its setpoint is at or between 426.8 psig and 453.2 psig, as verified by required testing.
L
An RCS vent is OPERABLE when open with an area of 2 5.4 square inche.s.
Each of these methods of overpressure prevention is capable of mitigating the limiting cold overpressure transient.
l MILLSTONE - UNIT 3 B 3/4 4-23 Amendscnt No. #7 0595
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