ML19347B526
| ML19347B526 | |
| Person / Time | |
|---|---|
| Site: | Vermont Yankee File:NorthStar Vermont Yankee icon.png |
| Issue date: | 10/07/1980 |
| From: | VERMONT YANKEE NUCLEAR POWER CORP. |
| To: | |
| Shared Package | |
| ML19347B525 | List: |
| References | |
| NUDOCS 8010150271 | |
| Download: ML19347B526 (6) | |
Text
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.1.1 SAFETY LIMIT 2.1 LIMITING SAFETY SYSTEM SETTING
' C3 4-4 In the event.of operation with the ratio of MFLPD.to CR FRP greater than 1.0, the APRM gain shall be increased'
_: C3.
by the ratio: MFLPD h-FRP j$$
where: MFLPD = maximum fraction of limiting power 7
density where the limiting power density is 18 5 KW/ft for 7 x 7 fiel
-and 13 4 KW/ft for 8 x 8 fuel.=
= fraction of rated power.(1593 Mwt)
In-the event of operation with the ratio of MFLPD to FRP equal to or less than 1.0, the.APRM gain shall be.
equal to or greater than 1.0.
For no combination of loop recirculation flow' rate and -
core. thermal power shall the APRM flux scram trip setting be allowed to exceed 120% of rated thermal power.
b.
Flux Scram Trip Setting,(Refuel or Startup and Hot Standby Mode)
When the reactor mode switch is in the REFUEL or STARTUP position, average power range monitor (APRM):
scram shall be set down to less' than or equal to 15%
of rated neutron flux. The IRM flux scram setting shall be set at.less than or equal to 120/125 of full scale.
B.
Core Thermal Power Limit (Reactor Pressure B.
APRM Rod Block Trip Setting 800 psia or Core Flow 10% cf Rated)
When the reactor pressure is 800 psia or The APRM rod block trip setting shall be as core flow 10% of rated, the core thermal shown in Figure-2.1.1 and shall be:
power shall not exceed 25% of rated thermal power.
SRB = 0.66W + 42%
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UYNPS
'I 1.1 - SAFETY LIMIT 2.1 LIMITING SAFETY SYSTEM SETTING-C.
Power Transient where-
'To ensure that'the Safety Limit SRB = Rod block setting in percent of-established in Specification 1.lA and rated thermal power 1593 MWt and 1.lB is not exceeded, each
. required scram shall be initiated by
- i = percent rated drive flow where 100%
its expected scram signal. The Safety rated drive flow is that. flow.
-Limit-'shall be assumed to be exceeded equivalent to 48 x 106 lbs/hr core I
when scram'is accomplished by a'means flow.
other than'the expected scram signal.
In the event of operation with the ratio of MFLPD to FRP greater than 1.0, the APRM gain shall be increased by the.
ratio: MFLPD FRP where: MFLPD = maximum fraction of limiting power density where the. limiting power density is 18.5 KW/ft for 7 x 7 fuel and 13 4 Kw/ft for 8 -
x 8 fuel.
= fraction of rated power (1593 Mwt)
In the event of operation,with the ratio of MFLPD to FRP equal to or less than 1.0, the-APRM gain shall be equal to or greater-than 1.0.
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1
UYNPS APRM Flux Scram Trip Setting (Run Mode)
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The scram trip setting must be adjusted to ;nem o that the LHGR transient peak is not increased for any combination of MFLPD and reactor core thermal power.
If the scram requires a change due to an abnormal peaking condition, it will be accomplished by increasing the APRM gain by the ratio in Specification 2.1.A.l.a. thus assuring a reactor scram at lower than design overpower conditions.
Analyses of the limiting transients show that no scram adjustment is required to assdre fuel cladding integrity when the transient is initiated from the operating limit MCPR (Specification 3. llc).
Flux Scram Trip Setting (Refuel or Startup & Hot Standby Mode)
For operation in the startup mode while the reactor is at low pressure, the reduced APRM scram setting to 15 percent of rated power provides adequate thermal margin between the setpoint and the safety limit, 25 percent of the rated. The margin is adequate to accomodate anticipated maneuvers associated with station startup. Effects of increasing pressure at zero or low void content are minor, cold water from sources available during startup is not much colder than that already in the system, temperature coefficients are small, and control rod patterns are constrained to be uniform by operating procedures backed up by the rod worth minimzer. Worth of individual rods is very low in a uniform rod pattern. Thus, of all possible sources of reactivity input, uniform control rod withdrawal is the most probable cause of significant power rise. Because the flux distribution associated with uniform rod withdrawals does not involve high local peaks, and because several rods must be moved to change power by a significant percentage of rated power, the rate of power rise is very slow. Generally, the heat flux is in near equilibrium with the fission rate.
In an assumed uniform rod withdrawal approach to the scram level, the rate of power rise is no more than 5 percent of rated power per minute, and the APRM system would be more than adequate to assure a scram before the power could exceed the safety limit. The reduced APRM scram remains active until the mode switch is placed in the RUN position. This switch can occur when reactor pressure is greater than 850 psig.
