SALEM
SALEM STUDY SYSTEM
Systems > Control Rod Drive

Control Rod Drive

⚠️ DRAFT

Control Rod Drive

Function

The Control Rod Drive Mechanism (CRDM) provides the means for inserting, withdrawing, and holding control rod assemblies. Rod motion is controlled by energizing and de-energizing the lift coil, moveable gripper coil, and stationary gripper coil in a programmed sequence. Rod control interlocks (rod stops) prevent unsafe rod withdrawal conditions. (UFSAR 7.7)

Rod Control Interlocks (Rod Stops)

These interlocks are duplicated here from RPS/SSPS (the master reference) for study convenience.

DesignationDerivationFunctionSource
C-11/2 intermediate range above setpointBlocks auto and manual rod withdrawalUFSAR T7.2-2
C-21/4 power range above setpointBlocks auto and manual rod withdrawalUFSAR T7.2-2
C-32/4 OT Delta-T above setpointBlocks rod withdrawal; actuates turbine runbackUFSAR T7.2-2
C-42/4 OP Delta-T above setpointBlocks rod withdrawal; starts turbine runbackUFSAR T7.2-2
C-51/1 turbine pressure below setpointBlocks automatic rod withdrawalUFSAR T7.2-2

Note: Automatic rod withdrawal is disabled at Salem. (UFSAR T7.2-2)

Rod Bank Selector Switch Speeds

Exam — 2018 Q30
Rod speeds by ROD BANK SELECTOR SWITCH position:
• AUTO: 8 SPM (minimum) to 72 SPM (maximum) — speed varies linearly with temperature error
• SBA, SBB, SBC, SBD (Shutdown Banks): 64 SPM
• CBA, CBB, CBC, CBD (Control Banks): 48 SPM
Trap: 72 SPM is the maximum speed in AUTO only — Control Bank positions (CBA) move at 48 SPM, NOT 72. Shutdown Bank positions (SBA) move at 64 SPM, NOT 48.

