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RCS

⚠️ DRAFT

RCS

Function

The Reactor Coolant System (RCS) transfers heat generated in the reactor core to the steam generators, where steam is produced to drive the turbine-generator. The RCS provides a pressure boundary for containing coolant under operating temperature and pressure conditions, confines radioactive material, and limits releases to the secondary system and other parts of the plant. During transient operation, the system’s heat capacity attenuates thermal transients generated by the core or steam generators. (UFSAR 5.1)

System Configuration

  • Type: Westinghouse 4-loop PWR
  • Number of heat transfer loops: 4, connected in parallel to the reactor vessel
  • Each loop contains: one steam generator, one reactor coolant pump, loop piping, and instrumentation
  • Pressurizer surge line: connected to one loop (hot leg)
  • All piping and fittings: austenitic stainless steel
  • All joints: welded (except pressurizer relief and safety valves — flanged)

Key Design Parameters

ParameterValueSource
Design Pressure2485 psigUFSAR T5.1-1
Nominal Operating Pressure2235 psigUFSAR T5.1-1
Design Temperature650°FUFSAR T5.2-3
Hydrostatic Test Pressure3107 psigUFSAR T5.2-3
Total Heat Output (100% power)11844 x 10⁶ Btu/hrUFSAR T5.1-1
Plant Design Life40 yearsUFSAR T5.1-1
Total System Volume (ambient), Unit 112071 ft³UFSAR T5.1-1
Total System Volume (ambient), Unit 213011 ft³UFSAR T5.1-1
System Liquid Volume (ambient), Unit 111351 ft³UFSAR T5.1-1
System Liquid Volume (ambient), Unit 212291 ft³UFSAR T5.1-1

Reactor Coolant Temperatures (Full Power)

ParameterUnit 1Unit 2Source
Inlet Temp (High Tavg)542.7°F542.8°FUFSAR T5.1-1
Inlet Temp (Low Tavg)530.2°F530.3°FUFSAR T5.1-1
Outlet Temp (High Tavg)613.1°F613.1°FUFSAR T5.1-1
Outlet Temp (Low Tavg)601.8°F601.7°FUFSAR T5.1-1
Coolant Temp Rise (High Tavg)70.4°F70.3°FUFSAR T5.1-1
Coolant Temp Rise (Low Tavg)71.6°F71.4°FUFSAR T5.1-1

Design Coolant Flow

ParameterUnit 1Unit 2Source
Total Design Flow (High Tavg)125.3 x 10⁶ lb/hr125.8 x 10⁶ lb/hrUFSAR T5.1-1
Total Design Flow (Low Tavg)127.3 x 10⁶ lb/hr125.9 x 10⁶ lb/hrUFSAR T5.1-1
Best Estimate Flow per Loop, Unit 194200 gpm—UFSAR T5.2-2
Best Estimate Flow per Loop, Unit 294800 gpm—UFSAR T5.2-2

Pressure Control Setpoints

ParameterValue (psig)Source
Design Pressure2485UFSAR T5.2-1
Operating Pressure2235UFSAR T5.2-1
Safety Valves (lift)2485UFSAR T5.2-1
PORVs (lift / reset)2335 / 2315UFSAR T5.2-1, T5.2-8
High Pressure Trip2385UFSAR T5.2-1
High Pressure Alarm2385UFSAR T5.2-1
Low Pressure Trip1865UFSAR T5.2-1
Low Pressure Alarm1865UFSAR T5.2-1
Pressurizer Spray (begin to open)2260UFSAR T5.2-1
Pressurizer Spray (full open)2310UFSAR T5.2-1
Proportional Heaters (begin)2250UFSAR T5.2-1
Proportional Heaters (full operation)2220UFSAR T5.2-1
Backup Heaters On2210UFSAR T5.2-1
Hydrostatic Test Pressure3107UFSAR T5.2-1

Key Components

  • Rx Vessel & Internals: Cylindrical vessel with hemispherical heads, contains core and internals. 4 inlet/4 outlet nozzles. (UFSAR 5.1, 5.4)
  • Pressurizer & PRT: Vertical cylindrical vessel, 84” ID. Maintains liquid-vapor equilibrium for pressure control. 1800 kW heaters, 800 gpm max spray. Connected to one hot leg via 14” surge line. (UFSAR 5.1, T5.2-4)
  • Steam Generator & Blowdown: 4 vertical shell and U-tube evaporators with integral moisture separation. Unit 1: Model F (5626 U-tubes, Inconel). Unit 2: AREVA NP Model 61/19T (5048 U-tubes, Inconel 690 TT). (UFSAR 5.1, T5.2-5)
  • RCPs: 4 vertical single-stage mixed-flow pumps, Model 93A. 6000 HP motors, 88500 gpm capacity, 277 ft developed head, 1180 RPM. Controlled leakage seal assembly. (UFSAR 5.1, T5.2-6)
  • Pressurizer Relief Tank: Carbon steel, 1800 ft³ total volume. Design pressure 100 psig. Two rupture discs (100 psig) discharge to containment. Normal water temperature at containment ambient (120°F max). (UFSAR 5.1, T5.2-4)

