PWR Operator Manual · Open the simulator

05 — Mode Transition Procedures

Document: PWR-MT-01
Title: Plant MODE Transitions (Mode 1, At Power through Mode 6, Refueling)
Revision: 24

1.0 Purpose

Define commercial-style plant MODES (Mode 1, At Power … Mode 6, Refueling) for this trainer and provide procedures to move the unit between them — including the full path Mode 5, Cold Shutdown → Mode 1, At Power and Mode 1, At Power → Mode 5, Cold Shutdown.

Also covers turbine load modes (Follow / Manual / Disconnected), AUTO/MAN control, ESF arms, and NIS / trip blocks. Those are not plant MODE numbers.

Related: 04_NORMAL_OPERATIONS.md (N procedures referenced below).

2.0 Plant MODE definitions

These MODES follow the commercial PWR (Westinghouse-style Tech Spec) structure, adapted to SI units and this trainer’s physics.

2.1 Simulator initial-condition mapping

2.2 What this trainer can actually step through

THE COLD END OF THIS PLANT IS MODE 5 AGAIN. The water-property floor moved from 14.5 psi (0.1 MPa) to 0.29 psi (0.002 MPa), so a steam generator at or below Mode 5's 200 °F (93.3 °C) boundary is representable and the Mode 5 ↔ Mode 1 story is driveable end to end on the board, from the cold_shutdown initial condition at 122 °F (50 °C) — or from hot_shutdown (Mode 4) to skip the first leg.

2.3 Turbine load modes (not plant MODES)

Do not say “Mode 1, At Power” for Follow or “Mode 5, Cold Shutdown” for Disconnected.

3.0 Master path — Mode 5, Cold Shutdown to Mode 1, At Power

Procedure ID: PWR-T20
Title: Startup from Mode 5, Cold Shutdown to Mode 1, At Power
Purpose: Take the unit from cold shutdown conditions up to power operation.

Overview

Mode 5, Cold Shutdown          [sim] — cold_shutdown
        │  heatup / pressurize / draw PZR bubble
        ▼
Mode 4, Hot Shutdown           [sim]
        │  continue heatup to NOP
        ▼
Mode 3, Hot Standby           [sim]  ← Free Play starts here
        │  approach to criticality
        ▼
Mode 2, Startup                 [sim]
        │  low-power ops, turbine roll, raise power past 5 %
        ▼
Mode 1, At Power         [sim]
        │  ascend to desired power (e.g. full power)
        ▼
Mode 1, At Power at power (watchstanding)

Phase A — Mode 5, Cold Shutdown → Mode 4, Hot Shutdown → Mode 3, Hot Standby [sim — real rates] → PWR-N01