Automatic Rod Control Speed

Exam — 2019 Q29
With rods in AUTO, control rod speed varies based on total temperature error magnitude:
• 1.5 °F to 3 °F error: rods move at 8 SPM (minimum)
• 3 °F to 5 °F error: rods move from 8 SPM to 72 SPM (linearly) — rate of increase is 32 SPM/°F
• Above 5 °F error: rods move at 72 SPM (maximum)
Calculation: with a temperature error of 4 °F, rod speed = 8 + (32 x (4 - 3)) = 40 SPM. Trap: 72 SPM is the maximum (at 5 °F), and 8 SPM is the minimum (at 3 °F). The speed is LINEAR between these points, not a step function.
Exam — 2023 Q55
Urgent Failure alarm prevents ALL rod motion in both Manual and Automatic. When Urgent Failure is present, rods are held in position by the stationary gripper coil energized at a reduced current. The moveable gripper coil is de-energized. Rods cannot be inserted even in Manual. (Ref: S2.OP-AR.ZZ-0012)
JPM — 2023 Sim-g
Failed high Tavg channel (AB.ROD-0003): recognize unwarranted rod insertion, take rods to manual. Adjust rods to control Tavg within 1.5F of Tprogram. Stabilize PZR level (Master Flow Controller to manual). Defeat BOTH Differential Temperature AND Average Temperature deviation alarms on 2CC2. Select valid recorder channel (2, 3, or 4) for both parameters.
JPM — 2023 IP-j
Rod Drive M-G Set paralleling (S1.OP-SO.RCS-0001): incoming set voltage adjusted 0-2.5V higher than running set via DMM (not installed voltmeter). Move single SYNCHRONIZE handle from running to incoming set, place in ON — generator breaker auto-closes. Do NOT manually close generator breaker.
Exam — 2022 Q56
OHA E-8 (ROD INSERT LMT LO) alarms when rods are 10 steps from the Rod Insertion Limit. The COLR Rod Insertion Limit has NOT been exceeded when E-8 alarms — it is a warning. Per the alarm response procedure, the crew initiates a normal boration per S2.OP-SO.CVC-0006 (Boron Concentration Control) to restore rods to their normal band. Emergency boration (Rapid Borate) was previously required for OHA E-16 (ROD INSERT LO-LO) but this requirement has been removed from the OHA alarm procedures.
Exam — 2022 Q92
Misaligned rod response per AB.ROD-0001: with one rod misaligned >=12 steps from the group demand counter and reactor power >85% RTP, per TS 3.1.3.1 action c.3.d: reduce power to <75% RTP. The misaligned rod is aligned to the BANK position (not the other way around). You do NOT insert the bank to match the misaligned rod. If two or more rods are misaligned, the action is Hot Standby within 6 hours.
Exam — 2020 Q19
Dropped rod recovery (AB.ROD-0002): before withdrawing the dropped rod, reset the group step counter to zero so it matches actual rod position and the rod is withdrawn to the proper height. For Control Bank D Group 1 rods, also reset the P/A converter to zero locally at the RPI-2 cabinet — this ensures bank overlap is maintained. Group step counter does NOT input into the P/A converter; P/A input is from the Group 1 Data Logging card. After P/A converter reset: OHAs E-8 (RIL LO) and E-16 (RIL LO-LO) WILL annunciate (expected alarm). OHA E-40 (ROD BANK URGENT FAILURE) annunciates after rod withdrawal begins.
Exam — 2020 Q20
Misaligned rods (AB.ROD-0001): more than one rod stuck/misaligned = place unit in Hot Standby. Only one rod misaligned = reduce power to < 75% RTP. Do not confuse Hot Standby (>1 rod) with Hot Shutdown or power reduction (1 rod).
Exam — 2020 Q56
Rod Drive MG Set power supplies: 11 MG set is powered from the 1E 460V bus; 12 MG set is powered from the 1G 460V bus. Loss of 1E AND 1F 460V buses does NOT trip the reactor because one MG set (12, on 1G bus) is sufficient to maintain power to the Rod Control System. Trap: 1E and 1F buses do NOT each power one MG set — both MG sets are NOT lost on loss of 1E + 1F.
Exam — 2020 Q68
Manual rod movement communication (OP-AP-300-1001 step 4.4.3): during non-transient conditions, the RO shall STATE the selected control rod bank initial position, target control rod position, and the direction of movement. Minimum info is bank/target/direction — NOT TAVG, NOT NIS power, NOT number of steps.
JPM — 2020 RO-A1
SDM calculation with inoperable rod (SC.RE-ST.ZZ-0002 Att. 3): rod bank penalty (step 4.2.6) is based on the position of the rod BANK, not the individual misaligned rod. With all banks at ARO (225 steps), penalty = 0 pcm even though rod 1D5 is at 214 steps. Step 4.1.5 = 0 untrippable rods (rod is trippable). Calculated SDM = -2398 pcm vs acceptance of -1300 pcm — SAT. Time critical JPM (60-minute limit per TSAS 3.1.3.1 Action c.3).
JPM — 2020 SRO-A1
SRO review of SDM calculation with two untrippable rods (1D4 and 1D2): key error was treating rods as dropped/misaligned rather than untrippable — different penalty section (4.2.4 vs 4.2.5, 4330 pcm) and different rod worth formula ("Trippable Rod Worth with Untrippable RCCA(s)" = -2327.8 pcm). Correct SDM = -269.5 pcm (UNSAT) — requires power reduction to ~19% RTP. Control Bank D at 200 steps (not ARO).
JPM — 2020 Sim-c
Rod control surveillance (S2.OP-ST.RCS-0001): insert Control Bank D 15 steps, verify each rod moved at least 10 steps on P-250, record SAT, restore to ARO. Alternate path: when rod bank selector switch placed in AUTO at Step 5.1.11.C, uncontrolled rod insertion occurs. Rods continue inserting in both AUTO and MANUAL. Operator must recognize continuous insertion and manually trip the reactor.
Exam — 2019 Q67
AB.ROD-0003 (Continuous Rod Motion) entry conditions: rods withdraw OR insert a MINIMUM of 3 steps at steady state. Process noise can cause up to 2 steps of rod motion — this is normal and does NOT warrant AB.ROD-0003 entry.

