ML14339A790
Text
McGuire Units 1 and 2 B 3.4.4-1 Revision No. 115 RCS LoopsMODES 1 and 2 B 3.4.4 B 3.4 REACTOR COOLANT SYSTEM (RCS)
B 3.4.4 RCS LoopsMODES 1 and 2 BASES BACKGROUND The primary function of the RCS is removal of the heat generated in the fuel due to the fission process, and transfer of this heat, via the steam generators (SGs), to the secondary plant.
The secondary functions of the RCS include:
- a.
Moderating the neutron energy level to the thermal state, to increase the probability of fission;
- b.
Improving the neutron economy by acting as a reflector;
- c.
Carrying the soluble neutron poison, boric acid;
- d.
Providing a second barrier against fission product release to the environment; and
- e.
Removing the heat generated in the fuel due to fission product decay following a unit shutdown.
The reactor coolant is circulated through four loops connected in parallel to the reactor vessel, each containing an SG, a reactor coolant pump (RCP), and appropriate flow and temperature instrumentation for both control and protection. The reactor vessel contains the clad fuel. The SGs provide the heat sink to the isolated secondary coolant. The RCPs circulate the coolant through the reactor vessel and SGs at a sufficient rate to ensure proper heat transfer and prevent fuel damage. This forced circulation of the reactor coolant ensures mixing of the coolant for proper boration and chemistry control.
APPLICABLE Safety analyses contain various assumptions for the design bases SAFETY ANALYSES accident initial conditions including RCS pressure, RCS temperature, reactor power level, core parameters, and safety system setpoints. The important aspect for this LCO is the reactor coolant forced flow rate, which is represented by the number of RCS loops in service.
Both transient and steady state analyses have been performed to establish the effect of flow on the departure from nucleate boiling (DNB). The transient and accident analyses for the plant have been performed
RCS Loops - MODES 1 and 2 B 3.4.4 BASES McGuire Units 1 and 2 B 3.4.4-2 Revision No. 115 APPLICABLE SAFETY ANALYSES (continued) assuming the number of RCS loops in operation is consistent with the Technical Specifications. The majority of the plant safety analyses are based on initial conditions at high core power or zero power. The primary coolant flowrate, and thus the number of RCPs in operation is an important assumption in all accident analyses (Ref. 1).
Steady state DNB analysis has been performed for the four RCS loop operation. For four RCS loop operation, the steady state DNB analysis, which generates the pressure and temperature Safety Limit (SL) (i.e., the departure from nucleate boiling ratio (DNBR) limit) assumes a maximum power level of 118% RTP. This is the design overpower condition for four RCS loop operation. The DNBR limit defines a locus of pressure and temperature points that result in a minimum DNBR greater than or equal to the critical heat flux correlation limit.
The plant is designed to operate with all RCS loops in operation to maintain DNBR above the SL, during all normal operations and anticipated transients. By ensuring heat transfer in the nucleate boiling region, adequate heat transfer is provided between the fuel cladding and the reactor coolant.
RCS LoopsMODES 1 and 2 satisfy Criterion 2 of 10 CFR 50.36 (Ref.
2).
LCO The purpose of this LCO is to require an adequate forced flow rate for core heat removal. Flow is represented by the number of RCPs in operation for removal of heat by the SGs. To meet safety analysis acceptance criteria for DNB, four pumps are required in MODES 1 and 2.
An OPERABLE RCS loop consists of an OPERABLE RCP in operation providing forced flow for heat transport and an OPERABLE SG.
APPLICABILITY In MODES 1 and 2, the reactor is critical and thus has the potential to produce maximum THERMAL POWER. Thus, to ensure that the assumptions of the accident analyses remain valid, all RCS loops are required to be OPERABLE and in operation in these MODES to prevent DNB and core damage.
The decay heat production rate is much lower than the full power heat rate. As such, the forced circulation flow and heat sink requirements are reduced for lower, noncritical MODES as indicated by the LCOs for MODES 3, 4, and 5.
RCS Loops - MODES 1 and 2 B 3.4.4 BASES McGuire Units 1 and 2 B 3.4.4-3 Revision No. 115 APPLICABILITY (continued)
Operation in other MODES is covered by:
LCO 3.4.7, "RCS LoopsMODE 5, Loops Filled";
LCO 3.4.8, "RCS LoopsMODE 5, Loops Not Filled";
LCO 3.4.17, "RCS LoopsTest Exceptions";
LCO 3.9.5, "Residual Heat Removal (RHR) and Coolant CirculationHigh Water Level" (MODE 6); and LCO 3.9.6, "Residual Heat Removal (RHR) and Coolant CirculationLow Water Level" (MODE 6).
ACTIONS A.1 If the requirements of the LCO are not met, the Required Action is to reduce power and bring the plant to MODE 3. This lowers power level and thus reduces the core heat removal needs and minimizes the possibility of violating DNB limits.
The Completion Time of 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> is reasonable, based on operating experience, to reach MODE 3 from full power conditions in an orderly manner and without challenging safety systems.
SURVEILLANCE SR 3.4.4.1 REQUIREMENTS This SR requires verification that each RCS loop is in operation.
Verification includes flow rate, temperature, or pump status monitoring, which help ensure that forced flow is providing heat removal while maintaining the margin to DNB. The Surveillance Frequency is based on operating experience, equipment reliability, and plant risk and is controlled under the Surveillance Frequency Control Program.
REFERENCES
- 1.
UFSAR, Section 15.
- 2.
10 CFR 50.36, Technical Specifications, (c)(2)(ii).