RCS Piping

ParameterValueSource
Reactor Inlet (Cold Leg) ID27.5 inUFSAR T5.2-7
Reactor Outlet (Hot Leg) ID29 inUFSAR T5.2-7
Pump Suction Piping ID31 inUFSAR T5.2-7
Surge Line ID, Unit 111.500 inUFSAR T5.2-7
Surge Line ID, Unit 211.188 inUFSAR T5.2-7
Total Water Volume (all 4 loops + surge line)1455 ft³UFSAR T5.2-7

Pressure Relief

  • Safety Valves: 3 per unit, Crosby HB-BP-86, set at 2485 psig, rated capacity 420000 lb/hr each (saturated steam). 6” inlet x 6” outlet. (UFSAR T5.2-8)
  • PORVs: 2 per unit, Copes-Vulcan diaphragm-operated, set at 2335 psig (reset 2315 psig), rated capacity 210000 lb/hr each. 2” valve with 3” connections. (UFSAR T5.2-8)
  • PORV Block Valves: 2 per unit, Velan 3” motor-operated gate valves with Limitorque operators. (UFSAR T5.2-8)
  • All safety/relief valves discharge to the pressurizer relief tank.

RCS Pressure Drop

LegUnit 1 (psi)Unit 2 (psi)Source
Pump Discharge Leg3.13.1UFSAR T5.2-2
Across Vessel (incl. nozzles)49.349.3UFSAR T5.2-2
Hot Leg1.11.2UFSAR T5.2-2
Across Steam Generator35.834.4UFSAR T5.2-2
Pump Suction Leg2.92.9UFSAR T5.2-2
Total92.290.9UFSAR T5.2-2

RCS High Point Venting

Three principal high points: pressurizer, reactor vessel head, and steam generator tube bundle invert.

  • Pressurizer vent: PORV serves as safety-grade vent, operable from control room, meets single failure criterion.
  • Reactor vessel head vent: Dedicated ¾” Schedule 160 vent tap with 3/8” restricting orifice. Vents to PRT or containment via redundant solenoid valves. Remote-manually actuated from control room via key lock switch. Powered from two redundant vital DC buses. Designed to NUREG-0737 requirements. Vent size within LOCA definition (3/8” orifice) — inadvertent opening does not require ECCS actuation. Can vent ½ gas volume of RCS in 1 hour. (UFSAR 5.1)
  • Steam generator tube invert: Cannot be vented at that location. Westinghouse study (WCAP-9600/9601) concluded small amount of noncondensables would not significantly impact natural circulation.
Exam — 2023 Q17
During RCS depressurization without RCPs running (e.g., EOP-LOCA-5), upper head voiding IS expected. Without forced circulation, the upper head contains hotter liquid that flashes to steam during depressurization, displacing water into the pressurizer and causing a rapidly rising PZR level. Monitor PZR level to stop depressurization before going solid.

Tech Spec LCOs

  • TS 3/4.4 — Reactor Coolant System — RCS pressure, temperature, flow limits
  • TS 3/4.4 — Reactor Coolant System|TS 3/4.4.1 — Reactor Coolant Loops (Modes 1-4)
  • TS 3/4.4 — Reactor Coolant System|TS 3/4.4.2 — Safety Valves
  • TS 3/4.4 — Reactor Coolant System|TS 3/4.4.3 — PORVs and Block Valves
  • TS 3/4.4 — Reactor Coolant System|TS 3/4.4.4 — Pressurizer
  • TS 3/4.4 — Reactor Coolant System|TS 3/4.4.5 — Steam Generators
  • TS 3/4.4 — Reactor Coolant System|TS 3/4.4.6 — RCS Leakage Detection
  • TS 3/4.4 — Reactor Coolant System|TS 3/4.4.7 — RCS Leakage
  • TS 3/4.4 — Reactor Coolant System|TS 3/4.4.8 — Chemistry
  • TS 3/4.4 — Reactor Coolant System|TS 3/4.4.9 — Specific Activity
JPM — 2023 Sim-d
EOP-FRHS-1 Bleed and Feed: actuate SI, open PZR PORVs. If 2PR2 fails to open, open reactor head vent valves 2RC40 through 2RC43 (key-locked on 2RP2 backpanel) as alternate bleed path. Requires going to backpanel to insert key and turn each valve individually.

