ML020640603
| ML020640603 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 03/04/2002 |
| From: | NRC/NRR/DLPM |
| To: | |
| References | |
| TAC MB3935 | |
| Download: ML020640603 (3) | |
Text
2.0 SAFETY LIMITS AND LIMITING SAFETY SYSTEM SETTINGS 2.1 SAFETY LIMITS 2.1.1 REACTOR CORE DNBR 2.1.1.1 The DNBR of the reactor core shall be maintained Ž 1.25.
APPLICABILITY:
MODES 1 and 2.
ACTION:
Whenever the DNBR of the reactor core has decreased to less than 1.25, be in HOT STANDBY within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />.
PEAK FUEL CENTERLINE TEMPERATURE 2.1.1.2 The peak fuel centerline temperature shall be maintained <5080'F (decreasing by 58'F per 10,000 MWD/MTU for burnup and adjusting for burnable poisons per CENPD-275-P, Revision 1-P-A and CENPD-382-P-A).
APPLICABILITY:
MODES 1 and 2.
ACTION:
Whenever the peak fuel centerline temperature has equaled or exceeded 5080'F (decreasing by 58'F per 10,000 MWD/MTU for burnup and adjusting for burnable poisons per CENPD-275-P, Revision 1-P-A and CENPD-382-P-A),
be in HOT STANDBY within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />.
Amendment No.
4-,,-&ý- 238 ARKANSAS
- UNIT 2 2-1
2.1.1 SAFETY LIMITS AND LIMITING SAFETY SYSTEM SETTINGS BASES 2.1.1 REACTOR CORE The restrictions of these safety limits prevent overheating of the fuel cladding and possible cladding perforation which would result in the release of fission products to the reactor coolant.
Overheating of the fuel cladding is prevented by (1) restricting fuel operation to within the nucleate boiling regime where the heat transfer coefficient is large and the cladding surface temperature is slightly above the coolant saturation temperature, and (2) maintaining the dynamically adjusted peak linear heat rate of the fuel at or less than 21 kw/ft which will not cause fuel centerline melting in any fuel rod.
First, by operating within the nucleate boiling regime of heat transfer, the heat transfer coefficient is large enough so that the maximum clad surface temperature is only slightly greater than the coolant saturation temperature.
The upper boundary of the nucleate boiling regime is termed "departure from nucleate boiling" (DNB).
At this point, there is a sharp reduction of the heat transfer coefficient, which would result in higher cladding temperatures and the possibility of cladding failure.
Correlations predict DNB and the location of DNB for axially uniform and non-uniform heat flux distributions.
defined as the ratio of the predicted DNB heat flux at a particular core location to the actual heat flux at that location, is indicative of the margin to DNB.
The minimum value of DNBR during normal operational occurrences is limited to 1.25 for the CE-l correlation and is established as a Safety Limit.
Second, operation with a peak linear heat rate
- 21 kw/ft setpoint will ensure that the peak fuel centerline temperature safety limit protects fuel rod and cladding integrity.
Above this peak linear heat rate level (i.e.,
with some melting in the center),
fuel rod integrity would be maintained only if the design and operating conditions are appropriate throughout the life of the fuel rods.
Volume changes which accompany the solid to liquid phase change are significant and require accommodation.
Another consideration involves the redistribution of the fuel which depends on the extent of the melting and the physical state of the fuel rod at the time of melting.
Because of the above factors, the steady state value of the peak linear heat rate which would not cause fuel centerline melting is established as a Limiting Safety System Setting.
To account for fuel rod dynamics (lags), the directly indicated linear heat rate is dynamically adjusted.
TS 2.1.1.2 establishes a peak fuel centerline temperature of 5080'F with adjustments for burnup and burnable poison. An adjustment for burnup of 58'F per 10,000 MWD/MTU has been established in NRC approved Topical Report CEN-386-P-A, "Verification of the Acceptability of a 1-Pin Burnup Limit of 60 MWD/kgU for Combustion Engineering 16x16 PWR Fuel," August 1992. Adjustments for burnable poisons are established based on NRC approved Topical Reports CENPD-275-P, Revision 1-P-A, "CE Methodology for Core Designs Containing Gadolinia-Urania Burnable Absorbers",
May 1988 and CENPD-382-P-A, "Methodology for Core Designs Containing Erbium Burnable Absorbers",
August 1993.
A steady state peak linear heat rate of 21 kw/ft has been established as the Limiting Safety System Setting to prevent fuel centerline melting during normal operation.
Following design basis anticipated operational occurrences, the transient linear heat rate may exceed 21 kw/ft as long as the fuel centerline melt temperature is not exceeded.
Amendment No.
-4,
,38 ARKANSAS UNIT 2 R 2-1
SAFETY LIMITS AND LIMITING SAFETY SYSTEM SETTINGS BASES Steam Generator Level-Low The Steam Generator Level-Low trip provides protection against a loss of feedwater flow incident and assures that the design pressure of the Reactor Coolant System will not be exceeded due to loss of the steam generator heat sink.
This specified setpoint provides allowance that there will be sufficient water inventory in the steam generator at the time of the trip to provide sufficient margin before emergency feedwater is required.
Local Power Density-High The Local Power Density-High trip is provided to prevent the linear heat rate (kw/ft) in the limiting fuel rod in the core from exceeding the fuel design limit in the event of any'anticipated operational occurrence.
The local power density is calculated in the reactor protective system utilizing the following information:
- a.
Nuclear flux power and axial power distribution from the excore flux monitoring system;
- b.
Radial peaking factors from the position measurement for the CEAs;
- c.
AT power from reactor coolant temperatures and coolant flow measurements.
The local power density (LPD),
the trip variable, calculated by the CPC incorporates uncertainties and dynamic compensation routines.
These uncertainties and dynamic compensation routines ensure that a reactor trip occurs when the actual core peak LPD is sufficiently less than the fuel design limit such that the increase in actual core peak LPD after the trip will not result in a violation of the peak fuel centerline temperature Safety Limit.
CPC uncertainties related to peak LPD are the same types used for DNBR calculation.
Dynamic compensation for peak LPD is provided for the effects of core fuel centerline temperature delays (relative to changes in power density),
sensor time delays, and protection system equipment time delays.
238 ARKANSAS -
UNIT 2 B 2-5 Amendment No.
244