ML17334A170
| ML17334A170 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 11/02/2017 |
| From: | Tennessee Valley Authority |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML17334A154 | List: |
| References | |
| Download: ML17334A170 (308) | |
Text
WBN 5.1-1 5.0 REACTOR COOLANT SYSTEM
5.1
SUMMARY
DESCRIPTION
The reactor coolant system (RCS), shown in Figure 5.1-1-1, consists of four similar heat transfer
loops connected in parallel to the reactor pressure vessel. Each loop contains a reactor coolant
pump, steam generator and associated piping and va lves. In addition, the system includes a pressurizer, a pressurizer relief tank, interconnecting piping and instrumentation necessary for
operational control. The above components are located in the Containment Building.
During operation, the RCS transfers the heat generated in the core to the steam generators
where steam is produced to drive the turbine generator. Borated demineralized light water is
circulated in the RCS at a flow rate and temperature consistent with achieving the reactor core
thermal-hydraulic performance presented in Section 4.4. The water also acts as a neutron
moderator and reflector, and as a solvent for the neutron absorber used in chemical shim
control.
The RCS pressure boundary provides a barrier against the release of radioactivity generated
within the reactor, and is designed to ensure a high degree of integrity throughout the life of the
plant.
RCS pressure is controlled by the use of the pressurizer where water and steam are maintained
in equilibrium by electrical heaters and water sprays. Steam can be formed (by the heaters) or
condensed (by the pressurizer spray) to minimize pressure variations due to contraction and
expansion of the reactor coolant. Three spring-loaded safety valves and two power-operated
relief valves are mounted on the pressurizer and discharge to the pressurizer relief tank, where
the steam is condensed and cooled by mixing with water.
The extent of the RCS is defined as:
- 1. The reactor vessel including control rod drive mechanism housings.
- 2. The reactor coolant side of the steam generators.
- 3. Reactor coolant pumps.
- 4. A pressurizer attached to one of the reactor coolant loops.
- 5. Safety and relief valves.
- 6. The interconnecting piping, valves and fittings between the principal components listed above.
- 7. The piping, fittings and valves leading to connecting auxiliary or support systems up to and including the second isolation valve (from the high pressure side) on each line.
WBN 5.1-2 Reactor Coolant System Components Reactor Vessel
The reactor vessel is cylindrical, with a welded hemispherical bottom head and a removable, flanged and gasketed, hemispherical upper head. The vessel contains the core, core
supporting structures, control rods and other parts directly associated with the core.
The vessel has inlet and outlet nozzles located in a horizontal plane just below the reactor
vessel flange but above the top of the core. Coolant enters the vessel through the inlet nozzles
and flows down the core barrel-vessel wall annulus, turns at the bottom and flows up through
the core to the outlet nozzles.
The vessel head contains four vertical penetrations which are capped. One of the capped
penetrations contains the Reactor Vessel Head Vent System (RVHVS) connection.
The steam generators are vertical shell and U-tube evaporators with integral moisture
separating equipment. The reactor coolant flows through the inverted U-tubes, entering and
leaving through the nozzles located in the hemispherical bottom head of the steam generator.
Steam is generated on the shell side and flows upward through the moisture separators to the
outlet nozzle at the top of the vessel.
Reactor Coolant Pumps
The reactor coolant pumps are identical single-speed centrifugal units driven by air-water
cooled, three-phase induction motors. The shaft is vertical with the motor mounted above the
pump. A flywheel on the shaft above the motor provides additional inertia to extend pump
coastdown. The inlet is at the bottom of the pump; discharge is on the side. Reverse rotation is
prevented by a ratchet mechanism.
Reactor Vessel Head Vent System (RVHVS)
The RVHVS is a subsystem of the RCS designed to provide a means for venting
noncondensibles and/or steam from the reactor vessel in a remotely operable, controlled
manner. The active portion of this subsystem consists of four 1-inch solenoid-operated valves.
Two parallel solenoid isolation valves are arranged in series with two parallel throttle valves.
The RVHVS is connected to piping originating at the upper head of the reactor vessel and
discharges to the pressurizer relief tank.
The RVHVS provides additional flexibility to the plant operator during both normal as well as
accident mitigation operations. Air venting duri ng plant startup, system venting during plant shutdown, or postaccident mitigation of nondesign basis events are all possible periods of
RVHVS usage.