Thermal-Hydraulic Design

Function

The thermal-hydraulic design ensures adequate heat transfer between the fuel cladding and reactor coolant so the core thermal output is not limited by fuel temperature or DNB considerations. The design takes into account local variations in fuel rod dimensions, power generation, flow distribution, and mixing. (UFSAR 4.4)

Core Thermal Parameters

ParameterValueSource
Reactor Core Heat Output3459 MWtUFSAR T4.4-1
Reactor Core Heat Output, Unit 111806 x 10⁶ Btu/hrUFSAR T4.4-1
Reactor Core Heat Output, Unit 211844 x 10⁶ Btu/hrUFSAR T4.4-1
Heat Generated in Fuel97.4%UFSAR T4.4-1
Nominal System Pressure2250 psiaUFSAR T4.4-1
Minimum Steady State Pressure (STDP)2218 psiaUFSAR T4.4-1

Coolant Flow

ParameterValueSource
Total Thermal Design Flow Rate125.3 x 10⁶ lb/hrUFSAR T4.4-1
Effective Flow Rate (Unit 1)116.3 x 10⁶ lb/hrUFSAR T4.4-1
Effective Flow Rate (Unit 2)115.7 x 10⁶ lb/hrUFSAR T4.4-1
Effective Flow Area (V5H, V+)51.3 ft²UFSAR T4.4-1
Effective Flow Area (RFA)51.1 ft²UFSAR T4.4-1
Average Velocity Along Fuel Rods (V5H, V+)14.1 ft/secUFSAR T4.4-1
Average Velocity Along Fuel Rods (RFA)14.2 ft/secUFSAR T4.4-1
Average Mass Velocity (V5H, V+)2.27 x 10⁶ lb/hr-ft²UFSAR T4.4-1
Average Mass Velocity (RFA)2.28 x 10⁶ lb/hr-ft²UFSAR T4.4-1

Coolant Temperatures

ParameterValueSource
Nominal Inlet Temperature542.7°FUFSAR T4.4-1
Average Rise in Vessel70.4°FUFSAR T4.4-1
Average Rise in Core75.2°FUFSAR T4.4-1
Average in Core (Unit 1)582.4°FUFSAR T4.4-1
Average in Core (Unit 2)582.6°FUFSAR T4.4-1
Average in Vessel577.9°FUFSAR T4.4-1

Heat Transfer

ParameterValueSource
Active Heat Transfer Surface Area59700 ft²UFSAR T4.4-1
Average Heat Flux192470 Btu/hr-ft²UFSAR T4.4-1
Maximum Heat Flux (normal operation)461930 Btu/hr-ft²UFSAR T4.4-1
Average Thermal Output5.52 kW/ftUFSAR T4.4-1
Maximum Thermal Output (normal operation)13.3 kW/ftUFSAR T4.4-1
Peak Linear Power (protection setpoints)≤22.4 kW/ftUFSAR T4.4-1
Peak Fuel Center Temp (max overpower trip)<4700°FUFSAR T4.4-1
Heat Flux Hot Channel Factor (FQ)2.40UFSAR T4.4-1
Fuel Clad Outer Surface Temp (hot spot, steady state)~660°FUFSAR 4.4.2.2.5

Core Pressure Drop

ConfigurationPressure Drop (psi)Source
Full core V5H, V+22.2UFSAR T4.4-1
Full core RFA with DFBN24.7UFSAR T4.4-1
Full core RFA with SDFBN24.5UFSAR T4.4-1

Based on best estimate flow of 93300 gpm/loop (DFBN) or 94800 gpm/loop (SDFBN). (UFSAR T4.4-1)

Departure from Nucleate Boiling (DNB)

DNBR Design Limits

Fuel TypeCell TypeDNBR Design LimitCorrelationSource
V5H, V+Typical1.24 (RTDP)WRB-1UFSAR T4.4-1
V5H, V+Thimble1.24 (RTDP)WRB-1UFSAR T4.4-1
RFATypical1.24 (RTDP)WRB-2UFSAR T4.4-1
RFAThimble1.22 (RTDP)WRB-2UFSAR T4.4-1

DNBR correlation limit: 1.17 for both WRB-1 and WRB-2. (UFSAR T4.4-1)