Simulator: Phase A is driveable from Mode 5 — load the cold_shutdown initial condition (Mode 5, Cold Shutdown, 122 °F (50 °C); restored 2026-08-31), or hot_shutdown to start at Mode 4 — and perform the heatup: start the RCPs (set_rcp), raise the pressurizer setpoint to draw up to NOP pressure (set_pressure_setpoint), and keep the turbine off line with the dumps shut so the SG bottles, then hand the bottle to the dumps at the top of the ride — set_steam_dump {mode:'auto'}, which with the turbine tripped selects steam-pressure control on the dump setpoint (03 §12.3). That is the whole evolution — heat crossing the tubes has no steam sink on the way up, so it goes into secondary pressure, and the plant rides up on pump heat with the control bank never leaving its cold-shutdown position. Re-measured full stack 2026-08-07 with no rod motion (real rates end to end): ~12.3 plant-hours cold to 567.0 °F (297.2 °C) — the no-load anchor — arriving at ρ = −2828 pcm on 856.8 ppm, control bank still at 0 of 912 steps (the retired engine's bank; the shipped plant is 627 steps — see the note that closes this paragraph). (This paragraph read −3377 pcm on 907 ppm until then, a figure that predates the second moderator re-fit; PWR-N01's own acceptance had the right numbers and this one did not.) It ends less subcritical than the −6287 pcm recorded in older revisions because the old model charged a moderator defect over three times too large on the way up. The thermal ride barely moved across the re-clocks — 545 °F (285.0 °C) arrives at ~11.3 plant-hours and the steady rate is 30.0 °F/hr (16.7 °C/hr) — because pump heat depends on neither the moderator coefficient nor the pressurizer clock. The endpoint reads 567.0 °F rather than the older 548 °F because 567 °F is where the retired engine's Tavg-mode dump program opened and Tavg stopped, and this run was carried to that settling point; it is a measurement-window difference, not a 19 °F physics gain. On the shipped plant that endpoint is a different number and it belongs to a different valve — see the note that closes this paragraph. Or skip Phase A by loading Hot Standby (hot_zero_power) = Mode 3, Hot Standby. (The plant-hour and ρ/ppm figures in this paragraph were measured on the RETIRED engine. On the shipped plant — pwr2 walkthrough replay, 2026-08-31 — the pump-heat class runs ~87–114 °F/hr, arrival is at cold-shutdown boron ~918 ppm / ρ ≈ −3,418 pcm with a 627-step bank, the pressurization is STAGED around the P-11 permissive (complete to 2235 psi only with the secondary hot, or a standing low-steam-pressure SI signal sheds the heaters at the crossing), and the startup walkthrough begins with the dilution 918 → 719 ppm — criticality then sits at 223 of 627 steps (36 % withdrawn), re-measured 2026-09-03. The heat sink at the top of the ride is an operator action, not a given: the cold plant boots with the dump controller out of service, and pressing STEAM DUMP AUTO — which with the turbine tripped selects steam-pressure mode — parks the plant at 547.2 °F (286.2 °C) / 1005 psig with the condenser dumps carrying 0.4–2.9 %. Without it the secondary climbs past the anchor and the atmospheric dump valve relieves at 1042 psig, parking the plant at 551.6 °F (288.7 °C), 4.4 °F (2.4 °C) above the no-load band and venting to atmosphere for the rest of the heatup. Neither figure is the retired engine's 567 °F. The live walkthroughs are the measured authority; see PWR-N01/PWR-T03.)

The approach to criticality is not part of this phase — it is Phase C, and in the campaign it lives in pwr_startup_challenge and pwr_return_to_mode1. Heating a plant to Hot Standby on fission is what the simulator used to do, because the steam generator netted pump heat out of its own steam balance and a pump-heat heatup stalled at 218.69 °F (103.72 °C); that was a modelling fudge and it is gone.

Phase B — Mode 3, Hot Standby lineup [sim] → PWR-N02

Acceptance: Board is Mode 3, Hot Standby (Hot Standby).

Phase C — Mode 3, Hot Standby → Mode 2, Startup [sim] → PWR-N03

Acceptance: Critical; power ≤ 5 %; SUR controlled → Mode 2, Startup.

Phase D — Mode 2, Startup → Mode 1, At Power [sim] → PWR-N04, N05, N06

Acceptance: Critical, power > 5 %, turbine supporting load as intended → Mode 1, At Power.

Phase E — Mode 1, At Power at full power (optional)

4.0 Master path — Mode 1, At Power to Mode 5, Cold Shutdown

Procedure ID: PWR-T21
Title: Shutdown from Mode 1, At Power to Mode 5, Cold Shutdown
Purpose: Take the unit from power operation down to cold shutdown conditions.

Overview

Mode 1, At Power         [sim]
        │  reduce power / off-load turbine
        ▼
Mode 2, Startup                 [sim]  (optional plateau ≤ 5 %)
        │  SCRAM or full insertion
        ▼
Mode 3, Hot Standby           [sim]
        │  decay heat removal; begin cooldown story
        ▼
Mode 4, Hot Shutdown           [sim]
        │  continue cooldown / depressurize; RHR
        ▼
Mode 5, Cold Shutdown          [sim]  ← genuine cold end state

Phase A — Mode 1, At Power → Mode 2, Startup / Mode 3, Hot Standby [sim]

Acceptance: Reactor shut down; RCS still hot → Mode 3, Hot Standby.

Phase B — Mode 3, Hot Standby hold [sim]

This is the deepest fully simulated shutdown state.