Reactivity Management — Power Defect and Rod Worth

Exam — 2018 Q41
Using the Curve Book (S2-RE-RA.ZZ-0016), for a power reduction from 100% to 80% at 8000 EFPH and 900 ppm boron: power defect adds (+) 396 pcm to core reactivity. If boration absorbs (-) 216 pcm, Control Bank D must absorb the remaining (-) 180 pcm by inserting from 228 steps to approximately 186 steps (IAW Figure 2C, Integral Rod Worth). Trap: requires reading multiple curves/tables — power defect from Figure 17A or Table 2-1, then rod worth from Figure 2C. Errors in any step give different (wrong) rod positions.

CRDM Vent Fans

JPM — 2018 Sim-a / 2019 Sim-a
Loss of all CRD Vent Fans (alternate path JPM, tested both 2018 and 2019): 21 CRD Vent Fan damper fails causing AIR FLOW LO alarm and SEQUENCE COMPLETE bezel extinguished on 2CC1 console. ARP S2.OP-AR.ZZ-0011 page 13 directs swapping to standby fan. After standby fan starts, both remaining running fans trip within seconds. With NO CRDM Vent Fans in operation, ARP step 3.2.D directs TRIP the Reactor. Operating with less than two Rod Drive Vent Fans could degrade the Rod Drive Coils. CRD Vent Fan Outlet Temperature alarm points: 21 T2602A/160F, 22 T2603A/160F, 23 T2604A/160F. Step 3.2.D was initially marked N/A when the standby started — it becomes applicable when all fans trip. The operator must re-evaluate and trip the reactor. S2.OP-SO.CBV-0001 provides direction for starting/stopping CRDM Vent Fans but provides NO direction for a loss of all fans — the ARP is the governing procedure.

Mode 2 Entry and Rod Withdrawal

Exam — 2018 Q39
Per S2.OP-IO.ZZ-0003 (Hot Standby To Minimum Load), MODE 2 entry is recorded in the Control Room Narrative Log when withdrawal of Control Bank "A" is imminent. Mode 2 entry is NOT when the reactor is critical, NOT when Shutdown Bank A withdrawal begins (Shutdown Banks are withdrawn first but remain in Mode 3), and NOT when IR power stabilizes at 2E-3% (that is for ICRR data collection).

Tech Spec LCOs

JPM — 2019 IP-j
Startup and parallel 11 RDMG set per S1.OP-SO.RCS-0001 Section 5.4 (same task as 2023 JPM IP-j using Section 4.4 — procedure was revised between exams). Key values: running set GENERATOR LINE VOLTS 260V (247V-273V), GENERATOR LINE AMPS ~80 DC AMPS. VOLTAGE ADJUST range check: ~220V (LOW) to ~300V (HIGH). DMM readings at back of installed voltmeter are approximately half of meter face readings (~130V). Must adjust incoming set voltage 0-2.5V higher than running set on DMM before paralleling.
Scenario — 2018 #2
22 RC Loop Tavg Channel fails high at 40% power causing rods to step in (automatic control sees Tavg higher than Tref). RO places rod control in Manual, stopping rod motion. OHA E-8 (Rod Insertion Lo) and E-16 (Rod Insertion Lo-Lo). CRS enters AB.ROD-0003. RO adjusts rods to maintain Tavg within 1.5 deg of program. After defeating failed channel and restoring rods to ARO, RO returns rod control to Auto.

Connections