WBN 5.1-3 Piping The reactor coolant loop piping is specified in the smallest sizes consistent with system
requirements.
The hot leg inside diameter is 29 inches and the inside diameter of the cold leg return line to the
reactor vessel is 27.5 inches. The piping between the steam generator and the pump suction is
increased to a 31 inch inside diameter to reduce pressure drop and improve flow conditions to
the pump suction.
Pressurizer
The pressurizer is a vertical, cylindrical vessel with hemispherical top and bottom heads.
Electrical heaters are installed through the bottom head of the vessel while the spray nozzle, relief and safety valve connection are located in the top head of the vessel.
Pressurizer Relief Tank
The pressurizer relief tank is a horizontal, cylindrical vessel with hemispherical ends. Steam
from the pressurizer safety and relief valves is discharged into the pressurizer relief tank
through a sparger pipe under the water level. This condenses and cools the steam by mixing it
with water that is near ambient temperature.
Safety and Relief Valves
The pressurizer safety valves are system pr essure actuated, enclosed bonnet pop type valves with balancing bellows for maintaining full system pressure across the seat independent of back
pressure.
The power-operated relief valves limit system pr essure for large power mismatch. They are operated automatically or by remote manual cont rol. Remotely operated valves are provided to isolate the inlet to the power-operated relief valves if excessive leakage occurs.
Reactor Coolant System Performance Characteristics
Tabulations of important design and performance characteristics of the RCS are provided in
Table 5.1-1.
Reactor Coolant Flow
The reactor coolant flow, a major parameter in the design of the system and its components, is established with a detailed design procedure supported by operating plant performance data, by
pump model tests and analysis, and by pressure drop tests and analyses of the reactor vessel
and fuel assemblies. Data from operating plants have indicated that the actual flow has been
well above the flow specified for the thermal design of the plant. By applying the design
procedure described below, it is possible to specify the expected operating flow with reasonable accuracy.
WBN 5.1-5 For Unit 1 with the change in steam generators and the resulting decrease in best estimate flow to 101,100 gpm per loop at 0% steam generator tube plugging, the margin to mechanical design
flow is increased to 3.9%.
Pump overspeed, due to a turbine generator overspeed of 20%, results in a peak reactor
coolant flow of 120% of the mechanical design flow. The overspeed condition is applicable only
to operating conditions when the reactor and turbine generator are at power.
The plant design system thermal and hydraulic data in Table 5.1-1 is based on plant operation
with 0% steam generator tube plugging.
In parallel with the Unit 1 RSG program, the safety analyses also addressed the impact of
reducing the reactor coolant vessel average temperature Tavg setpoint by 2 o F from the current value of 588.2 o F to 586.2 o F . In the event the actual steam pressure with the RSGs is higher than desired, this Tavg reduction strategy supports a reduction in steam pressure while maintaining the ability to operate at full NSSS power (3,474.21 MWt).
When evaluating or analyzing the accidents, the NSSS accident analyses used the parameters
with the higher Tavg (588.2 o F) if these operating conditions were more limiting/less conservative.
Most of the analyses fall into this category. Conversely, if the lower Tavg (586.2 o F) operating conditions were more limiting/less conservative, the lower temperature parameters were used.
The analyses that used the lower temperature parameters are: LOCA hydraulic forces (Appendix 3.6B), short-term LOCA mass and energy release (Section 6.2.1), SGTR margin to
overfill (Section 15.4.3), and inadvertent operation of the ECCS (Section 15.2.1.4)
Interrelated Performance and Safety Functions
The interrelated performance and safety functions of the RCS and its major components are
listed below:
- 1. The RCS provides sufficient heat transfer capability to transfer the heat produced during power operation and when the reactor is subcritical, including the initial phase of plant
cooldown, to the steam and power conversion system.
- 2. The system provides sufficient heat transfer capability to transfer the heat produced during the subsequent phase of plant cooldown and cold shutdown to the residual heat removal system.
- 3. The system heat removal capability under power operation and normal operational transients, including the transition from forced to natural circulation, shall assure no fuel
damage within the operating bounds permitted by the reactor control and protection systems.
- 4. The RCS provides the water used as the core neutron moderator and reflector and as solvent for-chemical shim control.
WBN 5.1-6 5. The system maintains the homogeneity of soluble neutron poison concentration and rate of change of coolant temperature such that uncontrolled reactivity changes do not occur.