Phase C — Mode 3, Hot Standby → Mode 4, Hot Shutdown → Mode 5, Cold Shutdown [sim, rate-compressed]

C3 is not optional. The accumulators are passive: nitrogen behind a check valve. Nothing automatic shuts them, and the SI block set at the start of the cooldown blocks pumps, not these tanks. Carry the discharge isolation valve open past 665 psi (4.585 MPa) and all four dump into the RCS — the plant arrives at Mode 5 water-solid, with the accumulators empty and RCS boron dragged up toward the 2500 ppm tank charge. Watch the SIT fill and cover-gas indications on the ECCS side of the board. Isolating at 1000 psig leaves 400 psi (2.76 MPa) of margin. Real plants isolate here too, under administrative control rather than on an automatic signal (NUREG-1431 Rev 4.0 LCO 3.5.1, applicable in *"MODE 3 with RCS pressure

[1000] psig"*; LTOP SR 3.4.12.3 "Verify each accumulator is isolated."). **Re-align them

on the way back up** — Phase A step A5.

Simulator: This is driveable to the Mode 5 end state — the cold plant on RHR — and there is a walkthrough for it: Main Menu → Walkthroughs → Shutdown Part 3, which reproduces N15's performance table. Borate for shutdown margin, block SI before the pressure setpoint moves (the depressurization crosses the 1798 psi (12.4 MPa) SI actuation setpoint; measured with SI armed the pumps inject, the pressurizer goes solid and the plant trips), and block BOTH low-pressure reactor trips — the 1800 psi (12.41 MPa) low-pressure trip and the reactor-trip-on-SI are two separate entries on the same channel, and HPI/LPI OFF blocks neither (measured: the plant scrams in the first cooling leg with either one left armed). Then — with the dumps in steam-pressure mode, which is what AUTO selects on a tripped turbine and the only mode in which the setpoint is read at all (03 §12.3) — lower the steam-dump setpoint (set_steam_dump_setpoint) to cool the secondary and with it the primary — one entry of 120 psi (0.83 MPa); the controller paces the walk at 60 °F/hr (33.3 °C/hr) (03 §12.3, declared departure 2026-09-28), depressurize in step (set_pressure_setpoint, spray) keeping subcooling positive, isolate the accumulators at 1000 psi (6.895 MPa) (discharge valve on the board's ECCS side, close_accumulator_valve), place RHR On below the 425 psig (440 psi / 3.03 MPa) block-open permissive, and secure the RCPs (set_rcp) so RHR draws the RCS to cold. Once RHR carries the cooldown, set its pace with RHR Cooldown Rate (HX flow split) (set_rhr_hx) — walk it up to hold the ~90 °F/h (50 °C/h) cooldown limit rather than opening full-through-the-exchanger on a hot plant. The cold end state matches the cold_shutdown IC. Cooldown rate is time-compressed. PWR-N15 is the companion procedure.

5.0 Individual MODE transition procedures

PWR-T01 — Enter Free Play in Mode 3, Hot Standby

PWR-T02 — Enter Free Play in Mode 1, At Power

PWR-T03 — Mode 3, Hot Standby → Mode 1, At Power (sim path)

Abort: Uncontrolled SUR → insert rods; any RPS trip → PWR-T06 (remain/return Mode 3, Hot Standby).

PWR-T04 — Mode 1, At Power → Mode 3, Hot Standby (controlled)

PWR-T05 — Mode 1, At Power power increase (stay in Mode 1, At Power)

Raise power within Mode 1, At Power (e.g. 50 % → 100 %). See PWR-N06 / N07. MODE number does not change.

PWR-T06 — Mode 1, At Power (or Two) → Mode 3, Hot Standby by trip

Simulator: there is a runnable walkthrough for this — pwr_post_trip, the one you run on the board. Measured full stack from hot_full_power (scram at t = 60 s): the reset is refused at t+1 s with power still at 33 %, and accepted at t+3 s once the rods seat; Mode 3 is reached inside a minute; main feedwater isolates and AFW auto-starts by about t+3 min, holding SG level near 37 % after it falls from 65 %; the plant settles hot and subcritical at 547.2 °F (286.2 °C) and 2235 psi (15.41 MPa). **Step 2 did not exist in this procedure until 2026-08-03** — authoring the walkthrough found it missing, and reset_rps had been reachable on the board since #75 while being named by no procedure, mission or walkthrough at all.

