ML031350645
| ML031350645 | |
| Person / Time | |
|---|---|
| Site: | Vermont Yankee File:NorthStar Vermont Yankee icon.png |
| Issue date: | 05/15/2003 |
| From: | NRC/NRR/DLPM |
| To: | |
| References | |
| TAC MB8727 | |
| Download: ML031350645 (1) | |
Text
VYNPS BASES:
3.4 & 4.4 REACTOR STANDBY LIQUID CONTROL SYSTEM A. Normal Operation The design objective of the Reactor Standby Liquid Control System is to provide the capability of bringing the reactor from full power to a cold, xenon-free shutdown assuming that none of the withdrawn control rods can be inserted.
To meet this objective, the Standby Liquid Control System is designed to inject a quantity of boron which produces a concentration of 800 ppm of natural boron in the reactor core in less than 138 minutes.
An 800 ppm natural boron concentration in the reactor core is required to bring the reactor from full power to a subcritical condition. An additional margin (25% of boron) is added for possible imperfect mixing of the chemical solution in the reactor water. A minimum quantity of 3850 gallons of solution having a 10.1%
natural sodium pentaborate concentration is required to meet this shutdown requirement.
The time requirement (138 minutes) for insertion of the boron solution was selected to override the rate of reactivity insertion due to cooldown of the reactor following the xenon poison peak.
For a required minimum pumping rate of 35 gallons per minute, the maximum net storage volume of the boron solution is established as 4830 gallons.
In addition to its original design basis, the Standby Liquid Control System also satisfies the requirements of OCFR50.62(c)(4) on anticipated transients without scram (ATWS) by using enriched boron.
The ATWS rule adds hot shutdown and neutron absorber (i.e., boron-10) injection rate requirements that exceed the original Standby Liquid Control System design basis.
However, changes to the Standby Liquid Control System as a result of the ATWS rule have not invalidated the original design basis.
With the reactor mode switch in the "Run" or "Startup/Hot Standby" position, shutdown capability is required.
With the mode switch in "Shutdown," control rods are not able to be withdrawn since a control rod block is applied. This provides adequate controls to ensure that the reactor remains subcritical.
With the mode switch in "Refuel,"
only a single control rod can be withdrawn from a core cell containing fuel assemblies.
Determination of adequate shutdown margin by Specification 3.3.A ensures that the reactor will not become critical.
Therefore, the Standby Liquid Control System is not required to be operable when only a single control rod can be withdrawn.
Pump operability testing (by recirculating demineralized water to the test tank)in accordance with Specification 4.6.E is adequate to detect if failures have occurred.
Flow, relief valve, circuitry, and trigger assembly testing at the prescribed intervals assures a high reliability of system operation capability.
Recirculation of the borated solution is done during each operating cycle to ensure one suction line from the boron tank is clear. In addition, at least once during each operating cycle, one of the standby liquid control loops will be initiated to verify that a flow path from a pump to the reactor vessel is available by pumping demineralized water into the reactor vessel.
B. Operation With Inoperable Components Only one of the two standby liquid control pumping circuits is needed for proper operation of the system.
If one pumping circuit is found to Amendment No. -10, 114, 12-,
7-S-,
BVY 03-38 97