- 6. The reactor vessel is an integral part of the RCS pressure boundary and is capable of accommodating the temperatures and pressures associated with the operational
transients. The reactor vessel functions to support the reactor core and control rod drive
mechanisms.
- 7. The pressurizer maintains the system pressure during operation and limits pressure transients. During the reduction or increase of plant load, reactor coolant volume
changes are accommodated in the pressurizer via the surge line.
- 8. The reactor coolant pumps supply the coolant flow necessary to remove heat from the reactor core and transfer it to the steam generators.
- 9. The steam generators provide high quality steam to the turbine. The tube and tubesheet boundary are designed to prevent or control to acceptable levels the transfer of activity
generated within the core to the secondary system.
- 10. The RCS piping serves as a boundary for containing the coolant under operating temperature and pressure conditions and for limiting leakage (and activity release) to the
containment atmosphere. The RCS piping contains demineralized borated water which
is circulated at the flow rate and temperature consistent with achieving the reactor core
thermal and hydraulic performance.
5.1.1 Schematic Flow Diagram The reactor coolant system is shown schematically in Figure 5.1-3. Included on this figure is a
tabulation of principal pressures, temperatures, and the flow rate of the system under normal
steady state full power operating conditions. These parameters are based on the best estimate
flow at the pump discharge. Reactor coolant system volume under the above conditions is
presented in Table 5.1-1.
5.1.2 Piping and Instrumentation Diagrams A piping and instrumentation diagram of the reacto r coolant system is shown on Figure 5.1-1-1.
The diagram shows the extent of the systems lo cated within the containment, and the points of separation between the reactor coolant system , and the secondary (heat utilization) system.
5.1.3 Elevation Drawing Reference information only: Figures 1.2-11, 1.2-12, 1.2-13 and 1.2-14 are cross sectional and elevation drawings providing principal dimensions of the reactor coolant system in relation to
surrounding concrete structures.
REFERENCES None
WBN 5.3-1 5.3 THERMAL HYDRAULIC SYSTEM DESIGN
5.3.1 Analytical Methods and Data
The thermal and hydraulic design bases of the Reactor Coolant System are described in
Section 5.5, "Component and Subsystem Design" and in Section 4.4, "Thermal and Hydraulic
Design" in terms of core heat generation rates, DNBR, analytical models, peaking factors and
other relevant aspects of the reactor.
5.3.2 Operating Restrictions On Pumps
The minimum Net Positive Suction Head (NPSH) and minimum seal injection flow rate must be established before operating the reactor coolant pumps. With the minimum 6 gpm labyrinth seal
injection flow rate established, the operator will hav e to verify that the system pressure satisfies
NPSH requirements. See Section 5.5.1, "Reactor Coolant Pumps."
5.3.3 Power-Flow Operating Map (BWR)
Not applicable to Pressurized Water Reactors.
5.3.4 Temperature-Power Operating Map
The relationship between reactor coolant system temperature and power is shown in Figure 5.3-1.
The effects of reduced core flow due to inoperative pumps is discussed in Section 5.5.1. 15.2.5, and 15.3.4. Natural circulation is discussed in Section 15.4.2.
5.3.5 Load Following Characteristics
The reactor coolant system is designed on the basis of steady state operation at full power heat
load. The reactor coolant pumps utilize constant speed drives as described in Section 5.5.1
and the reactor power is controlled to maintain average coolant temperature at a value which is
a linear function of load, as described in Section 7.7.1. Operation with one pump out of service
requires adjustment only in reactor trip system setpoints as discussed in Section 7.2. However, Technical Specifications do not allow continued operation with one pump out of service.
5.3.6 Transient Effects
Transient effects on the reactor coolant system are evaluated in Chapter 15.
5.3.7 Thermal and Hydraulic Characteristics Summary Table
The thermal and hydraulic characteristics are given in Table 4.4-1.
AMENDMENT 4
(,)= ()()
=+
=[1]+=() 2
COMPONENT AND SUBSYSTEM DESIGN5.5-95WATTS BARWBNP-114Figure 5.5-3 Unit 2 Steam Generator
WATTSBARNUCLEARPLANTFINALSAFETYANALYSISREPORTCROSSOVERLEGVERTICALRUNRESTRAINTFIGURE5.5-11
.