PWR-T20 — Mode 5, Cold Shutdown → Mode 1, At Power

See §3.0 (full master path).

PWR-T21 — Mode 1, At Power → Mode 5, Cold Shutdown

See §4.0 (full master path).

PWR-T22 — Mode 2, Startup → Mode 3, Hot Standby (reject startup)

PWR-T23 — Mode 3, Hot Standby → Mode 2, Startup only (criticality practice)

6.0 Turbine load mode transitions (at Mode 1, At Power)

These change grid/turbine state, not plant MODE (you remain Mode 1, At Power if still critical > 5 %).

PWR-T07 — Follow → Manual

1. Select Manual on Turbine-Generator.
2. Set MWe near current output, then ramp slowly.
3. Match reactor power; keep feed AUTO.

CAUTION: Large step cuts can trip the plant back toward Mode 3, Hot Standby.

PWR-T08 — Manual → Follow

1. Stabilize power.
2. Select Follow.
3. Confirm load tracks reactor power.

PWR-T09 — → Disconnected

1. Prefer power reduction first if controlled.
2. Disconnect / turbine trip / or SCRAM.
3. If SCRAM: plant → Mode 3, Hot Standby; load Disconnected.

7.0 Control and ESF mode transitions

PWR-T10 — Normal AUTO suite in Mode 1, At Power

PWR-T11 — Single channel AUTO → MANUAL

Operate underlying control or select MAN; re-engage AUTO when stable.

PWR-T12 — ESF MANUAL → re-arm AUTO

Press AUTO on Emergency card after manual intervention. Standing start conditions may fire immediately.

PWR-T13 — SR → IR handoff (Mode 3, Hot Standby / Mode 2, Startup startup)

IR ≥ 1e-10 A (P-6) → confirm INTER RANGE is reading, then block SR HIGH FLUX on the Trip Blocks panel: it blocks the source-range trip and switches the detector off, and the intermediate range becomes the instrument to read. Miss it and the source range trips the reactor at 1e5 cps. On the way down the block clears itself below 5e-11 A and the detector comes back on.

PWR-T14 — Startup trip blocks (entering Mode 1, At Power)

Above P-10 (8 %) — already Mode 1, At Power — block IR high (25 %) first, then PR low setpoint (35 %). Two separate presses; the IR one also clears the 20 % rod stop. Both auto-reinstate below P-10.

Take them with power comfortably above 10 %, not at 8 %. The permissive is graded on the indicated power-range channel, which carries about ± 0.3 % with no 2-of-4 coincidence behind it, so a block taken just over 8 % is revoked within seconds: measured, one press survives 1 s at 8.19 % power, 4 s at 8.72 %, 105 s at 9.10 %, and indefinitely from 9.36 %. Below the permissive the BLOCK control does not accept the press at all; above it the press is always accepted and it is the block that does not hold.

PWR-T15 — Learning ↔ Realistic display

Training display only; does not change plant MODE.

8.0 Transition map

 Mode 6, Refueling     OUT OF SCOPE
        │
 Mode 5, Cold Shutdown ──────── [sim] heatup ─────────┐
        ▲                                                 │
        │ [sim] cooldown                                  ▼
 Mode 4, Hot Shutdown ◄─────────────────────── Mode 3, Hot Standby  [sim]
        ▲                                                 │
        │                                                 │ N02 [sim]
        │                                                 ▼
        │                                          Mode 2, Startup  [sim]
        │                                                 │
        │ N14 / trip                                      │ N04–N06 [sim]
        │                                                 ▼
        └─────────────── Mode 1, At Power  [sim] ── full power

Operator goal paths:

9.0 Related documents

• 04_NORMAL_OPERATIONS.md — N01–N15 aligned to MODES
• 01_GENERAL_DESCRIPTION.md — MODE table
• 09_SETPOINTS_LIMITS.md — Mode 1, At Power / Mode 3, Hot Standby normals
• 10_GLOSSARY.md — MODE definitions

These manuals are licensed CC BY 4.0 — see Legal. Training documents for an educational simulator, not licensing-basis documents for a real plant.