The IRM system consists of 6 chambers, 3 in each of the reactor protection system logic channels. The IRM is a 5-decade instrument which covers the range of power level between that covered by the SRM and the APRM. The 5 decades are covered by the IRM by means of a range switch and the 5 decades are broken down into 10 ranges, each being one-half of a decade in size. The IRM scram trip setting of 120/125 of full scale is active in each range of the IRM. For example, if the instrument were on range 1, the scram setting would be a 120/125 of full scale fo that range; likewise, if the instrument were on range 5, the scram would be 120/125 of full scale on that range. Thus, as the IRM is ranged up to accomodate the increase in power level, the scram trip setting is also ranged up. The most significant sources of reactivity change during the power increase are due to control rod withdrawal. For insequence control rod withdrawal, the rate of change of power is slow enough due to the physical limitation of withdrawing control rods, that heat flux is in equilibrium with the neutron flux and an IRM scram would result in a reactor shutdown well before any Safety Limit is exceeded, l
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UYNPS In order to ensure that the IRM provided adequate protection against the single rod withdrawal error, a range of rod withdrawal accidents was analyzed. This analysis included starting the accident at various power levels. The most severe case invloves an initial condition in which the reactor is just suboritical and the IRM system is not yet on scale. This condition exists at quarter rod density. additional conservatism was taken in this analysis by assuming that the IRM channel closest to the withdrawn rod is by passed. The results of this analysis show that the reactor is scrammed and peak power limited to one -
percent of rated power, thus maintaining MCPR above the fuel cladding integrity safety limit. Based on the above analysis, the IRM provides protection against local control rod withdrawal errors and continuous withdrawal of control rods in sequence.
B.
APRM Rod Block Trip Setting Reactor power level may be varied by moving control rods or by varying the recirculation flow rate. The APRM system provides a control rod block to prevent rod withdrawal beyond a given point at the fuel cladding integrity safety limit. This rod block trip setting, which is automatically varied with recirculation loop flow rate, prevents an increase in the reactor power level to excessive values due to contrl rod withdrawal.
The flow variable trip setting provides substantial margin from fuel damage, assuming a steady-state operation at the trip setting, over the entire recirculation flow range. The margin to the Safety Limit increases as the flow decreases for the specified trip setting versus flow relationship, therefore the worst case MCPR which could occur during steady-state operation is at 108% of rated thermal power because of the APRM rod block trip setting. The actual power distribuiton in the core is established by specified control rod sequences and is monitored continuously by the incore LPRM system. As with the APRM scram trip setting, the APRM rod block trip setting must be adjusted downward if the ratio of MFLPD to FRP exceeds the specified value.
If the APRM rod block requires a change due to abnormal peaking conditions, it will be accomplished by increasing the APRM gain by the ratio in Specification 2.lB, thus ensuring a rod block at lower than design overpower conditions.
C.
Reactor Low Water Level Scram The reactor low water level scram is set at a point which will prevent reactor operation with the steam separators uncovered, thus limiting carry-under to the recirculation loops.
In addition, the safety limit is based on a water level below the scram point and therefore this setting is provided.
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VYNPS 31 LIMITING CONDITIONS FOR OPERATION
'.1 SURVEILLANCE REQUIREMENTS 4
31 REACTOR PROTECTION SYSTEM 4.1 REiCTOR PROTECTION SYSTEM Applicability:
Applicability:
Applies to the operbility of plant instrumen-Applies to the surveillance of the plant instrumen-tation and control systems required for reactor tion and control systems required for reactor safety.
safety.
Objective:
Objective:
To specify the limits imposed on plant operation To specify the type and frequency of surveillance by those instrument and control systems required to be applied to those instrument and control for reactor safety.
systems required for reactor safety.
Specification:
Specification:
A.
Plant operation at any power level shall be A.
Instrumentation systems shall be functionally permitted in accordance with Table 31.1.
tested and calibrated as indicated in Tables The system response time from the opening of 4.1.1 and 4.1.2, respectively.
the sensor contact up to and including the opening of the scram solenoid relay shall not exceed 50 milliseconds.
B.
During operation with the ratio of MFLPD to B.
Once a day during reactor power operation the FRP greater than 1.0 either:
maximum fraction of limiting power density and fraction of rated power shall be determined a.
The APRM System gains shall be adjusted and the APRM system gains shall be adjusted by by the ratios given in Technical the ratios given in Technical Specifications Specifications 2.1.A.1 and 2.1.B or 2.1.A.l.a and 2.1.B.
b.
The power distribution shall be changed tc reduce the ratio of MFLPD to FRP.
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VYNPS Bases:
-4.1 ; REACTOR PROTECTION SYSTEM A.
The scram sensor channels listed in Tables 4.1.1 and 4.1.2 are divided into three groups:
A, B and C.
' Sensors that make up Group A are of the on-off type and will be tested and calibrated at _the indicated
' intervals. -Initially the tests are more frequent than Yankee experience indicates. necessary. However, by testing more frequently, the _ confidence level with this instrumentation will increase and testing will provide data to justify extending the test intervals as experience is accruel.
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' Group B devices utilize an analog sensor followed by an amplifier and bi-stable trip circuit. This type of equipment incorporates control room mounted indicators and annunciator alarms. A failure in the sensor or-amplifier may be detected by an alarm or by an operator who observes that one indicator. does not track the others in similar channels. The bi-stable trip circuit failures are detected by the periodic testing.
Group C devices are active only during a given portion of the operating-cycle. For er. ample, the IRM is active _ during start-up and inactive during full-ppower operation. Testing of these instruments is only meaningful within a reasonable period prior to their use.
i B.
The ratio of MFLPD to FRP shall be checked once per day to determine if the APRM Eains require adjustment. ~
Because few control rod movements or power changes occur, checking these parameters daily is adequate.
i a.
1 31-