DNBR at Normal Conditions

Fuel TypeCell TypeDNBRSource
V5H, V+Typical2.44UFSAR T4.4-1
V5H, V+Thimble2.32UFSAR T4.4-1
RFATypical2.64UFSAR T4.4-1
RFAThimble2.62UFSAR T4.4-1

Design Basis

The design basis for DNB is that there is at least a 95% probability at the 95% confidence level that the limiting fuel rod in the core does not experience DNB during Condition I and II events. This is met by ensuring the minimum DNBR remains above the design limit DNBR. (UFSAR 4.4.1.1)

RTDP (Revised Thermal Design Procedure): Plant operating parameter uncertainties (pressure, temperature, power, flow) are statistically combined into the DNBR design limit, allowing safety analyses to use nominal values. (UFSAR 4.4.1.1)

STDP (Standard Thermal Design Procedure): Used when RTDP is not applicable. Minimum steady state pressure of 2218 psia is assumed. (UFSAR T4.4-1)

Key Exam Concepts

  • DNB = transition from nucleate boiling to film boiling on fuel rod surface; results in rapid temperature increase
  • DNBR = ratio of heat flux required to cause DNB to actual local heat flux; must stay above design limit
  • The fuel design basis is that centerline fuel melt does not occur during normal operation or AOOs
  • Core average void fraction is less than 0.5% (due to local/statistical boiling only)
  • RFA fuel has higher DNBR margins than V5H fuel (better mixing from IFM grids)

Thermal-Hydraulic Tech Spec LCOs

  • TS 3/4.2.1 — Axial Flux Difference (delta-I)
  • TS 3/4.2.2 — Heat Flux Hot Channel Factor (FQ)
  • TS 3/4.2.3 — Nuclear Enthalpy Rise Hot Channel Factor (F-delta-H)
  • TS 3/4.2.5 — DNB Parameters (pressurizer pressure, RCS Tavg, RCS flow)
Exam — 2023 Q73
Steam table subcooling calculation: at 1805 psig (1820 psia), RCS Tsat = 623°F. For 100°F subcooling margin, cold leg temperature must be ≤523°F. Since SGs are saturated and negligible delta-T exists across SG tubes, SG temperature must also be ~523°F. Saturation pressure at 523°F is approximately 820 psia (805 psig). Common trap: failing to convert psia to psig (off by ~15 psi).
Exam — 2023 Q74
Condensate depression effects: decreasing condensate depression (from 5°F to 2°F) means the condensate is closer to saturation temperature. This produces less NPSH at the condensate pump suction (closer to cavitation) because there is less subcooling to prevent flashing. However, because the feedwater is hotter, the SGs must add less sensible heat to reach saturation — improving steam cycle thermal efficiency. Trap: lower condensate depression means higher efficiency but worse pump cavitation margin — effects go in opposite directions.
JPM — 2018 Sim-d
EOP-FRHS-1 Bleed and Feed (Steps 21-25): loss of all AFW, 3 SG WR levels < 32% → Bleed and Feed criteria met. Initiate SI on both trains. Verify charging/SI pumps running, BIT flow ~150-160 gpm, Table C valves open. Open both PORV stop valves (2PR6, 2PR7). Open both PORVs in Manual — 2PR2 fails to open (alternate path). At 2RP2 backpanel, use four keys to open reactor head vent valves 2RC40 through 2RC43 as alternate bleed path. Reactor Head Vents are NOT the standard bleed path — only used when a PORV fails to open.
JPM — 2022 RO-A1
EOP-FRCI-3 Attachment 1 maximum vent time calculation for reactor vessel upper head voiding: at 1600 psig RCS pressure, hydrogen flow rate from Figure 1 is 3333 cfm. Maximum containment hydrogen increase limited to keep total below 3.0%. With 2.3% current H2 and 140F containment temperature, maximum vent time = 4.5 minutes.
Exam — 2019 Q69
Subcooling calculation when subcooling monitor is NOT functional: convert psig to PSIA by adding 14.7 (rounded to 15), use steam tables to find TSAT. Subcooling = TSAT - Hottest CET (not TAVG). Two traps: (1) subtracting 15 instead of adding gives wrong TSAT, (2) using TAVG instead of Hottest CET gives wrong subcooling. Hottest CET is always the correct reference for subcooling calculation.
JPM — 2018 RO-A2
Control room log OTDT setpoint channel check: each RC loop's OTDT console reading compared against REM FIGURE 5 (A-D) setpoint band. Tavg Channel Check requires all 4 loop average temperatures within 3 degrees F (S/R 4.3.2.1.1, Modes 1-3). Loop 24 OTDT of 72 deg F found outside the REM FIGURE 5D setpoint band.

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